title: Effect of amiloride on inotropic and toxic actions of ouabain in guinea-pig left atria.
abstract: The effect of amiloride on the positive inotropic and toxic effects of ouabain in guinea-pig left atria has been studied. In atria driven at 1 Hz, amiloride (0.3 and 0.5 mM) decreased the EC50 but did not affect the maximal tension developed by ouabain. At 0.1 Hz, amiloride did not change either the EC50 or the maximal tension developed by ouabain. Ouabain toxicity (onset of arrhythmias) was not changed by amiloride at either frequency of stimulation. Therefore, amiloride did not antagonize either the positive inotropic or the toxic effect of ouabain. The positive inotropic effect of amiloride has been ascribed to the inhibition of the Na+/Ca2+ exchanger. Since amiloride inhibits also the Na+/H+ exchanger, 5-(N-ethyl-N-isopropyl)amiloride (EIPA), an amiloride derivative which selectively inhibits the Na+/H+ exchange, has been tested to evaluate the role of the Na+/H+ exchange in the amiloride-ouabain interaction. EIPA increased the EC50 values of ouabain and decreased the maximal developed tension by the glycoside in atria driven at 0.1 and 1 Hz, but did not antagonize the toxic response (arrhythmias) of atria to ouabain. It is suggested that the inhibition of Ca2+ exit through the Na+/Ca2+ exchange by amiloride and ouabain may explain the observation that the positive inotropic effects of amiloride and ouabain are additive.
authors.affiliation: Department of Pharmacology, University of Padua, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Cargnelli
authors.forename: S
authors.initials: S
authors.lastname: Bova
authors.forename: S
authors.initials: S
authors.lastname: Cannas
authors.forename: P
authors.initials: P
authors.lastname: Debetto
authors.forename: S
authors.initials: S
authors.lastname: Luciani
mesh_terms: D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology*; D000818:Animals; D000889:Anti-Arrhythmia Agents / Q000494:pharmacology; D016279:Atrial Function, Left / Q000187:drug effects*; D004357:Drug Synergism; D004558:Electric Stimulation; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects*; D006325:Heart Atria / Q000187:drug effects; D006331:Heart Diseases / Q000139:chemically induced*; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000187:drug effects*; D010042:Ouabain / Q000494:pharmacology* / Q000633:toxicity*; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors; D013268:Stimulation, Chemical
chemical_list: D000889:Anti-Arrhythmia Agents; D017923:Sodium-Hydrogen Exchangers; D010042:Ouabain; D000584:Amiloride; C039614:ethylisopropylamiloride