Evidence for protein kinase C as a regulator of intestinal electrolyte transport

We examined the possibility that the Ca2+-and phospholipid-dependent protein kinase, protein kinase C, may regulate intestinal electrolyte transport. By use of two active phorbol esters, known activators of protein kinase C, we studied the secretory response in rat small intestine. In the in vivo enteropooling assay, 4 beta-phorbol 12-myristate 13-acetate given by gavage produced intestinal fluid secretion and accumulation comparable with that of known secretagogues. The response was dose dependent and only partially blunted at higher doses by atropine. Known inactive phorbol esters failed to elicit a secretory response. In Ussing chamber preparations, 4 beta-phorbol 12,13-dibutyrate applied on the serosal side produced a dose-dependent increase in short-circuit current (Isc). This response was totally dependent on the presence of Cl ion, and as seen in vivo atropine only partially attenuated the Isc response to high concentrations of phorbol ester. Only a minimal increase in Isc was observed when the ester was applied to the mucosal side. Protein kinase C was found to phosphorylate the microvillus membrane of the enterocyte, and the phosphorylation was stimulated by phorbol ester in a concentration-dependent manner. Two membrane proteins of Mr 51,000 and 46,000 were the primary substrates of the enzyme. These studies demonstrate that phorbol esters, specific activators of protein kinase C, elicit the secretory response in rat small intestine both in vivo and in vitro and that this response is mediated by an increase in Cl ion secretion. Furthermore, protein kinase C-mediated phosphorylation of specific microvillus membrane proteins suggests that protein kinase C may modify the functional state of the microvillus membrane of the enterocyte.