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Disparities in longitudinal growth within a species can be partly explained by endocrinological differences. We hypothesized that regulatory networks acting locally in the growth plate may also be important. We tested this hypothesis by evaluating the IGF/IGFBP expression, the vitamin D pathway, and the PTHrP-Indian hedgehog (IHH) feedback loop in rib growth plates from 10- and 21-wk-old small- (Miniature Poodles, MP) and large-breed dogs (Great Danes, GD) using immunohistochemistry and quantitative (q)PCR. The rib growth plates of GD were 1.7 times thicker compared with those of MP, with larger proliferative (in absolute terms) and larger hypertrophic (in absolute and relative terms) zones. IGF/IGFBP gene expression profiling of the growth plates revealed decreased gene expression of igfbp2, -4, and -6 and an unaltered expression of igf-I and igf-II and their respective receptors in GD vs. MP. Immunohistochemistry and qPCR findings showed that the vitamin D pathway was more active in GD than in MP. Staining for 1[alpha]- and 24-hydroxylase was more abundant and intense in GD and the gene expressions of 1[alpha]-hydroxylase and the vitamin D receptor-driven 24-hydroxylase were six- and eightfold higher in GD vs. MP, respectively. Consistent with the immunohistochemistry findings, the expression of mRNA for components of the parathyroid hormone-related peptide (PTHrP)-IHH loop was different in GD compared with MP, with there being a relative threefold downregulation of Pthrp and a tenfold upregulation of Ihh in GD vs MP. These differences suggest that the effects of IHH in the regulation of chondrocyte proliferation and hypertrophy, both independently of PTHrP, can become more dominant during rapid growth rates. In conclusion, our data suggest that, in addition to modest endocrine differences, more pronounced changes in the expression of locally acting regulatory networks, such as the IGF system, vitamin D pathway, and PTHrP-IHH feedback loop are important contributors to within-species disparities in growth rates. canine; growth plate; vitamin D; Indian hedgehog; parathyroid hormone-related peptide; insulin-like growth factor doi: 10.1152/ajpendo.00170.2010.

N-methyl-D-aspartate (NMDA) receptors (NMDAR) are tetrameric amino acid receptors that act as membrane calcium channels. The presence of the receptor has been detected in the principal organs responsible for calcium homeostasis (kidney, bone, and parathyroid gland), pointing to a possible role in mineral metabolism. The aim of this study was to test the effect of NMDAR activation in the kidney and on 1,25[(OH).sub.2][D.sub.3] synthesis. We determined the presence of NMDAR subunits in HK-2 (human kidney cells) cells and proved its functionality. NMDA treatment for 4 days induced a decrease in 1[alpha]-hydroxylase levels and 1,25[(OH).sub.2][D.sub.3] synthesis through the activation of the MAPK/ERK pathway in HK-2 cells. In vivo administration of NMDA for 4 days also caused a decrease in blood 1,25[(OH).sub.2][D.sub.3] levels in healthy animals and an increase in blood PTH levels. This increase in PTH induced a decrease in the urinary excretion of calcium and an increase in urinary excretion of phosphorous and sodium as well as in diuresis. Bone turnover markers also increased. Animals with 5/6 nephrectomy showed low levels of renal 1[alpha]-hydroxylase as well as high levels of renal glutamate compared with healthy animals. In conclusion, NMDAR activation in the kidney causes a decrease in 1,25[(OH).sub.2][D.sub.3] synthesis, which induces an increase on PTH synthesis and release. In animals with chronic kidney disease, high renal levels of glutamate could be involved in the downregulation of 1[alpha]-hydroxylase expression. parathyroid hormone; mitogen-activated protein kinase; hyperparathyroidism doi: 10.1152/ajpendo.00428.2010.

Valdivielso JM, Cannata-Andia J, Coll B, Fernandez E. A new role for vitamin D receptor activation in chronic kidney disease. Am J Physiol Renal Physiol 297: F1502-F1509, 2009. First published July 22, 2009; doi: 10.1152/ajprenal.00130.2009.--Vitamin D has proven to be much more than a simple 'calcium hormone.' The fact that the vitamin D receptor has been found in cells not related to mineral metabolism supports that statement. The interest of nephrologists in vitamin D and its effects beyond mineral metabolism has increased over the last few years, evidencing the importance of this so-called 'sunshine hormone.' In the present review, we highlight the most recent developments in the traditional use of vitamin D in chronic kidney disease (CKD) patients, namely, the control of secondary hyper- parathyroidism (sHPT). Furthermore, we also explore the data available regarding the new possible therapeutic uses of vitamin D for the treatment of other complications present in CKD patients, such as vascular calcification, left ventricular hypertrophy, or proteinuria. Finally, some still scarce but very promising data regarding a possible role of vitamin D in kidney transplant patients also are reviewed. The available data point to a potential beneficial effect of vitamin D in CKD patients beyond the control of mineral metabolism. doi: 10.1152/ajprenal.00130.2009

1[alpha],25[(OH).sub.2] vitamin D3 [1,25[(OH).sub.2][D.sub.3]] increases serum [Ca.sup.2+] concentration in vivo, an action counteracted by activation of the [Ca.sup.2+]-sensing receptor (CaSR), which decreases parathyroid hormone (PTH) secretion and increases renal [Ca.sup.2+] excretion. Relatively little is known of the role the CaSR plays in this response through its potentially direct actions in kidney, gut, and bone independently of PTH. We report PTH-independent roles of the CaSR in modulating the response to exogenous 1,25[(OH).sub.2][D.sub.3] in mice with targeted disruption of both the CaSR and PTH genes ([C.sup.-][P.sup.-]) compared with that in mice with disruption of the PTH gene alone ([C.sup.+][P.sup.-]) or wild-type mice ([C.sup.+][P.sup.+]). After intraperitoneal injection of 0.5 ng/g body wt 1,25[(OH).sub.2][D.sub.3], peak calcemic responses were observed at 24 h in all three genotypes in association with 1) a greater increase in serum [Ca.sup.2+] in [C.sup.-][P.sup.-] mice than in the other genotypes on a [Ca.sup.2+]-replete diet that was attenuated by a [Ca.sup.2+]-deficient diet and pamidronate, 2) increased urinary [Ca.sup.2+]-to-creatinine ratios (UCa/Cr) in the [C.sup.+][P.sup.-] and [C.sup.+][P.sup.+] mice but a lowered ratio in the [C.sup.-][P.sup.-] mice on a [Ca.sup.2+]-replete diet, and 3) no increase in calcitonin (CT) secretion in the [C.sup.+][P.sup.+] and [C.sup.+][P.sup.-] mice and a small increase in the [C.sup.-][P.sup.-] mice. PTH deficiency had the anticipated effects on the expression of key genes involved in [Ca.sup.2+] transport at baseline in the duodenum and kidney, and injection of 1,25[(OH).sub.2][D.sub.3] increased gene expression 8 h later. However, the changes in the genes evaluated did not fully explain the differences in serum [Ca.sup.2+] seen among the genotypes. In conclusion, mice lacking the full-length CaSR have increased sensitivity to the calcemic action of 1,25[(OH).sub.2][D.sub.3] in the setting of PTH deficiency. This is principally from enhanced 1,25[(OH).sub.2][D.sub.3]-mediated gut [Ca.sup.2+] absorption and decreased renal [Ca.sup.2+] excretion, without any differences in bone-related release of [Ca.sup.2+] or CT secretion among the three genotypes that could explain the differences in their calcemic responses. vitamin D; kidney

The possibility of pathological calcium metabolism is a critical health concern introduced by long-term space travel. Because vitamin D plays an important role in calcium homeostasis, we evaluated the effects of hypergravity on the expression of genes involved in vitamin D and calcium metabolism in ICR mice. When exposed to 2G hypergravity for 2 days, the mRNA expression of renal 25-hydroxyvitamin D 24-hydroxylase (Cyp24a1) was increased and that of 25-hydroxyvitamin D1[alpha]-hydroxylase (Cyp27b1) was decreased. Although hypergravity decreased food intake and increased the expression of starvation-induced genes, the changes in Cyp24a1 and Cyp27b1 expression were not due to starvation, suggesting that hypergravity affects these genes directly. Hypergravity decreased plasma 1[alpha], 25-dihydroxyvitamin [D.sub.3] levels in ICR mice, suggesting a consequence of decreased Cyp27b1 and increased Cyp24a1 expression. Although 1[alpha]-hydroxyvitamin [D.sub.3] [1[alpha](OH)[D.sub.3]] treatment induced the expression of vitamin D receptor (VDR) target genes in the kidney of 2G-exposed ICR mice to similar levels as controls, l[alpha](OH)[D.sub.3] increased the intestinal expression of Cyp24a1 in ICR mice. Hypergravity-dependent changes of Cyp24a1 and Cyp27b1 expression were diminished in mice exposed to hypergravity for 14 days, which may represent an adaptation to hypergravity stress. Hypergravity exposure also increased Cyp24a1 expression in the kidney of C57BL/6J mice. We examined the effects of hypergravity on VDR-null mice and found that renal Cyp27b1 expression in VDR-null mice was decreased by hypergravity while renal Cyp24a1 expression was not detected in VDR-null mice. Thus hypergravity modifies the expression of genes involved in vitamin D metabolism. vitamin D; vitamin D receptor; hypergravity; kidney; transcription

The available evidence suggests that vitamin D has cardiovascular effects besides regulating calcium homeostasis. To examine the effect of 1,25-dihydroxyvitamin D3, the major metabolite of vitamin D, on endothelium-dependent contractions, aortic rings of spontaneously hypertensive rats (SHR) were suspended in organ chambers for isometric force measurements. Rings were incubated with [N.sup.[omega]]-nitro-L-arginine methyl ester (LNAME) and then exposed to increasing concentrations of acetylcholine, ATP, or the calcium ionophore to trigger contractions. This was done in the absence or presence of 1,25-dihydroxyvitamin D3. The release of prostacyclin after acetylcholine or A-23187 stimulation was also measured. The cytosolic-free calcium concentration was measured by confocal microscopy after incubation with the fluorescent dyes fluo-4 and fura red. The presence of vitamin D receptors was confirmed using immunohistochemistry. Acetylcholine- and ATP-induced endothelium-dependent contractions were significantly reduced compared with those obtained in the absence of the drug. This effect was not present if A-23187 was used as an agonist. The acetylcholine- but not the A-23187-induced release of prostacyclin was reduced by the acute administration of 1,25-dihydroxyvitamin [D.sub.3]. Exposure to 1,25-dihydroxyvitamin D3 reduced the increase in cytosolic-free calcium concentration caused by acetylcholine but not by A-23187 in cells. Vitamin D receptors were densely distributed in the endothelium. Inecalcitol (19-nor-14-epi-23-yne-l,25-dihydroxy-vitamin [D.sub.3]), a synthetic analog of vitamin D, caused a comparable depression of endothelium-dependent contractions as 1,25-dihydroxy-vitamin D3. These results demonstrate that vitamin D3 modulates vascular tone by reducing calcium influx into the endothelial cells and hence decreasing the production of endothelium-derived contracting factors. calcium; endothelium-derived contracting factors; inecalcitol

A new, completely in vivo method of measuring the rate of intestinal phosphate absorption has been developed. As expected from previous in vitro and ex vivo measurements, intestinal phosphate absorption is potently and rapidly stimulated by 1,25-dihydroxyvitamin [D.sub.3]. The response is saturated with as little as 11.3 ng of 1,25-dihydroxyvitamin [D.sub.3] per day, consistent with a genomic mechanism. The effect of 1,25-dihydroxyvitamin [D.sub.3] disappears when the dosing solution of phosphate is at 2 M, suggesting that 1,25-dihydroxyvitamin D3 stimulates active transport of phosphate but not diffusion of phosphate. Finally, unlike findings resulting from in vitro or ex vivo experiments, no evidence in vivo was obtained that phosphate absorption requires sodium or is inhibited by potassium. vitamin D; phosphorus; intestine doi: 10.1152/ajpendo.00654.2006

FGF23 suppresses both serum phosphate and 1,25-dihydroxyvitamin D [1,25D] levels in vivo. Because 1,25D itself is a potent regulator of phosphate metabolism, it has remained unclear whether FGF23-induced changes in phosphate metabolism were caused by a 1,25D-independent mechanism. To address this issue, we intravenously administered recombinant FGF23 to vitamin D receptor (VDR) null (KO) mice as a rapid bolus injection and evaluated the early effects of FGF23. Administration of recombinant FGF23 further decreased the serum phosphate level in VDR KO mice, accompanied by a reduction in renal sodium-phosphate cotransporter type IIa (NaPi2a) protein abundance and a reduced renal 25-hydroxyvitamin D-1[alpha]-hydroxylase (1[alpha]OHase) mRNA level. Thus FGF23-induced changes in NaPi2a and 1[alpha]OHase expression are independent of the 1,25D/VDR system. However, 24-hydroxylase (24OHase) mRNA expression remained undetectable by the treatment with FGF23. We also analyzed the regulatory mechanism for FGF23 expression. The serum FGF23 level was almost undetectable in VDR KO mice, whereas dietary calcium supplementation significantly increased circulatory levels of FGF23 and its mRNA abundance in bone. This finding indicates that calcium is another determinant of FGF23 production that occurs independently of the VDR-mediated mechanism. In contrast, dietary phosphate supplementation failed to induce FGF23 expression in the absence of VDR, whereas marked elevation in circulatory FGF23 was observed in wild-type mice fed with a high-phosphate diet. Taken together, FGF23 works, at least in part, in a VDR-independent manner, and FGF23 production is also regulated by multiple mechanisms involving VDR-independent pathways. VDR; phosphate metabolism; sodium phosphate cotransporter

Fibroblast growth factor-23 (FGF23) has been recently identified as playing an important pathophysiological role in phosphate homeostasis and vitamin D metabolism. To elucidate the precise physiological regulation of FGF-23, we characterized the mouse FGF-23 5'-flanking region and analyzed its promoter activity. The 5'-flanking region of the mouse FGF-23 gene contained a TFIID site (TATA box) and several putative transcription factor binding sites, including MZF1, GATA-1 and c-Ets-1 motifs, but it did not contain the typical sequences of the vitamin D response element. Plasmids encoding 554-bp (pGL/-0.6), 364-bp (pGL/-0.4) and 200-bp (pGL/-0.13) promoter regions containing the TFIID element and + 1-bp fragments drove the downstream expression of a luciferase reporter gene in transfection assays. We also found that FGF-23 mRNA was expressed in K-562 erythroleukemia cell lines but not in MC3T3-E1, Raji, or Hep G2 human carcinoma cells. Treatment with 1,25-dihydroxyvitamin [D.sub.3] in the presence of high phosphate markedly stimulated pGL/-0.6 activity, but calcium had no effect. In addition, the plasma FGF-23 levels were affected by the dietary and plasma inorganic phosphate concentrations. Finally, the levels of plasma FGF-23 in vitamin D receptor-null mice were significantly lower than in wild-type mice. The presents study demonstrated that vitamin D and the plasma phosphate level are important regulators of the transcription of the mouse FGF-23 gene. gene regulation; vitamin D receptor; phosphate homeostasis

Studies were performed to further characterize the male-specific hepatic recombinant microsomal vitamin D 25-hydroxlase CYP2C11, expressed in baculovirus-infected insect cells, and determine whether it is also a vitamin D 24-hydroxylase. 25- and 24-hydroxylase activities were compared with those of 10 other recombinant hepatic microsomal cytochrome P-450 enzymes expressed in baculovirus-infected insect cells. Each of them 25-hydroxylated vitamin [D.sub.2], vitamin [D.sub.3], 1[alpha]-hydroxyvitamin [D.sub.2] (1[alpha]OH[D.sub.2]), and 1[alpha]-hydroxyvitamin [D.sub.3] (1[alpha]OH[D.sub.3]). CYP2C11 had the greatest activity with these substrates, except vitamin [D.sub.3], which had the same activity as four of the other enzymes. The descending order of 25-hydroxylation by CYP2C11 was 1[alpha]OH[D.sub.3] > 1[alpha]OH[D.sub.2] > vitamin [D.sub.2] > vitamin [D.sub.3]. Each of the recombinant cytochrome P-450 enzymes 24-hydroxylated 1[alpha]OH[D.sub.2]. CYP2C11 had the greatest activity. 24-Hydroxylation of 1[alpha]OH[D.sub.3] was very low, and there was none with vitamin [D.sub.3]. Only CYP2C11 24-hydroxylated vitamin [D.sub.2]. Structures of vitamin D metabolites, including 24-hydroxyvitamin [D.sub.2], 1,24(S)-dihydroxyvitamin [D.sub.2], and 1,24-dihydroxyvitamin [D.sub.3], were confirmed by HPLC and. gas chromatography retention times and characteristic mass spectrometric fragmentation patterns. In male rats, hypophysectomy significantly reduced body weight, liver weight, hepatic CYP2C11 mRNA expression, and 24- and 25-hydroxylation of 1[alpha]OH[D.sub.2]. Expression of CYP2J3 and CYP2R1 mRNA did not change. In male rat hepatocytes, CYP2C11 mRNA expression and 24-and 25-hydroxylation were significantly reduced after culture for 24 h compared with uncultured cells. Expression of CYP2J3 and CYP2R1 either increased or did not change. It is concluded that CYP2C11 is a male-specific hepatic microsomal vitamin D 25-hydroxylase that hydroxylates vitamin [D.sub.2], vitamin [D.sub.3], 1[alpha]OH[D.sub.2], and 1[alpha]OH[D.sub.3]. CYP2C11 is also a vitamin D 24-hydroxylase. 25-hydroxyvitamin D; 24-hydroxyvitamin D; liver microsomes; cytochrome P-450 enzymes

Recent studies suggest that vitamin D signaling regulates bone formation. However, the overall effect of 1,25-dihydroxyvitamin [D.sub.3] [1,25[(OH).sub.2][D.sub.3]] on bone turnover in vivo is still unclear. In this study, our aim was to examine the effect of 1,25[(OH).sub.2][D.sub.3] on bone turnover in SAM-P/6, a hormone-independent mouse model of senile osteoporosis characterized by a decrease in bone formation. Male and female 4-too-old SAM-P/6 mice were treated with 1,25[(OH).sub.2][D.sub.3] (18 pmol/24 h) or vehicle for a period of 6 wk, and a group of age- and sex-matched nonosteoporotic animals was used as control. Bone mineral density (BMD) at the lumbar spine increased rapidly by >30 [+ or -] 5% (P < 0.001) in 1,25[(OH).sub.2][D.sub.3]- treated SAM-P/6 animals, whereas BMD decreased significantly by 18 [+ or -] 2% (P < 0.01) in vehicle-treated SAM-P/6 animals and remained stable in control animals during the same period. Static and dynamic bone histomorphometry indicated that 1,25[(OH).sub.2][D.sub.3] significantly increased bone volume and other parameters of bone quality as well as subperiosteal bone formation rate compared with vehicle-treated SAM-P/6 mice. However, no effect on trabecular bone formation was observed. This was accompanied by a marked decrease in the number of osteoclasts and eroded surfaces. A significant increase in circulating bone formation markers and a decrease in bone resorption markers was also observed. Finally, bone marrow cells, obtained from 1,25[(OH).sub.2][D.sub.3]-treated animals and cultured in the absence of 1,25[(OH).sub.2][D.sub.3], differentiated more intensely into osteoblasts compared with those derived from vehicle-treated mice cultured in the same conditions. Taken together, these findings demonstrate that 1,25[(OH).sub.2][D.sub.3] acts simultaneously on bone formation and resorption to prevent the development of senile osteoporosis. 1,25-dihydroxyvitamin [D.sub.3]; osteoporosis; bone turnover; histomorphometry; bone markers

Intake of a low-phosphate diet stimulates transepithelial transport of [P.sub.i] in small intestine as well as in renal proximal tubules. In both organs, this is paralleled by a change in the abundance of the apically localized Na[P.sub.i] cotransporters Na[P.sub.i] type IIa (Na[P.sub.i]-IIa) and Na[P.sub.i] type IIb (Na[P.sub.i]-IIb), respectively. Low-[P.sub.i] diet, via stimulation of the activity of the renal 25-hydroxyvitamin-[D.sub.3]-1[alpha]-hydroxylase (1[alpha]OHase), leads to an increase in the level of 1,25-dihydroxy-vitamin [D.sub.3] [1,25[(OH).sub.2]D]. Regulation of the intestinal absorption of [P.sub.i] and the abundance of Na[P.sub.i]-IIb by 1,25[(OH).sub.2]D has been supposed to involve the vitamin D receptor (VDR). In this study, we investigated the adaptation to a low-[P.sub.i] diet of Na[P.sub.i]-IIb in small intestine as well as Na[P.sub.i]-IIa in kidneys of either VDR- or 1[alpha]OHase-deficient mice. In both mouse models, upregulation by a low-[P.sub.i] diet of the Na[P.sub.i] cotransporters Na[P.sub.i]-IIa and Na[P.sub.i]-IIb was normal, i.e., similar to that observed in the wild types. Also, in small intestines of VDR- and 1[alpha]OHase-deficient mice, the same changes in Na[P.sub.i]-IIb mRNA found in wild-type mice were observed. On the basis of the results, we conclude that the regulation of Na[P.sub.i] cotransport in small intestine (via Na[P.sub.i]-IIb) and kidney (via Na[P.sub.i]-IIa) by low dietary intake of [P.sub.i] cannot be explained by the 1,25[(OH).sub.2]D-VDR axis. Na[P.sub.i] type IIb; vitamin [D.sub.3]

Critical role of vitamin D in sulfate homeostasis: regulation of the sodium-sulfate cotransporter by 1,25-dihydroxyvitamin [D.sub.3]. Am J Physiol Endocrinol Metab 287: E744-E749, 2004. First published May 27, 2004; 10.1152/ajpendo.00151.2004.--As the fourth most abundant anion in the body, sulfate plays an essential role in numerous physiological processes. One key protein involved in transcellular transport of sulfate is the sodium-sulfate cotransporter NaSi-1, and previous studies suggest that vitamin D modulates sulfate homeostasis by regulating NaSi-1 expression. In the present study, we found that, in mice lacking the vitamin D receptor (VDR), NaSi-1 expression in the kidney was reduced by 72% but intestinal NaSi-1 levels remained unchanged. In connection with these findings, urinary sulfate excretion was increased by 42% whereas serum sulfate concentration was reduced by 50% in VDR knockout mice. Moreover, levels of hepatic glutathione and skeletal sulfated proteoglycans were also reduced by 18 and 45%, respectively, in the mutant mice. Similar results were observed in VDR knockout mice after their blood ionized calcium levels and rachitic bone phenotype were normalized by dietary means, indicating that vitamin D regulation of NaSi-1 expression and sulfate metabolism is independent of its role in calcium metabolism. Treatment of wild-type mice with 1,25-dihydroxyvitamin [D.sub.3] or vitamin D analog markedly stimulated renal NaSi-1 mRNA expression. These data provide strong in vivo evidence that vitamin D plays a critical rote in sulfate homeostasis. However, the observation that serum sulfate and skeletal proteoglycan levels in normoealcemic VDR knockout mice remained low in the absence of rickets and osteomalacia suggests that the contribution of sulfate deficiency to development of rickets and osteomalacia is minimal. vitamin D receptor; sulfate

Recent studies suggest that vitamin D may play a role in intestinal [Na.sup.+]-dependent phosphate transport adaptation to variable levels of dietary [P.sub.i]. Therefore, the goal of the current study was to assess [Na.sup.+]-dependent [P.sub.i] cotransport activity in transgenic mice to determine whether vitamin D is an essential mediator of this process. Intestinal brush-border membrane (BBM), [Na.sup.+]-dependent [P.sub.i] cotransport activity was significantly decreased in vitamin D receptor (VDR) null [VDR (-/-)] mice compared with wild-type (VDR+/+) mice. While intestinal Na-[P.sub.i] cotransporter (type IIb) mRNA levels were similar in VDR (-/-) and VDR (+/+) mice, type IIb Na-[P.sub.i] cotransporter protein expression was markedly suppressed in VDR (-/-) mice compared with VDR (+/+) mice. Furthermore, Na-[P.sub.i] cotransport activity in renal BBM was similar in VDR (-/-) and VDR (+/+) mice, but type IIa Na-[P.sub.i] cotransporter protein expression was decreased in VDR (-/-) mice. After administration of a low-[P.sub.i] diet, type IIb protein expression was significantly increased in VDR (+/+) and VDR (-/-) mice, and type IIb protein expression was present in the intestinal BBM of VDR (-/-) mice. These data demonstrate that intestinal Na-[P.sub.i] cotransport adaptation to a low-[P.sub.i] diet occurs independently of vitamin D. dietary phosphate; phosphate transport; intestine

I. METABOLIC ACTIVATION OF VITAMIN D A. Introduction An appreciation that vitamin [D.sub.3] represents only a precursor to its functionally active form, 1[Alpha],25-dihydroxyvitamin [D.sub.3] [1,25-[(OH).sub.2][D.sub.3]], is arguably one of the [...]

Western blot analysis helps detect two specific zeta and alpha isoforms of protein kinase C (PKC) in Caco-2 cells and indicates that 1, 25-dihydroxyvitamin D3 (1,25(OH)2D3) induces PKC-alpha activity to suppress secosteroid-activated intracellular Ca(2+) (Ca(2+)i) increase in the cells. Vitamin D3 stimulates PKC-alpha but not PKC-zeta activity. Pretreatment of Caco-2 cells with PKC activator 12-O-tetradecanoylphorbol 13-acetate suppresses 1,25(OH)2D3-induced (Ca2+)i increase, but has no effect on Ins(1,4, 5)P3 production in cells. PKC-alpha isoforms regulate Ca(2+)i increase at a level distal to InS(1,4,5)-P3 production.

Analysis of homologous modulation of the vitamin D receptor (VDR) in the duodenum of rats with severe renal failure reveals that decrease in the homologous upregulation of VDR is due to damage at a posttranslational or translational step. Northern blot analysis detects the baseline extent of VDR mRNA in uremic rats which is similar to that in sham-operated rats.

Experiments to examine if NaCl assimilation was influenced by hormones which control calcium revealed that calcium, calcitonin, parathyroid hormone or 1,25-di-hydroxyvitamin D3 do not mediate enhanced NaCl assimilation generated by calcium deprivation. The important hormones which regulate calcium are not associated with the monitoring of instantaneous NaCl assimilation in the rat.

Balesaria S, Sangha S, Waiters JR. Human duodenum responses to vitamin D metabolites of TRPV6 and other genes involved in calcium absorption. Am J Physiol Gastrointest Liver Physiol 297: G1193-G1197, 2009. First published September 24, 2009; doi: 10.1152/ajpgi.00237.2009.--Calcium absorption by the intestine is necessary for bone mineralization. Much has been learned about this process and the role of vitamin D metabolites in gene transcription from animal studies, but the molecular mechanisms in humans are less well understood. We have used samples of normal human duodenal mucosa, obtained at endoscopy, to investigate the effects of the vitamin D metabolites, 1[alpha]-dihydroxycholecalciferol [1,25[(OH).sub.2][D.sub.3]] and 25-hydroxycholecalciferol (25OHD), on transcripts on genes involved in calcium absorption and vitamin D metabolism. TRPV6 transcripts were significantly higher after incubation for 6 h with 1,25[(OH).sub.2][D.sub.3] ([10.sup.-9] mol/l) than after control incubations (median difference 3.1-fold, P < 0.001). Unexpectedly, TRPV6 expression was also higher (2.4-fold, P < 0.02) after incubation with 25OHD ([10.sup.-7] mol/l). Transcripts for the calcium-ATPase, PMCA1, were significantly higher with 1,25[(OH).sub.2][D.sub.3]; CYP24 transcripts were reliably detected after incubation with either metabolite, but calbindin-D9k transcripts were unaffected. The response of TRPV6 to 25OHD and the expression of transcripts for CYP27B1, the 25OHD-1[alpha]-hydroxylase, were significantly correlated (r = 0.82, P < 0.02). Basal duodenal expression of TRPV6 and CYP27B1 were significantly associated (r = 0.72, P < 0.001) in a separate previously reported series of subjects. Multiple regression analysis of the associations with basal duodenal TRPV6 expression identified CYP27B1 expression and serum 1,25[(OH).sub.2]D as major factors. Expression of the CYP27B1 protein was demonstrated immunohistochemically in duodenal mucosa. This study has shown that human duodenal TRPV6, PMCA1, and CYP24 transcripts respond rapidly to 1,25[(OH).sub.2][D.sub.3] and provides evidence suggesting that local duodenal production of 1,25[(OH).sub.2][D.sub.3] by 25OHD-1[alpha]-hydroxylase may have a role in human calcium absorption. cholecalciferol; calcium transporter; intestine; organ culture doi: 10.1152/ajpgi.00237.2009

Christensen, Erik Ilso, and Henrik Birn, Megalin and cubilin: synergistic endocytic receptors in renal proximal tubule. Am J Physiol Renal Physiol 280: F562-F573, 2001.--The multiligand, endocytic receptors megalin and cubilin are colocalized in the renal proximal tubule. They are heavily expressed in the apical endocytic apparatus. Megalin is a 600-kDa transmembrane protein belonging to the low-density lipoprotein-receptor family. The cytoplasmic tail contains three NPXY motifs that mediate the clustering in coated pits and are possibly involved in signaling functions. Cubilin, also known as the intestinal intrinsic factor-cobalamin receptor, is a 460-kDa receptor with no transmembrane domain and no known signal for endocytosis. Because the two receptors bind each other with high affinity and colocalize in several tissues, it is highly conceivable that megalin mediates internalization of cubilin and its ligands. Both receptors are important for normal tubular reabsorption of proteins, including albumin. Among the proteins normally filtered in the glomeruli, cubilin has been shown to bind albumin, immunoglobulin light chains, and apolipoprotein A-I. The variety of filtered ligands identified for megalin include vitamin-binding proteins, hormones, enzymes, apolipoprotein H, albumin, and [[Beta].sub.2]; and [[Alpha].sub.1]-microglobulin. Loss of these proteins and vitamins in the urine of megalin-deficient mice illustrates the physiological importance of this receptor. proteinuria; vitamin D; vitamin [B.sub.12]; retinol; low-density lipoprotein-receptor family

Hoenderop, Joost G. J., Peter H. G. M. Willems, and Rene J. M. Bindels. Toward a comprehensive molecular model of active calcium reabsorption. Am. J. Physiol. Renal Physiol. 278: F352-F360, 2000.--The fine tuning of [Ca.sup.2+] excretion in the kidney takes place in the distal nephron, which consists of the distal convoluted tubule, connecting tubule, and initial portion of the cortical collecting duct. In these segments, [Ca.sup.2+] is reabsorbed through an active transcellular pathway. The apical influx of [Ca.sup.2+] into the distal renal cell is presumably the rate-limiting step in this process, and its molecular identity has remained obscure so far. The recently discovered epithelial [Ca.sup.2+] channel (ECaC) exhibits the expected properties for being the gatekeeper in transcellular [Ca.sup.2+] reabsorption. The characteristics and potential physiological role of ECaC will be discussed in this review. Our knowledge of the mechanisms involved in the regulation of transcellular [Ca.sup.2+] transport has advanced rapidly since the development of cell models originating from distal tubular cells. Studies using these models indicate that hormones including arginine vasopressin, [PGE.sub.2], adenosine, ATP, and atrial natriuretic peptide should be considered as calciotropic hormones controlling renal [Ca.sup.2+] handling. Evidence is now beginning to emerge that the stimulating calciotropic hormones utilize new cAMP-independent pathways to stimulate [Ca.sup.2+] reabsorption. These new findings allow the development of a comprehensive and detailed model of the process of transcellular calcium transport in the kidney whereby the individual contribution of the participating transporters can now be fully appreciated. calcium channel; parathyroid hormone; vitamin [D.sub.3]; atrial natriuretic peptide; arginine vasopressin; hypercalciuria; guanosine 3'5'-cyclic monophosphate; adenosine 3'5'-cyclic monophosphate

Several studies on vitamin D receptor (VDR)-ablated mice revealed that the activation of genes that control intestinal calcium transport is the most important role of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), the active form of vitamin D. Moreover, 1,25(OH)2D3 has also been found to be capable of controling the expression of many genes involved in various biological actions. These actions have suggested several new therapeutic applications for 1,25(OH)2D3 and its analogs in immune dysfunction, endocrine disorders and hyperproliferative disordes.