Nitric oxide is an autocrine feedback inhibitor of vascular smooth muscle contraction

Background, Substances that increase intracellular calcium ([Ca2+](i)), such as potassium chloride and serotonin, are known to induce vascular smooth muscle (VSM) contraction. One form of nitric oxide synthase, which converts L-arginine to nitric oxide, exists as a Ca2+-calmodulin dependent enzyme. The objective of this study was to determine whether agonists that induce VSM contraction by increasing [Ca2+](i) might also activate Ca2+-calmodulin dependent nitric oxide synthase in VSM. Methods, Strips of bovine carotid arterial smooth muscle denuded of endothelium were equilibrated in a physiologic muscle bath. A maximal contractile response to high extracellular potassium chloride and serotonin was established. The strips were then preincubated with N-G-monomethyl-L-arginine (L-NMMA), a structural analog of L-arginine and specific inhibitor of nitric oxide synthase, and again treated with either KCl or 5-hydroxytryptamine. Results. The contractile responses of muscle strips to KCI or 5-hydroxytryptamine were significantly greater in muscle strips pretreated with L-NMMA than responses in the absence of L-NMMA (p < 0.02, Student's t test). To determine whether this response was Ca2+ dependent, phorbolester-induced contractions in Ca2+-free conditions were examined. No difference was noted in the magnitude of Ca2+-free, phorbol ester-induced contractions in the presence and absence of L-NMMA. Conclusions. These data thus suggest that Ca2+-calmodulin dependent nitric oxide synthase is functionally present in VSM and may function as an autocrine regulatory mechanism of VSM contraction.