Induction of a sodium ion influx by progesterone in human spermatozoa

Abstract

In human spermatozoa, progesterone (P4) induces a depolarization of the plasma membrane, a rapid calcium (Ca2+) influx, and a chloride efflux. The sodium ion (Na+) was partly responsible for the P4-induced depolarizing effect but was not required for calcium influx. We used fluorescent probes for spectrofluorometry to investigate whether P4 induced a Na+ influx and whether voltage-operated channels were involved in Na+ and/or Ca2+ entries. We found that 10 μM P4 significantly increased intracellular Na+ concentration from 17.8 ± 2.0 mM to 27.2 ± 1.6 mM (P < 0.001). Prior incubation of spermatozoa with 10 μM flunarizine, a Na+ and Ca2+ voltage- dependent channel blocker, inhibited the sodium influx induced by 10 μM P4 by 84.6 ± 15.4%. The Ca2+ influx induced by 10 μM P4 was also significantly inhibited in a Na+-containing medium by 10 μM flunarizine or 10 μM pimozide (P < 0.01). In contrast, flunarizine had no inhibitory effect on the Ca2+ influx induced by 10 μM P4 in spermatozoa incubated in Na+- depleted medium. The P4-promoted acrosome reaction (AR) was significantly higher when spermatozoa were incubated in Na+-containing medium as compared to Na+-depleted medium. These data demonstrate that P4 stimulates a Na+ influx that could be involved in the AR completion. They also suggest that voltage-dependent Na+ and Ca2+ channels are implicated in P4-mediated signaling pathway in human spermatozoa.