Caprine sperm acrosome reaction: promotion by progesterone and homologous zona pellucida.

Experiments were designed to characterize the effect of progesterone and the zona pellucida (ZP) on the goat sperm acrosome reaction (AR) through a comparative study. Goat spermatozoa were incubated for 4h in Krebs-Ringer bicarbonate media (KRB) for capacitation. Progesterone and ZP stimulated exocytosis of capacitated spermatozoa in a dose-dependent manner. EGTA and La(3+), added 10min before the addition of the agonists, completely abolished the stimulatory effects. Ca(2+) influx was observed to occur through a calcium phosphate transporter. Picrotoxin and bicuculline, two GABA(A)/Cl(-) channel antagonists, also inhibited progesterone-induced AR when added 10min before steroid addition. ZP-induced AR was unaffected by these antagonists. Studies using pertussis toxin (PTX) showed that, unlike ZP, progesterone acts without the involvement of a G-protein. Progesterone-3-(O-carboxymethyl) oxime: BSA conjugate (P-BSA) also induced AR in capacitated sperm suspension. Results suggest that progesterone and ZP induce AR via specific membrane receptors through different mechanisms, both requiring an influx of Ca(2+). It is assumed that both the mechanisms reconcile at some stages of the cascade and elicits a similar physiological response.