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Abstract: Aims: Both natriuretic peptides and nitric oxide may be protective in cardiac hypertrophy, although their functional effects are diminished in hypertrophy. Hypoxia inducible factor-1 (HIF-1) may also protect in cardiac hypertrophy. We hypothesized that upregulation of HIF-1 would protect the functional effects of cyclic GMP (cGMP) signaling in hypertrophied ventricular myocytes. Main methods: A cardiac hypertrophy model was created in mice by transverse aorta constriction. HIF-1 was increased by deferoxamine (150mg/kg for 2days). HIF-1α protein levels were examined. Functional parameters were measured (edge detector) on freshly isolated myocytes at baseline and after BNP (brain natriuretic peptide, 10−8–10−7 M) or CNP (C-type natriuretic peptide, 10−8–10−7 M) or SNAP (S-nitroso-N-acetyl-penicillamine, a nitric oxide donor, 10−6–10−5 M) followed by KT5823 (a cyclic GMP-dependent protein kinase (PKG) inhibitor, 10−6 M). We also determined PKG expression levels and kinase activity. Key findings: We found that under control conditions, BNP (−24%), CNP (−22%) and SNAP (−23%) reduced myocyte shortening, while KT5823 partially restored function. Deferoxamine treated control myocytes responded similarly. Baseline function was reduced in the myocytes from hypertrophied heart. BNP, CNP, SNAP and KT5823 also had no significant effects on function in these myocytes. Deferoxamine restored the negative functional effects of BNP (−22%), CNP (−18%) and SNAP (−19%) in hypertrophic cardiac myocytes and KT5823 partially reversed this effect. Additionally, deferoxamine maintained PKG expression levels and activity in hypertrophied heart. Significance: Our results indicated that the HIF-1 protected the functional effects of cGMP signaling in cardiac hypertrophy through preservation of PKG. [Copyright &y& Elsevier]

We tested the hypothesis that the negative functional effects of cyclic GMP on cardiac myocytes were mediated through phospholamban (PLB) and activation of sarcoplasmic reticulum Ca2+-ATPase. Using ventricular myocytes from wild type (WT, n=10) and PLB knockout (PLB-KO, n=10) mouse hearts, functional changes were measured using a video edge detector at baseline and after 10-6, 10-5M 8-bromo-cyclic GMP (cGMP), 10-8, 10-7M C-type natriuretic peptide (CNP), or 10-6, 10-5M S-nitroso-N-acetyl-penicillamine (SNAP, nitric oxide donor). Changes in cytosolic Ca2+ concentration were assessed in fura 2-loaded WT and PLB-KO myocytes. Cyclic GMP dependent phosphorylation analysis was also performed in WT and PLB-KO myocytes. 8-bromo-cGMP 10-5M caused a significant decrease in %shortening (3.6±0.2% to 2.3±0.1%) in WT, but little change in PLB-KO myocytes (3.4±0.1% to 3.2±0.2%). Similarly, CNP and SNAP reduced %shortening of WT, but not PLB-KO myocyte. Changes in other contractile parameters such as maximum rate of shortening and relaxation were consistent with the changes in % shortening. Intracellular Ca2+ transients changed similarly to cell contractility in WT and PLB-KO myocytes treated with cGMP and CNP; i.e. Ca2+ transients decreased with cGMP or CNP in WT myocytes, but were unchanged in PLB-KO myocytes. cGMP dependent phosphorylation analysis showed that some proteins were phosphorylated by cGMP to a lesser extent in PLB-KO compared with WT myocytes, suggesting impaired cGMP-kinase function in PLB-KO cardiac myocytes. These results indicated that cGMP-induced reductions in cardiac myocyte function were at least partially mediated through the action of phospholamban. Copyright © 2007 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Baseline function and signal transduction are depressed in hearts with hypertrophic failure. We tested the hypothesis that the effects of cGMP and its interaction with cAMP would be reduced in cardiac myocytes from hypertrophic failing hearts. Ventricular myocytes were isolated from control dogs, dogs with aortic valve stenosis hypertrophy, and dogs with pacing hypertrophic failure. Myocyte function was measured using a video edge detector. Cell contraction data were obtained at baseline, with 8-bromo-cGMP (10-7, 10-6, and 10-5 M), with erythro-9-(2-hydroxy-3-nonyl)adenine [EHNA; a cAMP phosphodiesterase (PDE2) inhibitor] plus 8-bromo-cGMP, or milrinone (a PDE3 inhibitor) plus 8-bromo-cGMP. Baseline percent shortening and maximal rates of shortening (Rmax) and relaxation were slightly reduced in hypertrophic myocytes and were significantly lower in failing myocytes (Rmax: control dogs, 95.3 ± 17.3; hypertrophy dogs, 88.2 ± 5.5; failure dogs, 53.2 ± 6.4 µm/s). 8-Bromo-cGMP dose dependently reduced myocyte function in all groups. However, EHNA (10-6 M) and milrinone (10-6 M) significantly reduced the negative effects of cGMP on cell contractility in control and hypertrophy but not in failing myocytes (Rmax for control dogs: cGMP, -46%; +EHNA, -21%; +milrinone, -19%; for hypertrophy dogs: cGMP, -40%; +EHNA, -13%; +milrinone, -20%; for failure dogs: cGMP, -40%; +EHNA, -29%; +milrinone, -32%). Both combinations of EHNA-cGMP and milrinone-cGMP significantly increased intracellular cAMP in control, hypertrophic, and failing myocytes. These data indicated that the cGMP signaling pathway was preserved in hypertrophic failing cardiac myocytes. However, the interaction of cGMP with the cAMP signaling pathway was impaired in these failing myocytes. [ABSTRACT FROM AUTHOR]

Hearts undergoing cardiopulmonary arrest and resuscitation have depressed function and may have changes in signal transduction. We hypothesized that the cyclic GMP (cGMP) signaling pathway would be altered in the post-resuscitation heart. This was studied in ventricular myocytes from 7 anesthetized open-chest rabbits. Cardiopulmonary arrest was achieved for 10 min through ventricular fibrillation and respirator shutdown. After cardiopulmonary arrest, respiration was resumed, the heart was defibrillated, and the heart recovered for 15 min. Seven additional rabbits served as controls. Myocyte function was measured via a video edge detector. Myocytes were treated with 8-bromo-cGMP (10–5–10–6 mol/L) followed by KT5823 (10–6 mol/L, cGMP protein kinase inhibitor). The baseline percent shortening was significantly depressed in the cardiac arrest myocytes compared with control (3.3 ± 0.1 vs. 5.5 ± 0.3%). Treatment with 8-Br-cGMP similarly and dose-dependently reduced cell contraction in both cardiac arrest (–24%) and control (–25%) myocytes. The negative effect of 8-Br-cGMP was partially reversed by KT5823 in control myocytes, but not in the arrest group, indicating reduced involvement of cGMP protein kinase. Multiple proteins were specifically phosphorylated when cGMP was present, but the degree of phosphorylation was significantly less in myocytes after cardiac arrest. The data suggested that the basal contraction was reduced, but the functional response to 8-Br-cGMP was preserved in myocytes from cardiopulmonary arrested hearts. The results also indicated that the action of cGMP appeared to be mainly through non-cGMP protein kinase pathways in the post-resuscitation heart. [ABSTRACT FROM AUTHOR]

1. We tested the hypothesis that the cGMP-dependent protein kinase has major negative functional effects in cardiac myocytes and that the importance of this pathway is reduced in thyroxine (T4; 0.5 mg/kg per day for 16 days) hypertrophic myocytes. 2. Using isolated ventricular myocytes from control ( n = 7) and T4-treated ( n = 9) rabbit hypertrophic hearts, myocyte shortening was studied with a video edge detector. Oxygen consumption was measured using O2 electrodes. Protein phosphorylation was measured autoradiographically. 3. Data were collected following treatment with: (i) 8-(4-chlorophenylthio)guanosine-3′,5′-monophosphate (PCPT; 10−7 or 10−5 mol/L); (ii) 8-bromo-cAMP (10−5 mol/L) followed by PCPT; (iii) β-phenyl-1, N2-etheno-8-bromoguanosine-3′,5′-monophosphorothioate, SP-isomer (SP; 10−7 or 10−5 mol/L); or (iv) 8-bromo-cAMP (10−5 mol/L) followed by SP. 4. There were no significant differences between groups in baseline percentage shortening (Pcs; 4.9 ± 0.2 vs 5.6 ± 0.4% for control and T4 groups, respectively) and maximal rate of shortening (Rs; 64.8 ± 5.9 vs 79.9 ± 7.1 µm/ s for control and T4 groups, respectively). Both SP and PCPT decreased Pcs (−43 vs−21% for control and T4 groups, respectively) and Rs (−36 vs−22% for control and T4 groups, respectively), but the effect was significantly reduced in T4 myocytes. 8-Bromo-cAMP similarly increased Pcs (28 vs 23% for control and T4 groups, respectively) and Rs (20 vs 19% for control and T4 groups, respectively). After 8-bromo-cAMP, SP and PCPT decreased Pcs (−34%) and Rs (−29%) less in the control group. However, the effects of these drugs were not altered in T4 myocytes (Pcs −24%; Rs −22%). Both PCPT and cAMP phosphorylated the same five protein bands. In T4 myocytes, these five bands were enhanced less. 5. We conclude that, in control ventricular myocytes, the cGMP-dependent protein kinase exerted major negative functional effects but, in T4-induced hypertrophic myocytes, the importance of this pathway was reduced and the interaction between cAMP and the cGMP protein kinase was diminished. [ABSTRACT FROM AUTHOR]

Leptin is a regulator of body weight and affects nitric oxide (NO) production. This study was designed to determine whether the myocardial NO-cGMP signal transduction system was altered in leptin-deficient obese mice. Contractile function, guanylyl cyclase activity, and cGMP-dependent protein phosphorylation were assessed in ventricular myocytes isolated from genetically obese (B6.V-Lep[sup ob]) and age-matched lean (C57BL/6J) mice. There were no differences in baseline contraction between the lean and obese groups. After stimulation with the NO donor S-nitroso-N-acetyl-penicillamine (SNAP, 10[sup -6] and 10[sup -5] M) or a membrane-permeable cGMP analog 8-bromo-cGMP (8Br-cGMP, 10[sup -6] and 10[sup -5] M), cell contractility was depressed. However, 8-Br-cGMP had significantly greater effects in obese mice than in lean controls with percent shortening reduced by 47 vs. 39% and maximal rate of shortening decreased by 46 vs. 36%. The negative effects of SNAP were similar between the two groups. Soluble guanylyl cyclase activity was not attenuated. This suggests that the activity of the cGMP-independent NO pathway may be enhanced in obesity. The phosphorylated protein profile of cGMP-dependent protein kinase showed that four proteins were more intensively phosphorylated in obese mice, which suggests an explanation for the enhanced effect of cGMP. These results indicate that the NO-cGMP signaling pathway was significantly altered in ventricular myocytes from the leptin-deficient obese mouse model. [ABSTRACT FROM AUTHOR]

We tested the hypothesis that the mechanism through which cyclic GMP reduces cardiac function is mediated by activation of the sarcoplasmic reticulum Ca[sup 2+] -ATPase (SERCA). Cardiac myocytes were isolated from New Zealand white rabbits (n = 11). Individual ventricular cells were stimulated by electrical field stimulation. The maximal rate of cell shortening and percentage shortening were measured with a video edge detector. Thapsigargin (10[sup –8] mol/l) was used as a specific inhibitor of SERCA. When 8-bromo-cyclic GMP (8-Br-cGMP, 10[sup –7, –6, –5] mol/l) was added to cells, the maximal rate of myocyte shortening (R[sub max] , μm/s) and percentage shortening were both decreased in a concentration-dependent manner. R[sub max] decreased 27% from 117 ± 12 at baseline to 85.2 ± 13 when 10[sup –5] mol/l of 8-Br-cGMP was present, and percent shortening was reduced 28% from 6.0 ± 0.5 to 4.3 ± 0.5%. Thapsigargin (10[sup –8] mol/l) increased the maximal rate of myocyte shortening and percent shortening. Addition of thapsigargin prior to 8-Br-cGMP reduced the negative effects of cGMP on myocyte function. The percent shortening decreased only 11% and R[sub max] decreased 14% with 10[sup –5] mol/l 8-Br-cGMP, which was not significant. Cyclopiazonic acid, another SERCA inhibitor, was also used to test whether 8-Br-cGMP reduced myocyte function through SERCA. The results were similar to those when thapsigargin was used. These results indicated that the cyclic GMP-induced reduction in cardiac myocyte function was partially mediated through the action of the sarcoplasmic reticulum Ca[sup 2+] -ATPase.Copyright © 2002 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

We tested the hypothesis that in isolated cardiac myocytes, the negative functional effects of cyclic GMP would be blunted when the level of cyclic AMP was increased and that this interaction would be altered in renal hypertensive (One-Kidney-One-Clip, 1K1C) cardiac hypertrophic rabbits. Using isolated control and 1K1C ventricular myocytes, cyclic AMP and cell shortening (%) data were collected: 1) at baseline, 2) after the addition of 8-Br-cGMP 10-7, -6, -5 M, and 3) after forskolin (10-6 M), and adenylate cyclase activator, followed by 8-Br-cGMP 10-7, -6,-5 M. Basal levels of cyclic AMP were similar in control vs. 1K1C myocytes (10.2 ± 1.6 vs. 11.3 ± 2.6 pmol/105 myocytes). We found that 8-Br-cGMP decreased the percent shortening in a dose related manner in both control myocytes (5.1 ± 0.6 to 3.2 ± 0.4%) and hypertrophic myocytes (5.2 ± 0.4 to 3.6 ± 0.5). The level of cyclic AMP significantly increased after the addition of 8-Br-cGMP in control myocytes (14.1 ± 2.1), but not in 1K1C myocytes. Forskolin increased the percent shortening in the control myocytes (3.8 ± 0.1 to 4.8 ± 0.4), but no significant increase was noted in the hypertrophic myocytes (3.6 ± 0.3 to 3.7 ± 0.3). The level of cyclic AMP significantly increased after the addition of forskolin in both control (13.9 ± 2.0), and 1K1C cells (14.6 ± 3.8). Forskolin attenuated the negative functional effects of 8-Br-cGMP in the control (4.8 ± 0.4 to 3.2 ± 0.1) and 1K1C myocytes (3.7 ± 0.3 to 2.7 ± 0.3). The adition of 8-Br-cGMP did not affect the level of cyclic AMP after forskolin in either control (13.9 ± 2.0 to 14.8 ± 2.5) or 1K1C myocytes (14.6 ± 3.8 to 13.8 ± 1.9). These data indicated that in hypertrophic cardiac myocytes the negative functional effects of 8-Br-cGMP were similar to control, but the positive functional effects of cyclic AMP were blunted. There was an increase in cyclic AMP levels after addition of 8-Br-cGMP in control but not 1K1C cells. We conclude that in control and hypertrophic myocytes, the effects of cyclic GMP were blunted after forskolin, but this did not seem to be related to cyclic AMP phosphodiesterase activity. [ABSTRACT FROM AUTHOR]

Chronic exposure to nitrates causes tachyphylaxis to nitric oxide (NO), which reduces the effects of cyclic GMP. We tested the hypothesis that prolonged exposure to NO would not only blunt the effects of NO, but also natriuretic peptides. Cardiac myocytes were isolated from control (N=7) and chronic nitroglycerin (patched, N=7) rabbits. Patched animals received a transdermal nitroglycerin patch (0.3mg/hr for 5 days). Myocyte function was determined at baseline, CNP (C-type natriuretic peptide) or BNP (Brain natriuretic peptide) 10-8M, 10-7M, followed by KT5823 (cyclic GMP protein kinase inhibitor) 10-6M; baseline, SNAP (S-nitroso-N-acetyl-penicilliamine, NO donor) 10-6M, 10-5M, followed by KT5823. Soluble and particulate guanylyl cyclase activities were measured and in vitro phosphoprotein analysis was performed. In control animals, CNP 10-8M, 10-7M significantly reduced percentage shortening from baseline 6.1±1.6% to 5.14±0.5 % and 4.4±0.7%. KT5823 restored percentage shortening to 4.9±0.8%. Similar data were obtained with BNP and SNAP. In patched animals, CNP, BNP, SNAP had no significant effects on percentage shortening. The data on maximal rate of shortening and relaxation were consistent with these results. Guanylyl cyclase activities were not different in the control and patched animals. The myocytes from control and patched animals had similar protein phosphorylation patterns. Our data suggested that in addition to NO; the responses to both natriuretic peptides were downregulated after chronic exposure to nitroglycerin, but these effects were not due to changes in either guanylyl cyclase or cyclic GMP protein kinase, suggesting altered downstream effects. [ABSTRACT FROM AUTHOR]

Abstract: Lack of endothelial nitric oxide synthase (eNOS) may affect the sensitivity of cyclic GMP signaling through soluble guanylyl cyclase (sGC). We hypothesized that in eNOS knockout (eNOS–/–) mice, stimulation of guanylyl cyclase would have enhanced effects inhibiting cardiac contraction. We measured cell shortening and calcium transients in isolated ventricular myocytes from adult eNOS–/– and wild-type (WT) mice after stimulating particulate guanylyl cyclase (pGC) with C-type natriuretic peptide (CNP, 10–8 and 10–7 M) or sGC with S-nitroso-N-acetyl-penicillamine (SNAP, NO donor, 10–6 and 10–5 M). Although sGC activity was increased by +71% in eNOS–/–, SNAP had similar effects in the two groups (%shortening –39% control vs. –37% eNOS–/–), suggesting that the cyclic GMP pathway was desensitized in eNOS–/– myocytes. CNP had significantly smaller effects on cell contraction (%shortening –34% control vs. –14% eNOS–/–) and pGC activity was not changed in eNOS–/– myocytes. Similar effects were also produced by guanylin and carbon monoxide, stimulators of pGC and sGC. CNP''s effects on Ca2+ transients were also attenuated in eNOS–/– myocytes. SNAP did not alter Ca2+ transients in eNOS–/– or control cells. In the eNOS–/– mice, cyclic GMP-dependent protein kinase and cyclic AMP phosphodiesterase activity were reduced. This study demonstrated that the downstream cyclic GMP pathway was attenuated in eNOS–/– mice and this was partially compensated for by increased sGC, but not pGC activity in ventricular myocytes. [Copyright &y& Elsevier]

Abstract: Exposure to nitrates causes tachyphylaxis to nitric oxide (NO), which reduces the effects of the second messenger cyclic guanosine-3′,-5′-monophosphate (cyclic GMP). We tested the hypothesis that prolonged exposure to NO would also blunt the effects of natriuretic peptides. Cardiac myocytes were isolated from control (N =7) and chronic nitroglycerin (patched, N =7) rabbits. Patched animals received a transdermal nitroglycerin patch (0.3mg/h for 5 days). Myocyte function was determined at baseline, after C-type natriuretic peptide (CNP, 10−8 and 10−7 M) or brain natriuretic peptide (BNP, 10−8 and 10−7 M) or S-nitroso-N-acetyl-penicilliamine (SNAP, a NO donor, 10−6 and 10−5 M) followed by KT5823 (a cyclic GMP protein kinase inhibitor, 10−6 M). Soluble and particulate guanylyl cyclase activities were measured in vitro and phosphoprotein analysis was performed. In control animals, CNP 10−8 M (5.14±0.5%) and 10−7 M (4.4±0.7%) significantly reduced percentage shortening from baseline (6.1±1.6%). KT5823 restored percentage shortening to 4.9±0.8%. Similar data were obtained with BNP and SNAP. In patched animals, CNP, BNP, SNAP had no significant effects on percentage shortening. The data on maximal rate of shortening and relaxation were consistent with these results. Guanylyl cyclase activities were not different in the control and patched animals. The myocytes from control and patched animals had similar protein phosphorylation patterns. Our data suggested that in addition to NO, the responses to both natriuretic peptides were downregulated after chronic exposure to nitroglycerin, but these effects were not due to changes in either guanylyl cyclase or cyclic GMP protein kinase, suggesting an altered downstream pathway. [Copyright &y& Elsevier]

Abstract: Brain natriuretic peptide (BNP) affects the regulation of myocardial metabolism through the production of cGMP and these effects may be altered by cardiac hypertrophy. We tested the hypothesis that BNP would cause decreased metabolism and function in the heart and cardiac myocytes by increasing cGMP and that these effects would be disrupted after thyroxine-induced cardiac hypertrophy (T4). Open-chest control and T4 rabbits were instrumented to determine local effects of epicardial BNP (10−3 M). Function of isolated cardiac myocytes was examined with BNP (10−8–10−7 M) with or without KT5823 (10−6 M, cGMP protein kinase inhibitor). Cyclic GMP levels were measured in myocytes. In open-chest controls, O2 consumption was reduced in the BNP area of the subepicardium (6.6±1.3mlO2/min/100g versus 8.9±1.4mlO2/min/100g) and subendocardium (9.4±1.3 versus 11.3±0.99). In T4 animals, functional and metabolic rates were higher than controls, but there was no difference between BNP-treated and untreated areas. In isolated control myocytes, BNP (10−7 M) reduced percent shortening (PSH) from 6.5±0.6 to 4.3±0.4%. With KT5823 there was no effect of BNP on PSH. In T4 myocytes, BNP had no effect on PSH. In control myocytes, BNP caused cGMP levels to rise from 279±8 to 584±14fmol/105 cells. In T4 myocytes, baseline cGMP levels were lower (117±2l) and were not significantly increased by BNP. Thus, BNP caused decreased metabolism and function while increasing cGMP in control. These effects were lost after T4 due to lack of cGMP production. These data indicated that the effects of BNP on heart function operated through a cGMP-dependent mechanism, and that this mechanism was disrupted in T4-induced cardiac hypertrophy. [Copyright &y& Elsevier]

Abstract: Natriuretic peptides, including atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) act through different receptors and at different potencies to affect cardiac myocyte function. We tested the hypothesis that these three peptides would differentially reduce cardiomyocyte function through their effects on the cyclic GMP signaling pathway. Rabbit ventricular myocytes were isolated and stimulated by electrical field stimulation. Cell function was measured using a video edge detector. ANP BNP or CNP at 10−9, 10−8, 10−7 M were added to the myocytes. Intracellular cyclic GMP was determined using a radioimmunoassay in the absence or presence of ANP, BNP or CNP. All natriuretic peptides decreased myocyte contractility in a similar concentration dependent manner. Myocyte percentage shortening was significantly decreased with all peptides at 10−7 M compared with baseline (ANP from 5.4±0.4 to 3.9±0.2%; BNP from 5.0±0.2 to 3.5±0.1%; CNP from 5.6±0.3 to 4.0±0.3%). Maximum rate of shortening and relaxation were also decreased similarly and significantly. Intracellular cyclic GMP was significantly increased in myocytes treated with ANP, BNP or CNP (Baseline 1.0±0.2, ANP 2.1±0.2, BNP 2.3±0.3, CNP 2.0±0.2 pmol/105 myocytes). Furthermore, inhibition of the cyclic GMP protein kinase with KT5823 caused a reversal in the functional effects of CNP. We concluded that all natriuretic peptides had similar negative effects on ventricular myocyte function and their effects were accompanied by increased cyclic GMP. Blockade the effect of CNP by a cyclic GMP protein kinase inhibitor demonstrated that effects were mediated through the cyclic GMP signaling pathway. [Copyright &y& Elsevier]

In this study, we tested the hypothesis that the negative functional effects of cyclic GMP on cardiac myocytes would be affected by the actions of cyclic GMP on cyclic AMP phosphodiesterases. Ventricular myocytes from eight rabbits were used to determine the functional and cyclic AMP changes caused by 10−7, 10−6, 10−5 M 8-Bromo-cGMP alone and after the administration of 10−6 M milrinone (cyclic GMP-inhibited cyclic AMP phosphodiesterase inhibitor) or 10−6 M erythro-9-(2-Hydroxy-3-3-nonyl)adenine (EHNA, cyclic GMP-stimulated cyclic AMP phosphodiesterase inhibitor). 8-Br-cGMP dose-dependently reduced %shortening by 35±4% of baseline at 10−5 M. This effect was significantly blunted by EHNA at all doses. The maximum rate of shortening was reduced by 31±3% by 10−5 M 8-Br-cGMP. This effect of 8-Br-cGMP was significantly enhanced (42±4%) in the milrinone group. A similar pattern was observed in the maximum rate of relaxation data. Cyclic AMP levels were significantly increased from a baseline level of 4.0±0.8 pmol/105 myocytes by milrinone (+60%), EHNA (+61%) and 8-Br-cGMP (+47%). The combination of EHNA plus 8-Br-cGMP increased cyclic AMP levels significantly more that the combination of milrinone plus 8-Br-cGMP. Exogenous cyclic GMP reduces myocyte function, while raising cyclic AMP possibly through cyclic GMP-inhibited cyclic AMP phosphodiesterase effects. Blocking cyclic GMP-inhibited cyclic AMP phosphodiesterase enhances the functional effects cyclic GMP, while blocking cyclic GMP-stimulated cyclic AMP phosphodiesterase reduced these effects. The study demonstrated a functional interaction between cyclic GMP and cyclic AMP related to the cyclic GMP affected cyclic AMP phosphodiesterases. [Copyright &y& Elsevier]