Effects of phorbol esters on sodium and chloride transport in rat colon

To determine the role of protein kinase C in the regulation of active electrolyte transport in rat descending colon, the effects of phorbol dibutyrate (PDB) were studied using the Ussing chamber/voltage-clamp technique. PDB added to the serosal surface increased the short-circuit current in a concentration dependent manner with a EC50 of 3 X 10(-8) M and a maximal effect at 10(-7) M PDB. The effect was not seen with the inactive alpha-phorbol analogue but was reproduced with 1-oleoyl-2-acetylglycerol, a more permeable analogue of diacylglycerol. PDB caused a decrease in mucosal-to-serosal and net fluxes of Na and Cl and an increase in serosal-to-mucosal Cl flux, indicating inhibition of Na and Cl absorption and stimulation of Cl secretion. The PDB-induced increase in Cl secretion was virtually abolished by both indomethacin and ibuprofen, indicating a dependence on arachidonic acid metabolism via the cyclooxygenase pathway. The Cl secretion was inhibited by verapamil and Ca2+-free bathing solution on the serosal surface but not by dantrolene, suggesting the importance of extracellular Ca2+ but not intracellular stored Ca2+ in the PDB-induced secretion. The Cl secretory effect was also inhibited by tetrodotoxin and atropine, suggesting involvement of cholinergic nerves. In contrast, the PDB-induced decrease in Na and Cl absorption was not dependent on metabolites of the cyclooxygenase pathway, not dependent on extracellular Ca2+, and not blocked by tetrodotoxin. It appears likely that protein kinase C is involved in the regulation of rat colonic active Na and Cl absorption and electrogenic Cl secretion but that the pathways involved are different in the two transport systems.