Regulation of active amino acid transport by growth-related changes in membrane potential in a human fibroblast.

Indirect measurements of membrane potential in human diploid fibroblast cells indicate that there is a growth-related difference in membrane potential between subconfluent, growing cells (-49 mV) and serum-deprived, quiescent cells (-22 mV). The elevated membrane potential in growing cells appears to be solely responsible for the a-fold higher a-aminoisobutyric acid accumulating capacity that we observed previously in growing cells, as compared to quiescent cells (Villereal, M. L., and Cook, J. S. (1977) J. Supramol. Struct. 6, 179-189). When the membrane potentials of quiescent and growing cells are set equal, using valinomycin and varying [K+],, a-aminoisobutyric acid accumulation is dependent on the magnitude of the potential, but it is independent of the growth state of the cell. Initial a-aminoisobutyric acid influx measurements in growing and quiescent cells show that when cells become quiescent the K,,, increases and the V,,,,, of the transport system decreases. For growing cells, K, = 1.2 + 0.09 mM and V,,, = 15.6 + 0.9 pmol/g of protein/min. For quiescent cells, K, = 2.1 f 0.18 mM and V,,,,, = 8.5 f 0.8 pmol/g of protein/min. Serum stimulation of quiescent cells for 1 h results in a decrease in K,,, (to 1.2 + 0.13 mM) and no change in V,,,,, (7.8 + 0.8 pmol/g of protein/min). Hyperpolarization of quiescent cells by valinomycin also induces a decrease in K, (to 1.2 rf: 0.13 mM) without a change in Vm,,. This suggests that the immediate response of initial a-aminoisobutyric acid transport to serum stimulation is mediated by a membrane hyperpolarization. These data indicate that both the accumulation of a-aminoisobutyric acid in human diploid fibroblast cells and the K,,, for its transport are sensitive to experimentally induced changes in membrane potential, and furthermore, that the observed growth-related difference in membrane potential between growing and quiescent cells can account for their growth-associated differences in a-aminoisobutyric acid accumulation and K,,,.