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 microliters 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 4 alpha-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+.(ABSTRACT TRUNCATED AT 250 WORDS)

We investigated the role of serum albumin in the hepatic uptake of organic anions by determining the effect of added bovine albumin on sulfobromophthalein (BSP) uptake in skates, an animal which naturally lacks this protein. A single-pass perfused liver model was used to determine steady-state net uptake rates as the BSP or BSP and albumin concentrations were systematically varied. Results indicated that two different steps in the uptake process are capable of limiting the uptake rate depending on the albumin and BSP concentrations selected. At higher rates of uptake corresponding to higher BSP concentrations (up to 112 microM), saturation kinetics were observed as the BSP concentration was varied, with apparent Km and Vmax values which were independent of the albumin concentration (0.05 to 0.75%). These data suggest that under these conditions uptake is limited by a saturable step intrinsic to the liver. In contrast, for lower BSP concentrations and albumin concentrations below about 0.5%, a different kinetic pattern was seen which suggested that the rate-limiting step in uptake was transfer of BSP from albumin to the liver. The latter data were found to be consistent with a model in which the limiting step in the transfer process is spontaneous dissociation of the BSP from binding sites on albumin within the sinusoid. These results suggest that skate liver clears BSP from albumin solutions by a two-step mechanism in which dissociation from albumin is followed by a saturable process consistent with carrier-mediated transport. The albumin concentration and uptake velocity are important factors in determining which of these steps limits the uptake rate. Clearance is efficient and irreversible even though elasmobranch liver lacks high-affinity cytosolic binding proteins for BSP, and elasmobranch plasma does not normally contain albumin.

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