Sodium-gradient-stimulated transport of l-alanine by plasma-membrane vesicles isolated from liver parenchymal cells of fed and starved rats. Crucial role of the adrenal glucocorticoids

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Abstract
The ability of liver to efficiently take up amino acids, particularly L-alanine, during starvation was studied in a cell-free system by isolating plasma-membrane vesicles in a transport-competent state from rat liver parenchymal cells. These membrane vesicles have the capacity to accumulate L-alanine against an apparent concentration gradient when exposed to an artificial and transient transmembrane Na+ gradient (extravesicular Na+ concentration greater than inside). The rate of accumulation of L-alanine is dependent on the plasma-membrane vesicle concentration, and the steady-state concentration attained is inversely related to the osmolarity of the medium. The Na+-mediated stimulation is not exhibited if the membrane vesicles are pre-equilibrated with NaCl, if K+ or Li+ are substituted for Na+, or if SO42- replaces Cl- as the counter ion. The apparent active transport of L-alanine into the membrane vesicles appears to occur by an electrogenic mechanism: the use of NaSCN significantly heightens the early concentrative phase of transport when compared with the effect of NaCl; an enhanced active transport is also observed when a valinomycin-induced K+ efflux occurs concomitant with Na+ and L-alanine influx. Plasma-membrane vesicles isolated from liver parenchymal cells of a 24 h-starved rat exhibit an initial L-alanine transport rate that is 3- to 4-times that for membrane vesicles derived from a fed animal. The increased rate of L-alanine transport by plasma-membrane vesicles from starved animals can be obliterated by adrenalectomy and restored by administration of glucocorticoid. Stimulation of the gluconeogenic pathway by starvation involves a plasma-membrane-localized change affecting L-alanine transport which is regulated in part by the glucocorticoid hormones.

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