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L Zhang and XQ Hu
Department of Pharmacology, Loma Linda University School of Medicine, California, USA.
Serotonin (5-HT)-mediated inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) synthesis and contractions were examined in isolated sheep uterine arteries. 5-HT stimulated a rapid increase of Ins(1,4,5)P3 production with the peak at 30 sec. The accumulation of Ins(1,4,5)P3 was transient and declined to a steady state slightly above the basal level at 4 min. The increase of inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) was also rapid, reaching the peak at 60 sec, and subsequently declining to the steady state at 4 min. Comparison of the time courses of 5-HT- induced Ins(1,4,5)P3 production with the force development indicated that increase of Ins(1,4,5)P3 content preceded the force development in the initial phasic component, but subsequently decreased, whereas the maximal tension was maintained. Consistent with the time courses, there was a nonlinear temporal relationship between Ins(1,4,5)P3 production and the force development measured simultaneously in the same tissue stimulated by 10 microM 5-HT. 5-HT-stimulated Ins(1,4,5)P3 was concentration-dependent with EC50 of 0.48 microM. In accordance, 5-HT produced concentration-dependent contractions. The dissociation constant (KA) of 5-HT in the uterine artery was 0.52 +/- 0.08 microM. Plotting the relative responses as a function of the fractional receptor occupancy indicated a hyperbolic relationship for contractions, but a linear relationship for Ins(1,4,5)P3 production. Simultaneous measurement of contractions and Ins(1,4,5)P3 productions elicited by 5-HT (0.1-3 microM) revealed a significant linear correlation between these two events. The 5-HT-mediated Ins(1,4,5)P3 response was blocked by ketanserin (0.1 microM), but not by prazosin (0.1 microM). Pretreatment of tissues with pertussis toxin (200 ng/ml, 3 hr) failed to block 5-HT-induced inositol phosphates accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)
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