Effects of N(G)-monomethyl-L-arginine on Ca(2+) current and nitric-oxide synthase in rat ventricular myocytes

The effects of N(G)-monomethyl-L-arginine (L-NMMA), a nitric-oxide synthase (NOS) inhibitor, on the L-type Ca(2+) current (ICa) and NO effects on NOS were determined in rat ventricular myocytes. L-NMMA (10 and 100 microM) had no significant effect on basal ICa, but in a cAMP-stimulated condition due to forskolin (1 microM) or milrinone (10 microM), a cGMP-inhibited cAMP-phosphodiesterase (PDE), L-NMMA (10 and 100 microM) concentration dependently augmented ICa. The enhancing effects of L-NMMA (10 and 100 microM) on ICa were not seen in the presence of either a nonselective inhibitor of PDE, 3-isobutyl-1-methylxanthine (20 microM), resulting in a stimulated ICa condition or a cGMP-dependent protein kinase activator, 8-bromo-cGMP (200 microM). 8-Bromo-cGMP (200 microM) inhibited 100 microM L-NMMA-induced ICa increase in the simultaneous application of forskolin (1 microM). Acetylcholine (ACh; 1 and 3 microM) inhibited 1 microM 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 microM). In the continuing presence of both L-NMMA (100 microM) and forskolin (1 microM), ACh (6 microM) 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.