Membrane Hyperpolarization by Sperm-Activating and -Attracting Factor Increases cAMP Level and Activates Sperm Motility in the Ascidian Ciona intestinalis

摘要

In the ascidian Ciona intestinalis (and C. savignyi), sperm-activating and -attracting factor (SAAF) is released from the egg at fertilization and stimulates both Ca~(2+) influx and a transient increase in cAMP level of the sperm, leading to the activation of sperm motility (M. Yoshida et al., 1994 Dev. Growth Differ. 36, 589-595). In this paper we show in C. intestinalis that valinomycin, a potassium-selective ionophore, as well as SAAF, activated sperm motility, and this activation was suppressed by extracellular high K~+. Membrane potential measurements showed that both SAAF and valinomycin increase K~+ permeability of sperm and induce membrane hyperpolarization, the amplitude of which depends on the external K~+ concentration. The membrane potential and intracellular K~+ concentration of Ciona sperm without SAAF were estimated to be about -50 m V and 560 +- 40 mM, respectively. After treatment with SAAF or valinomycin the membrane potential became almost equal to the equilibrium potential of K~+ (-100 mV), and the cAMP level increased in artificial seawater. A potent voltage-dependent K~+ channel blocker, MCD peptide, at the concentration of 10 #mu#M blocked SAAF-induced hyperpolarization of the cells, increase in cAMP, and sperm motility. These results suggest that membrane hyperpolarization produced by the opening of K~+ channels elevates cAMP synthesis and leads to the activation of sperm motility in Ciona.