Protein kinase C agonists inhibit bile secretion independently of effects on the microcirculation in the isolated perfused rat liver

Abstract

The role of hormones in the regulation of bile secretion is not known; however vasoactive agents, which act via the phosphoinositide signal transduction pathway, may mediate changes in bile flow by altering the hepatic microvasculature. We therefore examined the effects of phorbol esters and diacylglycerol, agonists of the protein kinase C branch of the phosphoinositide cascade, on perfusion pressure and bile flow in a single-pass, hemoglobin-free, isolated perfused rat liver system with constant perfusate flow. The active phorbol ester, 12,13-phorbol dibutyrate, produced a dose-dependent (maximal effect at 10⁻⁶ M), sustained and reversible decrease in bile flow from 1.09 ± 0.18 to 0.61 ± 0.09 μl per min per gm liver (37.2 ± 5.9%) while simultaneously increasing perfusion pressure from 12.3 ± 0.7 to 21.5 ± 2.5 cm H₂O (74.0 ± 4.3%). Both effects were inhibited by the synthetic protein kinase C antagonist H-7. 1,2-Dioctanoyl-sn-glycerol, a diacylglycerol, produced changes in bile flow and perfusion pressure that were similar to, but more marked than, those caused by 12,13-phorbol dibutyrate, whereas the inactive phorbol ester 4α-phorbol didecanoate and the vehicle dimethyl sulfoxide had no effects on either parameter. 12,13-Phorbol dibutyrate infusion resulted in reversible decreases in oxygen consumption (23.3%) and a reversible vascular redistribution of trypan blue dye but did not alter hepatic venous effluent concentrations of K⁺. Increases in perfusion pressure produced by mechanical constrictions of the hepatic venous cannulae did not reproduce phorbol-induced changes in bile flow, suggesting that the cholestasis is not mediated by postsinusoidal increases in hepatic outflow resistance. Furthermore, the nonspecific vasodilators sodium nitroprusside (10⁻³ M) and papaverine (40 μM) prevented the phorbol-induced increases in perfusion pressure (p < 0.01) but did not block 12,13-phorbol dibutyrate’s cholestatic effects. We conclude from this study that protein kinase C agonists have both vascular and nonvascular effects in the isolated perfused rat liver and that the cholestasis caused by these agents is independent of effects on the hepatic microcirculation.