Activation of voltage-dependent calcium channels of mammalian sperm is required for zona pellucida-induced acrosomal exocytosis

Abstract

Previous work indicates that antagonists of the L-type voltage-dependent Ca 2+ channel (VDCC) prevent the Ca i increase in mammalian sperm that is promoted by incubation in alkaline, K +-based media. Here, we provide additional evidence that sperm possess VDCC and show that their activation is required for the Ca 2+ entry that mediates acrosomal exocytosis in both the presence and the absence of egg agonists. Specifically, we report that: (1) Sperm membrane potential changes, Ca i elevation, and acrosomal exocytosis have similar K + dose dependencies, consistent with a characteristic requirement of a large depolarization for activation of the sperm VDCC; (2) High affinity binding sites ( K d ∼0.35 ± 0.03 and 0.45 ± 0.06 n M; B max = 16.0 ± 1.4 and 5.8 ± 0.8 fmole/mg protein) for the VDCC antagonist, PN200-110, respectively, are present in membrane preparations from sperm of the ram and bull; (3) PN200-110 and the other VDCC antagonists nitrendipine, nisoldipine, verapamil, diltiazem, Ni 2+, or Co 2+ inhibit (IC 50 = 0.1, 0.4, 0.6, 0.8, 1.0, 60, and 110 μ M, respectively) the acrosomal exocytosis produced by combined elevation of pH 0 and membrane depolarization; (4) Exocytosis induced by the ZP3 agonist of the mammalian egg also is inhibited by VDCC antagonists with similar dose dependencies; (5) Depolarizing treatments that presumably activate the sperm VDCC bypass the blockade of ZP3-induced exocytosis imposed by pertussis toxin. These results indicate that activation of the sperm VDCC is sufficient to induce sperm acrosomal exocytosis and that VDCC activation is necessary in the ZP3 signal transduction pathway. They also indicate that the presumed G-protein targets of pertussis toxin probably produce a required but indirect activation of the putative sperm VDCC. Possible intervening events include alteration of the voltage sensitivity of the VDCC, membrane depolarization, or both. We suggest that the depolarization-induced acrosome reaction may provide a useful system to investigate subsequent events in the exocytotic process.