title: Effects of isoproterenol on the mechanical activity of isolated papillary muscles and perfused rat hearts in various calcium concentrations.
abstract: The aim of the study was to examine, in papillary muscles and in a whole heart preparation, the effects of isoproterenol on the myocardial mechanical activity at different extracellular Ca2+ concentrations. Papillary muscles from left ventricles, contracting isometrically, and rat hearts perfused by the Langendorff technique developing isovolumetric pressure at a fixed rate (200 bpm) and diastolic pressure of 5 mmHg were studied at different Ca2+ concentrations for analysis of the effects of increasing doses of isoproterenol. Papillary muscles were treated with isoproterenol (0.5 to 8 ng ml-1) at four extracellular Ca2+ concentrations (0.25, 0.5, 1.25 and 2.5 mM) and Langendorff perfused hearts were stimulated by isoproterenol (0.05 ng ml-1 to 0.8 ng ml-1), also at four extracellular Ca2+ concentrations (0.5, 1.25, 2.5 and 3.75 mM). Both papillary muscles and perfused hearts showed that force and isovolumetric systolic pressure increase in response to isoproterenol at low Ca2+ concentrations. As Ca2+ concentration is increased, isoproterenol's positive inotropic effect subsides. However, papillary muscle isometric contractions showed a similar time to peak tension decrease in response to isoproterenol at all external Ca2+ concentrations used. The results suggest that the positive inotropic response to isoproterenol, in isolated preparations, changes as a function of extracellular Ca2+ concentration decreasing as external Ca2+ increases. Time to peak tension reduction reinforces the idea that this small positive inotropic response to isoproterenol of the rat myocardium, at the physiological Ca2+ concentration, is due to the Ca2+ saturation of the mechanical activity.
authors.affiliation: Department of Physiological Sciences, Federal University of Espirito Santo, Vitoria, Brazil.
authors.forename: D V
authors.initials: DV
authors.lastname: Vassallo
authors.forename: E Q
authors.initials: EQ
authors.lastname: Lima
authors.forename: P
authors.initials: P
authors.lastname: Campagnaro
authors.forename: I
authors.initials: I
authors.lastname: Stefanon
authors.forename: C M
authors.initials: CM
authors.lastname: Leite
authors.forename: J G
authors.initials: JG
authors.lastname: Mill
mesh_terms: D000818:Animals; D002118:Calcium / Q000008:administration & dosage* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005260:Female; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects; D007545:Isoproterenol / Q000008:administration & dosage* / Q000494:pharmacology; D008297:Male; D010210:Papillary Muscles / Q000187:drug effects / Q000502:physiology*; D010477:Perfusion; D051381:Rats; D017208:Rats, Wistar
chemical_list: D007545:Isoproterenol; D002118:Calcium