Calcium Uptake in Human Spermatozoa: Characterization and Mechanisms

Abstract

Basal ⁴⁵Ca²⁺ influx was analyzed in human seminal spermatozoa using a method that allows these highly reactive cells to be easily and safely handled. The uptake was a time-dependent process, with its maximum at 400 s. The kinetics of ⁴⁵Ca^{2 +} transport was saturating as a function of extracellular Ca^{2 +} concentration with a Km of 429 μM and a V_max of 1.6 nmol ⁴⁵Ca^{2 +} /mg protein/2.5 min. Depolarizing conditions and the calcium channel blocker verapamil did not affect the uptake; based on this, the presence of operating calcium channels in seminal spermatozoa is excluded. The independence of ⁴⁵Ca²⁺ uptake on external concentration of both Na⁺ and Ca²⁺ suggests that Na⁺ /Ca²⁺ exchange does not occur in these cells. The anticalmodulin drug trifluoperazine, the mitochondrial inhibitor antimycin A, and the SH reagents N-ethylmaleimide and mersalyl all inhibited the ion transport. A calmodulin-regulated, energy-requiring, proteinaceous Ca²⁺ transporter seems to be the main operating mechanism of calcium uptake in human seminal gametes.