Extracellular adenosine triphosphate stimulates acrosomal exocytosis in bovine spermatozoa via P2 purinoceptor

The presence of ATP in the genital tract fluid of mammals provokes questions regarding its function in the fertilization process. We investigated the effect of extracellular ATP (ATPe) on the activation of bovine spermatozoa. A signal transduction mechanism for ATP involving the receptor-mediated release of second messengers is described. Treatment of spermatozoa with ATP, uridine triphosphate (UTP), or 2-methylthio-ATP resulted in a concentration-dependent increase of acrosomal exocytosis, whereas treatment with either AMP or adenosine induced little exocytosis. This suggested that the receptor involved is of the P-2 and not the P-1 type. Several lines of evidence also suggest that the ATP purinoceptor is of the P-2y and not the P-2x type. First, the acrosome reaction was induced by the P-2y-agonists ATP, UTP, or 2-methylthio-ATP, but no effects were shown by the P-2x-agonists alpha,beta-methylene-ATP or beta,gamma-methylene-ATP. Second, ATP-induced acrosomal exocytosis was inhibited by the P-2y antagonists, but not by the P-2x antagonists. Third, enhanced Ca2+ uptake into the cells was observed with ATP and 2-methylthio-ATP, but not with beta,gamma-methylene-ATP. Additionally, ATP induced elevation of intracellular Ca2+ and cAMP, and the effect on cAMP was predominantly enhanced by including Ca2+ and the Ca2+-ionophore A23187 in the incubation medium. Extracellular ATP also activates protein kinase Calpha (PKCalpha), and the acrosome reaction, stimulated by ATPe, is inhibited by a PKC-specific inhibitor. in summary, we suggest that ATPe activates the P-2 purinoceptor that elevates [Ca2+](i), which leads to PKCalpha activation and culminates in acrosomal exocytosis.