Voltage dependent calcium channels in mammalian spermatozoa

Abstract

Calcium influx is an absolute requirement for the physiological acrosome reaction in sperm from all sources examined, both invertebrate and mammalian. Pharmacological studies suggest that the major channel in the sperm head plasma membrane responsible for modulating calcium entry and intracellular ionized calcium levels could be either an L-type (a class of high voltage-activated) or a T-type (low voltage-activated) voltage-dependent calcium channel. Patch clamp analysis of calcium currents in immature spermatogenic cells demonstrates the presence of T-type currents. Therefore, an argument has been put forth that the acrosome reaction of ejaculated sperm is regulated by a T-type calcium channel. However, indirect analysis of calcium currents in mature sperm after transfer of ion channels to planar lipid bilayers detects three current types, including that similar, but not identical, to an L-type channel, but no T-type currents. Molecular cloning of the alpha-1 pore forming subunit of calcium channels expressed in the male reproductive tract and in ejaculated sperm has resolved this controversy, demonstrating the existence of only high voltage-activated channels. Further analysis of the alpha-1 subunit isoform from rat and human testis and sperm suggests that, as a result of alternate splicing, this L-type alpha-1 subunit could produce calcium currents that were T-like, e.g., transient, rapidly inactivating with slow deactivation. Multiple splice variants of this isoform were detected in human testis, suggesting a correlation with intra-individual variation in the ability of sperm to undergo an induced acrosome reaction and with male infertility. These variants could be developed as useful biomarkers for susceptibility to environmental and occupational toxicants. Knowledge of calcium channels structure will also contribute to design of new male contraceptives based on existing calcium channel antagonists.