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Elevated levels of plasma homocysteine (Hcy) causes severe cardiac dysfunction, which is closely associated with oxidative stress. Emodin, a naturally occurring anthraquinone derivative, has been shown to exert antioxidant and anti-apoptosis activities. However, whether emodin could protect against Hcy-induced cardiac dysfunction remains unknown. The current study aimed to investigate the effects of emodin on the Hcy-induced cardiac dysfunction and its molecular mechanisms. Rats were fed a methionine diet to establish the animal model of hyperhomocysteinemia (HHcy). H9C2 cells were incubated with Hcy to induce a cell model of Hcy-injured cardiomyocytes. ELISA, HE staining, carotid artery and left ventricular cannulation, MTT, fluorescence staining, flow cytometry and western blotting were used in this study. Emodin significantly alleviated the structural damage of the myocardium and cardiac dysfunction from HHcy rats. Emodin prevented apoptosis and the collapse of MMP in the Hcy-treated H9C2 cells in vitro. Further, emodin reversed the Hcy-induced apoptosis-related biochemical changes including decreased Bcl-2/Bax protein ratio, and increased protein expression of Caspase-9/3. Moreover, emodin suppressed oxidative stress in Hcy-treated H9C2 cells. Mechanistically, emodin significantly inhibited the Hcy-activated MAPK by reducing ROS generation in H9C2 cells. Furthermore, emodin upregulated NO production by promoting the protein phosphorylation of Akt and eNOS in injured cells. The present study shows that emodin protects against Hcy-induced cardiac dysfunction by inhibiting oxidative stress via MAPK and Akt/eNOS/NO signaling pathways.

Cardiac hypertrophy is a common myocardial structural abnormality which may cause heart failure. Many studies have shown that cardiac hypertrophy can be induced by hyperthyroidism. Ligand-gated potassium channels have been reported to be involved in various biological processes in the cardiovascular system, such as GPCR coupled K(ACh) and metabolism sensor K(ATP) channel. It is unclear whether the gene expression of K(ACh) and K(ATP) was altered in hyperthyroid rabbit atria. We aimed to investigate the expression of K(ACh) and K(ATP) genes in rabbit atria in our experimental model. We established an effective hyperthyroidism-induced cardiac hypertrophy animal model through an injection of T4. H&E staining and RT-PCR were used to observe the histomorphological damages and alteration of gene expression. The results showed that the heart weight, heart rate significantly increased in T4-treated rabbits. The systolic pressure increased from 115.60 mmHg to 152.6 mmHg in T4-treated rabbits. The expression of K(ACh) and K(ATP) genes was decreased in the atria of hyperthyroidism-induced cardiac hypertrophied rabbits. These findings indicated that the decreased gene expression of K(ACh) and K(ATP) may be related to hyperthyroidism-induced cardiac hypertrophy and atrial fibrillation.

Homocysteine (Hcy) induced vascular endothelial dysfunction is known to be closely associated with oxidative stress and impaired NO system. 1,8-Dihydroxy-3-methoxy-6-methylanthracene-9,10-dione (physcion) has been known to has antioxidative and anti-inflammatory properties.The purpose of the present study was to define the protective effect of physcion on Hcy-induced endothelial dysfunction and its mechanisms involved.Hyperhomocysteinemia (HHcy) rat model was induced by feeding 3% methionine. A rat thoracic aortic ring model was used to investigate the effects of physcion on Hcy-induced impairment of endothelium-dependent relaxation. Two doses, low (L, 30 mg/kg/day) and high (H, 50 mg/kg/day) of physcion were used in the present study. To construct Hcy-injured human umbilical vein endothelial cells (HUVECs) model, the cells treated with 3 mM Hcy. The effects of physcion on Hcy-induced HUVECs cytotoxicity and apoptosis were studied using MTT and flow cytometry. Confocal analysis was used to determine the levels of intracellular CaIn the HHcy rat model, plasma levels of Hcy and malondialdehyde (MDA) were elevated (20.45 ± 2.42 vs. 4.67 ± 1.94 μM, 9.42 ± 0.48 vs. 3.47 ± 0.59 nM, p0.001 for both), whereas superoxide dismutase (SOD) and nitric oxide (NO) levels were decreased (77.11 ± 4.78 vs. 115.02 ± 5.63 U/ml, 44.51 ± 4.45 vs. 64.18 ± 5.34 μM, p0.001 and p0.01, respectively). However, treatment with physcion significantly reversed these changes (11.82 ± 2.02 vs. 20.45 ± 2.42 μM, 5.97 ± 0.72 vs. 9.42 ± 0.48 nM, 108.75 ± 5.65 vs. 77.11 ± 4.78 U/ml, 58.14 ± 6.02 vs. 44.51 ± 4.45 μM, p0.01 for all). Physcion also prevented Hcy-induced impairment of endothelium-dependent relaxation in HHcy rats (1.56 ± 0.06 vs. 15.44 ± 2.53 nM ECPhyscion prevents Hcy-induced endothelial dysfunction by activating Ca

Background Homocysteine (Hcy) induced vascular endothelial dysfunction is known to be closely associated with oxidative stress and impaired NO system. 1,8-Dihydroxy-3-methoxy-6-methylanthracene-9,10-dione (physcion) has been known to has antioxidative and anti-inflammatory properties. Purpose The purpose of the present study was to define the protective effect of physcion on Hcy-induced endothelial dysfunction and its mechanisms involved. Study Design and Methods Hyperhomocysteinemia (HHcy) rat model was induced by feeding 3% methionine. A rat thoracic aortic ring model was used to investigate the effects of physcion on Hcy-induced impairment of endothelium-dependent relaxation. Two doses, low (L, 30 mg/kg/day) and high (H, 50 mg/kg/day) of physcion were used in the present study. To construct Hcy-injured human umbilical vein endothelial cells (HUVECs) model, the cells treated with 3 mM Hcy. The effects of physcion on Hcy-induced HUVECs cytotoxicity and apoptosis were studied using MTT and flow cytometry. Confocal analysis was used to determine the levels of intracellular Ca2+. The levels of protein expression of the apoptosis-related markers Bcl-2, Bax, caspase-9/3, and Akt and endothelial nitric oxide synthase (eNOS) were evaluated by western blot. Results In the HHcy rat model, plasma levels of Hcy and malondialdehyde (MDA) were elevated (20.45 ± 2.42 vs. 4.67 ± 1.94 μM, 9.42 ± 0.48 vs. 3.47 ± 0.59 nM, p Conclusion Physcion prevents Hcy-induced endothelial dysfunction by activating Ca2+- and Akt-eNOS-NO signaling pathways. This study provides the first evidence that physcion might be a candidate agent for the prevention of cardiovascular disease induced by Hcy.

Homocysteine (Hcy) is an independent risk factor in the development of cardiovascular diseases (CVD). Hyperhomocysteinemia (HHcy), induces the injury of vascular endothelial cells via oxidative stress. Oxymatrine (OMT), one of the main components of Sophora flavescens, has displayed anti-inflammatory, anti-oxidant and anti-apoptotic activity. However, the effect of OMT on the Hcy-induced endothelial injury is not clearly defined yet. The aim of this study was to determine the protective effect of OMT on the Hcy-induced endothelial injury and its mechanisms involved. Human umbilical vein endothelial cells (HUVECs) were cultured in vitro. Methyl thiazolyl tetrazolium assay (MTT), fluorescence staining, flow cytometry and western blotting were used in this study. OMT prevented the Hcy-induced toxicity and apoptosis in HUVECs. Moreover, OMT suppressed Hcy-induced increases in reactive oxygen species, lactate dehydrogenase, malondialdehyde levels and increased superoxide dismutase levels. OMT reversed the Hcy-induced decrease in the protein expression of nuclear factor erythroid-2-related factor 2 (Nrf2). In addition, OMT reversed the Hcy-induced apoptosis related biochemical changes such as decreased mitochondrial membrane potential and Bcl-2/Bax protein ratio, and increased protein expression of caspase-9 and caspase-3. Furthermore, OMT elevated the phosphorylation levels of Akt and eNOS, and the formation of nitric oxide (NO) in injured cells. These results suggest that OMT prevents Hcy-induced endothelial injury by regulating mitochondrial-dependent apoptosis and Akt-eNOS-NO signaling pathways concomitantly with accentuation of Nrf2 expression.

Ethnopharmacological relevance Radix Paeoniae Rubra (RPR) is an important traditional Chinese medicine (TCM) commonly used in clinic for a long history in China. RPR is the radix of either Paeonia lactiflora Pall. or Paeonia veitchii Lynch. RPR has a wide variety of pharmacological actions such as anti-thrombus, anti-coagulation, and anti-atherosclerotic properties, protecting heart and liver. However, the mechanisms involved are to be defined. Aim of the study The aim of the present study was to define the effect of Paeonia lactiflora Pall. extracts on vascular tension and responsible mechanisms in rat thoracic aortic rings. Materials and methods Ethanol extract of Paeonia lactiflora Pall. (EPL) was examined for their vascular relaxant effects in isolated phenylephrine-precontracted rat thoracic aorta. Results EPL induced relaxation of the phenylephrine-precontracted aortic rings in a concentration-dependent manner. Vascular relaxation induced by EPL was significantly inhibited by removal of the endothelium or pretreatment of the rings with N G -nitro- l -arginine methylester ( l -NAME) or 1 H -[1,2,4]-oxadiazolo-[4,3- α ]-quinoxalin-1-one (ODQ). Extracellular Ca 2+ depletion or diltiazem significantly attenuated EPL-induced vasorelaxation. Modulators of the store-operated Ca 2+ entry (SOCE), thapsigargin, 2-aminoethyl diphenylborinate and Gd 3+ , and an inhibitor of Akt, wortmannin, markedly attenuated the EPL-induced vasorelaxation. Further, the EPL-induced vasorelaxation was significantly attenuated by pretreatment with tetraethylammonium, a non-selective K Ca channels blocker, or glibenclamide, an ATP-sensitive K + channels inhibitor, respectively. Inhibition of cyclooxygenases with indomethacin, and adrenergic and muscarinic receptors blockade had no effects on the EPL-induced vasorelaxation. Conclusions The present study suggests that EPL relaxes vascular smooth muscle via endothelium-dependent and Akt- and SOCE-eNOS-cGMP-mediated pathways through activation of both K Ca and K ATP channels and inhibition of L-type Ca 2+ channels.

Aim of the study The aim of the present study was to define the effect of Xanthoceras sorbifolia extracts (XS) on vascular tension and responsible mechanisms in rat thoracic aortic rings. Materials and methods Ethanol extract of the leaves of XS (EXS) was examined for their vascular relaxant effects in isolated phenylephrine-precontracted rat thoracic aorta. Results EXS (0.1–100 μg/ml) induced relaxation of the phenylephrine-precontracted aortic rings in a concentration-dependent manner. Endothelium-denudation abolished EXS-induced vasorelaxation. Pretreatment of the endothelium-intact aortic rings with N G -nitro- l -arginine methylester ( l -NAME) and 1H-[1,2,4]-oxadiazolo-[4,3-α]-quinoxalin-1-one (ODQ) inhibited EXS-induced vasorelaxation. Inhibition of Ca 2+ entry via l -type Ca 2+ channels failed to block the EXS-induced vasorelaxation. Extracellular Ca 2+ depletion significantly attenuated EXS-induced vasorelaxation. Modulators of the store-operated Ca 2+ entry (SOCE), thapsigargin, 2-aminoethyl diphenylborinate (2-APB) and Gd 3+ , and an inhibitor of Akt, wortmannin, markedly attenuated the EXS-induced vasorelaxation. EXS increased cGMP levels of the aortic rings in a concentration-dependent manner and the effect was blocked by l -NAME, ODQ, thapsigargin, Gd 3+ , 2-APB, and wortmannin. Further, EXS-induced vasorelaxation was significantly attenuated by tetraethylammonium, a non-selective K ca channels blocker, but not by glibenclamide, an ATP-sensitive K + channels inhibitor. Inhibition of cyclooxygenase with indomethacin, and adrenergic and muscarinic receptors blockade had no effects on EXS-induced vasorelaxation. Conclusions The present study suggests that EXS relaxes vascular smooth muscle via endothelium-dependent NO-cGMP signaling through activation of the Akt- and SOCE-eNOS-sGC pathways, which may, at least in part, be related to the function of K + channels.

It has been shown that histamine inhibits atrial natriuretic peptide (ANP) release. Because cardiac mast cells are the principal source of histamine in the heart, we hypothesized that cardiac mast cells are involved in the regulation of atrial ANP release. To test the hypothesis, experiments were performed in perfused beating rabbit atria allowing atrial pacing and measurements of changes in atrial stroke volume, intraatrial pulse pressure and myocyte ANP release. Mast cell degranulation with Compound 48/80 decreased atrial myocyte ANP release, and the response was blocked by a selective histamine H 2 receptor blocker, cimetidine, indicating that histamine was responsible for the decrease in ANP release. Mast cell stabilization with cromolyn blocked the Compound 48/80-induced decrease in ANP release. These data suggest that mast cell-derived histamine is involved in the regulation of cardiac ANP release. Thus, the cardiac mast cell–cardiomyocyte communication via the histamine–ANP pathway may implicate in the cardiac disorder associated with mast cell degranulation such as in acute coronary syndrome or cardiac hypertrophy.

Hyperosmolality has been known to increase ANP release. However, its physiological role in the regulation of atrial myocytic ANP release and the mechanism by which hyperosmolality increases ANP release are to be defined. The purpose of the present study was to define these questions. Experiments were performed in perfused beating rabbit atria. Hyperosmolality increased atrial ANP release, cAMP efflux, and atrial dynamics in a concentration-dependent manner. The osmolality threshold for the increase in ANP release was as low as 10 mosmol/kgH2O (∼3%) above the basal levels (1.55 ± 1.71, 17.19 ± 3.11, 23.15 ± 5.49, 54.04 ± 11.98, and 62.00 ± 13.48% for 10, 20, 30, 60, and 100 mM mannitol, respectively; all P < 0.01). Blockade of sarcolemmal L-type Ca2+channel activity, which increased ANP release, attenuated hyperosmolality-induced increases in ANP release (−13.58 ± 4.68% vs. 62.00 ± 13.48%, P < 0.001) and cAMP efflux but not atrial dynamics. Blockade of the Ca2+release from the sarcoplasmic reticulum, which increased ANP release, attenuated hyperosmolality-induced increases in ANP release (13.44 ± 7.47% vs. 62.00 ± 13.48%, P < 0.01) and dynamics but not cAMP efflux. Blockades of Na+-K+-2Cl−cotransporter, Na+/H+exchanger, and Na+/Ca2+exchanger had no effect on hyperosmolality-induced increase in ANP release. The present study suggests that hyperosmolality regulates atrial myocytic ANP release and that the mechanism by which hyperosmolality activates ANP release is closely related to the cross-talk between the sarcolemmal L-type Ca2+channel activity and sarcoplasmic reticulum Ca2+release, possibly inactivation of the L-type Ca2+channels.

Emodin, an active anthraquinone constituent isolated from the rhubarb, a traditional Chinese herbal medicine which is widely used in clinical treatment, has cardiovascular protective properties. However, it remains unclear whether the cardiovascular protective actions of emodin are related to an activation of cardiac natriuretic hormone secretion. The purpose of the present study was to explore the effect of emodin on the secretion of ANP, a member of the family of cardiac natriuretic hormones, and its mechanisms involved. Experiments were performed in isolated perfused beating rabbit atria allowing measurement of ANP secretion, atrial pulse pressure, and stroke volume. Emodin increased ANP secretion concomitantly with a decrease in atrial pulse pressure and stroke volume in a concentration-dependent manner. These effects were reversible. Inhibition of K(+) channels with tetraethylammonium and glibenclamide attenuated the emodin-induced changes in ANP secretion and atrial dynamics. Furthermore, the emodin-induced changes in ANP secretion and atrial dynamics were attenuated by inhibition of L-type Ca(2+) channels with nifedipine. Atropine, methoctramine, tertiapin-Q, and pertussis toxin had no significant effect on the emodin-induced changes in ANP secretion and mechanical dynamics. The present study demonstrates that emodin increases ANP secretion via inhibition of L-type Ca(2+) channels through an activation of K(+)ATP channel in isolated beating rabbit atria. The results also provide a rationale for the use of emodin in the treatment of impairment of the regulation of the cardiovascular homeostasis.

Ethnopharmacological relevance Euphorbia humifusa Willd. (EH) is an important traditional Chinese medicine that has commonly been used for treating bacillary dysentery and enteritis in many Asian countries for thousands of years. EH has a wide variety of pharmacological actions such as antioxidant, hypotensive, and hypolipidemic effects. However, the mechanisms involved are to be defined. Aim of the study The present study was performed to evaluate the cardiovascular effects of EH in rats. Materials and methods Methanol extract of EH (MEH) and ethylacetate fraction of the MEH (EEH) was examined for their vascular relaxant effects in phenylephrine-precontracted aortic rings. Effects of EEH on systolic blood pressure and heart rate were tested in Sprague–Dawley rats. Results MEH and EEH induced vasorelaxation in a concentration-dependent manner. Endothelium-denudation abolished the EEH-induced vasorelaxation. Pretreatment of the endothelium-intact aortic rings with N G -nitro- l -arginine methylester ( l -NAME) and 1 H -[1,2,4]-oxadiazolo-[4,3-α]-quinoxalin-1-one (ODQ) significantly inhibited the EEH-induced vasorelaxation. EEH increased cGMP levels of the aortic rings in a concentration-dependent manner and the effect was blocked by l -NAME or ODQ. Extracellular Ca 2+ depletion and treatments with thapsigargin, Gd 3+ , and 2-aminoethyl diphenylborinate significantly attenuated the EEH-induced vasorelaxation. Wortmannin markedly attenuated the EEH-induced vasorelaxation. In addition, tetraethylammonium, iberiotoxin, and charybdotoxin, but not apamin, attenuated the EEH-induced vasorelaxation. Glibenclamide, indomethacin, atropine, and propranolol had no effects on the EEH-induced vasorelaxation. Furthermore, EEH decreased systolic blood pressure and heart rate in a concentration-dependent manner in rats. Conclusions The present study demonstrates that EEH induces endothelium-dependent vasorelaxation via eNOS-NO-cGMP signaling through the modification of intracellular Ca 2+ , Ca 2+ entry, and large- and intermediate-conductance K Ca channel homeostasis. The data also suggest that the Akt-eNOS pathway is involved in the EEH-induced vasorelaxation. EEH induces hypotension and bradycardia in vivo .

Ursolic acid is reported to have beneficial effects on the regulation of cardiovascular homeostasis. However, the effects of ursolic acid on cardiac hormone secretion are yet to be defined. The present study was designed to test the effects of ursolic acid on the secretory and contractile functions of the atria. Experiments were conducted in isolated perfused beating rabbit atria. We measured the changes in atrial dynamics, pulse pressure, stroke volume, cAMP efflux, as well as the secretion of atrial natriuretic peptide (ANP). Ursolic acid increased ANP secretion and mechanical dynamics in a concentration-dependent manner. The inhibition of L-type Ca(2+) channels with nifedipine attenuated the ursolic acid-induced increase in ANP secretion but not mechanical dynamics. The inhibition of K(+)(ATP) channels with glibenclamide attenuated the ursolic acid-induced increase in ANP secretion-but not atrial dynamics-in a concentration-dependent manner. The selective Na(+)-K(+)-ATPase inhibitor ouabain blocked the ursolic acid-induced increase in atrial dynamics but not ANP secretion. These findings show that ursolic acid increases ANP secretion via its activation of K(+)(ATP) channels and subsequent inhibition of Ca(2+) entry through L-type Ca(2+) channels in rabbit atria. These data also suggest that ursolic acid increases atrial dynamics via its inhibition of Na(+)-K(+)-ATPase activity.

Aims Although a β-adrenoceptor (β-AR) blockade-induced increase in plasma atrial natriuretic peptide (ANP) levels is implicated in the therapeutic significance of β-AR antagonists, the role of β-AR in the regulation of ANP release is not clearly defined. The purpose of the present study was to define the role of β-AR subtypes and the mechanisms responsible for regulation of atrial ANP release. Main methods Experiments were performed in isolated perfused beating rabbit atria, including measurement of atrial contractile response, cAMP efflux, and atrial myocyte ANP release. Key findings β-AR activation with (–)-isoproterenol decreased ANP release concomitantly with increases in cAMP efflux concentration, atrial dynamics, stroke volume and pulse pressure in a concentration-dependent manner. The ANP response was inversely related to the change in cAMP efflux concentrations. The isoproterenol-induced decrease in ANP release was inhibited by β 1 -AR blockade with CGP 20712A but not by β 2 -AR blockade with ICI 118551. The isoproterenol-induced decrease in ANP release was attenuated by the L-type Ca 2+ channel antagonist nifedipine and the cAMP-dependent protein kinase inhibitor KT5720. Significance These findings suggest that β 1 -AR activation decreases ANP release via cAMP- and Ca 2+ -dependent mechanisms.

2,3-Butanedione monoxime (BDM) is a chemical phosphatase and has been known to dissociate mechanical contraction in the excitation-contraction coupling via inhibition of myofibrillar ATPase. BDM has also been found to decrease sarcolemmal L-type Ca(2+) channel activity and intracellular Ca(2+) in cardiac myocytes. It has been shown that Ca(2+) entry via L-type Ca(2+) channels decreased atrial myocyte atrial natriuretic peptide (ANP) release. The purpose of the present study was to address the effects of BDM in the regulation of ANP release. Experiments were performed in perfused beating rabbit atria. BDM accentuated atrial myocyte ANP release concomitantly with a decrease in atrial stroke volume and pulse pressure in a concentration-dependent manner. The BDM-induced activation of ANP release was attenuated by the treatment with nifedipine, an inhibitor of L-type Ca(2+) channels. BDM further decreased atrial stroke volume and pulse pressure in the presence of nifedipine. Blockade of function of the sarcoplasmic reticulum with thapsigargin plus ryanodine slightly but not significantly attenuated the BDM-induced activation of ANP release. These data show that BDM is a potent stimulator for the ANP release and also suggest that the mechanism by which BDM activates atrial myocyte ANP release is related to inhibition of the L-type Ca(2+) channel activity. The present finding also suggests that the effects of ANP released may be considered in an occasion of uncoupling by BDM of the excitation-contraction coupling of cardiomyocytes.

Although it has been known that atrial natriuretic peptide (ANP) release is regulated through muscarinic acetylcholine receptors (mAChR), the mechanism by which this neurotransmitter regulates atrial ANP release is largely unknown. This study tested the hypothesis that K(+)(ACh) channels mediate the action of mAChR on atrial myocyte ANP release. Experiments were performed in perfused beating rabbit atria. Carbachol (CCh), an agonist of cardiac mAChR, increased atrial myocyte ANP release concomitantly with a decrease in stroke volume and intra-atrial pulse pressure in a concentration-dependent manner. Isoproterenol, a beta-adrenoceptor agonist, decreased ANP release concomitantly with an increase in cAMP and mechanical dynamics. In the presence of isoproterenol, the CCh-induced increase in ANP release and decrease in cAMP efflux levels and mechanical dynamics were able to be repeated. The CCh-induced changes were blocked by selective M(2) mAChR antagonists. Tertiapin, a selective G-protein-gated K(+)(ACh) channel blocker, attenuated the CCh-induced increase in ANP release and decrease in mechanical dynamics in a concentration-dependent manner, but without a significant effect on the CCh-induced decrease in cAMP efflux levels. The CCh-induced changes in ANP release and atrial dynamics were inhibited in the atria from pertussis toxin-pretreated rabbits. These findings demonstrate that G-protein-gated K(+)(ACh) channels regulate atrial myocyte ANP release. The present study also shows that mAChR and adrenoceptors have opposing roles in the regulation of ANP release.

The role of C-type natriuretic peptide (CNP) in the pathophysiology of atrial function in hyperthyroidism has not been defined. This study was to define the role of CNP-activated particulate (p) guanylyl cyclase (GC)-cGMP-phosphodiesterase (PDE)3 signaling in the regulation of cAMP levels and contractile and secretory functions in the atria from hyperthyroid rabbits. Experiments were performed in perfused beating rabbit atria. CNP was used to activate pGC. In euthyroid atria from sham-treated rabbits, CNP (100 nM) increased cGMP and cAMP efflux by 176.7 ± 17.7 and 55.3 ± 10.0%, respectively. CNP decreased stroke volume and pulse pressure and ANP release by 51 ± 7 and 41 ± 2 and 60.4 ± 3.2%, respectively. Pretreatment with milrinone blocked the CNP-induced increase of cAMP but without significant changes in decrease of atrial dynamics and ANP release. In hyperthyroid atria, CNP-induced increase of cGMP levels was accentuated, while CNP-induced increase of cAMP was attenuated. The gain of cAMP, i.e., change in cAMP efflux concentration in terms of cGMP was attenuated in the hyperthyroid compared to euthyroid atria. CNP rather increased atrial dynamics in hyperthyroid atria instead of decrease. CNP-induced decrease in atrial ANP release was attenuated. Pretreatment with milrinone blocked the CNP-induced increase of cAMP levels concomitantly with a decrease of atrial dynamics. The present study demonstrates that altered role of CNP-activated pGC–cGMP–PDE3–cAMP signaling is involved in the pathophysiology of hyperthyroid heart.

This study tests the hypothesis that particulate (p) guanylyl cyclase (GC) and soluble (s) GC are involved in the distinct roles for the regulation of cGMP-PDE-cAMP signaling and of mechanical and secretory functions in the heart. Experiments were performed in perfused beating rabbit atria. C-type natriuretic peptide (CNP) and SIN-1, an NO donor, or BAY 41-2272 (BAY), a direct activator for sGC, were used to activate pGC and sGC, respectively. CNP and SIN-1 increased cGMP and cAMP efflux in a concentration-dependent manner. Increase in cAMP was a function of cGMP. The changes in cAMP efflux concentration in terms of cGMP were much more prominent in the atria treated with CNP than in the atria treated with SIN-1. Increase in cAMP efflux concentration was blocked by milrinone but not changed by EHNA. BAY increased cGMP but not cAMP in a concentration-dependent manner. CNP and SIN-1 decreased atrial stroke volume and myocytic ANP release. The decreases in terms of cGMP efflux concentration were much more prominent in the atria treated with CNP than in the atria treated with SIN-1 or BAY. Milrinone accentuated GC agonist–induced decreases in atrial stroke volume and ANP release. In the presence of ODQ, SIN-1 or BAY induced effects were not observed. These data suggest that pGC and sGC activations have distinct roles via cGMP-PDE3-cAMP signaling in the cardiac atrium: high and low gain switches, respectively, for the regulation of cAMP levels and contractile and secretory functions.

Changes in cyclic nucleotide production and atrial dynamics have been known to modulate atrial natriuretic peptide (ANP) release. Although cardiac atrium expresses histamine receptors and contains histamine, the role of histamine in the regulation of ANP release has to be defined. The purpose of the present study was to define the effect of histamine on the regulation of ANP release in perfused beating rabbit atria. Histamine decreased ANP release concomitantly with increases in cAMP efflux and atrial dynamics in a concentration-dependent manner. Histamine-induced decrease in ANP release was a function of an increase in cAMP production. Blockade of histamine H2receptor with cimetidine but not of H1receptor with triprolidine abolished the responses of histamine. Cell-permeable cAMP analog, 8-Br-cAMP, mimicked the effects of histamine, and the responses were dose-dependent and blocked by a protein kinase A (PKA)-selective inhibitor, KT5720. Nifedipine failed to modulate histamine-induced decrease in ANP release. Protein kinase nonselective inhibitor staurosporine blocked histamine-induced changes in a concentration-dependent manner. KT5720 and RP-adenosine 3′,5′-cyclic monophosphorothioate, another PKA-selective inhibitor, attenuated histamine-induced changes. These results suggest that histamine decreases atrial ANP release by H2receptor-cAMP signaling via PKA-dependent and -independent pathways.

Regulation of atrial release of atrial natriuretic peptide (ANP) is coupled to changes in atrial dynamics. However, the mechanism by which mechanical stretch controls myocytic ANP release must be defined. The purpose of this study was to define the mechanism by which cAMP controls myocytic ANP release in perfused, beating rabbit atria. The cAMP-elevating agents forskolin and 3-isobutyl-1-methylxanthine (IBMX) inhibited myocytic ANP release. The activation of adenylyl cyclase with forskolin inhibited ANP release, which was a function of an increase in cAMP production. Inhibitors for L-type Ca2+channels and protein kinase A (PKA) attenuated a minor portion of the forskolin-induced inhibition of ANP release. Gö-6976 and KN-62, which are specific inhibitors for protein kinase C-α and Ca2+/calmodulin kinase, respectively, failed to modulate forskolin-induced inhibition of ANP release. The nonspecific protein kinase inhibitor staurosporine blocked forskolin-induced inhibition of ANP release in a dose-dependent manner. Staurosporine but not nifedipine shifted the relationship between cAMP and ANP release. Inhibitors for L-type Ca2+channels and PKA and staurosporine blocked forskolin-induced accentuation of atrial dynamics. These results suggest that cAMP inhibits atrial myocytic release of ANP via protein kinase-dependent and L-type Ca2+-channel-dependent and -independent signaling pathways.

Atrial secretion of atrial natriuretic peptide (ANP) has been shown to be regulated by atrial workload. Although modulating factors for the secretion of ANP have been reported, the role for intracellular Ca2+on the secretion of ANP has been controversial. The purpose of the present study was to define roles for L- and T-type Ca2+channels in the regulation of ANP secretion in perfused beating rabbit atria. BAY K 8644 (BAY K) increased atrial stroke volume and pulse pressure. BAY K suppressed ANP secretion and ANP concentration in terms of extracellular fluid (ECF) translocation concomitantly with an increase in atrial dynamics. BAY K shifted the relationship between ANP secretion and ECF translocation downward and rightward. These results indicate that BAY K inhibits myocytic release of ANP. In the continuous presence of BAY K, diltiazem reversed the effects of BAY K. Diltiazem alone increased ANP secretion and ANP concentration along with a decrease in atrial dynamics. Diltiazem shifted relationships between ANP secretion and atrial stroke volume or ECF translocation leftward. The T-type Ca2+channel inhibitor mibefradil decreased atrial dynamics. Mibefradil inhibited ANP secretion and ANP concentration in contrast with the L-type Ca2+channel inhibitor. These results suggest that activation of L- and T-type Ca2+channels elicits opposite effects on atrial myocytic release of ANP.