Mitochondrial transmembrane pH and electrical gradients: evaluation of their energy relationships with respiratory rate and ATP synthesis

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Abstract
Mitochondria from rat liver were suspended in ionic media, and parallel measurements were made of the respiratory rate, intramitochondrial volumes, and the phosphorylation state [[ATP]/([ADP][Pi])], as well as transmembrane pH gradients (from the distribution of labeled weak acids and bases) and electrical gradients (from the distribution of labeled lipid-soluble ions and of K+ in the presence of valinomycin). A decrease in the pH gradient was observerd with additions of sodium propionate, acetate, or lactate. In the case of propionate, the pH gradient decreased from -0.94 .+-. 0.10 (control) to -0.09 .+-. 0.08 unit (with 3 mM propionate) without change in any of the other measured parameters. Variation of the intramitochondrial volumes was achieved by changing [K+] (+valinomycin) and by diluting the choline chloride media. The transmembrane electrical potential decreased from -150 mV under control conditions to .apprx. -75 mV in a 70 mosM [milliosmolar] medium or when 8 mM K+ (+valinomycin) was present. The changes in [ATP]/([ADP][Pi]) were as follows: from 1.8 .times. 104 (control) to 3.9 .times. 102 M-1 with 8 mM K+ (+valinomycin) or 1.0 .times. 104 M-1 when the osmolarity was decreased from 300 to 70 mosM. This corresponds to a decrease in the energy utilized for ATP synthesis from 14.1 to 11.9 kcal/mol (from 58.9 to 49.7 kJ/mol) with added K+ and to 13.8 kcal/mol (57.7 kJ/mol) with decreased osmolarity. No direct correlation was found between the changes in transmembrane electrical potential, pH gradient, or total proton electrochemical gradient and either the respiratory rate or [ATP]/([ADP][Pi]).