Does the guanine nucleotide regulatory protein Ni mediate progesterone inhibition of Xenopus oocyte adenylate cyclase?

In Xenopus laevis oocytes progesterone is able to inhibit directly the plasma membrane adenylate cyclase activity and induce reinitiation of meiotic maturation. To determine whether progesterone inhibition is mediated by the inhibitory guanine nucleotide-binding regulatory component of adenylate cyclase, Ni, the effect of the Bordetella pertussis toxin (IAP) and limited proteolysis on progesterone action in oocytes was investigated. Treatment of oocyte membranes with islet activating protein (IAP) in the presence of [32P]NAD led to incorporation of radiolabel into a 41 000-dalton membrane protein. However, exposure of isolated oocytes to 100 ng/ml IAP for up to 24 h, or oocyte membranes with concentrations of toxin as high as 100 micrograms/ml, had no effect on either progesterone inhibition of adenylate cyclase or induction of maturation. Similarly, limited alpha-chymotrypsin proteolysis of oocyte membranes failed to modify progesterone-induced inhibition of adenylate cyclase. In contrast, inhibition of human platelet adenylate cyclase by epinephrine, acting via a GTP-dependent, alpha 2-adrenergic receptor-mediated pathway, is almost completely abolished by both IAP treatment and limited proteolysis of platelet membranes. These data indicate that unlike attenuation of platelet enzyme activity, the inhibition of adenylate cyclase in oocyte membranes by progesterone does not occur via a classical Ni-mediated pathway.