AJP - Cell Physiology, Vol 258, Issue 2 C327-C335, Copyright © 1990 by American Physiological Society
Stimulation of glucose transport in Clone 9 cells by exposure to alkaline pH
F. Ismail-Beigi, C. L. Mercado and J. N. Loeb
Department of Medicine, College of Physicians and Surgeons, Columbia University, New York City, New York 10032.
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.
