Bicarbonate-carbon dioxide buffer system: a determinant of the mitochondrial pH gradient

The influence of the bicarbonate-carbon dioxide buffer system on the pH gradient (delta pH) across the inner membrane of mitochondria from rabbit renal cortex was studied with and without phosphate in the medium. delta pH with bicarbonate buffer or phosphate in the medium was greater at low than at high medium pH so that the difference (delta delta pH) between delta pH at pH 7.1 and at 7.6 was positive. Varying the concentration of phosphate from 0 to 10 mM had little effect on delta delta pH produced by bicarbonate buffer. Inhibition of the phosphate-hydroxyl carrier with N-ethylmaleimide abolished delta delta pH when phosphate was present in non-bicarbonate-containing media. With bicarbonate buffer present, N-ethylmaleimide increased delta delta pH. Similar effects were observed in mitochondria from liver and heart as well as from kidney. The effects of the bicarbonate buffer system on delta pH may result either from an inner membrane permeable to carbon dioxide but not to bicarbonate ion or from an active carrier for bicarbonate ion in the inner membrane. In intact kidney cells, the influence of the bicarbonate buffer system on delta pH may provide a mechanism for regulating substrate metabolism in response to acid-base changes. It may also serve in many organs to reduce fluctuations in matrix pH when alterations in cytoplasmic pH occur.