Expression and differential cell distribution of low-threshold Ca2+ channels in mammalian male germ cells and sperm

Numerous sperm functions including the acrosome reaction (AR) are associated with Ca2+ influx through voltage-gated Ca2+ (Ca-V) channels. Although the electrophysiological characterization of Ca2+ currents in mature sperm has proven difficult, functional studies have revealed the presence of lowthreshold (Ca(V)3) channels in spermatogenic cells. However, the molecular identity of these proteins remains undefined. Here, we identified by reverse transcription polymerase chain reaction the expression of Ca(V)3.3 mRNA in mouse male germ cells, an isoform not previously described in these cells. Immunoconfocal microscopy revealed the presence of the three Ca(V)3 channel isoforms in mouse spermatogenic cells. In mature mouse sperm only Ca(V)3.1 and Ca(V)3.2 were detected in the-head, suggesting its participation in the AR. Ca(V)3.1 and Ca(V)3.3 were found in the principal and the midpiece of the flagella. All Ca(V)3 channels are also present in human sperm, but only to a minor extent in the head. These findings were corroborated by immunogold transmission electron microscopy. Tail localization of Ca(V)3 channels suggested they may participate in motility, however, mibefradil and gossypol concentrations that inhibit Ca(V)3 channels did not significantly affect human sperm motility. Only higher miliefradil doses that can block high-threshold (HVA) Cav channels caused small but significant motility alterations. Antibodies to HVA channels detected Ca(V)1.3 and Ca(V)2.3 in human sperm flagella. (C) 2004 Published by Elsevier B.V. on behalf of the Federation of European Biochemical Societies.