NITRIC-OXIDE SYNTHASES AND CARDIOVASCULAR SIGNALING

Nitric oxide (NO) synthesized from L-arginine is a ubiquitous intercellular chemical messenger involved in signal transduction in diverse mammalian cells, including vascular endothelium and neuronal tissues. The recent isolation of molecular clones for NO synthases has permitted the characterization of several distinct enzyme isoforms and has allowed us to identify a family of related genes. NO synthesized in vascular endothelial cells appears to play an important role in the control of vascular tone and platelet aggregation, apparently through the activation of guanylate cyclase activity in target tissues mediated by NO. The role of the NO signaling pathway in the direct modulation of cardiac function is less well characterized. We have found that inhibitors of NO synthase can modulate the response of neonatal or adult rat ventricular myocytes exposed to muscarinic or adrenergic agonists. The effects of carbachol on the inhibition of the spontaneous beating rate of cultured neonatal rat cardiac myocytes are blocked by L-N-monomethylarginine, an L-arginine analog that inhibits NO synthase, and by methylene blue, an inhibitor of NO; these agents have no effect on the basal beating rate of these cells. The negative chronotropic effect of carbachol is also mimicked by analogs of cyclic guanosine monophosphate (cGMP), a second messenger implicated in mediating the action of NO in other cell types. Production of NO can be detected directly in carbachol-stimulated neonatal myocytes using a reporter cell bioassay. In studies of adult cardiac myocyte contractility, the NO synthase inhibitor L-nitro-arginine was found to increase the inotropic effect of the beta-adrenergic agonist isoproterenol on adult rat ventricular myocytes but had no effect on basal contractility. Thus, the physiologic response of isolated neonatal and adult ventricular myocytes to both muscarinic cholinergic and beta-adrenergic stimulation is mediated, at least in part, by products of an endogenous NO synthase.