Membrane hyperpolarization by sperm-activating and -attracting factor increases cAMP level and activates sperm motility in the ascidian Ciona intestinalis

In the ascidian Ciona intestinalis (and C. savignyi), sperm-activating and -attracting factor (SAAF) is released from the egg at fertilization and sti mulates both Ca2+ influx and a transient increase in cAMP level of the sper m, leading to the activation of sperm motility (M. Yoshida et al., 1994, De v. Growth Differ. 36, 589-595). In this paper we show in C. intestinalis th at 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 in crease RC permeability of sperm and induce membrane hyperpolarization, the amplitude of which depends on the external K+ concentration. The membrane p otential and intracellular K+ concentration of Ciona sperm without SAAF wer e estimated to be about -50 mV and 560 +/- 40 mM, respectively. After treat ment 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 RC channel blocker, MCD pe ptide, at the concentration of 10 mu M blocked SAAF-induced hyperpolarizati on of the cells, increase in cAMP, and sperm motility. These results sugges t that membrane hyperpolarization produced by the opening of K+ channels el evates cAMP synthesis and leads to the activation of sperm motility in Cion a. (C) 1999 Academic Press.