Intracellular acid-base metabolism of rat smooth and skeletal muscle

The intracellular pH (pHi) of smooth muscles has been studied for the first time. In addition, the relationship between intracellular pH and a variety of extracellular acid-base distortions has been compared in smooth and skeletal muscle. Extracellular acid-base distortions were produced in vivo in animals mechanically respired with either 1) atmospheres containing CO2 (respiratory distortions) or 2) room air after the injection of either NaHCO3 or NH4Ci solution (metabolic distortions). The following general relationships between pHi and pHe were found to apply to both smooth and skeletal muscle. 1) Intracellular pH decreases in metabolic or respiratory acidosis, and increases in metabolic or respiratory alkalosis. However, there are certain limits of pHe within which intracellular pH changes very little when extracellular pH is varied by either respiratory or metabolic means. 2) Intracellular pH is related to extracellular CO2 tension, pH and bicarbonate ([HCO3-])oncentration, but, at any given pHe, CO2 tension has a greater influence on p1^ than does [HCO3-]. The following discrete differences between smooth and skeletal muscle pHj in relation to the extracellular acid-base status were observed. 1) Intracellular pH of smqoth muscle is higher than that of skeletal muscle. 2) Smooth muscle pHi is stable over a smaller range of pHe than is skeletal muscle pHj. 3) Smooth muscle pHi is more sensitive than is skeletal muscle pHi to extracellular metabolic acidosis. 4) Smooth muscle pHi is affected less than is skeletal muscle pH^ by respiratory distortions of pHe. These results, and other known differences between smooth and skeletal muscle, suggest that smooth muscle pHi regulation is related to extracellular HCO3- as well as to H+ ion extrusion, while skeletal muscle pHi regulation is primarily a function of H^ extrusion.