The role of protein phosphorylation in the regulation of pancreatic function by carbachol, cholecystokinin-octapeptide (CCK8), and insulin was investigated using isolated pancreatic acini of the mouse. Carbachol and CCK8 increased the phosphorylation of a Mr = 32,500 particulate protein and Mr = 16,000 and 23,000 soluble proteins. These agents also caused the dephosphorylation of Mr = 21,000 and 20,500 soluble proteins. Alterations in phosphorylation produced by carbachol were dose-dependent (maximal at 1-3 microM) and consistent with the dose-response relationship for carbachol-induced amylase secretion. Maximal dephosphorylation of the Mr = 21,000 and 20,500 proteins occurred within 1 min of addition of carbachol and was concurrent with the first detectable stimulation of amylase cyclase by carbachol. Moreover, atropine inhibition of carbachol-induced amylase secretion was concurrent with reversal of dephosphorylation of these two proteins. The calcium ionophore A23187, which mimicks the actions of carbachol and CCK8 on the pancreas, also mimicked the effects of these agents on protein phosphorylation, suggesting that pancreatic protein phosphorylation is regulated by calcium. Insulin, which enhances many of the actions of carbachol and CCK8 on the pancreas, alone increased the phosphorylation of the Mr = 16,000, 23,000, and 32,500 proteins and enhanced in an additive manner the effects of carbachol and CCK8 on the latter two proteins. By contrast, phosphorylation of the Mr = 21,000 or 20,500 proteins was not altered by insulin either in the absence or presence of carbachol or CCK8. In conclusion, the results suggest that alterations in phosphorylation of specific acinar cell proteins may provide a mechanism by which hormones and neurotransmitters interact in the regulation of pancreatic function.