Vol. 294, Issue 1, 216-223, July 2000

Effects of N^G-Monomethyl-L-arginine on Ca²⁺ Current and Nitric-Oxide Synthase in Rat Ventricular Myocytes

Shigeji Matsumoto, Toshiaki Takahashi, Mizuho Ikeda, Toshimi Nishikawa, Shinki Yoshida and Tomoyuki Kawase

Department of Physiology, Nippon Dental University, School of Dentistry at Tokyo, Tokyo (S.M., T.T., M.I., T.N., S.Y.); and Department of Pharmacology, Niigata University, School of Dentistry, Niigata, Japan (T.K.)

The effects of N^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 µM L-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.