In rat aorta, 5-hydroxytryptamine (5-HT) stimulated phosphoinositide (PI) turnover and contraction (EC50 = 10 +/- 3 microM); these two responses were highly correlated (r = 0.95; P less than .01). We have characterized the inhibitory potency of a variety of 5-HT-antagonists against the stimulation of PI turnover elicited by 5-HT. Classic 5-HT2 antagonists mianserin, ketanserin, metergoline and pizotifen were found to inhibit this response in the low nanomolar range; amitryptiline and haloperidol were 10- to 20-fold less potent. The alpha-1 receptor antagonist, prazosin, was inactive in micromolar concentrations. The potency of the 5-HT2 antagonists was correlated with their ability to displace [3H] ketanserin binding from rat frontal cortex membranes (r = 0.90; P less than .05). The tumor promoter phorbol dibutyrate was found to inhibit 5-HT-stimulated PI turnover at low nanomolar concentrations whereas the biologically inactive substance 4-alpha-phorbol was ineffective. Pretreatment of rat aorta with phorbol dibutyrate at concentrations that inhibited 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 may desensitize 5-HT2-receptor-mediated PI turnover and contraction of rat aorta, possibly via an activation of protein kinase C.