CALCIUM REQUIREMENT FOR 5-HYDROXYTRYPTAMINE-INDUCED MATURATION OF SPISULA OOCYTES

5-Hydroxytryptamine (5-HT) triggers dissolution of the germinal vesicle (GVBD) in Spisula oocytes suspended in artificial sea water (ASW) but not in Ca2+-free ASW. The maturation-inducing potency of 5-HT increases with increasing concentrations of extracellular Ca2+ up to 50% of natural SW content (9.27 mM). Of the various Ca2+ channel antagonists tested, verapamil was the most potent blocker of 5-HT-induced maturation. 5-HT stimulated Ca-45(2+) uptake revealed a lag period of two min followed by an accelerated uptake. The 5-HT agonists, 8-OH-DPAT (5-HT1A) and c(-methyl-5-HT (5-HT2), stimulated Ca-45(2+) uptake by Spisula occytes. The 5-HT antagonist, mianserin, and the Ca2+ channel antagonist, verapamil, blocked 5-HT-stimulated Ca-45(2+) uptake. Taken together, the present results suggest that an early action of 5-HT on Spisula oocytes is to promote gating of Ca2+ channels.