Control of the low voltage-activated calcium channel of mouse sperm by egg ZP3 and by membrane hyperpolarization during capacitation

Abstract

Sperm adhesion to egg zonae pellucidae initiates sperm acrosome 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 reactions, based on the inhibitory effects of Ca²⁺ channel antagonists. However, analysis of this channel is limited by the inability to apply electrophysiological methods directly to sperm. In this report, optical methods of determining 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 for opening in uncapacitated sperm because of voltage-dependent, steady state inactivation; (iii) membrane hyperpolarization during sperm capacitation is sufficient to recruit channels into a closed state, from which they are available for opening during fertilization; and (iv) channel conductance state may be a factor in determines the efficacy with which channel antagonists 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 capacitation through the control of channel availability. Finally, these results suggest that channel conductance state may be a central feature in the design of channel antagonists that inhibit sperm function.