On the mechanism by which bupivacaine conducts protons across the membranes of mitochondria and liposomes.
Bupivacaine and etidocaine possess the remarkable property of stimulating mitochondrial respiration to levels comparable with those observed with classical anionic protonophores (Dabadie, P., Bendriss, P., Erny, P., and Mazat, J.P. (1987) FEBS Lett. 226, 77-82). We show that these amphiphilic amines conduct protons across the membranes of mitochondria and liposomes and stimulate respiration by a true protonophoretic mechanism. The kinetics of drug-induced H+ flux exhibited integer Hill coefficients that were greater than two under all conditions, suggesting that multimers are required for H+ transport. When the energy barrier for ion transport was lowered in mitochondria, by increasing the membrane potential, or in liposomes, by adding phloretin, the Hill coefficients decreased to lower integer numbers. Protonophoretic activity depended exclusively on medium concentration of free base, leading us to conclude that bupivacaine and etidocaine conduct protons as associated, intramembrane multimers of the free base. Bupivacaine-induced H+ leak was ohmic rather than nonohmic, as would be expected of a mobile charged carrier. This kinetic behavior seems improbable for a multimeric mobile carrier mechanism and suggests a channel mechanism, in which ohmicity results from splitting of the energy barrier by energy wells along the transport pathway (Garlid, K. D., Beavis, A. D., and Ratkje, S. K. (1989) Biochim. Biophys. Acta 976, 109-120). We hypothesize that bupivacaine and etidocaine act by a novel "flickering channel" mechanism, in which transient linear complexes of free base molecules provide weak binding sites (energy wells) for protons within lipid bilayer membranes.
Sun
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Department of Pharmacology, Medical College of Ohio, Toledo 43699.
Garlid
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D000818:Animals; D002045:Bupivacaine / Q000737:chemistry / Q000494:pharmacology*; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D005041:Etidocaine / Q000737:chemistry / Q000494:pharmacology*; D066298:In Vitro Techniques; D007425:Intracellular Membranes / Q000187:drug effects; D007473:Ion Channels / Q000502:physiology; D007476:Ionophores; D008081:Liposomes / Q000737:chemistry; D008564:Membrane Potentials / Q000187:drug effects; D008930:Mitochondria, Liver / Q000187:drug effects* / Q000502:physiology; D010101:Oxygen Consumption / Q000187:drug effects; D010693:Phloretin / Q000494:pharmacology; D011522:Protons; D051381:Rats; D011919:Rats, Inbred Strains; D014475:Uncoupling Agents; D014634:Valinomycin / Q000494:pharmacology
D007473:Ion Channels; D007476:Ionophores; D008081:Liposomes; D011522:Protons; D014475:Uncoupling Agents; D014634:Valinomycin; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D005041:Etidocaine; D010693:Phloretin; D002045:Bupivacaine