GLUTATHIONE AS A PRIMARY OSMOTIC DRIVING FORCE IN HEPATIC BILE FORMATION

Indirect evidence suggests that transport of glutathione (GSH) across the canalicular plasma membrane into bile contributes to the formation of the bile acid-independent fraction of bile flow. To directly test this hypothesis, the present study measured bile flow in isolated perfused rat livers whose biliary GSH excretion rate was selectively modulated by administration of GSH monoethyl ester (50, 100, and 200 mumol infused over a 20-min interval), a high dose of GSH itself (550 mumol over 20 min), and the three amino acid components of GSH (70 mumol each) with and without methionine (35 mumol). Animals were starved overnight to decrease hepatic GSH levels, and livers were pretreated with acivicin to inhibit gamma-glutamyl transferase. Livers perfused single pass with Krebs-Henseleit buffer excreted bile acids at a relatively low rate of 1-3 nmol. min-1 . g-1, and this rate was unaffected by agents used to alter biliary GSH efflux. In comparison, basal biliary GSH efflux rates were 8-13 nmol-min-1. g-1. Administration of the GSH ester produced a dramatic dose-dependent choleresis, a stimulation of biliary GSH excretion, and resulted in the biliary excretion of intact GSH ester. Changes in total biliary GSH excretion and bile flow were temporally and quantitatively related. Infusion of GSH and amino acid supplementation also resulted in higher rates of bile flow and biliary GSH excretion, but their effects were more modest. A plot of the changes in bile flow (mul-min-1 .g-1; ordinate) vs. the accompanying changes in biliary GSH excretion (nmol . min-1 . g-1) demonstrated a relation described by the line y = 0.034x + 0.067 (r = 0.88), suggesting that biliary excretion of 1 mumol of GSH is accompanied by the excretion of 34 mul of bile. These findings provide direct evidence for GSH as an osmotic driving force in hepatic bile formation, and provide an estimate of the choleretic potency of this tripeptide.