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Intact rat diaphragms were exposed in vitro to varying CO(2) tensions and bicarbonate concentrations, and the steady-state citrate content of diaphragm muscle was measured to investigate the relationship between metabolism and extracellular pH, P(CO2), and (HCO(3) (-)). In addition, rat hemidiaphragms were incubated with 1,5-citrate-(14)C under different acid-base conditions, and (14)CO(2) production was determined as a measure of citrate oxidation. Acidification of the bathing medium achieved by raising CO(2) tension or lowering (HCO(3) (-)) was associated with a decrease in muscle citrate content. On the other hand, alkalinization of the medium induced by lowering CO(2) tension or raising (HCO(3) (-)) caused tissue citrate content to rise. At a physiologic extracellular pH value of approximately 7.40, citrate content was decreased or normal depending on the CO(2)/HCO(3) (-) combination employed to attain the pH. Under low bicarbonate and low P(CO2) conditions, citrate content was reduced. A similar result was found at external pH values of 7.15, implying that at these two extracellular pH levels (HCO(3) (-)) primarily determines citrate content. When changes in citrate content were compared with intracellular pH data reported earlier using the same intact diaphragm preparation, no simple relation between citrate content and intracellular pH was found. The effect of acidity on citrate content seems related to a change in citrate oxidation since the latter increased progressively with increasing degrees of medium acidity. These results show that cellular metabolism is not a simple function of extracellular pH but is dependent on the particular combination of P(CO2) and bicarbonate employed to achieve the pH value. These studies also suggest that accumulation or disposal of organic acids, such as citric acid, helps to regulate cellular acidity thereby contributing to the cells' defense against external acid-base disorders.

The role of pH, PCO2, and bicarbonate in regulating rat diaphragm citrate content