Progesterone inhibits adenylate cyclase in Xenopus oocytes. Action on the guanine nucleotide regulatory protein.

Previous studies have shown that ripe Xenopus 00cytes, 1.4 mm in diameter and suitable for microinjection, undergo meiotic cell division in response to progesterone by a mechanism involving a decrease in the level of CAMP. In order to investigate the mechanism by which the level of CAMP is reduced by progesterone treatment, adenylate cyclase activity was measured in manually dissected plasma membranes of oocytes in the presence of agents known to activate adenylate cyclase in other systems. Manually dissected oocyte plasma membranes express a basal adenylate cyclase activity of 1-5 pmol/mg/h, and this activity is reduced by 50% in the presence of progesterone. The oocyte adenylate cyclase is activated by pretreatment of intact oocytes with cholera toxin as well as by microinjection of the A subunit of toxin into the cell. Activation by holotoxin is concentration dependent with half-maximal stimulation at a concentration of 1 PM toxin, and the time course of toxin activation is sigmoidal with a lag of approximately 30 min before significant elevation of activity. Up to 60% of cholera toxin activation of adenylate cyclase is prevented by progesterone in a dose-dependent manner, with half-maximal inhibition at 1 PM progesterone, and this effect is not observed with steroids unable to induce meiotic cell division such as 17P-estradiol. The activation of adenylate cyclase by microinjected A subunit is prevented totally by progesterone, suggesting that progesterone effects some step of toxin action subsequent to binding and A subunit internalization. Sodium fluoride (F-), guanyl-5’-yl imidodiphosphate (Gpp(NH)p), and manganese each stimulate the oocyte adenylate cyclase, and while progesterone significantly inhibits both Fand Gpp(NH)p-stimulated activity, there is no inhibition of activity measured in the presence of manganese. These results indicate that progesterone inhibits the oocyte adenylate cyclase by directly or indirectly acting on the guanine nucleotide regulatory protein subunit of adenylate cyclase.