Expression and differential cell distribution of low-threshold Ca ²⁺ 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 low-threshold (Ca_V3) 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_V3.3 mRNA in mouse male germ cells, an isoform not previously described in these cells. Immunoconfocal microscopy revealed the presence of the three Ca_V3 channel isoforms in mouse spermatogenic cells. In mature mouse sperm only Ca_V3.1 and Ca _V3.2 were detected in the head, suggesting its participation in the AR. Ca_V3.1 and Ca_V3.3 were found in the principal and the midpiece of the flagella. All Ca_V3 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 _V3 channels suggested they may participate in motility, however, mibefradil and gossypol concentrations that inhibit Ca_V3 channels did not significantly affect human sperm motility. Only higher mibefradil doses that can block high-threshold (HVA) Ca_V channels caused small but significant motility alterations. Antibodies to HVA channels detected Ca _V1.3 and Ca_V2.3 in human sperm flagella.