A fast transient hyperpolarization occurs during the sea urchin sperm acrosome reaction induced by egg jelly.

The egg jelly-induced sea urchin sperm depolarization, assayed with the membrane potential-sensitive dye diS-C3-(5), is preceded by a fast quenching, which was initially taken as an artifact. Here we show that part of this quenching results from a K+-dependent transient hyperpolarization (approximately 4 s). Seawater containing 25-35 mM KCl inhibits this hyperpolarization, the depolarization and the acrosome reaction induced by egg jelly, or by the acrosome reaction inducing factor purified therefrom. These results suggest that egg jelly induces a transient hyperpolarization mediated by an increase in K+ permeability, which may be involved in triggering the acrosome reaction.