The Simultaneous Determination of Muscle Cell pH Using a Weak Acid and Weak Base

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Abstract
Should significant pH heterogeneity exist within cells then the simultaneous calculation of intracellular pH from the distribution of a weak acid will give a value closest to the highest pH in the system, whereas calculation from the distribution of a weak base will give a value closer to the lowest pH. These two values should then differ significantly. Intact rat diaphragms were exposed in vitro to varying bicarbonate concentrations (pure metabolic) and CO2 tensions (pure respiratory), and steady-state cell pH was measured simultaneously either by distribution of the weak acid 5,5-dimethyloxazolidine-2,4-dione-14C (pH DMO) or by distribution of the weak base nicotine-14C (pH nicotine). The latter compound was found suitable to measure cell pH since it was neither metabolized nor bound by rat diaphragms. At an external pH of 7.40, pH DMO was 7.17 while pH nicotine was 6.69—a pH difference of 0.48 pH units (P < 0.001). In either respiratory or metabolic alkalosis both DMO and pH nicotine rose so that differences between them remained essentially constant. Metabolic acidosis induced a decrease in both values though they fell more slowly than did extracellular pH. In contradistinction, in respiratory acidosis, decreasing extracellular pH from 7.40 to 6.80 resulted in 0.35 pH unit drop in pH DMO while pH nicotine remained constant. In every experiment, under all external conditions, pH DMO exceeded pH nicotine. These results indicate that there is significant pH heterogeneity within diaphragm muscle, but the degree of heterogeneity may vary under different external conditions. The metabolic implications of these findings are discussed. In addition, the data show that true overall cell pH is between 6.69 and 7.17—a full pH higher than would be expected from thermodynamic considerations alone. This implies the presence of active processes to maintain cell pH.