Effect of intracellular pH on depolarization-evoked calcium influx in human sperm

Human sperm are endowed with putative voltage-dependent calcium channels (VDCC) that produce measurable increases in intracellular calcium concentration ([Ca2+](i)) in response to membrane depolarization with potassium. These channels are blocked by nickel, inactivate in 1-2 min in calcium-deprived medium, and are remarkably stimulated by NH4Cl, suggesting a role for intracellular pH (pH(i)). In a previous work, we showed that calcium permeability through these channels increases approximately onefold during in vitro "capacitation," a calcium-dependent process that sperm require to fertilize eggs. In this work, we have determined the pH(i) dependence of sperm VDCC. Simultaneous depolarization and pH(i) alkalinization with NH4Cl induced an [Ca2+](i) increase that depended on the amount of NH4Cl added. VDCC stimulation as a function of pH(i) showed a sigmoid curve in the 6.6-7.2 pH(i) range, with a half-maximum stimulation at pH similar to7.00. At higher pH(i) (greater than or equal to7.3), a further stimulation occurred. Calcium release from internal stores did not contribute to the stimulating effect of pH(i) because the [Ca2+](i) increase induced by progesterone, which opens a calcium permeability pathway that does not involve gating of VDCC, was unaffected by ammonium. The ratio of pH(i)-stimulated-to-nonstimulated calcium influx was nearly constant at different test depolarization values. Likewise, depolarization-induced calcium influx in pH(i)-stimulated and nonstimulated cells was equally blocked by nickel. In our capacitating conditions pH(i) increased 0.11 pH units, suggesting that the calcium influx stimulation observed during sperm capacitation might be partially caused by pH(i) alkalinization. Additionally, a calcium permeability pathway triggered exclusively by pH(i) alkalinization was detected.