Temperature dependence of intracellular Ca²⁺ homeostasis in rat meiotic and postmeiotic spermatogenic cells

The hypothesis that intracellular [Ca²⁺] is a cell parameter responsive to extreme temperatures in rat meiotic and postmeiotic spermatogenic cells was tested using intracellular fluorescent probes for Ca²⁺ and pH. In agreement with this hypothesis, extreme temperatures induced a rapid increase of cytosolic [Ca²⁺] in rat pachytene spermatocytes and round spermatids. Oscillatory changes in temperature can induce oscillations in cytosolic [Ca²⁺] in these cells. Intracellular [Ca²⁺] homeostasis in round spermatids was more sensitive to high temperatures compared with pachytene spermatocytes. The calculated activation energies for SERCA ATPase-mediated fluxes in pachytene spermatocytes and round spermatids were 62 and 75 kJ mol^-1, respectively. The activation energies for leak fluxes from intracellular Ca²⁺ stores were 55 and 68 kJ mol^-1 for pachytene spermatocytes and round spermatids, respectively. Together with changes in cytosolic [Ca²⁺], round spermatids undergo a decrease in pH_i at high temperatures. This temperature-induced decrease in pH_i appears to be partially responsible for the increase in cytosolic [Ca²⁺] of round spermatids induced by high temperatures. This characteristic of rat meiotic and postmeiotic spermatogenic cells to undergo an increment in cytosolic Ca²⁺ at temperatures > 33°C can be related to the induction of programmed cell death by high temperatures in these cells.