Control of the low voltage-activated calcium channel of mouse sperm by eggZP3 and by membrane hyperpolarization during capacitation

Sperm adhesion to egg zonae pellucidae initiates sperm aerosome reactions, an exocytotic event that is an early step during fertilization. Previously, it was suggested that zona pellucida-evoked Ca2+ entry into sperm through low voltage-activated Ca2+ channels is an essential step in acrosome reacti ons, based on the inhibitory effects of Ca2+ channel antagonists. However, analysis of this channel is limited by the inability to apply electrophysio logical methods directly to sperm. In this report, optical methods of deter mining membrane potential and internal Ca2+ levels were used to demonstrate that (i) contact with zonae pellucidae activates a transient Ca2+ response in sperm that has a time course and antagonist sensitivity anticipated of low voltage-activated Ca2+ channels; (ii) these channels are unavailable fo r opening in uncapacitated sperm because of voltage-dependent, steady state inactivation; (iii) membrane hyperpolarization during sperm capacitation i s sufficient to recruit channels into a closed state, from which they are a vailable for opening during fertilization; and (iv) channel conductance sta te may be a factor in determines the efficacy with which channel antagonist s inhibit fertilization. This study provides evidence for the activation of sperm Ca2+ channels during gamete adhesion and offers a mechanism that may account for aspects of the regulation of sperm fertility during capacitati on through the control of channel availability. Finally, these results sugg est that channel conductance state may be a central feature in the design o f channel antagonists that inhibit sperm function.