Stimulation of glucose transport in Clone 9 cells by exposure to alkaline pH

Abstract

Incubation of a rat liver cell line (Clone 9) for 2 h at pH 8.5 was found to result in a profound (5- to 8-fold) stimulation of cytochalasin B-inhibitable glucose transport. The enhancement of glucose transport after exposure to elevated external pH (achieved by lowering the CO2 tension in a bicarbonate-containing medium) was demonstrable within 15 min, was half-maximal at pH 8.0, and was near-maximal at pH 8.6. Intracellular pH rose linearly with incremental changes in external pH, from pH 7.45 to 8.6 with a slope of 0.6. The increase in transport activity in response to incubation at alkaline pH was accompanied by a parallel increase in lactate production and persisted for more than an hour after external pH was restored to normal. During the latter period, intracellular glucose concentration (less than 10% of that of the external medium under control conditions) increased greater than 10-fold to approximate that in the extracellular medium. Incubation of these cells at pH 8.5 for 2 h resulted in a complete resistance of cell ATP levels to challenge with 5 mM cyanide, suggesting that the adaptive facilitation of glucose transport was of sufficient magnitude to permit a marked stimulation of glycolytic ATP synthesis on inhibition of oxidative phosphorylation. The enhancement of glucose transport was attributable to an increase in the maximum velocity (Vmax) rather than to any change in the Michaelis constant (Km) for transport and was not prevented by cycloheximide. It is concluded that the marked stimulation of glucose transport resulting from exposure of these "low-glucose" cells to alkaline pH reflects either an increase in the abundance of functional glucose transporters in the plasma membrane or an increase in their catalytic turnover rate.