Voltage-operated calcium channels in male germ cells

The acrosome reaction is a key event in fertilization. Current models for induction of the acrosome reaction incorporate a necessary influx of Ca(2+), which is mediated by agonist-induced gating of ion channels in the sperm plasma membrane. The difficulty of applying electrophysiological techniques to spermatozoa has severely hampered studies on the expression of functional ion channels in these cells. However, during the last few years, a combination of molecular and physiological techniques (applied to immature spermatogenic cells) has elucidated both the expression of Ca(2+) channels in male germ cells and their role in induction of the acrosome reaction. It now appears that a range of voltage-operated Ca(2+) channels, similar to those that occur in somatic cells, is expressed in spermatozoa. Male rodent germ cells express a low-voltage activated (T-type) channel that is regulated by membrane potential and provides the primary Ca(2+) influx mechanism in zona pellucida-stimulated spermatozoa. In human spermatozoa, similar channels are apparently expressed, but their function in induction of the acrosome reaction has yet to be established. A range of other, high voltage-activated channels also appear to be present in rodent and human spermatozoa, but their roles are not yet known. In this review, the structure and characteristics of voltage-operated Ca(2+) channels are outlined and the evidence for their expression and function in male germ cells is assembled and discussed.