Functional interaction between mouse spermatogenic LVA and thapsigargin-modulated calcium channels.

The acrosome reaction in mouse is triggered by a long-lasting calcium signaling produced by a chain of openings of several calcium channels, a low-voltage-activated (LVA) calcium channel, an inositol trisphosphate receptor (IP(3)R), and the store-operated calcium channel TRP2. Since mature sperm cells are refractory to patch clamp experiments, we study the functional interactions among those sperm calcium channels in spermatogenic cells. We have studied the role of cytosolic calcium in voltage-dependent facilitation of low voltage-activated calcium channels. Calcium concentration was modified through the inclusion of the calcium buffers, EGTA and BAPTA, in the recording pipette solution, and by addition of calcium modulators like thapsigargin and the calcium ionophore A23187. We demonstrate that lowering calcium concentration below resting level allows to evidence a voltage-dependent facilitation. We also show that LVA calcium channels present strong voltage-dependent inhibition by thapsigargin. This effect is independent of cytosolic calcium elevation secondary to calcium store depletion and to the activation of TRP channels. Our data evidence an interesting functional relationship, in this cell type, between LVA channels and proteins whose activity is related to calcium filling state of the endoplasmic reticulum (presumably TRP channels and inositol triphosphate receptor). These relationships may contribute to the regulation of calcium signaling during acrosome reaction of mature sperm cell.