title: Alterations in mitochondrial Ca2+ flux by the antibiotic X-537A (lasalocid-A).
abstract: A previous communication (Pereira da Silva, L., Bernardes, C.F. and Vercesi, A.E. (1984) Biochem. Biophys. Res. Commun. 124, 80-86) presented evidence that lasalocid-A, at concentrations far below those required to act as a Ca2+ ionophore, significantly inhibits Ca2+ efflux from liver mitochondria. In the present work we have studied the mechanism of this inhibition in liver and heart mitochondria. It was observed that lasalocid-A (25-250 nM), like nigericin, promotes the electroneutral exchange of K+ for H+ across the inner mitochondrial membrane and as a consequence can cause significant alterations in delta pH and delta psi. An indirect effect of these changes that might lead to inhibition of mitochondrial Ca2+ release was ruled out by experiments showing that the three observed patterns of lasalocid-A effect depend on the size of the mitochondrial Ca2+ load. At low Ca2+ loads (5-70 nmol Ca2+/mg protein), under experimental conditions in which Ca2+ release is supposed to be mediated by a Ca2+/2H+ antiporter, the kinetic data indicate that lasalocid-A inhibits the efflux of the cation by a competitive mechanism. The Ca2+/2Na+ antiporter, the dominant pathway for Ca2+ efflux from heart mitochondria, is not affected by lasalocid-A. At intermediate Ca2+ loads (70-110 nmol Ca2+/mg protein), lasalocid-A slightly stimulates Ca2+ release. This effect appears to be due to an increase in membrane permeability caused by the displacement of a pool of membrane bound Mg2+ possibly involved in the maintenance of membrane structure. Finally, at high Ca2+ loads (110-140 nmol Ca2+/mg protein) lasalocid-A enhances Ca2+ retention by liver mitochondria even in the presence of Ca2(+)-releasing agents such as phosphate and oxidants of the mitochondrial pyridine nucleotides. The maintenance of a high membrane potential under these conditions may indicate that lasalocid-A is a potent inhibitor of the Ca2(+)-induced membrane permeabilization. Nigericin, whose chemical structure resembles that of lasalocid-A, caused similar results.
authors.affiliation: Departamento de Bioquimica, Instituto de Biologia, UNICAMP, Campinas, SP, Brasil.
authors.forename: R V
authors.initials: RV
authors.lastname: Antonio
authors.forename: L P
authors.initials: LP
authors.lastname: da Silva
authors.forename: A E
authors.initials: AE
authors.lastname: Vercesi
mesh_terms: D000085:Acetates / Q000378:metabolism; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007832:Lasalocid / Q000494:pharmacology*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D008929:Mitochondria, Heart / Q000187:drug effects* / Q000378:metabolism; D008930:Mitochondria, Liver / Q000187:drug effects* / Q000378:metabolism; D008933:Mitochondrial Swelling / Q000187:drug effects; D009550:Nigericin / Q000494:pharmacology; D011188:Potassium / Q000502:physiology; D051381:Rats; D011919:Rats, Inbred Strains; D012430:Ruthenium Red / Q000494:pharmacology; D012964:Sodium / Q000502:physiology
chemical_list: D000085:Acetates; D012430:Ruthenium Red; D012964:Sodium; D009550:Nigericin; D011188:Potassium; D002118:Calcium; D007832:Lasalocid