Amino acid transport in plasma membrane vesicles from isolated rat liver parenchymal cells

A rapid method for the functional reconstitution of amino acid transport systems from rat liver plasma membranes. Partial purification of System A

A rapid method for the functional reconstruction of amino acid transport from liver plasma-membrane vesicles using the neutral detergent decanoyl-N-glucamide ('MEGA-10') is described. The method is a modification of that previously employed in this laboratory for reconstitution of amino acid transport systems from kidney brush-border membranes [Lynch & McGivan (1987) Biochem. J. 244, 503-508]. The transport activities termed 'System A', 'System N', and 'System L' are all reconstituted. The reconstitution procedure is rapid and efficient and is suitable as an assay for transport activity in studies involving membrane fractionation. By using this reconstitution procedure, System A transport activity was partially purified by lectin-affinity chromatography.

Maintenance of glucagon-stimulated system A amino acid transport activity in rat liver plasma membrane vesicles

Plasma membrane vesicles prepared from intact rat liver or isolated hepatocytes retain transport activity by systems A, ASC, N, and Gly. Selective substrates for these systems showed a Na+-dependent overshoot indicative of energy-dependent transport, in this instance, driven by an artificially-imposed Na+ gradient. Greater than 85% of Na+-dependent 2-aminoisobutyric acid (AIB) uptake was blocked by an excess of 2-(methylamino)isobutyric acid (MeAIB) with an apparent Ki of 0.6 mM. Intact hepatocytes obtained from glucagon-treated rats exhibited a stimulation of system A activity and plasma membrane vesicles isolated from those same cells partially retained the elevated activity. Transport activity induced by substrate starvation of cultured hepatocytes was also evident in membrane vesicles prepared from those cells. The membrane-bound glucagon-stimulated system A activity decays rapidly during incubation of vesicles at 4 degrees C (t1/2 = 13 h), but not at -75 degrees C. Several different inhibitors of proteolysis were ineffective in blocking the decay of transport activity. Hepatic system N transport activity was also elevated in plasma membrane vesicles from glucagon-treated rats, whereas system ASC was essentially unchanged. The results indicate that both glucagon and adaptive regulation cause an induction of amino acid transport through a plasma membrane-associated protein.

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

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.

Plasma membrane vesicles from isolated hepatocytes retain the increase of amino acid transport induced by dibutyryl cyclic AMP in intact cells

We have investigated the effect of cyclic AMP on hepatic amino acid transport by measuring the uptake of L-alanine in plasma membrane vesicles purified from hepatocytes incubated without or with dibutyryl cyclic AMP. The application of an Na+ gradient to vesicles from hepatocytes exposed to dibutyryl cyclic AMP, compared to control, resulted in a greater transient accumulation of L-alanine, whereas in the presence of a K+ gradient a similar slow equilibration of L-alanine was observed. Kinetic analysis of L-alanine influx revealed that the increased uptake resulted from an increased capacity (Vmax) with no change in the affinity (Km) of the carriers for L-alanine. These results strongly suggest that dibutyryl cyclic AMP induces stable changes at the membrane level probably by increasing the number of amino acid carrier molecules.

Amino acid-dependent sodium transport in plasma membrane vesicles from rat liver

The L-alanine-dependent transport of sodium ions across the plasma membrane of rat-liver parenchymal cells was studied using isolated plasma membrane vesicles. Sodium uptake is stimulated specifically by the L-isomer of alanine and other amino acids, whose transport is sodium-dependent in rat-liver plasma membrane vesicles. The L-alanine-dependent sodium flux across the membrane is inhibited by an excess of Li+ ions, but not by K+ or choline ions. Sodium transport is sensitive to -SH reagents and ionophores, and is an electrogenic process: a membrane potential (negative inside) can enhance L-alanine-dependent sodium accumulation. The data presented provide further evidence for a sodium-alanine cotransport mechanism.