Nitric Oxide Synthase (NOS3)–Mediated Cholinergic Modulation of Ca 2+ Current in Adult Rabbit Atrioventricular Nodal Cells

Abstract

Abstract We examined the role of endogenous NO in the autonomic regulation of atrioventricular (AV) nodal function by studying spontaneous action potentials (SAPs) and L-type Ca 2+ current (I Ca-L ) in isolated single AV nodal cells from adult rabbit hearts. Both the perforated and the membrane-ruptured patch-clamp techniques in the whole-cell configuration were used under conditions known to alter NO production. Three NO donors, 3-morpholinosydnonimine (SIN-1, 0.1 mmol/L), S -nitroso-acetylcysteine (0.1 mmol/L), and sodium nitroprusside (0.1 mmol/L), suppressed the β-adrenergic agonist isoproterenol (ISO, 1 μmol/L)–stimulated increase in I Ca-L . SIN-1 also decreased the frequency and amplitude of SAPs. In cells in which I Ca-L had been previously attenuated by the muscarinic agonist carbamylcholine (CCh, 1 μmol/L), SIN-1 had no additive effect. CCh activated an acetylcholine-sensitive outward K + current (I K(ACh) ) in AV nodal cells, in addition to the I Ca-L inhibition. Intracellular dialysis with the NO synthase inhibitor N -monomethyl- l -arginine (L-NMMA, 0.5 mmol/L) blocked CCh-induced, but not SIN-1–induced, I Ca-L attenuation. However, intracellular dialysis with methylene blue (20 μmol/L), which inhibits NO-mediated activation of guanylyl cyclase and cGMP production, blocked the effects of both CCh and SIN-1 on I Ca-L . In these cells, neither L-NMMA nor methylene blue affected the CCh-activated I K(ACh) . Direct application of cGMP (10 μmol/L) via internal dialysis significantly inhibited ISO-stimulated I Ca-L . In AV nodal cells internally perfused with either a nonhydrolyzable cAMP analogue, 8-Br-cAMP (0.5 mmol/L), or a high concentration of cAMP (0.5 mmol/L), CCh did not inhibit I Ca-L but still activated I K(ACh) . CCh-induced I Ca-L attenuation could be abolished or quickly reversed by the nonselective phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (20 μmol/L). However, CCh still significantly suppressed ISO-stimulated I Ca-L after the cGMP-inhibited PDE isozyme (PDE3) had been selectively inhibited by milrinone (5 μmol/L). Immunohistochemical staining identified the presence of the endothelial constitutive NO synthase (ecNOS or NOS3) in both single AV nodal cells in vitro and in cryostat sections of AV nodal tissue in situ. These results demonstrate that endogenous NO is involved in the muscarinic cholinergic attenuation of I Ca-L in AV nodal cells; the mechanism likely involves the cGMP-stimulated PDE.