Human sperm KSper is physiologically activated by intracellular pH alkalization and CatSper-mediated Ca2+ signaling

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The hyperpolarization of membrane potential (V m ) generated by the activation of sperm-specific K + channel (KSper) is considered as an important indicator for the evaluation of sperm fertilizing capacity. However, owing to the relatively low pH sensitivity and low Ca 2+ affinity of human KSper (hKSper), whether the changes of intracellular pH (pH i ) or cytosol Ca 2+ ([Ca 2+ ] i ) in response to physiological stimuli are sufficient to potentiate native hKSper is fairly obscure. Here, by utilizing quantitative V m fluorometry and current-clamp recordings on human sperm, our results reliably demonstrated that physiologically-relevant extracellular pH alkalization activated hKSper and evoked ∼20 mV hyperpolarization of V m . Given that sperm Ca 2+ influx is primarily mediated by the sperm-specific Ca 2+ channel (CatSper), fluorometric results showed that progesterone, a physiological agonist of CatSper, remarkably hyperpolarized V m in a dose-dependent manner. This hyperpolarizing effect (∼10−15 mV estimated by population V m measurements) was largely suppressed by pharmacological inhibition of hKSper or the loss of functional CatSper. Surprisedly, electrophysiological recordings failed to detect progesterone-elicited hyperpolarization of V m when employing Ca 2+ chelator-free pipette solution. However, ∼10 mV hyperpolarization could be detected when Ca 2+ chelator contained in the pipette. In addition, [Ca 2+ ] i alteration under 100 nM could potently enhance hKSper activity, suggesting that hKSper can sense resting [Ca 2+ ] i levels. Taken together, our results clearly illustrate the activating effect of pH i and [Ca 2+ ] i elevation on hKSper under physiological conditions and moreover, broaden the understanding of Ca 2+ -involved regulatory mechanism of hKSper.

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