Role of bile acid conjugation in hepatic transport of dihydroxy bile acids

The effect of conjugation and side chain length on dihydroxy bile acid unidirectional hepatic uptake and efflux was studied using the isolated perfused rat liver which was perfused prograde or retrograde in single pass fashion. Deoxycholic acid (DC) and its C23 (nor) derivative nor-DC, as well as the synthetically prepared taurine conjugate of DC, were administered at a constant dose of 1 mumol/min/kg (body weight), upon which a bolus tracer dose of labeled bile acid was superimposed. Analysis of radioactivity recovery in perfusate indicated that unidirectional uptake of all three bile acids was equally rapid, but that only nor-DC showed considerable and continuing efflux into the perfusate; this involved mostly the unchanged acid. Nor-DC was not amidated but was metabolized to mostly ester glucuronides and hydroxylated derivatives; the biotransformation products did not reflux and were secreted into bile; similarly, DC was amidated with taurine; its taurine conjugate did not efflux and was secreted into bile. When nor-DC-taurine was infused, it did not efflux and was secreted rapidly into bile. When the liver was perfused retrograde fashion to increase concentrations of bile acids pericentral cells, only nor-DC showed efflux, which again involved only the unchanged acid. All bile acids were partly 7 alpha-hydroxylated, the magnitude being greater during retrograde perfusion presumably because slower cellular transport exposed bile acid to hydroxylation enzymes for a longer period. It is concluded that bile acid conjugation, whether by esterification with CoA formation adn subsequent amidation or by esterification with glucuronate, restricts the movement of lipophilic dihydroxy bile acids to the hepatocyte and canalicular lumen.(ABSTRACT TRUNCATED AT 250 WORDS)