In vivo studies of cyclic AMP phosphodiesterase activity in microinjected amphibian oocytes

Abstract

Using the microinjection technique it has been possible to measure the hydrolysis of (H3) cAMP injected into Xenopus laevis oocytes. A very active cAMP phosphosidiesterase activity hydrolyzes its substrate to 5'AMP at a rate of 20 pmoles per minute per oocyte. An apparent K(m) of 5 x 10-4 M can be estimated for the in vivo reaction Endogenous levels of cAMP range from 2 to 6 x 10-6 M in full grown oocytes. Germinal vesicle (nuclear) concentrations are about 10 fold higher. Coinjected cGMP does not inhibit the hydrolysis of cAMP whereas 5 x 10-4 M theophylline inhibits 40%. Slow hydrolysis of microinjected cGMP was measurable at very low concentrations (10-6 M) but was significant at higher levels. The endogenous level of cGMP in full grown oocytes is below 10-7 M.