Dual effect of spermine on acrosomal exocytosis in capacitated bovine spermatozoa

The influence on spermine on the acrosomal exocytosis of capacitated bovine spermatozoa was studied. Dual effect of spermine was observed, depending on its concentration. It was shown that 10 microM spermine stimulated acrosomal exocytosis and prostaglandin F2 alpha production, whereas higher concentrations of spermine inhibited these processes. Acrosomal exocytosis induced by spermine was inhibited by staurosporine, a specific protein kinase C (PKC) inhibitor, indicating that PKC may be involved in this stimulation. Also, acrosomal exocytosis induced by the PKC activator phorbol 12-myristate-13-acetate was inhibited by 10 mM spermine. Therefore, these data indicate that spermine is involved in signal transduction events leading to exocytosis. We suggest that the concentration-dependent reversal of the stimulatory action of spermine could be explained by the existence of two binding sites for spermine: high affinity sites involved in inducing acrosomal exocytosis by low spermine concentration and low affinity sites mediating inhibition of acrosomal exocytosis by high concentration of spermine.