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 Ca 2ϩ 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-␣ and Ca 2ϩ /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 Ltype Ca 2ϩ 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 Ca 2ϩ -channeldependent and -independent signaling pathways. atrial natriuretic peptide; forskolin; phosphodiesterase; channels ENDOCRINE ATRIUM SYNTHESIZES and releases a family of natriuretic peptides including atrial natriuretic peptide (ANP) and brain natriuretic peptide (7,19). C-type natriuretic peptide, a third member of the natriuretic peptide family (32) that is synthesized in the atrium, may have paracrine/autocrine function for the regulation of ANP release (17).There have been reports on the variable modulators for the control of ANP release (23). However, the specific control mechanism for ANP release must be defined. The most prominent activator for atrial secretion of ANP has been shown to be the stretch and/or release of atrial wall (2, 10, 16). However, the intracellular mechanism responsible for the activation of ANP release by mechanical stimulation is unknown.The potential roles of cyclic nucleotides and Ca 2ϩ in the regulation of ANP release have been subjects of interest. Recently we found that both cGMP and Ca 2ϩ are negative regulators for atrial myocytic release of ANP (4, 14, 17). There are diverse reports on the effects of cAMP in the regulation of ANP secretion. Forskolin, an activator of adenylyl cyclase (AC), has been shown to decrease ANP release from cultured atrial myocytes (12,20,30) and perfused rat heart (25); 3-isobutyl-1-methylxanthine (IBMX), a nonselective inhibitor of cyclic nucleotide phosphodiesterase (PDE) (12), and 8-bromoadenosine 3Ј,5Ј-cyclic monophosphate (8-BrcAMP) (12, 30), have also been shown to inhibit ANP secretion. In contrast, it has also been shown that cAMP-elevating agents (1,5,24) and cell membranepermeant cAMP analogs (1,5,27...

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 Ca(2+) on the secretion of ANP has been controversial. The purpose of the present study was to define roles for L- and T-type Ca(2+) 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 Ca(2+) channel inhibitor mibefradil decreased atrial dynamics. Mibefradil inhibited ANP secretion and ANP concentration in contrast with the L-type Ca(2+) channel inhibitor. These results suggest that activation of L- and T-type Ca(2+) channels elicits opposite effects on atrial myocytic release of ANP.

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Abstract-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. Key Words: cGMP Ⅲ cAMP Ⅲ guanylyl cyclase Ⅲ phosphodiesterase 3 Ⅲ atrial natriuretic peptide B oth particulate (p) guanylyl cyclase (GC)-and soluble (s) GC-cGMP systems are known to be involved in the regulation of cardiac contractile and secretory functions. cGMP-phosphodiesterase (PDE)-cAMP signaling is one of the mechanisms responsible for the function. Both systems are present in cardiac myocytes. 1,2 However, significance of the presence of the dual systems in the regulation of cardiac function has to be defined. This study tests the hypothesis that pGC and sGC are involved in the distinct roles for the regulation of cGMP-PDE-cAMP signaling and of mechanical and secretory functions in the heart. Nitric oxide (NO) is an endogenous activator for sGC activity. The most important and main second messenger for NO is cGMP. The effects of NO on myocardial contractility and Ca 2ϩ current are variable. 3 In human atrial myocytes, NO donor (sodium nitroprusside, SNP) induced positive inotropic effect. 4 In rabbit atrial cells, NO donor increased Ca 2ϩ current. 5 In contrast, in human atrial and ventricular myocardium, SNP induced negative inotropic effect via cGMP signaling. 6 Also, it was shown that NO donors exhibited biphasic effects: an increase in contractile response of rat ventricular myocytes to a low increase in cGMP by NO donor and a decrease in contractile response to a high increase in cGMP. 7,8 Biphasic effect of NO was also observed with SIN-1 in the human atrial myocytes 4 : an increase in Ca 2ϩ current with a low do...

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