Effect of intracellular pH on depolarization-evoked calcium influx in human sperm |
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Authors: | Fraire-Zamora Juan J González-Martínez Marco T |
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Institution: | Departamento de Farmacología, Facultad de Medicina, Universidad Nacional Autónoma de México, Ciudad Universitaria, CP 04510, Apartado Postal 70-297 Mexico City, Mexico. |
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Abstract: | 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 12 min in calcium-deprived medium, and are remarkably stimulated by NH4Cl, suggesting a role for intracellular pH (pHi). 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 pHi dependence of sperm VDCC. Simultaneous depolarization and pHi alkalinization with NH4Cl induced an Ca2+]i increase that depended on the amount of NH4Cl added. VDCC stimulation as a function of pHi showed a sigmoid curve in the 6.67.2 pHi range, with a half-maximum stimulation at pH 7.00. At higher pHi (7.3), a further stimulation occurred. Calcium release from internal stores did not contribute to the stimulating effect of pHi 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 pHi-stimulated-to-nonstimulated calcium influx was nearly constant at different test depolarization values. Likewise, depolarization-induced calcium influx in pHi-stimulated and nonstimulated cells was equally blocked by nickel. In our capacitating conditions pHi increased 0.11 pH units, suggesting that the calcium influx stimulation observed during sperm capacitation might be partially caused by pHi alkalinization. Additionally, a calcium permeability pathway triggered exclusively by pHi alkalinization was detected. mammalian sperm; capacitation; intracellular calcium |
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