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

To investigate whether hepatobiliary transport of organic cations is u nder regulatory control, we studied transport of tri-n-butylmethylammo nium in the isolated perfused rat liver and in isolated rat hepatocyte s. Transport was investigated in the presence of modulators of the pro tein kinase C and the cyclic AMP second-messenger system. In the isola ted perfused rat liver, it was observed that compounds modulating prot ein kinase C activity clearly affected the biliary excretion process o f the cation tri-n-butylmethylammonium. Phorbol 12-myristate 13-acetat e, 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 pho rbol ester. The inactive derivative 4alpha-phorbol 12, 13-didecanoate (60 nmol/L) did not show any effect. Vasopressin (48 nmol/L), a recept or-mediated activator of protein kinase C, stimulated the excretion ra te of the cation by about 50%. Staurosporin (1 mumol/L), an inhibitor of protein kinase C, clearly decreased the biliary excretion rate of t he cation and also blocked its stimulation by phorbol 12-myristate 13- acetate. Neither phorbol 12-myristate 13-acetate nor vasopressin (at c oncentrations ranging from 10(-9) to 10(-6) mol/L) affected the initia l 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 transp ort of organic cations occurs at the level of carrier-mediated transpo rt in the canalicular membrane. Because bile flow was only slightly af fected by these agents, effects on biliary excretion rate of the catio n are unlikely to be caused by changes in bile flow. With regard to th e cyclic AMP second-messenger system, neither glucagon (concentration range of 10(-9) to 10(-6) mol/L), a receptor-mediated activator of ade nylate cyclase, nor forskolin (100 mumol/L), a direct activator of ade nylate cyclase and dibutyryl cyclic AMP (100 mumol/L), affected the bi liary excretion rate and the hepatic uptake rate of the cation in thes e preparations. In conclusion, cell-to-bile transport of the organic c ation tri-n-butylmethylammonium at the canalicular level is directly o r 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.