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

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Abstract
Numerous sperm functions including the acrosome reaction (AR) are associated with Ca2+ influx through voltage‐gated Ca2+ (CaV) channels. Although the electrophysiological characterization of Ca2+ currents in mature sperm has proven difficult, functional studies have revealed the presence of low‐threshold (CaV3) 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 CaV3.3 mRNA in mouse male germ cells, an isoform not previously described in these cells. Immunoconfocal microscopy revealed the presence of the three CaV3 channel isoforms in mouse spermatogenic cells. In mature mouse sperm only CaV3.1 and CaV3.2 were detected in the head, suggesting its participation in the AR. CaV3.1 and CaV3.3 were found in the principal and the midpiece of the flagella. All CaV3 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 CaV3 channels suggested they may participate in motility, however, mibefradil and gossypol concentrations that inhibit CaV3 channels did not significantly affect human sperm motility. Only higher mibefradil doses that can block high‐threshold (HVA) CaV channels caused small but significant motility alterations. Antibodies to HVA channels detected CaV1.3 and CaV2.3 in human sperm flagella.