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Adrenomedullin (AM), a peptide isolated from an extract of human pheochromocytoma, comprises 52 amino acids with an intramolecular disulfide bond and amidation at the carboxy-terminus. AM is present in various tissues and organs in rodents and humans, including the heart. The peptide concentration increases with cardiac hypertrophy, acute myocardial infarction, and overt heart failure in the plasma and the myocardium. The principal function of AM in the cardiovascular system is the regulation of the vascular tone by vasodilation and natriuresis via cyclic adenosine monophosphate-dependent or -independent mechanism. In addition, AM may possess unique properties that inhibit aldosterone secretion, oxidative stress, apoptosis, and stimulation of angiogenesis, resulting in the protection of the structure and function of the heart. The AM receptor comprises a complex between calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein (RAMP) 2 or 3, and the AM-CLR/RAMP2 system is essential for heart development during embryogenesis. Small-scale clinical trials have proven the efficacy and safety of recombinant AM peptide therapy for heart failure. Gene delivery and a modified AM peptide that prolongs the half-life of the native peptide could be an innovative method to improve the efficacy and benefit of AM in clinical settings. In this review, we focus on the pathophysiological roles of AM and its receptor system in the heart and describe the advances in AM and proAM-derived peptides as diagnostic biomarkers as well as the therapeutic application of AM and modified AM for cardioprotection.

Increased blood pressure variability has been shown to be associated with cardiovascular morbidity and mortality. Recently we reported that continuous infusion of angiotensin II not only elevated blood pressure level, but also increased blood pressure variability in a manner assumed to be independent of blood pressure elevation in rats. In the present study, the effects of the angiotensin type I receptor blocker losartan and the calcium channel blocker azelnidipine on angiotensin II-induced blood pressure variability were examined and compared with that of the vasodilator hydralazine in rats. Nine-week-old male Wistar rats were subcutaneously infused with 240 pmol/kg/min angiotensin II for two weeks without or with oral administration of losartan, azelnidipine, or hydralazine. Blood pressure variability was evaluated using a coefficient of variation of blood pressure recorded every 15min under an unrestrained condition via an abdominal aortic catheter by a radiotelemetry system. Treatment with losartan suppressed both blood pressure elevation and augmentation of systolic blood pressure variability in rats infused with angiotensin II at 7 and 14 days. Azelnidipine also inhibited angiotensin II-induced blood pressure elevation and augmentation of blood pressure variability; meanwhile, hydralazine attenuated the pressor effect of angiotensin II, but had no effect on blood pressure variability. In conclusion, angiotensin II augmented blood pressure variability in an angiotensin type 1 receptor-dependent manner, and azelnidipine suppressed angiotensin II-induced augmentation of blood pressure variability, an effect mediated by the mechanism independent of the blood pressure-lowering action.

Adrenomedullin (AM) is a potent hypotensive peptide that exerts a powerful variety of protective effects against multiorgan damage through the AM type 1 receptor (AM1 receptor), which consists of the calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 2 (RAMP2). Two β-arrestin (β-arr) isoforms, β-arr-1 and β-arr-2, play a central role in the agonist-induced internalization of many receptors for receptor resensitization. Notably, β-arr-biased agonists are now being tested in phase II clinical trials, targeting acute pain and acute heart failure. Here, we examined the effects of β-arr-1 and β-arr-2 on human AM1 receptor internalization. We constructed a V5-tagged chimera in which the cytoplasmic C-terminal tail (C-tail) of CLR was replaced with that of the β2-adrenergic receptor (β2-AR), and it was transiently transfected into HEK-293 cells that stably expressed RAMP2. The cell-surface expression and internalization of the wild-type or chimeric receptor were quantified by flow cytometric analysis. The [125I]AM binding and the AM-induced cAMP production of these receptors were also determined. Surprisingly, the coexpression of β-arr-1 or -2 resulted in significant decreases in AM1 receptor internalization without affecting AM binding and signaling prior to receptor internalization. Dominant-negative (DN) β-arr-1 or -2 also significantly decreased AM-induced AM1 receptor internalization. In contrast, the AM-induced internalization of the chimeric AM1 receptor was markedly augmented by the cotransfection of β-arr-1 or -2 and significantly reduced by the coexpression of DN-β-arr-1 or -2. These results were consistent with those seen for β2-AR. Thus, both β-arrs negatively control AM1 receptor internalization, which depends on the C-tail of CLR.

Augmented blood pressure (BP) variability over various time periods has been recognized as a risk factor for cardiovascular diseases. Both atrial and B-type natriuretic peptides (ANP and BNP) are secreted in response to volume or pressure overload to the heart, exerting natriuretic and vasodilator actions. In this study, we examined the relationships between year-by-year BP variability and plasma levels of ANP and BNP in the general population. Study subjects were local residents receiving an annual heath checkup, who had an estimated glomerular filtration rate of30 ml min

We introduce a simple and universal cloning plasmid system for gene expression in prokaryotic (Escherichia coli) and mammalian cells. This novel system has two expression modes: the (subcloning) prokaryotic and mammalian modes. This system streamlines the process of producing mammalian gene expression plasmids with desired genes. The plasmid (prokaryotic mode) has an efficient selection system for DNA insertion, multiple component genes with rare restriction sites at both ends (termed "units"), and a simple transformation to mammalian expression mode utilizing rare restriction enzymes and re-ligation (deletion step). The new plasmid contains the lac promoter and operator followed by a blunt-end EcoRV recognition site, and a DNA topoisomerase II toxin-originated gene for effective selection with isopropyl-β-D-thiogalactoside (IPTG) induction. This system is highly efficient for the subcloning of blunt-end fragments, including PCR products. After the insertion of the desired gene, protein encoded by the desired gene can be detected in E. coli with IPTG induction. Then, the lac promoter and operator are readily deleted with 8-nucleotide rare-cutter blunt-end enzymes (deletion step). Following re-ligation and transformation, the plasmid is ready for mammalian expression analysis (mammalian mode). This idea (conversion from prokaryotic to mammalian mode) can be widely adapted. The pgMAX system overwhelmingly simplifies prokaryotic and mammalian gene expression analyses.

Receptor activity-modifying protein 2 (RAMP2) enables the calcitonin receptor-like receptor (CLR, a family B GPCR) to form the type 1 adrenomedullin receptor (AM1 receptor). Here, we investigated the effects of the five non-visual GPCR kinases (GRKs 2 through 6) on the cell surface expression of the human (h)AM1 receptor by cotransfecting each of these GRKs into HEK-293 cells that stably expressed hRAMP2. Flow cytometric analysis revealed that when coexpressed with GRK4 or GRK5, the cell surface expression of the AM1 receptor was markedly decreased prior to stimulation with AM, thereby attenuating both the specific [(125)I]AM binding and AM-induced cAMP production. These inhibitory effects of both GRKs were abolished by the replacement of the cytoplasmic C-terminal tail (C-tail) of CLR with that of the calcitonin receptor (a family B GPCR) or β2-adrenergic receptor (a family A GPCR). Among the sequentially truncated CLR C-tail mutants, those lacking the five residues 449-453 (Ser-Phe-Ser-Asn-Ser) abolished the inhibition of the cell surface expression of CLR via the overexpression of GRK4 or GRK5. Thus, we provided new insight into the function of GRKs in agonist-unstimulated GPCR trafficking using a recombinant AM1 receptor and further determined the region of the CLR C-tail responsible for this GRK function.

The calcitonin (CT)/CT gene-related peptide (CGRP) family is conserved in vertebrates. The activities of this peptide family are regulated by a combination of two receptors, namely the calcitonin receptor (CTR) and the CTR-like receptor (CLR), and three receptor activity-modifying proteins (RAMPs). Furthermore, RAMPs act as escort proteins by translocating CLR to the cell membrane. Recently, CT/CGRP family peptides have been identified or inferred in several invertebrates. However, the molecular characteristics and relevant functions of the CTR/CLR and RAMPs in invertebrates remain unclear. In this study, we identified three CT/CGRP family peptides (Bf-CTFPs), one CTR/CLR-like receptor (Bf-CTFP-R), and three RAMP-like proteins (Bf-RAMP-LPs) in the basal chordate amphioxus (Branchiostoma floridae). The Bf-CTFPs were shown to possess an N-terminal circular region typical of the CT/CGRP family and a C-terminal Pro-NH2. The Bf-CTFP genes were expressed in the central nervous system and in endocrine cells of the midgut, indicating that Bf-CTFPs serve as brain and/or gut peptides. Cell surface expression of the Bf-CTFP-R was enhanced by co-expression with each Bf-RAMP-LP. Furthermore, Bf-CTFPs activated Bf-CTFP-R·Bf-RAMP-LP complexes, resulting in cAMP accumulation. These results confirmed that Bf-RAMP-LPs, like vertebrate RAMPs, are prerequisites for the function and translocation of the Bf-CTFP-R. The relative potencies of the three peptides at each receptor were similar. Bf-CTFP2 was a potent ligand at all receptors in cAMP assays. Bf-RAMP-LP effects on ligand potency order were distinct to vertebrate CGRP/adrenomedullin/amylin receptors. To the best of our knowledge, this is the first molecular and functional characterization of an authentic invertebrate CT/CGRP family receptor and RAMPs.

Background Augmented blood pressure (BP) variability is associated with cardiovascular diseases in some clinical conditions including hypertension. Drugs that effectively reduce BP variability need to be identified, while few animal models are currently available to study BP variability. Here, we report that hypertension induced by continuous infusion of angiotensin II (Ang II) was accompanied by increased BP variability in rats. Methods Ang II was subcutaneously infused at a rate of 240 pmol/kg/min into male Wistar rats undergoing intraperitoneal implantation of a transmitter connected to an abdominal aortic catheter. BP was continuously monitored via a telemetry system before and after the Ang II infusion in a conscious, unrestrained condition. BP variability was evaluated by coefficient of variation (CV) of BP levels measured every 15 minutes. In addition, spontaneously hypertensive and Wistar-Kyoto rats (SHR and WKY) were subjected to the BP monitoring experiment at 15 weeks of age. Results Both systolic and diastolic BP levels were significantly elevated following the Ang II infusion. Similarly, CVs of systolic and diastolic BP in the Ang II infusion group were significantly higher than in the vehicle group upon 1 and 2 weeks of the infusion. Meanwhile, CVs of systolic and diastolic BP of SHR were in a range similar to those of WKY despite significantly higher BP than in WKY. Conclusions Hypertension induced by the continuous infusion of Ang II was accompanied by augmented BP variability in rats, an effect assumed to be at least in part, independent of BP elevation.

The histamine system is involved in the regulation of the autonomic nervous system. We used gene-targeted mice to investigate the role of histamine receptors in the regulation of the sympathetic nervous system. Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed histamine H1, H2, and H3 receptor expression in the superior cervical ganglion, which contains sympathetic nerve cell bodies. We measured the heart rate variability (HRV), the changes in the beat-to-beat heart rate, which is widely used to assess autonomic activity in the heart. H1 blockade attenuated the baroreflex-mediated changes in heart rate in wild-type (WT) mice, whereas the heart rate response to H2- and H3-specific blockers was unaffected. l-Histidine decarboxylase (HDC) expression in the superior cervical ganglion of H1R-null mice was higher than that in WT controls, whereas the enzyme levels in H2R- and H3R-null mice were not significantly different from those in the WT. All mutant mice (H1R-, H2R-, and H3R-null mice) showed normal electrocardiogram (ECG) patterns with little modification in ECG parameters and the expected response to the β-adrenergic blocker propranolol. Similar to our findings in WT mice, H1 blockade attenuated the baroreflex-mediated heart rate change in H1R-null mice, whereas the heart rate response was unaffected in H2R- and H3R-null mice. The HRV analysis revealed relatively unstable RR intervals, an increased standard deviation of the interbeat interval (SDNN), and low-frequency (LF) component in H1R-null mice compared with the other groups, suggesting that sympathetic nerve activity was altered in H1R-null mice. Taken together, our findings indicate that H1 receptors play a major role in the regulation of sympathetic nerve activity.

The renin-angiotensin system (RAS), including angiotensin II (Ang II), plays an important role in the regulation of blood pressure and body fluid balance. Consequently, the RAS has emerged as a key target for treatment of kidney and cardiovascular disease. In a search for bioactive peptides using an antibody against the N-terminal portion of Ang II, we identified and characterized a novel angiotensin-related peptide from human urine as a major molecular form. We named the peptide Big angiotensin-25 (Bang-25) because it consists of 25 amino acids with a glycosyl chain and added cysteine. Bang-25 is rapidly cleaved by chymase to Ang II, but is resistant to cleavage by renin. The peptide is abundant in human urine and is present in a wide range of organs and tissues. In particular, immunostaining of Bang-25 in the kidney is specifically localized to podocytes. Although the physiological function of Bang-25 remains uncertain, our findings suggest it is processed from angiotensinogen and may represent an alternative, renin-independent path for Ang II synthesis in tissue.

At least one of three receptor activity-modifying proteins (RAMP1, RAMP2 and RAMP3) can interact with 10 G protein-coupled receptors (GPCRs; nine Family B GPCRs and a Family C GPCR). All three RAMPs interact with the calcitonin (CT) receptor (CTR), the CTR-like receptor (CLR), the vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating polypeptide (PACAP) 1 (VPAC1) and the VPAC2 receptor, which are all Family B GPCRs. Three RAMPs enable CTR to function as three heterodimeric receptors for amylin, which is a feeding suppression peptide. These RAMPs also transport the CLR to the cell surface, where they function as a CT gene-related peptide (CGRP) receptor (CLR/RAMP1 heterodimer) and two adrenomedullin (AM) receptors (CLR/RAMP2 and CLR/RAMP3 heterodimers). CGRP and AM are potent hypotensive peptides that exert powerful protective effects against multi-organ damage. We recently reported that the third extracellular loop (ECL3) of CLR governs the activation of AM, but not CGRP, signaling in the three CLR/RAMP heterodimers. Furthermore, we showed that in the presence of RAMP2, the eighth helix (helix 8) in the proximal portion of the cytoplasmic C-terminal tail of the CLR, which is thought to be present in all family B GPCRs, participates in receptor signaling. In addition, we demonstrated that overexpression of GPCR kinase (GRK) 2, GRK3 and GRK4 enhances the AM-induced internalization of the CLR/RAMP2 heterodimer. In this review, we describe these studies and consider their implications for other Family B GPCRs that can interact with RAMPs.

Calcitonin (CT) is a hormone that decreases serum calcium level by suppressing osteoclastic activity in the vertebrate bone. In vertebrates, the structure-function relationship of CTs has been studied extensively. We recently identified three CT superfamily peptides, Bf-CTFP1 to 3, and clarified the molecular and functional characteristics of their receptor and receptor activity-modifying protein in amphioxus, Branchiostoma floridae. However, the CT activity of Bf-CTFPs has yet to be investigated. In the present study, a functional analysis of Bf-CTFPs was performed using goldfish scales having both osteoclasts and osteoblasts. All Bf-CTFPs suppressed osteoclastic activity via a goldfish CT receptor. Although the primary amino acid sequences of the Bf-CTFPs showed low sequence similarity to vertebrate CTs, Bf-CTFP1 to 3 share three amino acids, Thr25, Thr27, and Pro32-NH2, that are required for receptor binding, with salmon CT. Moreover, homology model analysis revealed that the Bf-CTFPs form alpha-helical structures. The alpha-helical position and length of Bf-CTFP1 and 2 were conserved with those of a highly potent ligand, teleost CT. Interestingly, the composition of the alpha-helix of Bf-CTFP3 differed from those of teleost CT, despite that the action of Bf-CTFP3 on goldfish scales was the same as that of Bf-CTFP1 and 2. Collectively, the present study provides new insights into the structure-function relationship of CT and its functional evolution in chordates. © 2017 Elsevier Inc., Embargo Period 12 months

The carboxyl terminal-extended form of angiotensin I, proangotensin-12, was recently identified in rat tissues including the small intestine, cardiac ventricles, and kidneys. Single administration of proangiotensin-12 exerts vasoconstrictor and pressor effects, probably by conversion to angiotensin II; however, there are currently no data available about the subacute effects of proangiotensin-12. In the present study, we examined the effects of prolonged infusion of proangiotensin-12 in conscious rats. Continuous, subcutaneous infusion of 240 pmol/kg/min of proangiotensin-12 gradually elevated blood pressure over 14 days, as did the same dose of angiotensin II. The pressor effects of proangiotensin-12 were abolished by oral administration of losartan, an angiotensin II type 1 receptor blocker, or perindopril, an angiotensin converting enzyme (ACE) inhibitor. Meanwhile, angiotensin II-induced elevation of blood pressure was inhibited by losartan but not by perindopril. Both the plasma aldosterone level and heart weight/body weight ratio were increased by the prolonged infusion of proangiotensin-12, but these increases were attenuated by losartan and perindopril. The present results suggest that proangiotensin-12 infused continuously over 14 days exerts pressor effects accompanied with the elevation of plasma aldosterone and cardiac hypertrophy in an ACE- and angiotensin II type 1 receptor-dependent manner.

BACKGROUND AND PURPOSE The extracellular loops (ECLs) in Family A GPCRs are important for ligand binding and receptor activation, but little is known about the function of Family B GPCR ECLs, especially ECL3. Calcitonin receptor-like receptor (CLR), a Family B GPCR, functions as a calcitonin gene-related peptide (CGRP) and an adrenomedullin (AM) receptor in association with three receptor activity-modifying proteins (RAMPs). Here, we examined the function of the ECL3 of human CLR within the CGRP and AM receptors. EXPERIMENTAL APPROACH A CLR ECL3 chimera, in which the ECL3 of CLR was substituted with that of VPAC2 (a Family B GPCR that is unable to interact with RAMPs), and CLR ECL3 point mutants were constructed and transiently transfected into HEK-293 cells along with each RAMP. Cell-surface expression of each receptor complex was then measured by flow cytometry; [125I]-CGRP and [125I]-AM binding and intracellular cAMP accumulation were also measured. KEY RESULTS Co-expression of the CLR ECL3 chimera with RAMP2 or RAMP3 led to significant reductions in the induction of cAMP signalling by AM, but CGRP signalling was barely affected, despite normal cell-surface expression of the receptors and normal [125I]-AM binding. The chimera had significantly decreased AM, but not CGRP, responses in the presence of RAMP1. Not all CLR ECL3 mutants supported these findings. CONCLUSIONS AND IMPLICATIONS The human CLR ECL3 is crucial for AM-induced cAMP responses via three CLR/RAMP heterodimers, and activation of these heterodimers probably relies on AM-induced conformational changes. This study provides a clue to the molecular basis of the activation of RAMP-based Family B GPCRs. LINKED ARTICLES This article is part of a themed section on Secretin Family (Class B) G Protein-Coupled Receptors. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.166.issue-1

Two receptor activity-modifying proteins (RAMP2 and RAMP3) enable calcitonin receptor-like receptor (CLR) to function as two heterodimeric receptors (CLR/RAMP2 and CLR/RAMP3) for adrenomedullin (AM), a potent cardiovascular protective peptide. Following AM stimulation, both receptors undergo rapid internalization through a clathrin-dependent pathway, after which CLR/RAMP3, but not CLR/RAMP2, can be recycled to the cell surface for resensitization. However, human (h)RAMP3 mediates CLR internalization much less efficiently than does hRAMP2. Therefore, the molecular basis of the single transmembrane domain (TMD) and the intracellular domain of hRAMP3 during AM receptor internalization was investigated by transiently transfecting various RAMP chimeras and mutants into HEK-293 cells stably expressing hCLR. Flow cytometric analysis revealed that substituting the RAMP3 TMD with that of RAMP2 markedly enhanced AM-induced internalization of CLR. However, this replacement did not enhance the cell surface expression of CLR, [(125)I]AM binding affinity or AM-induced cAMP response. More detailed analyses showed that substituting the Thr(130)-Val(131) sequence in the RAMP3 TMD with the corresponding sequence (Ile(157)-Pro(158)) from RAMP2 significantly enhanced AM-mediated CLR internalization. In contrast, substituting the RAMP3 target sequence with Ala(130)-Ala(131) did not significantly affect CLR internalization. Thus, the RAMP3 TMD participates in the negative regulation of CLR/RAMP3 internalization, and the aforementioned introduction of the Ile-Pro sequence into the RAMP3 TMD may be a strategy for promoting receptor internalization/resensitization.

Adrenomedullin (AM), a biologically active peptide first detected in human pheochromocytoma, has been shown to be present in various tissues or organs, such as blood vessels, cardiac muscle, kidneys, and adipose tissue. AM exerts a wide range of actions, including those lessening cardiovascular damage associated with hypertension or obesityrelated disorders. This bioactive peptide was found to circulate in the human blood, and its plasma levels in hypertensive or obese patients were increased when compared with control subjects. Experiments with animal models of hypertension revealed a possible protective role of AM acting against elevation of blood pressure or hypertensive end-organ damage. In normotensive subjects, a rise in the plasma AM level was associated with future elevation of blood pressure. According to our recent study examining local residents, significant correlations were noted between plasma AM levels and body mass index, serum triglycerides, insulin, or high density lipoprotein cholesterol levels. Expression of AM in adipose tissue of an animal model of obesity was found to be higher than that of the control, and AM was produced and secreted from cultured adipocytes, where the AM receptor components were expressed. Animal experiments showed improved insulin resistance following AM infusion. Collectively, these findings suggest protective roles of AM acting against the progression of hypertensive organ damage and obesity-related metabolic disorders.

Adrenomedullin (AM), a hypotensive peptide, exerts powerful anti-oxidative, anti-inflammatory and antiatherosclerotic effects. In addition, AM strongly acts as an angiogenic and lymphangiogenic growth factor. Therefore, AM has attracted a great deal of attention as a potential therapeutic agent against cardiovascular diseases, such as acute myocardial infarction, heart failure, arteriosclerosis obliterans, pulmonary hypertension and secondary lymphedema. There are two receptor subtypes for AM, both of which are formed by the association of receptor activity-modifying proteins (RAMP2 or RAMP3), which are single transmembrane domain accessory proteins, with calcitonin receptor-like receptor (CLR), a G protein-coupled receptor. The CLR/RAMP2 and CLR/RAMP3 complexes form the AM1 and AM2 receptors, respectively. The specificity of AM for the AM1 receptor is higher than that for the AM2 receptor, which is conserved across multiple animal species. The AM1 receptor is essential for the development of the fetal cardiovascular system. Moreover, overexpression of the AM1 receptor in the vascular smooth muscle cell layers of the aorta can augment AM-induced blood pressure reduction and almost completely inhibit aortic vascular hypertrophy and inflammation caused by chronic angiotensin II infusion. Notably, the AM-AM1 receptor system was found to be crucially involved in both angiogenesis and lymphangiogenesis. In contrast, the cardiovascular functions of the AM2 receptor remain unclear, although there have been no reports showing differences in intracellular signaling via the two AM receptors. In this review, we focus on the molecular basis of AM1 receptor function and its roles in the cardiovascular system and also discuss the possibility of related drug discovery.

Plasma and tissue concentrations of proangiotensin-12 in rats treated with inhibitors of the renin-angiotensin system

Adrenomedullin (AM) is highly expressed in various cancer cell lines, suggesting a possible association with cancer growth. In the present study, we examined the expression and/or concentration of AM, its related peptide, adrenomedullin2/intermedin (AM2/IMD) and their receptors in human colorectal cancer and the surrounding normal tissue. In addition, we assessed the correlation between the expression of AM and AM2/IMD with that of vascular endothelial growth factor (VEGF)-A and matrix metalloproteinase (MMP)-9. Using a specific immunoradiometric assay, we found that AM concentrations were 2-11-fold higher in colorectal cancer tissues than in the surrounding normal tissues. Moreover, real-time quantitative RT-PCR showed that the expression levels of preproAM (+548%), preproAM2/IMD (+2674%), calcitonin receptor-like receptor (CLR) (+518%), receptor activity modifying protein (RAMP)2 (+281%), RAMP3 (+178%), VEGF-A (+277%) and MMP-9 (+864%) mRNAs were significantly higher in cancer tissues than in the surrounding normal tissues, and there was a positive correlation between the gene expressions of MMP-9 and preproAM (r=0.352; p=0.005), but not with preproAM2/IMD (r=0.041, p=0.406). Both AM and AM2/IMD immunoreactivity were detected mainly within cancer cells, whereas MMP-9 immunoreactivity was mostly seen in the surrounding stroma. These findings suggest that AM produced in colorectal tumors acts in concert with MMP-9 in the stroma to contribute to the pathogenesis of colorectal cancer.

Adrenomedullin (AM) is a novel hypotensive peptide that exerts a variety of strongly protective effects against multiorgan damage. AM-specific receptors were first identified as heterodimers composed of calcitonin-receptor-like receptor (CLR), a G protein coupled receptor, and one of two receptor activity-modifying proteins (RAMP2 or RAMP3), which are accessory proteins containing a single transmembrane domain. RAMPs are required for the surface delivery of CLR and the determination of its phenotype. CLR/RAMP2 (AM₁ receptor) is more highly AM-specific than CLR/RAMP3 (AM₂ receptor). Although there have been no reports showing differences in intracellular signaling via the two AM receptors, in vitro studies have shed light on their distinct trafficking and functionality. In addition, the tissue distributions of RAMP2 and RAMP3 differ, and their gene expression is differentially altered under pathophysiological conditions, which is suggestive of the separate roles played by AM₁ and AM₂ receptors in vivo. Both AM and the AM₁ receptor, but not the AM₂ receptor, are crucial for the development of the fetal cardiovascular system and are able to effectively protect against various vascular diseases. However, AM₂ receptors reportedly play an important role in maintaining a normal body weight in old age and may be involved in immune function. In this review article, we focus on the shared and separate functions of the AM receptor subtypes and also discuss the potential for related drug discovery. In addition, we mention their possible function as receptors for AM2 (or intermedin), an AM-related peptide whose biological functions are similar to those of AM.

Adrenomedullin 1 (AM 1 ) receptor is a heterodimer composed of calcitonin receptor-like receptor (CLR) – a family B G protein-coupled receptor (GPCR) – and receptor activity-modifying protein 2 (RAMP2). Both family A and family B GPCRs possess an eighth helix (helix 8) in the proximal portion of their C-terminal tails; however, little is known about the function of helix 8 in family B GPCRs. We therefore investigated the structure–function relationship of human (h)CLR helix 8, which extends from Glu430 to Trp439, by separately transfecting nine point mutants into HEK-293 cells stably expressing hRAMP2. Glu430, Val431, Arg437 and Trp439 are all conserved among family B GPCRs. Flow cytometric analysis revealed that Arg437Ala or Trp438Ala mutation significantly reduced cell surface expression of the receptor complex, leading to a ∼20% reduction in specific 125 I-AM binding but little change in their IC 50 values. Both mutants showed 6–8-fold higher EC 50 values for AM-induced cAMP production and ∼50% reductions in their maximum responses. Glu430Ala mutation also reduced AM signaling by ∼45%, but surface expression and 125 I-AM binding were nearly the same as with wild-type CLR. Surprisingly, Glu430Ala and Val431Ala mutations significantly enhanced AM-induced internalization of the mutant receptor complexes. Taken together, these findings suggest that within hCLR helix 8, Glu430 is crucial for Gs coupling, and Arg437 and Trp439 are involved in both cell surface expression of the hAM 1 receptor and Gs coupling. Moreover, the Glu430–Val431 sequence may participate in the negative regulation of hAM 1 receptor internalization, which is not dependent on Gs coupling.

Receptor activity-modifying protein 2 (RAMP2) enables calcitonin receptor-like receptor (CRLR) to form an adrenomedullin (AM)-specific receptor. Here we investigated the function of the cytoplasmic C-terminal tail (C-tail) of human (h)CRLR by co-transfecting its C-terminal mutants into HEK-293 cells stably expressing hRAMP2. Deleting the C-tail from CRLR disrupted AM-evoked cAMP production or receptor internalization, but did not affect [125I]AM binding. We found that CRLR residues 428–439 are required for AM-evoked cAMP production, though deleting this region had little effect on receptor internalization. Moreover, pretreatment with pertussis toxin (100 ng/mL) led to significant increases in AM-induced cAMP production via wild-type CRLR/RAMP2 complexes. This effect was canceled by deleting CRLR residues 454–457, suggesting Gi couples to this region. Flow cytometric analysis revealed that CRLR truncation mutants lacking residues in the Ser/Thr-rich region extending from Ser449 to Ser467 were unable to undergo AM-induced receptor internalization and, in contrast to the effect on wild-type CRLR, overexpression of GPCR kinases-2, -3 and -4 failed to promote internalization of CRLR mutants lacking residues 449–467. Thus, the hCRLR C-tail is crucial for AM-evoked cAMP production and internalization of the CRLR/RAMP2, while the receptor internalization is dependent on the aforementioned GPCR kinases, but not Gs coupling.

The three receptor activity-modifying proteins (RAMPs1, -2, and -3) associate with a wide variety of G protein-coupled receptors (GPCRs), including calcitonin receptor-like receptor (CRLR). In this study, we used flow cytometry to measure RAMP translocation to the cell surface as a marker of RAMP-receptor interaction. Because VPAC2 does not interact with RAMPs, although, like CRLR, it is a Family B peptide hormone receptor, we constructed a set of chimeric CRLR/VPAC2 receptors to evaluate the trafficking interactions between CRLR domains and each RAMP. We found that CRLR regions extending from transmembrane domain 1 (TM1) through TM5 are necessary and sufficient for the transport of RAMPs to the plasma membrane. In addition, the extracellular N-terminal domain of CRLR, its 3rd intracellular loop and/or TM6 were also important for the cell-surface translocation of RAMP2, but not RAMP1 or RAMP3. Other regions within CRLR were not involved in trafficking interactions with RAMPs. These findings provide new insight into the trafficking interactions between accessory proteins such as RAMPs and their receptor partners.

Receptor activity-modifying protein (RAMP)-2 and -3 chaperone calcitonin receptor-like receptor (CRLR) to the plasma membrane, where together they form heterodimeric adrenomedullin (AM) receptors. We investigated the contributions made by His residues situated in the RAMP extracellular domain to AM receptor trafficking and receptor signaling by co-expressing hCRLR and V5-tagged-hRAMP2 or -3 mutants in which a His residue was substituted with Ala in HEK-293 cells. Flow cytometric analysis revealed that hRAMP2-H71A mediated normal hCRLR surface delivery, but the resultant heterodimers showed significantly diminished [125I]AM binding and AM-evoked cAMP production. Expression of hRAMP2-H124A and -H127A impaired surface delivery of hCRLR, which impaired or abolishing AM binding and receptor signaling. Although hRAMP3-H97A mediated full surface delivery of hCRLR, the resultant heterodimers showed impaired AM binding and signaling. Other His residues appeared uninvolved in hCRLR-related functions. Thus, the His residues of hRAMP2 and -3 differentially govern AM receptor function.

Background Plasma levels of atrial and brain natriuretic peptides (ANP and BNP) are increased in patients with chronic kidney disease (CKD) complicated with deteriorated kidney function, but the relationship between the plasma level of ANP or BNP and the future development of CKD is unclear. Methods We measured the plasma ANP and BNP levels of 294 local residents without CKD in a Japanese community (56.5 ± 10.4 years, mean ± S.D.), who were followed up for the development of CKD over the next 7 years. Results Sixty-three residents developed CKD during the follow-up period, and the baseline level of plasma ANP of these residents was significantly higher than in those without CKD development. Kaplan-Meier analysis showed that the residents with higher ANP than the median value developed CKD more frequently than those with lower ANP. The association between plasma ANP level and CKD development was found to be independent of baseline estimated glomerular filtration rate by a Cox proportional hazards model, while this association became insignificant when adjusted by age; plasma ANP was significantly correlated with age. Compared with ANP, the relationship between plasma BNP and CKD development was unclear in these analyses. Conclusions Age-related elevation of plasma ANP levels preceded the development of CKD in the general population of Japan, raising a possibility for ANP being involved in the development of CKD.

Purpose: Adrenomedullin (AM) is a potent vasodilator peptide having pleiotropic effects including cardiovascular protection and angiogenesis. Because of these beneficial effects, AM appears to be a promising therapeutic tool for human diseases such as myocardial infarction or peripheral artery disease, while intravenous injection of AM stimulates sympathetic nerve activity due to the short-acting potent vasodilation resulting in increased heart rate and renin secretion. To lessen those acute unfavorable actions, we conjugated human adrenomedullin N-terminally with palmitic acid, and examined biological effects of palmitoylated AM in the present study. Methods: Synthesized human AM peptide was conjugated with palmitic acid, and then palmitoylation AM was purified by HPLC. Biological effects in vitro stimulating intracellular cAMP, a major second messenger of AM, were examined using cultured human embryonic (HEK)-239 cells stably expressing a specific AM receptor. Blood pressure-lowering effects in vivo were tested by intravenous injections of palmitoylated AM or native AM peptides into anesthetized rats. Plasma disappearance curve of peptides were evaluated by the two compartment model. Results: Palmitoylated AM stimulated intracellular accumulation of cAMP in cultured HEK-293 cells, as did native human AM peptide, in a dose-dependent manner. pEC50 of palmitoylatted AM was lower than natibe AM (8.49±0.12 vs. 9.17±0.12, mean±SEM, P Conclusions: N-terminally palmitoylated AM stimulated cAMP production in vitro, showing smaller acute hypotensive action and a prolonged plasma half-life in comparison of native AM peptide in vivo. The present results suggest a possibility for palmitoylated AM as a therapeutic tool with lessened unfavorable effect of acute hypotensin of native AM.

The calcitonin (CT)/CT gene-related peptide (CGRP) family is conserved in vertebrates. The activities of this peptide family are regulated by a combination of two receptors, namely the calcitonin receptor (CTR) and the CTR-like receptor (CLR), and three receptor activity-modifying proteins (RAMPs). Furthermore, RAMPs act as escort proteins by translocating CLR to the cell membrane. Recently, CT/CGRP family peptides have been identified or inferred in several invertebrates. However, the molecular characteristics and relevant functions of the CTR/CLR and RAMPs in invertebrates remain unclear. In this study, we identified three CT/CGRP family peptides (Bf-CTFPs), one CTR/CLR-like receptor (Bf-CTFP-R), and three RAMP-like proteins (Bf-RAMP-LPs) in the basal chordate amphioxus (Branchiostoma floridae). The Bf-CTFPs were shown to possess an N-terminal circular region typical of the CT/CGRP family and a C-terminal Pro-NH2. The Bf-CTFP genes were expressed in the central nervous system and in endocrine cells of the midgut, indicating that Bf-CTFPs serve as brain and/or gut peptides. Cell surface expression of the Bf-CTFP-R was enhanced by co-expression with each Bf-RAMP-LP. Furthermore, Bf-CTFPs activated Bf-CTFP-R·Bf-RAMP-LP complexes, resulting in cAMP accumulation. These results confirmed that Bf-RAMP-LPs, like vertebrate RAMPs, are prerequisites for the function and translocation of the Bf-CTFP-R. The relative potencies of the three peptides at each receptor were similar. Bf-CTFP2 was a potent ligand at all receptors in cAMP assays. Bf-RAMP-LP effects on ligand potency order were distinct to vertebrate CGRP/adrenomedullin/amylin receptors. To the best of our knowledge, this is the first molecular and functional characterization of an authentic invertebrate CT/CGRP family receptor and RAMPs.

Receptor activity-modifying proteins (RAMPs) enable calcitonin receptor-like receptor (CRLR) to function as a calcitonin gene-related peptide receptor (CRLR/RAMP1) or an adrenomedullin (AM) receptor (CRLR/RAMP2 or -3). Here we investigated the functions of the cytoplasmic C-terminal tails (C-tails) of human RAMP1, -2, and -3 (hRAMP1, -2, and -3) by cotransfecting their C-terminal deletion or progressive truncation mutants into HEK-293 cells stably expressing hCRLR. Deletion of the C-tail from hRAMP1 had little effect on the surface expression, function, or intracellular trafficking of the mutant heterodimers. By contrast, deletion of the C-tail from hRAMP2 disrupted transport of hCRLR to the cell surface, resulting in significant reductions in (125)I-hAM binding and evoked cAMP accumulation. The transfection efficiency for the hRAMP2 mutant was comparable with that for wild-type hRAMP2; moreover, immunocytochemical analysis showed that the mutant hRAMP2 remained within the endoplasmic reticulum. FACS analysis revealed that deleting the C-tail from hRAMP3 markedly enhances AM-evoked internalization of the mutant heterodimers, although there was no change in agonist affinity. Truncating the C-tails by removing the six C-terminal amino acids of hRAMP2 and -3 or exchanging their C-tails with one another had no effect on surface expression, agonist affinity, or internalization of hCRLR, which suggests that the highly conserved Ser-Lys sequence within hRAMP C-tails is involved in cellular trafficking of the two AM receptors. Notably, deleting the respective C-tails from hRAMPs had no effect on lysosomal sorting of hCRLR. Thus, the respective C-tails of hRAMP2 and -3 differentially affect hCRLR surface delivery and internalization.

Adrenomedullin is a potent hypotensive peptide that may act on myocytes to inhibit hypertrophy and on fibroblasts to inhibit growth in vitro induced by mechanical stretching and angiotensin II. Adrenomedullin is processed from the inactive intermediate adrenomedullin precursor with a glycine extension, which is subsequently converted to biologically active mature adrenomedullin by enzymatic amidation. Total adrenomedullin is the sum of intermediate and mature adrenomedullin. We examined the effect of a subpressor dose of angiotensin II on the production of left ventricular adrenomedullin and on protein levels of mature adrenomedullin in the left ventricle in vivo. We also investigated whether the effect is mediated by the angiotensin II type 1 receptor. Concentrations of total and mature adrenomedullin in the left ventricle and mature adrenomedullin-to-intermediate adrenomedullin ratio were significantly increased by angiotensin II infusion, regardless of pressure overload. Total and mature adrenomedullin concentrations significantly correlated with the weight of the left ventricle. Furthermore, increased adrenomedullin gene expression and protein levels were completely suppressed by a subdepressor dose of angiotensin II type 1 receptor blocker. In conclusion, angiotensin II stimulates the production of cardiac adrenomedullin and accumulates mature adrenomedullin in the left ventricle independently of hemodynamic stress. These processes are partially regulated through the angiotensin II type 1 receptor in vivo.

Three receptor activity modifying proteins (RAMPs) chaperone calcitonin-like receptor (CLR) to the cell surface. RAMP2 enables CLR to form an adrenomedullin (AM)-specific receptor that is sensitive to AM-(22-52) (AM(1) receptor). RAMP3 enables CLR to form an AM receptor sensitive to both calcitonin gene-related peptide (CGRP)-(8-37) and AM-(22-52) (AM(2) receptor), though rat and mouse AM(2) receptors show a clear preference for CGRP alpha-(8-37) over AM-(22-52). RAMP1 enables CRL to form the CGRP-(8-37)-sensitive CGRP(1) receptor, which can also be activated by higher concentrations of AM. Here we review the available information on the pharmacological features and possible pathophysiological roles of the aforementioned AM receptors.

We examined the effects of recombinant human C-reactive protein (rhCRP) on atherosclerosis-related factors in cultured human coronary artery endothelial and smooth muscle cells (HCAECs and HCASMCs). After removing endotoxin from commercial rhCRP preparations using the appropriate column, the purified (P)-rhCRP retained the ability to Ca 2+ -dependently bind to phosphorylcholine, but did not augment the secretion of interleukin-6 and MCP-1 from HCAECs, as non-purified (NP)-rhCRP did. By contrast, P-rhCRP elicited 2- to 3-fold increases in the secretion of both hormones from HCASMCs, though the effect was smaller than that obtained with NP-rhCRP. Production of PAI-1 and endothelin-1 was little affected by either rhCRP preparation in either cell type. In addition, P-rhCRP dose-dependently diminished adrenomedullin release from both cell types, but did not affect adrenomedullin receptor expression or function. Our findings highlight the importance of removing endotoxin from commercial rCRP preparations and show that hCRP elicits atherogenic responses from HCASMCs, but not HCAECs.

We tested whether heterodimers comprised of calcitonin (CT) receptor lacking the 16-amino acid insert in intracellular domain 1 (CTR(I1-)) and receptor activity-modifying protein (RAMP) can function not only as calcitonin gene-related peptide (CGRP) receptors but also as adrenomedullin (AM) receptors. Whether transfected alone or together with RAMP, human (h)CTR(I1-) appeared mainly at the surface of HEK-293 cells. Expression of CTR(I1-) alone led to significant increases in cAMP in response to hCGRP or hAM, though both peptides remained about 100-fold less potent than hCT. However, the apparent potency of AM, like that of CGRP, approached that of CT when CTR(I1-) was co-expressed with RAMP. CGRP- or AM-evoked cAMP production was strongly inhibited by salmon CT-(8-32), a selective amylin receptor antagonist, but not by hCGRP-(8-37) or hAM-(22-52), antagonists of CGRP and AM receptors, respectively. Moreover, the inhibitory effects of CT-(8-32) were much stronger in cells co-expressing CTR(I1-) and RAMP than in cells expressing CTR(I1-) alone. Co-expression of CTR(I1-) with RAMP thus appears to produce functional CT-(8-32)-sensitive AM receptors.

When co-expressed with receptor activity-modifying protein (RAMP) 2, calcitonin receptor-like receptor (CRLR) functions as an adrenomedullin (AM) receptor (CRLR/RAMP2). In the present study, we examined the function of the cysteine (C) residues in the extracellular loops of human (h)CRLR (C212, C225 and C282) and in the extracellular domain of hRAMP2 (C68, C84, C99 and C131). Using site-directed mutagenesis, the cysteine residues were substituted, one at a time, with alanine (A). Co-expression in HEK293 cells of hRAMP2 with the hCRLR C212A or C282A mutant significantly reduced the 50% of effective concentration (EC50) for AM-evoked cyclic adenosine monophosphate (cAMP) production, despite full cell surface expression of the receptor heterodimer. Co-expression of the C225A mutant had no effect on [125I]AM binding or receptor signaling. These results suggest that the cysteine residues in the first (C212) and the second (C282) extracellular loops form a disulfide bond that is important for stabilizing the receptor in the correct conformation for ligand binding and activation. Cells expressing hCRLR with an hRAMP2 mutant (C68A, C84A, C99A or C131A) showed no specific AM binding or AM-stimulated cAMP accumulation. Though abundant in the intracellular compartment, these receptors were not detected at the cell surface, suggesting that all four cysteine residues are essential for efficient transport to the plasma membrane. Cysteine residues in the extracellular loops of hCRLR and in the extracellular domain of hRAMP2 thus appear to play distinct roles in the cell surface expression and function of the receptor heterodimer. (Hypertens Res 2003; 26 (Suppl): S25-S31)

Genetic analyses have revealed an important association between P/Q-type calcium channel activities and hereditary neurological disorders. The P/Q-type channels are composed principally of heterologous multimeric subunits including CaV2.1 and CaVβ4. Of these, the β4 subunit is thought to play a significant role in channel physiology, because a mouse line mutant in that subunit (the lethargic mouse: lh) exhibits a severe ataxic phenotype. The aim of the present study was to elucidate the physiological importance of the β4 subunit. ECG analysis showed that the T wave was high in 8-week-old lh mutants; this may be associated with hyperkalemia. Upon pharmacological ECG analysis, 2-3-week-old lh mutants exhibited reduced responses to a β-blocker and a muscarinic receptor antagonist. Analysis of heart rate variability revealed that the R–R interval was unstable in lh mutants and that both the low- and high-frequency components had increased in extent, indicating that the tonus of both the sympathetic and parasympathetic nervous systems was modified. Thus, our present study revealed that the β4 subunit played a significant role in regulation of sympathetic and parasympathetic nerve activities.

Objective Both adrenomedullin (AM) and proadrenomedullin N-terminal 20 peptide (PAMP), processed from the same precursor of prepro-adrenomedullin (preproAM), have differential biological properties; AM dilates blood vessels and presumably affects the vascular remodeling, while PAMP inhibits catecholamine secretion. Since aldosterone has been shown to be involved in vascular remodeling, we examined the effects of aldosterone on AM and PAMP secretion and preproAM gene expression in human aortic vascular smooth muscle cells (VSMC). Methods AM and PAMP secreted from human VSMC incubated with aldosterone were measured by radioimmunoassay, and preproAM gene expression was evaluated by quantitative polymerase chain reaction. Results Cultured human VSMC secreted both AM and PAMP into the media, while the secretion rate of AM was much higher than that of PAMP. Aldosterone increased preproAM gene expression in the cultured VSMC in a dose-dependent fashion following incubation for 48 h, with a concomitant increase in AM secretion from the cells, but PAMP secretion remained unchanged. Aldosterone-stimulated AM secretion was significantly reduced by spironolactone. Reverse-phase high-performance liquid chromatography analyses showed that immunoreactive AM secreted from the VSMC untreated or treated with aldosterone emerged at the point of human AM(1-52)-NH 2 . Conclusions AM production was stimulated by aldosterone in cultured human VSMC without an increase in PAMP secretion, suggesting a possible role of AM in modulating vascular remodeling by aldosterone.

Adrenomedullin (AM) and atrial and brain natriuretic peptides (ANP and BNP) exert vasodilator and natriuretic actions and are thought to share roles in counteracting the progression of hypertension or heart failure as circulating or locally-acting hormones. However, little data is available with regard to their roles in subjects who have no apparent cardiovascular diseases. The present study was carried out to identify the factors that affect plasma levels of AM, ANP and BNP in the general population. We measured the plasma levels of AM, ANP and BNP in 184 local residents who had a scheduled regular health checkup, and compared the findings with those for other clinical parameters. Univariate analyses showed that the plasma levels of AM, ANP and BNP were significantly correlated with age. The plasma levels of ANP and BNP were also significantly correlated with systolic blood pressure (SBP) and with pulse pressure (PP), an indicator of the stiffness of the great vessels. Multivariate analyses conducted using a stepwise method revealed that age was a significant, independent variable for the plasma levels of AM, ANP and BNP. In addition, PP was a significant factor for the plasma levels of ANP and BNP, while the plasma AM was significantly associated with body mass index (BMI). Thus, the plasma levels of AM, ANP and BNP all increased in association with aging, and those of ANP and BNP increased in association with PP, suggesting possible relationships between the plasma levels and age-related changes in the cardiovascular system. (Hypertens Res 2002; 25: 887-892)

We investigated the anesthetic effects of propofol on the electrocardiogram (ECG) in mice. We also compared the effects of isoflurane (2%) inhalation anesthesia, intraperitoneal propofol (50 or 100 mg/kg), and pentobarbital (50 mg/kg) on ECG in mice. Isoflurane inhalation and pentobarbital anesthesia were both associated with an acceptable heart rate (HR) range (ca. 450 – 500 bpm). In contrast, high-dose propofol anesthesia significantly decreased the HR. Importantly, propofol anesthesia led to significantly reduced responses to propranolol, a β-blocker, suggesting that it affects sympathetic tonus and is not suitable for the evaluation of cardiovascular or sympathetic function. Propofol also reduced the response to atropine, indicative of suppression of mouse parasympathetic nerve activity. Our data suggest that propofol anesthesia should not be the first choice for cardiovascular analysis in mice. Keywords:: electrocardiogram, propofol, pentobarbital, inhalation anesthesia

Adrenomedullin (AM) is a vasodilator peptide with pleiotropic effects, including cardiovascular protection and anti-inflammation. Because of these beneficial effects, AM appears to be a promising therapeutic tool for human diseases, while intravenous injection of AM stimulates sympathetic nerve activity due to short-acting potent vasodilation, resulting in increased heart rate and renin secretion. To lessen these acute reactions, we conjugated the N-terminal of human AM peptide with polyethylene glycol (PEG), and examined the biological properties of PEGylated AM in the present study. PEGylated AM stimulated cAMP production, an intracellular second messenger of AM, in cultured human embryonic kidney cells expressing a specific AM receptor in a dose-dependent manner, as did native human AM. The pEC50 value of PEGylated AM was lower than human AM, but no difference was noted in maximum response (Emax) between the PEGylated and native peptides. Intravenous bolus injection of 10nmol/kg PEGylated AM lowered blood pressure in anesthetized rats, but the acute reduction became significantly smaller by PEGylation as compared with native AM. Plasma half-life of PEGylated AM was significantly longer than native AM both in the first and second phases in rats. In summary, N-terminal PEGylated AM stimulated cAMP production in vitro, showing lessened acute hypotensive action and a prolonged plasma half-life in comparison with native AM peptide in vivo.

Both endothelin (ET) and adrenomedullin (AM), produced by cardiac myocytes, are thought to be locally-acting hormones in the heart. Recently, calcitonin receptor-like receptor (CRLR) and receptor activity modifying proteins (RAMPs) have been shown to function together to serve as AM receptors stimulating cAMP production. In the present study, we examined the effects of ET on AM secretion, intracellular cAMP response to AM, and gene expressions of CRLR and RAMPs in cultured cardiac myocytes. Synthetic ET-1 dose-dependently increased AM secretion from the cardiomyocytes. AM increased the intracellular cAMP level in a dose-dependent manner and the cAMP accumulation by AM was significantly amplified by 24 h preincubation with ET-1. 10 nmol/L ET-1 significantly increased the CRLR mRNA level without any effect on RAMP1 mRNA. 1 μmol/L ET-1 significantly reduced the RAMP2 mRNA level, but ET-1 dose-dependently increased the RAMP3 mRNA level in the cardiac myocytes. These findings suggest that ET-1 not only stimulates AM secretion, but also modulates intracellular cAMP responses to AM probably by altering the expressions of CRLR and RAMPs in rat cardiomyocytes.

Proadrenomedullin N-terminal 20 peptide (PAMP) is generated from post-transcriptional enzymatic processing of a 185-amino acid precursor for adrenomedullin (AM), a potent vasodilator peptide. We have reported that AM is secreted from cultured neonatal rat cardiac myocytes and fibroblasts, and that secreted AM modulates the growth of these cells; however, it is unknown whether or not the cardiac cells produce PAMP. In this study, we examined the production of PAMP in cultured neonatal rat cardiac myocytes and fibroblasts. Both the cardiac myocytes and fibroblasts cultured with serum-free media secreted PAMP time-dependently at rates of 5.7+/-0.9 fmol/10(5) cells/40 h and 8.4+/-0.7 fmol/5x10(4) cells/48 h (mean+/-SD), respectively. Reverse-phase high performance liquid chromatography showed that immunoreactive PAMP secreted from these cells was identical to PAMP[1-20], a whole active molecule. PAMP and AM secretions were significantly (P0.01) stimulated by 10(-6) mol/L angiotensin II (Ang II) and 10% fetal bovine serum (FBS) in myocytes and fibroblasts, whereas the ratio of PAMP to AM secretion in the myocytes was smaller than that of the fibroblasts. These results suggest that PAMP is secreted along with AM from rat cardiac myocytes and fibroblasts, and the secretion is augmented by the growth-promoting stimuli of Ang II and FBS for these cells.

We studied whether a novel proadrenomedullin derived peptide was present and what was its physiological function in cultured bovine adrenal chromaffin cells. We found a high level of proadrenomedullin N-terminal 12 peptide (PAMP-12) which consists of a peptide from 9th amino acid to 20th amino acid of proadrenomedullin N-terminal 20 peptide (PAMP-20). PAMP-12 was released from the cells along with catecholamine upon stimulation of nicotinic cholinergic receptors. When PAMP-12 was added in the incubation medium, this peptide inhibited nicotinic receptor-mediated catecholamine release and influx of Na + and Ca 2+ into the cells. PAMP-12 did not affect catecholamine release evoked by histamine or by depolarization by high concentration of potassium. PAMP-12 also inhibited synthesis of catecholamines as well as the activation of tyrosine hydroxylase by nicotinic stimulation. Thus, PAMP-12 is an endogenous peptide that regulates release and synthesis of catecholamines by acting on nicotinic cholinergic receptors in an autocrine manner in adrenal chromaffin cells.

Expression of the calcitonin receptor-like receptor (CRLR) and its receptor activity modifying proteins (RAMPs) can produce calcitonin gene-related peptide (CGRP) receptors (CRLR/RAMP1) and adrenomedullin (AM) receptors (CRLR/RAMP2 or -3). A chimera of the CRLR and green fluorescent protein (CRLR-GFP) was used to study receptor localization and trafficking in stably transduced HEK 293 cells, with or without co-transfection of RAMPs. CRLR-GFP failed to generate responses to CGRP or AM without RAMPs. Furthermore, CRLR-GFP was not found in the plasma membrane and its localization was unchanged after agonist exposure. When stably coexpressed with RAMPs, CRLR-GFP appeared on the cell surface and was fully active in intracellular cAMP production and calcium mobilization. Agonist-mediated internalization of CRLR-GFP was observed in RAMP1/CGRP or AM, RAMP2/AM, and RAMP3/AM, which occurred with similar kinetics, indicating the existence of ligand-specific regulation of CRLR internalization by RAMPs. This internalization was strongly inhibited by hypertonic medium (0.45 m sucrose) and paralleled localization of rhodamine-labeled transferrin, suggesting that CRLR endocytosis occurred predominantly through a clathrin-dependent pathway. A significant proportion of CRLR was targeted to lysosomes upon binding of the ligands, and recycling of the internalized CRLR was not efficient. In HEK 293 cells stably expressing CRLR-GFP and Myc-RAMPs, these rhodamine-labeled RAMPs were co-localized with CRLR-GFP in the presence and absence of the ligands. Thus, the CRLR is endocytosed together with RAMPs via clathrin-coated vesicles, and both the internalized molecules are targeted to the degradative pathway.

Objective: The aim of the present study was to determine the role of adrenomedullin (AM) in cardiac fibroblasts. Methods: The production and secretion of AM were examined in cultured neonatal rat cardiac fibroblasts, and the effects of AM on proliferation and protein synthesis of these cells were assessed by [3H]thymidine and [3H]phenylalanine incorporation, respectively. Results: Cultured cardiac fibroblasts secreted AM into the medium time-dependently at a rate of 20.3±3.0 fmol/5×104 cells/48 h, mean±S.D. Northern blot analysis showed expression of preproAM mRNA of 1.6 kb in these cells. In addition, 10−6 mol/l of angiotensin II (Ang II) and endothelin-1 (ET-1) significantly increased the AM secretion by 55 and 48%, respectively. Synthetic AM significantly reduced 10−6 mol/l Ang II- or 10−7 mol/l ET-1-stimulated [3H]thymidine and [3H]phenylalanine incorporation in a dose-dependent manner, and these effects were attenuated by a calcitonin gene-related peptide (CGRP) type 1 receptor antagonist, CGRP(8-37). Synthetic AM also had a dose-dependent stimulatory effect on cAMP accumulation in these cells, which was significantly attenuated by CGRP(8-37). A cAMP analogue, 8-bromo-cAMP, mimicked the AM effects, inhibiting the Ang II-stimulated [3H]thymidine and [3H]phenylalanine incorporation. Blockage of the effect of endogenous AM by anti-AM monoclonal antibody not only significantly reduced the basal level of intracellular cAMP, but also enhanced the [3H]thymidine and [3H]phenylalanine incorporation into the cells. Conclusions: Cultured neonatal rat cardiac fibroblasts produce and secrete AM, and the secreted AM may inhibit proliferation and protein synthesis of these cells. AM may exert these inhibitory effects partly by elevating intracellular cAMP. It is suggested that AM has an important role in modulating the growth of cardiac fibroblasts in an autocrine or a paracrine manner.

Objective Adrenomedullin (AM) and proadrenomedullin N-terminal 20 peptide (PAMP) are novel hypotensive peptides produced from the same precursor. A relationship between AM and the renin-angiotensin-aldosterone (RAA) axis was reported in several studies, but the response of the above two peptides to short-term modulation of the RAA axis in humans is not yet clear. Here, we assessed the responses of AM and PAMP in patients with varying RAA system status, including renovascular hypertension (RVH) and primary aldosteronism (PA). Design and methods Essential hypertension (EHT), RVH and PA patients were hospitalized and maintained on a standard diet (NaCI 10 g/day). The patients underwent a captopril (25 mg) loading test A renin-secretion stimulating test (furosemide 1 mg/kg, i.v. +2 h of walking) and an ACTH loading test were performed for the PA patients. The plasma renin activity (PRA), plasma aldosterone concentration (PAC), and plasma AM and PAMP levels were monitored before and after the loadings. Results In the basal state, significantly higher concentrations of AM and PAMP were shown in the RVH patients compared to the other groups. AM and PAMP were significantly correlated with PRA but not PAC in all patients. The AM and PAMP levels were not affected by the captopril loading with or without a hypotensive reaction. The AM and PAMP levels were increased only slightly despite the large increase in PAC induced in the PA patients by the renin-secretion stimulating and ACTH loading tests. Conclusion The responses of plasma AM and PAMP to a short-term modulation of the RAA system were relatively small, despite the correlations observed between PRA and AM or PAMP.

Adrenomedullin (AM), a potent vasodilator peptide, exists in the cardiac ventricle; however, the role of AM in the ventricular tissue remains unknown. In the present study, we investigated the production and secretion of AM in cultured neonatal rat cardiomyocytes, and we examined the effect of AM on de novo protein synthesis in these cells by measuring [ 14 C]phenylalanine incorporation. The cardiomyocytes cultured with serum-free media secreted AM into the media in a time-dependent manner at the rate of 12.2±0.5 fmol/10 5 cells/48 hours (mean±SEM). Angiotensin II (1μmol/L) or 10% fetal bovine serum significantly ( P −6 mol/L angiotensin II-and 10% fetal bovine serum-stimulated [ 14 C]phenylalanine incorporation into the cells, by 16% ( P P 14 C]phenylalanine incorporation was abolished dose-dependently by a calcitonin gene-related peptide receptor antagonist, CGRP(8–37). Furthermore, blockade of the action of endogenous AM by either 10 −6 mol/L CGRP(8–37) or anti-AM monoclonal antibody significantly enhanced the basal and 10 −6 mol/L angiotensin II-stimulated [ 14 C]phenylalanine incorporation. In summary, cultured neonatal rat cardiomyocytes produce and secrete AM, and the secreted AM inhibits the protein synthesis of these cells. Thus, AM may act on cardiomyocytes as an autocrine or a paracrine factor modulating the cardiac growth.

Proadrenomedullin N-terminal 20 peptide (PAMP-20) is a potent hypotensive peptide processed from the adrenomedullin (AM) precursor. We developed a specific radioimmunoassay which recognizes the C-terminal region of PAMP-20. Using this radioimmunoassay, the distribution of immunoreactive (ir-) PAMP was determined in porcine tissues. High concentrations of ir-PAMP were observed in the adrenal medulla and in the atrium, and these values were comparable to the corresponding concentrations of ir-AM. The concentration of ir-PAMP was almost the same as that of ir-AM in the kidney, while ir-PAMP was significantly lower than ir-AM in the ventricle, lung, and aorta. Reversed-phase high performance liquid chromatography in each porcine tissue sample revealed that two major peaks of ir-PAMP existed: one emerged at a position identical to that of authentic porcine PAMP-20; the other unknown peak was eluted earlier. The unknown peptide was purified to homogeneity from porcine adrenal medulla, and its complete amino acid sequence was determined. This peptide was found to be PAMP[9–20] with a C-terminal amide structure, and was named PAMP-12. Intravenous injections of PAMP-12 in anesthetized rats showed a significant hypotensive effect in a dose-dependent fashion, and the effect was comparable to that of PAMP-20. These data indicate that PAMP-12, a major component of ir-PAMP, is processed from the AM precursor, as is PAMP-20, and may participate in cardiovascular control.