Intracellular sodium increase induced by external calcium removal in human sperm

In human sperm, removal of external calcium produces a fast Ne-dependent depolarization that is presumably due to sodium permeation through calcium channels. Calcium restoration produces a ouabain-sensitive hyperpolarization that brings the membrane potential to values frequently more negative than resting. In this work, we show evidence indicating that external calcium removal induces an increase in the intracellular sodium ([Na+](i)) and that this phenomenon is related to the Na+-dependent depolarization. Calcium restoration blocked the [Na+](i) increase and then produced a slow decrease that was inhibited by ouabain. The [Na+](i) increase was inhibited by nanomolar-micromolar calcium or by millimolar magnesium, which has been previously shown to inhibit the Na+-dependent depolarization. This evidence supports the hypothesis that, in zero-calcium medium, a calcium channel that would contribute to resting intracellular calcium levels allows sodium permeation, producing depolarization and a significant [Na+](i) increase. Sodium loading would stimulate the Na+,K+-ATPase, the activity of which contributes to the sperm hyperpolarization observed upon calcium restoration.