MODULATORS OF THE PROTEIN-KINASE-C SYSTEM INFLUENCE BILIARY-EXCRETION OF CATIONIC DRUGS

To investigate whether hepatobiliary transport of organic cations is under regulatory control, we studied transport of tri-n-butylmethylammonium in the isolated perfused rat liver and in isolated rat hepatocytes. Transport was investigated in the presence of modulators of the protein kinase C and the cyclic AMP second-messenger system. In the isolated perfused rat liver, it was observed that compounds modulating protein kinase C activity clearly affected the biliary excretion process of the cation tri-n-butylmethylammonium. Phorbol 12-myristate 13-acetate, a compound that directly stimulates protein kinase C, elevated the biliary excretion rate of tri-n-butylmethylammonium in a concentration-dependent manner, reaching a twofold increase at 60 nmol/L of the phorbol ester. The inactive derivative 4alpha-phorbol 12, 13-didecanoate (60 nmol/L) did not show any effect. Vasopressin (48 nmol/L), a receptor-mediated activator of protein kinase C, stimulated the excretion rate of the cation by about 50%. Staurosporin (1 mumol/L), an inhibitor of protein kinase C, clearly decreased the biliary excretion rate of the cation and also blocked its stimulation by phorbol 12-myristate 13-acetate. Neither phorbol 12-myristate 13-acetate nor vasopressin (at concentrations ranging from 10(-9) to 10(-6) mol/L) affected the initial uptake velocity of tri-n-butylmethylammonium in isolated hepatocytes and isolated perfused livers, whereas staurosporin (1 mumol/L) showed only a modest inhibition of the uptake of the cation. It is inferred that the effect of protein kinase C modulators on hepatobiliary transport of organic cations occurs at the level of carrier-mediated transport in the canalicular membrane. Because bile flow was only slightly affected by these agents, effects on biliary excretion rate of the cation are unlikely to be caused by changes in bile flow. With regard to the cyclic AMP second-messenger system, neither glucagon (concentration range of 10(-9) to 10(-6) mol/L), a receptor-mediated activator of adenylate cyclase, nor forskolin (100 mumol/L), a direct activator of adenylate cyclase and dibutyryl cyclic AMP (100 mumol/L), affected the biliary excretion rate and the hepatic uptake rate of the cation in these preparations. In conclusion, cell-to-bile transport of the organic cation tri-n-butylmethylammonium at the canalicular level is directly or indirectly regulated by protein kinase C. Neither the protein kinase C nor the cyclic AMP second-messenger systems seem to be involved in the hepatic uptake process of the cation.