Abstract
The ability of liver efficiently to 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 SO(4) (2-) replaces Cl(-) as the counterion. The apparent active transport of l-alanine into the membrane vesicles appears to occur by an electrogenic mechanism: (1) the use of NaSCN significantly heightens the early concentrative phase of transport when compared with the effect of NaCl; (2) 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 3-4 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. These results establish that 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.