Cyclic AMP-dependent protein phosphorylation in the rat anterior pituitary.

The activation of cyclic adenosine 3'5'-monophosphate (cAMP)-dependent protein kinases has been implicated as an integral mechanism in stimulus-secretion coupling in the anterior pituitary. Therefore, we have investigated phosphorylation of endogenous protein substrates both in the presence and absence of cAMP in cell-free extracts of the rodent anterior pituitary. Specific phosphoprotein substrates in the rat anterior pituitary, which are phosphorylated by a cAMP-dependent protein kinase in vitro, were identified. Cyclic AMP potentiated the phosphorylation of proteins with apparent molecular weights of 85,000, 77,000, 63,000, 53,000, 39,000, and 33,000 as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Proteins with apparent molecular weights of 124,000, 93,000, 48,000, and 43,000 were phosphorylated only in the presence of cAMP and not in the basal condition. The results highlight endogenous protein substrates that may potentially be involved in cAMP-dependent stimulus-secretion coupling in the anterior pituitary.