Interaction of ouabain and progesterone on induction of bull sperm capacitation.

The endogenous steroid hormone ouabain induces capacitation of bull sperm acting through its receptor Na/K-ATPase on the sperm plasma membrane. Progesterone (P4) is believed to act through the sperm membrane P4 receptor (mPR) to induce non-genomic signalling leading to capacitation and/or acrosome reaction (AR) in the sperm of some species, but the exact nature of this receptor molecule on bull sperm is not known. In amphibian oocytes, P4 acts through the low-affinity ouabain binding site on Na/K-ATPase to induce signalling highly reminiscent of ouabain's signalling that initiates capacitation. This study hypothesized that ouabain and P4 interact agonistically or antagonistically to induce bull sperm capacitation. Sperm were incubated with 0, 12.5, 25, 50 and 100 μM ouabain, P4 or ouabain + P4 (12.5, 25 and 50 μM each) under capacitating conditions, and capacitation was assessed microscopically looking at the acrosome status of sperm and as the amount of protein tyrosine phosphorylation (Tyr-P). Both steroids stimulated Tyr-P of certain sperm proteins, but ouabain caused tyrosine phosphorylation of more proteins than P4 and stimulated significantly more overall Tyr-P (P < 0.05). Ouabain also was the only steroid to stimulate significant microscopically-evident AR. When sperm were co-incubated with the two steroids, P4 partially inhibited ouabain-induced Tyr-P and AR. These results suggest that P4 and ouabain may both interact with Na/K-ATPase, but ouabain is the more effective hormone. Ouabain may, therefore, be the primary physiological inducer of bovine capacitation.