Caltrin, the calcium transport regulatory peptide of spermatozoa, modulates acrosomal exocytosis in response to the egg's zona pellucida.

Acrosomal exocytosis is initiated in mammalian sperm by stimulatory agonists in the zona pellucida. Recently, it was shown that exocytosis is modulated in bovine sperm by extrinsic factors present in seminal fluids (Florman, H.M., and First, N.L. (1988) Dev. Biol. 128, 464-474). Fractionation of bovine seminal fluids yields a M(r) approximately 6,500, basic (pI approximately 8.5) peptide that accounts for the positive modulation of zona pellucida-induced acrosome reaction (ED50 and maximal response at 0.2 and 1 micrograms/ml, respectively). In addition, application of purified peptide to fura 2-loaded, cauda epididymal sperm supported zona pellucida-promoted elevations of Ca2+i equivalent to those observed with sperm treated with unfractionated seminal fluids in vitro or in vivo. Finally, peptide treatment regulated gamete interaction in a manner consistent with the distinct behaviors of cauda epididymal and ejaculated bovine sperm. This purified seminal peptide was identified by sequence analysis as caltrin, a previously characterized regulator of Ca2+ transport in bovine sperm. We therefore propose that caltrin is a regulator of sperm signal transduction pathways activated by zona pellucida binding.

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

Previous work indicates that antagonists of the L-type voltage-dependent Ca2+ channel (VDCC) prevent the Ca(i) increase in mammalian sperm that is promoted by incubation in alkaline, K(+)-based media. Here, were provide additional evidence that sperm possess VDCC and show that their activation is required for the Ca2+ 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 (Kd approximately 0.35 +/- 0.03 and 0.45 +/- 0.06 nM; Bmax = 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, Ni2+, or Co2+ inhibit (IC50 = 0.1, 0.4, 0.6, 0.8, 1.0, 60, and 110 microM, respectively) the acrosomal exocytosis produced by combined elevation of pH0 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.