[System of cAMP-dependent phosphorylation in NIH 3T3 cells passing into the resting state]

The changes in the cAMP level and the activity of cAMP-dependent protein kinase in NIH 3T3 cells going into the resting state induced by cell cultivation in a medium with 0.5% serum were investigated. It was shown that under conditions when cells are going into the resting state, the slight increase in the intracellular concentration of cAMP observed within the first 24 hours after cell transfer onto a serum-deficient medium is correlated with a simultaneous decrease of the cAMP phosphodiesterase activity. A procedure was developed allowing to measure the activity of cAMP-dependent protein kinase in cell lysates in the presence of cAMP-independent protein kinases. The activity of cAMP-dependent protein kinase in the cells going into the resting state markedly increases (up to 5-fold) after 9 days as compared to that of dividing cells. This rise in activity is due to the decrease of the content of a thermostable protein inhibitor of cAMP-dependent protein kinase rather than to the elevated level of cAMP.