Na+/H+ exchange in mitochondria as monitored by BCECF fluorescence

Abstract

The recently developed method of loading isolated heart mitochondria with the fluorescent pH indicator, BCECF, was applied to monitor the Na⁺ ₀/H⁺ _i exchange process from the matrix side of the membrane. The Na⁺-induced changes in the pH of the matrix (pH_m) showed that: (i) the Na⁺ ₀/H⁺ _i exchange followed Michaelis-Menten kinetics with respect to external Na⁺ with a K _m of approx. 20 mM; (ii) in contrast to this, the dependence of the exchange rate on the matrix [H⁺] did not obey the Michaelian model. No Na⁺-induced alkalinization occurred above a pH_m of 7.45±0.09 (n=4). Below this value the reciprocal of the transport rate and that of the matrix [H⁺] deviated upwardly from the straight line. The results suggest that internal H⁺ might exert allosteric control on the mitochondrial Na⁺/H⁺ exchange process.