Temperature regulation of glucose metabolism in red blood cells of the freeze-tolerant wood frog

The low-temperature metabolism of erythrocytes from the freeze-tolerant frog Rana sylvatica was investigated by 13C and 31P NMR spectroscopy. Erythrocytes readily took up high concentrations of the natural cryoprotectant, glucose, at both high (12 and 17°C) and low (4°C) temperatures but glucose was apparently not metabolized at 4°C. Strong inhibition of glucose catabolism at low temperature would facilitate the maintenance of the very high concentrations of glucose (approximately 200 mM) that are accumulated to provide cryoprotection during freezing in wood frogs. Analysis of 13C labeling of glycolytic intermediates at 4°C showed mixing of label primarily in hexose (fructose) and hexose phosphate (glucose 6- phosphate, fructose 6-phosphate) pools but little label incorporation into triose phosphate intermediates. These data are consistent with a profound low-temperature-induced inhibition of phosphofructokinase (PFK). Investigations into potential PFK control mechanisms were undertaken. 31P NMR analysis showed that the intracellular pH of erythrocytes increased from 7.0 to 7.3 as temperature decreased from 17 to 4°C in a manner consistent with alphastat regulation. This change is exactly opposite to that expected if overall PFK activity was regulated by changes in cellular pH since PFK is less active at lower pH values in vitro. Other factors must, therefore, operate to regulate PFK at lower temperatures