The recently developed method of loading isolated heart mitochondria with the fluorescent pH indicator, BCECF, was applied to monitor the Na+,/H: exchange process from the matrix side of the membrane. The Na+-induced changes in the pH of the matrix (pH,) showed that: (i) the Nai/H: exchange followed Michaelis-Menten kinetics with respect to external Na+ with a K, 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 cccurred above a pH, 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. Na+/H+ exchange; Matrix pH; Biscarboxyethylcarboxyfluorescein; (Mitochondria)
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