SEROTONIN STIMULATES RAPID INCREASE OF INOSITOL 1,4,5-TRISPHOSPHATE IN OVINE UTERINE ARTERY - CORRELATION WITH CONTRACTILE STATE

Serotonin (5-HT)-mediated inositol 1,4,5-trisphosphate (Ins(1,4,5)P-3) synthesis and contractions were examined in isolated sheep uterine ar teries. 5-HT stimulated a rapid increase of Ins(1,4,5)P-3 production w ith the peak at 30 sec. The accumulation of Ins(1,4,5)P-3 was transien t and declined to a steady state slightly above the basal level at 4 m in. The increase of inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P- 4) was also rapid, reaching the peak at 60 sec, and subsequently decli ning to the steady state at 4 min. Comparison of the time courses of 5 -HT-induced Ins(1,4,5)P-3 production with the force development indica ted that increase of Ins(1,4,5)P-3 content preceded the force developm ent in the initial phasic component, but subsequently decreased, where as the maximal tension was maintained. Consistent with the time course s, there was a nonlinear temporal relationship between Ins(1,4,5)P-3 p roduction and the force development measured simultaneously in the sam e tissue stimulated by 10 mu M 5-HT. 5-HT-stimulated Ins(1,4,5)P-3 was concentration-dependent with EC(50) of 0.48 mu M. In accordance, 5-HT produced concentration-dependent contractions. The dissociation const ant (K-A) of 5-HT in the uterine artery was 0.52 +/- 0.08 mu M. Plotti ng the relative responses as a function of the fractional receptor occ upancy indicated a hyperbolic relationship for contractions, but a lin ear relationship for Ins(1,4,5)P-3 production. Simultaneous measuremen t of contractions and Ins(1,4,5)P-3 productions elicited by 5-HT (0.1- 3 mu M) revealed a significant linear correlation between these two ev ents. The 5-HT-mediated Ins(1,4,5)P-3 response was blocked by ketanser in (0.1 mu M), but not by prazosin (0.1 mu M). Pretreatment of tissues with pertussis toxin (200 ng/ml, 3 hr) failed to block 5-HT-induced i nositol phosphates accumulation. The results suggest that, in the uter ine artery of late pregnancy, 5-HT2 receptor-elicited contractions, at least the initial component, are predominantly mediated by the increa se of Ins(1,4,5)P-3, leading to release of Ca++ from intracellular sto res.