Ionic control of renal gluconeogenesis III. The effects of changes in pH, pCO 2, and bicarbonate concentration

Abstract

The effect of changing pH, pCO 2 and HCO 3 − concentration upon the formation of glucose from a variety of substrates, and the rates of decarboxylation of these substrates was studied in isolated rat renal tubules. A decrease in pH whether brought about by an increase in pCO 2 or a decrease in HCO 3 − concentration led to an increase in glucose production from citrate, glutamine, glutamate and α-ketoglutarate but not from malate or pyruvate. Conversely an increase in pH, whether due to an increase in HCO 3 − concentration or a decrease in pCO 2, led to an inhibition of glucose formation from all substrates. An increase in pH brought about by an increase in HCO 3 − concentration produced a greater inhibition than an increase in pH brought about by a decrease in pCO 2. In both instances alkalosis was associated with a decrease in cellular ATP content. In the case of both citrate and α-ketoglutarate, the rate of substrate disappearance followed the same pattern as glucose production, but in the case of malate and pyruvate their rates of disappearance decreased with a fall in pH even though glucose formation did not change, and their rates of disappearance did not change when pH was increased even though glucose production fell markedly. A change in total HCO 3 − concentration from 30 to 15 mM, maintaining a constant pH of 7.2±0.02, resulted in an increase in glucose formation from citrate, glutamine, and α-ketoglutarate but not from malate or pyruvate. This increase was less than that seen after the pH fell. When HCO 3 − concentration was raised from 30 to 60 mM, at constant pH, there was a fall in rate of glucose formation from all substrates. This fall was comparably to that seen in respiratory alkalosis but less than that seen in metabolic alkalosis. These data, plus data from measurements of metabolic concentrations under different conditions of pH, pCO 2 and HCO 3 − concentration and measurements of substrate decarboxylation in both tubules and isolated renal mitochondria, led to the conclusions that when pH is lowered either by a fall in HCO 3 − or an increase in pCO 2, the major regulator of gluconeogenesis is H + concentration which acts to inhibit citrate synthetase and activate both isocitrate and succinate dehydrogenase, all mitochondria enzymes. Conversely, a decrease in H + concentration could account for only part of the changes observed in alkalosis. The fall in H + concentration led to a decrease in ATP concentration which led in turn to an inhibition of phosphoglycerate kinase. In addition, an increase in HCO 3 − concentration has a specific inhibitory effect upon phosphoenolpyruvate carboxykinase, a cytosolic enzyme.