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

## TL;DR

This study shows how human sperm channels respond to changes in pH and calcium levels, which are important for fertility.

## Contribution

The study reveals that hKSper is physiologically activated by pH and CatSper-mediated Ca2+ signaling in human sperm.

## Key findings

- Intracellular pH alkalization activates hKSper and causes ∼20 mV hyperpolarization of membrane potential.
- Progesterone-induced hyperpolarization is mediated by CatSper and hKSper.
- Low Ca2+ levels enhance hKSper activity, indicating sensitivity to resting Ca2+ levels.

## Abstract

The hyperpolarization of membrane potential (Vm) generated by the activation of the 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 Ca2+ affinity of human KSper (hKSper), whether the changes of intracellular pH or cytosol Ca2+ ([Ca2+]i) in response to physiological stimuli are sufficient to potentiate native hKSper is fairly obscure. Here, by utilizing quantitative Vm 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 Vm. Given that sperm Ca2+ influx is primarily mediated by the sperm-specific Ca2+ channel (CatSper), fluorometric results showed that progesterone, a physiological agonist of CatSper, remarkably hyperpolarized Vm in a dose-dependent manner. This hyperpolarizing effect (∼10−15 mV estimated by population Vm measurements) was largely suppressed by pharmacological inhibition of hKSper or the loss of functional CatSper. Surprisingly, electrophysiological recordings failed to detect progesterone-elicited hyperpolarization of Vm when employing Ca2+ chelator-free pipette solution. However, ∼10 mV hyperpolarization could be detected when the Ca2+ chelator contained in the pipette. In addition, [Ca2+]i alteration under 100 nM could potently enhance hKSper activity, suggesting that hKSper can sense resting [Ca2+]i levels. Taken together, our results clearly illustrate the activating effect of intracellular pH and [Ca2+]i elevation on hKSper under physiological conditions, and moreover, broaden the understanding of the Ca2+-involved regulatory mechanism of hKSper.