Progesterone-mediated calcium influx and acrosome reaction of human spermatozoa: Pharmacological investigation of T-type calcium channels

The mechanisms of the progesterone (P-4)-activated Ca2+ influx and the relationship between the intracellular free Ca2+ concentration ([Ca2+],) and the acrosome reaction (AR) were investigated in this study. We compared the [Ca2+], of uncapacitated and capacitated human sperm populations in response to P-4 stimulation; characterized the effects of the pharmacological agents pimozide and mibefradil, inhibitors of T-type voltage-operated calcium channels (VOCCT), on the P-4-activated Ca2+ influx; and determined the effects of these drugs on the P-4-initiated AR. Since pimozide can also inhibit calmodulin-dependent enzymes, we examined the effects of the calmodulin antagonist, calmidazolium, on the above-mentioned events. The basal [Ca2+], and the amplitude of the P-4-activated Ca2+ influx were significantly (p < 0.05) higher in capacitated sperm populations. Also, in capacitated sperm populations, all three pharmacological agents significantly (p < 0.05) inhibited the P-4-activated Ca2+ influx (IC50): calmidazolium (0.7 mu M) > pimozide (8 mu M) > mibefradil (11 mu M). By contrast, the effects of these drugs on the P-4-initiated AR were varied: pimozide (10 and 20 mu M) significantly (p < 0.05) increased the percentage of AR spermatozoa, calmidazolium was without effect, and mibefradil (20 mu M) significantly (p < 0.05) inhibited the AR, These disparate results do not allow us to reach any definitive conclusion concerning the role of a sperm VOCCT in the mechanism of the P-4-initiated AR. However, the differences between the [Ca2+], and AR effects, in particular the inverse relationship in the case of pimozide, suggest a dissociation between the amplitude of the P-4-stimulated Ca2+ signal and the downstream biological effect of that signal, the AR.