Abstract

The effects ofN^G-monomethyl-l-arginine (l-NMMA), a nitric-oxide synthase (NOS) inhibitor, on the L-type Ca²⁺ current (ICa) and NO effects on NOS were determined in rat ventricular myocytes. l-NMMA (10 and 100 μM) had no significant effect on basal ICa, but in a cAMP-stimulated condition due to forskolin (1 μM) or milrinone (10 μM), a cGMP-inhibited cAMP-phosphodiesterase (PDE), l-NMMA (10 and 100 μM) concentration dependently augmented ICa. The enhancing effects of l-NMMA (10 and 100 μM) on ICa were not seen in the presence of either a nonselective inhibitor of PDE, 3-isobutyl-1-methylxanthine (20 μM), resulting in a stimulated ICa condition or a cGMP-dependent protein kinase activator, 8-bromo-cGMP (200 μM). 8-Bromo-cGMP (200 μM) inhibited 100 μMl-NMMA-induced ICa increase in the simultaneous application of forskolin (1 μM). Acetylcholine (ACh; 1 and 3 μM) inhibited 1 μM forskolin-stimulated ICa in a concentration-dependent manner, but this inhibitory action of ACh was significantly attenuated by the additional application of l-NMMA (100 μM). In the continuing presence of both l-NMMA (100 μM) and forskolin (1 μM), ACh (6 μM) had no inhibitory effect on ICa. In another series of experiments with isolated ventricular myocytes, we obtained both the positive staining of NADPH-diaphorase activity and the expression of the endothelial isoform of NOS. These data suggest that the effect of l-NMMA on ICa in a cAMP-stimulated condition with or without cholinergic inhibition is due to inhibition (acute effects) of a cGMP-stimulated cAMP-PDE via inhibition of the endothelial isoform of NOS.