Nitric oxide synthase does not participate in negative inotropic effect of acetylcholine in frog heart

In the heart, the parasympathetic neurotransmitter acetylcholine (ACh) reduces the force of contraction. Although the effect of ACh can be partly explained by an inhibition of adenylyl cyclase, some of the effects of ACh may also be mediated via stimulation of nitric oxide synthase (NOS) and production of guanosine 3',5'-cyclic monophosphate (cGMP). NOS inhibitors can prevent the negative chronotropic effect of ACh on spontaneously beating cardiomyocytes and suppress the inhibition of the L-type calcium current (I-Ca) by ACh in sinoatrial myocytes. This pathway may be relevant not only to the chronotropic effect of ACh but also to its inotropic effect, because ACh, NO, and cGMP regulate the force of contraction and I-Ca in the cardiac ventricle. Here we report the effects of L-arginine (L-Arg), the substrate of NOS, and N-G-monomethyl-L-arginine (L-NMMA) and N-G-nitro-L-arginine (L-NNA), two NOS inhibitors, on muscarinic effects in the cardiac ventricle. We found that L-Arg, L-NMMA, and L-NNA have no effect on the muscarinic inhibition of I-Ca in isolated frog myocytes. In addition, these compounds have no significant effects on basal I-Ca or beta-adrenergic stimulation of I-Ca. L-Arg and its analogues did not change the negative inotropic effect of ACh in frog ventricular fibers. Basal active tension and the positive inotropic effect of isoproterenol, a beta-adrenergic agonist, also were unaffected. We conclude that NOS is not involved in muscarinic inhibition of I-Ca in isolated frog ventricular myocytes or the negative inotropic effect of ACh in the frog ventricle.