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

Abstract

In the ascidian Ciona intestinalis (and C. savignyi), sperm-activating and -attracting factor (SAAF) is released from the egg at fertilization and stimulates both Ca2+ 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 mV 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 μM 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.