Role of the membrane potential in serum-stimulated uptake of amino acid in a diploid human fibroblast.

The Na+-dependent accumulation of alpha-aminoisobutyric acid (AIB), measured in normal growing and quiescent (serum-deprived) HSWP cells (human diploid fibroblast), was found to be twofold higher (AIB/in/AIBout = 20-25) under the normal growing conditions. Serum stimulation of quiescent cells increases their AIB concentrating capacity by approximately 70% within 1 hr. These observations suggest that the driving forces for AIB accumulation may be reversibly influenced by the serum concentration of the growth medium. Addition of valinomycin (Val) to cells preequilibrated with AIB causes an enhanced accumulation of AIB, suggesting that the membrane potential can serve as a driving force for AIB accumulation. After preequilibration with AIB in 6 mM K+, transfer to 94 mM K+ with Val results in a marked and rapid net loss of AIB. The effect of Val on the accumulation of AIB is greatest in quiescent cells, with the intracellular AIB concentrations reaching those seen both in Val-stimulated normal cells and in Val-stimulated serum-stimulated cells. By adjusting [K+]0, in the presence of Val, the membrane potential of growing cells can be matched to that of quiescent cells or vice versa. When this is done, the two accumulate AIB to the same extent. Hence the AIB accumulating capacity is characteristic of the membrane potential rather than of the growth state. In summary, these data suggest that the accumulation of AIB in HSWP cells is influenced by changes in membrane potential and that a serum-associated membrane hyperpolarization could be responsible for the increased capacity for AIB accumulation in serum-stimulated cells.