Abstract(#br)The sperm acrosome reaction occurs after the binding of the capacitated sperm to the egg zona pellucida. This study describes a novel mode of regulation of the sperm epidermal growth factor receptor (EGFR) under physiological conditions and its relevance to the acrosome reaction. Ouabain, a known Na/K ATPase blocker is present in the blood and in the female reproductive tract. We show here that physiological concentrations (nM) of ouabain enhance phosphorylation of EGFR on tyr-845, stimulate Ca 2+ influx and induce the acrosome reaction in sperm. These effects could be seen only in the presence of very low concentrations of EGF (0.1ng/ml or 0.016 nM) added together with nano-molar ouabain. Phosphorylation, Ca 2+ influx, and the acrosome reaction are... inhibited by an EGFR blocker, suggesting that trans-activation of the EGFR is involved. Moreover, our data revealed that protein kinase A and the family of tyrosine kinase, SRC, shown before to be involved in EGFR activation in sperm, mediate the acrosome reaction induced by ouabain. Ouabain alone (without EGF) at relatively high concentration (10µM) could enhance EGFR phosphorylation, Ca 2+ influx and acrosome reaction, and these processes were inhibited by EGFR blockers. Moreover, we show here that PKA and SRC family are involved in the activation of EGFR by 10µM ouabain, further demonstrating that ouabain induces the acrosome reaction by a mechanism mediated by the trans-activation of EGFR. In conclusion, this study describes an interesting regulatory path of EGFR by physiological concentrations of ouabain and EGF found in the female reproductive tract. Neither of these compounds can activate the EGFR alone at such low physiological levels; however, when both are present, the interaction of ouabain with the Na/K ATPase leads to the priming of the EGFR, which undergoes its full activation by EGF.