Progesterone inhibits membrane-bound adenylate cyclase in Xenopus laevis oocytes

Abstract

Recent experimental evidence indicates that progesterone acts at the cell surface to trigger protein synthesis and to reinitiate the first meiotic division in Xenopus laevis oocytes^1,2. The steroid hormone is physiologically released by follicle cells surrounding oocytes in the ovaries, and this naturally occurring event can be reproduced in vitro by adding progesterone to the incubation medium. Recently, cyclic AMP has been implicated in the mechanism of progesterone action in oocytes^3,4; there was an almost immediate decrease in cyclic AMP concentration in oocytes after addition of progesterone in vitro, whether or not the oocytes were pretreated with cholera toxin^5,6. Adenylate cyclase in X. laevis oocytes is compartmentalized, >50% soluble and ∼30% is found in the plasma membrane-containing fraction⁷. We report here that physiological concentrations of progesterone selectively inhibit membrane-bound adenylate cyclase activity, after addition to intact oocytes or in cell-free experiments; this specificity confirms the proposed membrane site of action for the hormone when reinitiating meiosis and is the first example of a ‘direct’ enzymatic action of a steroid (not by protein synthesis) related to a physiological function.