Hormonal regulation of glutathione efflux

The efflux of GSH has been shown previously to be a saturable process in both isolated rat hepatocytes and perfused liver, suggesting a carrier-mediated transport mechanism. The possibility in hormonal regulation of this process has been raised by recent reports. Our present work examined the role of hormones known to affect intracellular signal transduction mechanisms on GSH efflux in cultured rat hepatocytes and perfused rat livers. We found that cAMP-dependent factors, such as cholera toxin (CT), dibutyryl cAMP, forskolin, and glucagon all stimulated GSH efflux in cultured rat hepatocytes. The efflux kinetics were compared in cultured cells incubated with or without CT; the stimulation of GSH efflux was related to a near doubling of the Vmax while exhibiting no significant alteration of the Km. The increase in intracellular cAMP level associated with the threshold for this stimulatory effect was 25% above control. The stimulatory effect of CT could not be blocked by cyclohexamide pretreatment or reversed by colchicine treatment. The stimulatory effect of glucagon was abolished in the presence of ouabain but not in the presence of barium. On the other hand, hormones which act through Ca2+ and protein kinase C, such as phenylephrine and vasopressin, had no effect on GSH efflux in the cultured cells. In the perfused liver model, glucagon (10 nM) and dibutyryl cAMP (8 microM) stimulated sinusoidal GSH efflux to 130 and 144% of control values, respectively, and increased bile flow while not affecting biliary GSH efflux. Finally, the physiological significance of glucagon-mediated stimulation of sinusoidal GSH efflux was assessed by both plasma GSH and glucose levels in response to in vivo glucagon infusion. The threshold dose of glucagon for significant increase in plasma GSH (5.21 pmol/min) was lower than for glucose (15.61 pmol/min). At the highest glucagon infusion rate (261 pmol/min), plasma GSH level doubled while glucose level increased 80%. In conclusion, increased cAMP stimulates GSH efflux in cultured rat hepatocytes and perfused livers. The stimulatory effect of cAMP is exerted at the sinusoidal pole and appears to be mediated by hyperpolarization of hepatocytes by stimulation of Na(+)-K(+)-ATPase. In vivo studies confirmed the importance of cAMP-mediated stimulation of sinusoidal GSH efflux as it resulted in significant elevation of the plasma GSH level.