title: Positive inotropic effects of ouabain in isolated cat ventricular myocytes in sodium-free conditions.
abstract: The inotropic and toxic effects of cardiac steroids are thought to result from Na(+)-K(+)-ATPase inhibition, with elevated intracellular Na(+)(Na)causing increased intracellular Ca(2+)(Ca) via Na-Ca exchange. We studied the effects of ouabain on cat ventricular myocytes in Na(+)-free conditions where the exchanger is inhibited. Cell shortening and Ca transients (with fluo 4-AM fluorescence) were measured under voltage clamp during exposure to Na(+)-free solutions [LiCl or N-methyl-D-glucamine (NMDG) replacement]. Ouabain enhanced contractility by 121 +/- 55% at 1 micromol/l (n = 11) and 476 +/- 159% at 3 micromol/l (n = 8) (means +/- SE). Ca transient amplitude was also increased. The inotropic effects of ouabain were retained even after pretreatment with saxitoxin (5 micromol/l) or changing the holding potential to -40 mV (to inactivate Na(+) current). Similar results were obtained with both Li(+) and NMDG replacement and in the absence of external K(+), indicating that ouabain produced positive inotropy in the absence of functional Na-Ca exchange and Na(+)-K(+)-ATPase activity. In contrast, ouabain had no inotropic response in rat ventricular myocytes (10-100 micromol/l). Finally, ouabain reversibly increased Ca(2+) overload toxicity by accelerating the rate of spontaneous aftercontractions (n = 13). These results suggest that the cellular effects of ouabain on the heart may include actions independent of Na(+)-K(+)-ATPase inhibition, Na-Ca exchange, and changes in Na.
authors.affiliation: Division of Cardiology, Department of Medicine, Northwestern University Medical School, 303 E. Chicago Avenue, Chicago, IL 60611, USA.
authors.forename: Manabu
authors.initials: M
authors.lastname: Nishio
authors.forename: Stuart W
authors.initials: SW
authors.lastname: Ruch
authors.forename: J Andrew
authors.initials: JA
authors.lastname: Wasserstrom
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002316:Cardiotonic Agents / Q000494:pharmacology*; D002415:Cats; D006352:Heart Ventricles / Q000166:cytology; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009200:Myocardial Contraction / Q000187:drug effects*; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000378:metabolism; D010042:Ouabain / Q000494:pharmacology*; D018408:Patch-Clamp Techniques; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D012530:Saxitoxin / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*; D026941:Sodium Channel Blockers / Q000494:pharmacology; D015222:Sodium Channels / Q000378:metabolism
chemical_list: D002316:Cardiotonic Agents; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D012530:Saxitoxin; D010042:Ouabain; D012964:Sodium; D002118:Calcium