Regulation of mitochondrial K+/H+ antiport activity by hydrogen ions.

The effect of matrix pH (pHi) on the activity of the mitochondrial K+/H+ antiport has been studied using the fluorescence of 2,7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) to monitor pHi and passive swelling in K+ acetate to follow antiport activity. Heart mitochondria suspended in hypotonic K+ acetate in the absence of respiration show an initial delta pH of -0.4 (interior acid) that decays slowly. Addition of A23187 to deplete matrix Mg2+ results in a further acid shift in pHi followed by equilibration of delta pH. This equilibration appears to depend on K+/H+ antiport and is slow at acid pHi but very rapid when the matrix is alkaline. Swelling of Mg(2+)-depleted mitochondria in K+ acetate is multiphasic with a slow initial rate, a period of maximum swelling, and a final period in which the rate declines. At constant external pH (pH0), the initial rate of swelling is faster with increasing pHi and the time to the onset of the maximum swelling rate decreases. The maximum swelling rate is initiated at pHi 7.4 when pH0 is 7.8 and at pHi 7.1 when pH0 is 7.4. The maximum rate of swelling increases linearly with increasing pH0 in the range from 7.0 to 8.2. This rate also shows a linear relationship to the value of pHi at the time the maximum rate is attained. Dixon plots of the reciprocal of the maximum swelling rate vs [H+]0 suggest that external [H+] is a noncompetitive inhibitor of K+ entry on the antiport. It is concluded that K+/H+ antiport in Mg(2+)-depleted heart mitochondria can be regulated by matrix [H+] (see Beavis, A. D., and Garlid, K. D. (1990) J. Biol. Chem. 265, 2538-2545), but that this antiport is also sensitive to external [H+] or to delta pH when it acts in the direction of K+ uptake.