Cyclic nucleotide content and protein phosphorylation during maturation of Spisula oocytes.

Cyclic nucleotide levels in the oocytes of the surf clam Spisula solidissima were measured during germinal vesicle breakdown (GVBD) induced by fertilization. The level of cAMP and cGMP in untreated oocytes was 8.23 +/- 0.95 and 4.89 +/- 0.39 pmol/10(6) oocytes. The ratio of cAMP to cGMP ranged from 1.5 to 2.0. The cAMP level in Spisula oocytes fluctuated after fertilization and before GVBD. The cGMP level showed minimal fluctuation, with a tendency to decrease initially followed by a subsequent rise to the basal level in a nonsynchronous manner. These changes were not statistically significant. There was a general increase in protein phosphorylation during the period after fertilization and before GVBD. The greatest increase occurred with proteins of estimated molecular weights of 52, 18, and 12 kD, analyzed by gel electrophoresis and autoradiography.