Intracellular pH and distribution of weak acids across cell membranes. A study of D- and L-lactate and of DMO in rat diaphragm.

1. The steady-state distribution ratios of D- and L-lactate between fibre water and external fluid were measured in 'intact' rat hemidiaphragm preparations exposed for 2-5 hr to a variety of solutions of normal ionic strength and osmolarity. The studies were designed to minimize the effects, on these distributions, of conversion of lactate and of generation of lactic acid by the muscle. 2. At D-lactate concentrations between 2.3 and 118 mM, at normal pH and PCO2, the D-lactate distribution ratio, obtained from the distribution of [2-(3)H]D-lactate was independent of concentration; it averaged 0.349. As the concentration of D-lactate was reduced below 2.3 mM, its distribution ratio progressively fell to less than 0.1. 3. Radiochromatograms of extracts of incubated muscle showed that the tritium label was not attached to substances other than lactate. 4. At L-lactate concentrations of 59 and 108 mM, at normal pH and PCO2, the average L-lactate distribution ratios, obtained by enzymatic analysis, were respectively 0.395 and 0392. 5. At 19-89 mM D-lactate, depolarizing the muscle fibres by high K(49-127 mM), at normal pH, PCO2, and [K]0[Cl]0 product, only slightly affected the D-lactate distribution ratio which averaged 0.405. 6. The D-lactate distribution ratio and intracellular pH (pHi), obtained with the DMO method (5,5-dimethyl-2,4-oxazolidinedione), were measured in thirty sets of studies after exposure of the muscle to solutions buffered to pH values ranging between 5.99 and 8.13, and containing 18.5-118 mM D-lactate and 6-129 mM-K. 7. The relation between the distribution ratios of D-lactate ([TL]i/[TL]O) and of H ions ([Ho/[H]i) in these studies could be expressed by [TL]i/[T]O = 0.646 [H]o/[H]i+0.056. 8. It was concluded that it is predominantly the undissociated lactic acid molecules, rather than the much more numerous lactate ions, which permeate the fibre membrane; and that the steady-state lactate distribution ratio is determined by the transmembrane pH gradient, and not by membrane potential. 9. The expression of the steady-state lactate distribution ratio as function of relative membrane permeabilities of lactic acid molecule and lactate ion, membrane voltage, and internal and external H ion concentrations indicates that a finite permeability to the ion, three or four orders of magnitude less than that to the molecule, is compatible with the experimental data. When both ion and molecule of any weak acid are permeable, they act as a carrier system for the movement of protons down their electrochemical gradient. 10. Near-maintenance of pHi in the face of high fibre D-lactate (19-44 mM) and DMO (8-42 mM) indicates stimulation of proton extrusion by acid loans. 11. This extrusion is insensitive to ouabain, as judged from the lack of effect of the drug of pHi with acid loading.