Direct intracellular pH measurement in rat and crab muscle.

1. Conflicting values of intracellular pH (pH(i)) were obtained by Caldwell in crab muscle and by Carter et al. in rat muscle. To investigate this discrepancy, double and single barrelled pH sensitive micro-electrodes as described by Carter et al. were used both in rat muscle (in vivo and in vitro) and in large single fibres of crab (in vitro).2. The pH(i) values obtained in the superficial layers of muscle cells in rat and pH(i) values obtained in the superficial area of large single fibres in crab seemed in agreement with the Donnan equilibrium. Furthermore, when the membrane potential (E(m)) value of extracellular pH (pH(e)) value was changed, each new pH(i) value was obtained instantly.3. The pH(i) values obtained in deep layers of muscle cells in rat (pH(i) 6.68) and in a deep area of large single fibres in crab (pH(i) 6.96) did not agree with Donnan equilibrium; these pH(i) values were little influenced by modifications in E(m) or pH(e) values.4. These findings could be easily explained if the length of the sensitive portion of the pH micro-electrode exceeds the diameter of rat muscle cells and is smaller than the diameter of crab large fibres. The pH(i) values obtained superficially in the two preparations and which seem in agreement with the Donnan equilibrium are artificially low because the E(m) values recorded by the pH micro-electrodes are partly shunted.