Biliary excretion of glutathione and glutathione disulfide in the rat. Regulation and response to oxidative stress.

Regulation of the biliary excretion of reduced glutathione (GSH) and glutathione disulfide (GSSG) and responses to selected model toxins were examined in male Sprague-Dawley rats. In control and phenobarbital-pretreated rats in which the intrahepatic concentration of GSH was modulated by the administration of diethyl maleate or acetaminophen, the biliary concentration of GSH was consistently lower than, but directly proportional to, the intrahepatic concentration of GSH. Furthermore, increments in bile flow produced by the infusion of sulfobromophthalein (BSP)-glutathione were associated with proportional increases in the biliary excretion of GSH, suggesting that GSH passes into bile passively along a concentration gradient. In contrast, GSSG appears to be secreted into bile against a steep concentration gradient. An increased hepatic production and biliary excretion of GSSG resulted from the administration of t-butyl hydroperoxide. Measurement of biliary GSSG and BSP during a constant infusion of the GSH adduct of BSP indicated that GSSG shares a common excretory mechanism with GSH adducts. Diquat, nitrofurantoin, and paraquat also markedly stimulated the biliary excretion of GSSG. On a molar basis, these compounds generated much more GSSG than a direct substrate for glutathione peroxidase such as t-butyl hydroperoxide, indicating that the compounds undergo redox-cycling with concomitant production of hydrogen peroxide. Aminopyrine (0.8 mmol/kg) also significantly increased biliary GSSG. This increase, however, was associated with a proportional increase in bile flow and in the biliary excretion of GSH such that the GSSG/GSH ratio in bile did not change. Acetaminophen and chloroform, two compounds generating electrophilic metabolites that deplete intrahepatic GSH, led to a progressive decrease in the biliary excretion of GSH and GSSG. Furosemide and dimethylnitrosamine, the electrophilic metabolites of which do not deplete hepatic GSH, minimally altered biliary GSH and GSSG. Similarly, carbon tetrachloride and iproniazid, which yield organic radical metabolites that can peroxidize membrane lipids, did not increase the biliary excretion of GSSG. This finding indicates that membrane-bound lipid hydroperoxides may not be good substrates for glutathione peroxidases. The measurement of the biliary excretion of GSSG and of the GSSG/GSH ratio in bile is a sensitive index of oxidative stress in vivo and thus complements other in vivo parameters for the study of reactive intermediates of xenobiotics such as the determination of covalent binding, the formation of lipid hydroxy acids, and the depletion of intracellular GSH.

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