The smooth muscle of rat aorta was used as a model for the study of the molecular mechanisms of 5-HT2 receptor function. We have previously shown that 5-HT receptors in rat aorta are coupled to phosphoinositide (PI) - specific phospholipase C. Further, we showed that the mechanism of contraction elicited by 5-HT is a complicated scenario involving receptor-mediated activation of calcium channels and a phospholipase C. We now report that in rat aorta, the 5-HT-induced contraction and PI turnover are modulated by biologically active phorbol esters. In rat aorta the 5-HT-induced contraction and PI hydrolysis (EC50=10+3 uM) were highly correlated. Also, the inhibitory potency of a variety of 5-HT2 antagonists was correlated with binding to the brain 5-HT2 receptor. Further, the tumor-promoting phorbol ester, phorbol dibutyrate (PDB), inhibited 5-HT-induced PI turnover at low nM concentrations, while the biologically inactive substance 4-Alpha-phorbol was ineffective. Pretreatment of aortic rings with PDB at concentrations which desensitized 5-HT-induced PI turnover also attenuated the aortic contraction induced by 5-HT in the presence of a calcium channel blocker, nitrendipine. Our results suggest that phorbol esters desensitize 5-HT receptor-mediated PI turnover and contraction, probably by activation of protein kinase C. Studies of the physiological substrates for protein kinase C in aorta smooth cells are now in progress.