Intracellular pH of single crustacean muscle fibres by the DMO and electrode methods during acid and alkaline conditions.

1. The intracellular pH of intact single muscle fibres of the giant barnacle was measured directly with a glass micro-electrode following prolonged (2-5 hr) equilibration in one of three solutions: normal Ringer, CO2 Ringer and NH4+ Ringer. 2. The intracellular pH of identically-prepared fibres from the same specimen was measured indirectly from the distribution of DMO following prolonged equilibration in the same solutions. 3. The DMO-pH compared favourably with the electrode-pHi provided DMO-pHi was calculated from the values of the indicator compounds, [14C]DMO and [3H]inulin, obtained by extrapolating the slow uptake phase to time zero. 4. Following prolonged equilibration, the transmembrane H+ ion distribution was found to vary with the membrane potential but not in accordance with a simple Gibbs-Donnan equilibrium. 5. A model which recognizes the existence of two independent net fluxes for H+ across the membrane in developed to explain the results. One of the fluxes represents passive diffusion and the other represents the so called H+-pump. The model predicts the H+-pump rate increases by two orders of magnitude when pHi is reduced from 7-2 to 6-7.