On the Inhibition of the Mitochondrial Inner Membrane Anion Uniporter by Cationic Amphiphiles and Other Drugs

Depleting the mitochondrial matrix of divalent cations with the ionophore A23187 activates a pH-sensitive, anion uniport pathway which can transport many anions normally regarded as impermeant (Beavis, A. D., and Garlid, K. D. (1987) J. Biol. Chem. 262, 15085–15093). Addition of valinomycin to respiring mitochondria can also induce the uptake of a wide variety of anions; however, the mechanism of anion transport during this “respiration-induced” swelling is less certain. In this paper, I demonstrate that both of these processes are inhibited by a variety of cationic amphiphiles including propranolol, quinine, amiodarone, imipramine and amitriptyline, and the benzodiazepine R05-4864. Although the IC50 values for the two processes are not equal, the ratio of IC50 values for the two processes appears to be the same for all drugs. Measurements of net transmembrane proton fluxes that occur during the assays reveal that respiration-induced swelling is associated with extensive proton ejection, the peak of which coincides with the maximum rate of anion transport. Moreover, from measurements of matrix buffering power, it is estimated that the matrix pH is 3 units more alkaline during respiration-induced swelling than during A23187-induced swelling. It is also shown that the IC50 for A23187-induced transport is pH-dependent in a manner consistent with modulation of drug binding by protonation of two sites. These findings allow the difference in IC50 values for the two types of assay to be explained by the pH dependence of the binding constant for the drug. Furthermore, the pH gradient generated during respiration-induced swelling is so large that the electrical component of the proton-motive force will be negligible. Thus, despite the fact that the mitochondria are “energized,” rapid electrophoretic anion influx is possible. These data provide evidence that the transport of anions in these two types of assay occurs via the same pathway.