Intracellular pH

The estimated value of intracellular pH (pHi) of muscle cells has varied from 6.0 to 7.2, depending on the method of measurement used.The original estimates of pHi, using the partition of the physiologic weak acid system, HCO3-/CO2 were approximately 7.0 [1].Similar results have been obtained with the method of Waddell and Butler [2] using the weak acid DMO.Several different studies [4–7] in which pH-sensitive glass microelectrodes were placed into the cell have also yielded values of approximately 7.0 for pHi.In our laboratory, on the other hand, we have used a double-barreled pH-sensitive glass ultra microelectrode to measure pHiand have obtained values of approximately 6.0, which are in electrochemical equilibrium with pH of extracellular fluid [8, 9].According to our results the pHiis predictable from the transmembrane potential (Em) and extracellular pH(pH0): Em = RT1n 10(pH0-pHi)/ FIt has been pointed out that the partition of weak acids between intra- and extracellular phases would be disproportionately influenced by any alkaline compartment within the cell and would yield a value of pHi higher than the true integrated mean [3], This possibility, however, has to a large extent been ignored until only recently.Adler explored the possibility of intracellular compartments of differing pH by comparing the partition of the weak acid, DMO, with the partition of the weak base, nicotine, in rat diaphragm in vitro [10].The intracellular partition of nicotine would be disproportionately influenced by acid compartments, and thus would yield an estimated pH, lower than that obtained with DMO.He found that pHi estimated with nicotine was 0.48 pH units lower than that estimated from DMO, suggesting some degree of compartment-alization of pHi in skeletal muscle.The present studies undertaken in large muscle cells of barnacles were designed to further investigate the possibility of compartmentalization of pHi by means of glass pH electrodes and by the DMO and nicotine buffer partition methods.The results suggest the possibility that several compartments of differing pH value exist within the muscle cell