THE EFFECT OF pH ON CARBOHYDRATE CHANGES IN ISOLATED ANAEROBIC FROG MUSCLE

Proton Fall or Bicarbonate Rise: GLYCOLYTIC RATE IN MOUSE ASTROCYTES IS PAVED BY INTRACELLULAR ALKALINIZATION

Glycolysis is the primary step for major energy production in the cell. There is strong evidence suggesting that glucose consumption and rate of glycolysis are highly modulated by cytosolic pH/[H(+)], but those can also be stimulated by an increase in the intracellular [HCO3 (-)]. Because proton and bicarbonate shift concomitantly, it remained unclear whether enhanced glucose consumption and glycolytic rate were mediated by the changes in intracellular [H(+)] or [HCO3 (-)]. We have asked whether glucose metabolism is enhanced by either a fall in intracellular [H(+)] or a rise in intracellular [HCO3 (-)], or by both, in mammalian astrocytes. We have recorded intracellular glucose in mouse astrocytes using a FRET-based nanosensor, while imposing different intracellular [H(+)] and [CO2]/[HCO3 (-)]. Glucose consumption and glycolytic rate were augmented by a fall in intracellular [H(+)], irrespective of a concomitant rise or fall in intracellular [HCO3 (-)]. Transport of HCO3 (-) into and out of astrocytes by the electrogenic sodium bicarbonate cotransporter (NBCe1) played a crucial role in causing changes in intracellular pH and [HCO3 (-)], but was not obligatory for the pH-dependent changes in glucose metabolism. Our results clearly show that it is the cytosolic pH that modulates glucose metabolism in cortical astrocytes, and possibly also in other cell types.

The Pasteur effect in facultative anaerobic metazoa

The existence and the regulatory mechanisms of the Pasteur effect in facultative anaerobic metazoa are discussed. There are three reasons for the controversy surrounding this phenomenon. 1) The different definitions of the Pasteur effect, 2) the antagonistic effect of metabolic depression and its species specific response to hypoxia, as well as 3) the laboratory-specific differences in the experimental procedures for analyzing the Pasteur effect and its regulation. This review aims to clarify the confusion about the existence of the Pasteur effect in facultative anaerobic metazoa and to offer possible molecular mechanisms.

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

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.