Studies on the mechanism of uncoupling by amine local anesthetics. Evidence for mitochondrial proton transport mediated by lipophilic ion pairs.

The hydrophobic weak acid, carbonyl cyanide m-chlorophenylhydrazone (CCCP), induced rapid swelling of mitochondria suspended in NH4SCN, as predicted from the protonophoretic property of CCCP. Butacaine and other hydrophobic amines produced the same response as CCCP. As expected of a protonophore, CCCP (a) induced swelling in mitochondria suspended in potassium acetate and valinomycin, (b) induced rapid K+ efflux from mitochondria treated with valinomycin, and (c) caused an 8-fold stimulation of respiration. Butacaine alone was unable to mimic these effects of CCCP; however, all of these effects were observed with butacaine when SCN- or tetraphenylboron were added to the assay medium. We conclude that amine local anesthetics are not protonophores; rather, their uncoupler-like activity derives from their ability to form lipophilic ion pairs with certain anions. Depending on conditions, ion pair transport may result in electroneutral anion uptake or in electrophoretic proton transport. Uncoupling results from transmembrane cycling of neutral amine, charged anion, and neutral ion pair. Dose response studies were carried out for several pharmacologically important amines with local anesthetic properties. Their relative potencies in the NH4SCN swelling assay followed the order: chlorpromazine much greater than quinine greater than dibucaine greater than quinidine greater than butacaine greater than propranolol greater than tetracaine much greater than lidocaine, procaine, procainamide. Neutral anesthetics (progesterone and benzocaine) and an amine with low anesthetic potency (timolol) were found ineffective. Among hydrophobic drugs, swelling rates in NH4SCN correlate poorly with octanol:water partition coefficients, suggesting that the rate of ion pair transport is determined by the formation constant for lipophilic ion pairs.