Glucocorticoid regulation of splanchnic glutamine, alanine, glutamate, ammonia, and glutathione fluxes

Abstract

Interorgan glutamine and associated metabolite fluxes were measured across the gut and liver to delineate splanchnic bed fluxes secondary to enhanced arterial loads mobilized in the periphery by glucocorticoid. Experiments were performed on adrenalectomized rats since adrenalectomy doubled the hepatic glucocorticoid receptor population compared with intact animals. Under these conditions, triamcinolone supplement (40 micrograms.day-1.100 g body wt-1) enhanced the combined net glutamine uptake by gut and liver eightfold, whereas combined gut and liver unidirectional breakdown and synthesis fluxes both increased (3.4- and 7.4-fold, respectively). Triamcinolone supplement also altered the pattern of metabolite released; gut released predominantly ammonium and some alanine, whereas the liver removed more alanine along with glutamine and released more urea, glutamate, and glutathione. Mechanistically, enhanced cellular glutamine uptake could be attributed to a three- to fourfold acceleration of glutamine transport associated with a rise in intracellular glutamine content. However, uptake by isolated membrane vesicles revealed only a small (27%) increase in System N activity, whereas extraction and reconstitution of the transporter into proteoliposomes failed to demonstrate increased transporter activity. Similarly, activity of phosphate-dependent glutaminase and glutamate dehydrogenase increased in crude homogenates (2-fold), but the former disappears in completely disrupted preparations. Furthermore, whereas messenger RNA and assayable enzymic activity for glutamate dehydrogenase clearly increased with glucocorticoid, glutaminase message was less significantly increased. Thus glucocorticoid appears directly capable of accelerating hepatic glutamine extraction primarily by modulating transporter activity that is closely coupled to glutamine utilization.