The smooth muscle of rat aorta was used as a model system for the study of the molecular mechanisms of 5-HT2 receptor function. The 5-HT2 receptor-mediated contraction of rat aorta could be dissected into two distinct components (phasic and tonic) by the use of appropriate inhibitors; nifedipine, an inhibitor of voltage-dependent Ca2+ channels, inhibited only the phasic component of 5-HT-induced contraction while totally blocking the KCl-induced contraction. 2-Nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC), an inhibitor of phospholipase C, inhibited the tonic components of 5-HT-induced contraction. This component of contraction was mimicked by a protein kinase C activator, 12-0-tetradecanoylphorbol-13-acetate. The activity of phospholipase C was activated by 5-HT (500% increase) with an EC50 of about 10 MuM which was nearly identical to the dose response noted for 5-HT-induced contraction. The potency order of the antagonists was consistent with the view that 5-HT-induced activation of phospholipase C is mediated through 5-HT2 receptors. The phospholipase C inhibitor NCDC inhibited the 5-HT induced increase in phospholipase C activity in rat aorta. Phosphorylation in a cell-free system of rat aorta showed that addition of calcium and phosphatidylserine increased the phosphorylation of some aortic proteins. These protein phosphorylations were also increased by protein kinase C activators such as 12-0-tetradecanoylphorbol-13-acetate (TPA) and 1,2-diolein implying that these proteins are the physiological substrates for protein kinase C in rat aorta. This study suggests that 5-HT2 receptors differentially regulate a voltage-dependent Ca2+ channel and phospholipase C activity. This study may provide a model system for the study of the signal transduction of the brains 5-HT2 receptor.
