Phorbol ester and sperm activate mouse oocytes by inducing sustained oscillations in cell Ca²⁺

Abstract

The parthenogenetic activation of mouse oocytes by several agents is accompanied by a large rise in the intracellular calcium concentration ([Ca²⁺]_i)¹. The tumour-promoting phorbol ester 12-O-tetradecanoyl phorbol acetate (TPA) activates some cellular processes which are also activated by raised [Ca²⁺] (ref. 2). We therefore tested TPA on mouse oocytes and found it to be a potent parthenogenetic activating agent. Recent reports suggest that TPA mimics endogenous diacylglycerol³ and can stimulate cells by activating protein kinase C without raising [Ca²⁺]_i (refs 4, 5). We have now measured [Ca²⁺]_i with aequorin in mouse oocytes treated with TPA. Sustained oscillations in [Ca²⁺]_i appeared in those oocytes which were subsequently activated. We also measured [Ca²⁺]_i during fertilization. The response began with [Ca²⁺]_i transients as reported previously³, but the transients continued in a regular pattern for 4 h. There was no rise in [Ca²⁺]_i of the type induced by activating agents such as ethanol. One component of the fertilization response appears to be related to the TPA-induced oscillations.