Role of kinases and phosphatases in the regulation of fluid secretion and Cl-/HCO3- exchange in cholangiocytes

The role of protein kinase A (PKA), protein kinase C (PKC), and protein phosphatases in the process of secretin stimulation of fluid and bicarbonate 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 agonist, 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 stimulate both fluid secretion (111 vs. 25%; P < 0.01) and Cl-/HCO3-, exchanger activity (80 vs. 28%). Neither the PKC agonist phorbol 12-myristate 13-acetate, 10 mu M, nor the PKC antagonist staurosporine showed any effect on either basal or secretin-stimulated fluid secretion or Cl-/HCO3- exchange activity in IBDU. Okadaic acid, a specific inhibitor of protein phosphatases 1 and 2A, also had no effect on basal fluid secretion or on the basal activity of the Cl-/HCO3- exchanger. However, okadaic acid resulted in persistence of secretion after removal of secretin, in contrast to the reduction in secretion observed in controls. These findings indicate that PKA but not PKC is involved in the signal transduction of secretin-stimulated fluid secretion and Cl-/HCO3- exchange activity in rat bile duct epithelium, a process inactivated by dephosphorylation by protein phosphatases 1 and/or 2A.