ROLE OF KINASES AND PHOSPHATASES IN THE REGULATION OF FLUID SECRETIONAND CL- HCO3- EXCHANGE IN CHOLANGIOCYTES/

The role of protein kinase A (PKA), protein kinase C (PKC), and protei n phosphatases in the process of secretin stimulation of fluid and bic arbonate secretion from biliary epithelium was examined using a novel isolated bile duct unit (IBDU) model from rat liver. Sp-adenosine 3',5 '-cyclic monophosphothiolate (Sp-cAMPS), 100 mu M, a PKA-specific agon ist, significantly increased secretion during a 30-min perfusion (+61% , P < 0.01). In contrast, preincubation and perfusion of Rp-cAMPS, 100 FIM, a specific PKA inhibitor, reduced the ability of secretin to sti mulate both fluid secretion (111 vs. 25%; P < 0.01) and Cl-/HCO3-, exc hanger activity (80 vs. 28%). Neither the PKC agonist phorbol 12-myris tate 13-acetate, 10 mu M, nor the PKC antagonist staurosporine showed any effect on either basal or secretin-stimulated fluid secretion or C l-/HCO3- exchange activity in IBDU. Okadaic acid, a specific inhibitor of protein phosphatases 1 and 2A, also had no effect on basal fluid s ecretion or on the basal activity of the Cl-/HCO3- exchanger. However, okadaic acid resulted in persistence of secretion after removal of se cretin, in contrast to the reduction in secretion observed in controls . These findings indicate that PKA but not PKC is involved in the sign al transduction of secretin-stimulated fluid secretion and Cl-/HCO3- e xchange activity in rat bile duct epithelium, a process inactivated by dephosphorylation by protein phosphatases 1 and/or 2A.