Limited modulation of the mitogen-activated protein kinase pathway by cyclic AMP in rat parotid acinar cells

Treatment of rat parotid acinar cells, in vitro, with agents that elevate intracellular cAMP had only limited impact on the ability of EGF to subsequently stimulate MAP-kinase activity or phosphorylation. A time course of cAMP accumulation following in vivo administration of isoproterenol showed the greatest level of cAMP 15 min following the primary injection. Over a 72 hr injection regimen, agonist-stimulated cAMP levels were gradually reduced to control levels while cAMP-dependent protein kinase A (PKA) activity in immunoprecipitates of Raf-1 or MAP-kinase remained elevated. Raf-1 did not undergo phosphorylation following incubation with the catalytic subunit of PKA, suggesting that in normal rat acinar cells proliferation induced by isoproterenol or EGF involves the p21ras-Raf-MAP-kinase signaling cascade despite the presence of cAMP.