Mechanism of gamete shedding in starfish: Involvement of acetylcholine in extracellular Ca-dependent contraction of gonadal walls.

Starfish are suitable animals for the study of hormonal regulatory mechanism of oocyte maturation and ovulation. Although contraction of the gonadal walls is essential for the shedding gametes, little was known about the mechanism. When ovaries of starfish Patiria pectinifera were incubated in Ca-free seawater in the presence of 1-methyladenine (1-MeAde), the germinal vesicles in oocytes broke down, but no ovulation occurred. Verapamil, a potent inhibitor of voltage-dependent Ca channels, inhibited 1-MeAde-induced ovulation. These results suggest that extracellular Ca and its influx are indispensable for gamete shedding. Furthermore, acetylcholine (ACh) was involved in extracellular Ca-dependent contractions of gonadal walls. Although 1-MeAde failed to induce contraction of the gonadal walls in normal seawater containing L-glutamic acid, application of ACh or carbachol, an agonist for ACh receptor, could bring about shedding of mature oocytes. Atropine, a competitive antagonist of the muscarinic ACh receptor, inhibited 1-MeAde-induced ovulation, but a nicotinic ACh receptor antagonist mecamylamine had no effect. Furthermore, ACh was detected in the ovaries and testes in P. pectinifera. These findings suggest that ACh acts on muscarinic ACh receptors in gonadal walls to induce peristaltic contractions caused by Ca influx via Ca channels in the gonadal wall muscle for gamete shedding. The present study also provides new insight into the regulatory mechanism that 1-MeAde acts on secretion of ACh in ovaries and testes.