Differential effects of starvation on alanine and glutamine transport in isolated rat hepatocytes.
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ABSTRACT:
SUBMITTER: Hayes MR
PROVIDER: S-EPMC1158355 | biostudies-other | 1982 Apr
REPOSITORIES: biostudies-other
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Project description:The transport of glutamine and alanine into isolated rat colonocytes was studied. The transport of both amino acids appears to be dependent on a Na+ gradient. The apparent Km values for the transport of glutamine and alanine were 2.56 +/- 0.84 and 5.35 +/- 1.20 mM respectively, but with similar Vmax. values. Glutamine and alanine transport were mutually competitive, and the transport of both amino acids was competitively inhibited by 2-methylaminoisobutyrate. In contrast, histidine inhibited the transport of both glutamine and alanine non-competitively. It is concluded that glutamine and alanine are transported into rat colonocytes by a common carrier system similar to System A of other cells. It is suggested that the metabolic function of this transport system in rat colonocytes might be the partial exchange of extracellular glutamine for intracellular alanine.
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Project description:Na+-alanine cotransport across the cell membrane in isolated rat hepatocytes was studied. Changes in the cell membrane potential associated with the transport of alanine interfere with determination of the Na+-alanine coupling ratio of the cotransport. With valinomycin present to 'clamp' the cell membrane potential, a coupling ratio of 1:1 for the Na+-alanine influx was obtained.
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Project description:There is increasing evidence that membrane transporters for glutamine and glutamate are involved in control of liver metabolism in health and disease. We therefore investigated the effects of three catabolic states [starvation (60 h), diabetes (4 days after streptozotocin treatment) and corticosteroid (8-day dexamethasone) treatment] associated with altered hepatic amino acid metabolism on the activity of glutamine and glutamate transporters in sinusoidal membrane vesicles from livers of treated rats. In control preparations, L-[14C]glutamine uptake was largely Na(+)-dependent, but L-[14C]glutamate uptake was largely Na(+)-independent. Vmax. values for Na(+)-dependent uptake of glutamine and/or glutamate exceeded control values (by about 2- and 12-fold respectively) in liver membrane vesicles from starved (glutamine), diabetic (glutamate) or steroid-treated (glutamine and glutamate) rats. The Km values for Na(+)-dependent transport of glutamine or glutamate and the rates of their Na(+)-independent uptake were not significantly altered by any treatment. Na(+)-independent glutamate uptake appeared to include a dicarboxylate-exchange component. The patterns of inhibition of glutamine and glutamate uptake by other amino acids indicated that the apparent induction of Na(+)-dependent amino acid transport in catabolic states included increased functional expression of systems A, N (both for glutamine) and X-ag (for glutamate). The results demonstrate that conditions resulting in increased secretion of catabolic hormones (e.g. corticosteroid, glucagon) are associated with increased capacity for Na(+)-dependent transport of amino acids into liver cells from the blood. The modulation of hepatic permeability to glutamine and glutamate in these situations may control the availability of amino acids for intrahepatic metabolic processes such as ureagenesis, ammonia detoxification and gluconeogenesis.
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