Intracellular Acid-Base Regulation. I. The Response of Muscle Cells to Changes in CO2 Tension or Extracellular Bicarbonate Concentration *

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Abstract
C14-labeled DMO was used to measure the steady-state intracellular pH in rat diaphragm muscle while the latter was exposed in vitro either to varying CO2 tensions and a normal external bicarbonate concentration or to varying bicarbonate concentrations and a normal CO2 tension. Changes in water and electrolyte content were also studied. Under the conditions of these experiments, the relationship between internal and external hydrogen ion concentration could be described by 2 complex curves which, although generally similar in shape, had slightly different loci in the region of extreme extracellular acidity. Alkalinization of the extracellular medium, whether by reduction of PCO2 or by increase of bicarbonate, caused a similar and proportionate degree of alkalinization of muscle cells, together with a rise in potassium, sodium, and water content. On the other hand, acidification of the medium had no discernible effect on cell pH until either the pCO2 was raised above 70 mm Hg (pH 7.1) or the external bicarbonate concentration was reduced to less than 7 mEq per L (pH 6.9). Extracellular acidosis lowered the muscle potassium content, but muscle sodium was increased, and the water content of the cells was unchanged. These data show that, contrary to prevailing opinion, the pH of muscle cells is readily influenced by the external bicarbonate concentration as well as by CO2 tension. These studies also demonstrate that resting skeletal muscle resists external acidosis much more efficiently than it does external alkalosis.