title: Mechanism of negative lusitropic effect of alpha 1-adrenoceptor stimulation in cat papillary muscles.
abstract: Experiments were performed in cat papillary muscles to explore the mechanisms by which alpha 1-adrenoceptor stimulation affects myocardial relaxation. Phenylephrine (PE; 10 microM) + atenolol (1 microM; n = 8 experiments) produced a negative lusitropic effect, i.e., a prolongation of half-relaxation time (t1/2; time to 50% relaxation) by 30 +/- 10% (P < 0.05) and a proportionally smaller increase in maximal velocity of relaxation (-T) than in maximal velocity of contraction (+T), which significantly increased the ratio +T/-T. A similar increase in contractility, produced by increasing calcium, failed to significantly change t1/2 and +T/-T. PE-induced negative lusitropic effect was significantly inhibited by two protein kinase C (PKC) inhibitors, staurosporine (0.1 microM) and chelerythrine (10 microM). PE also increased intracellular pH by 0.18 +/- 0.05 pH units (P < 0.05, n = 4), as measured by the fluorescent dye 2'-7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. Intracellular alkalosis and the negative lusitropic effect of PE were prevented by the Na+/H+ exchanger inhibitor ethylisopropylamiloride (10 microM). No significant changes in calcium myofilament sensitivity and maximal tension were detected in trabeculae treated with PE either before or after chemical skinning. These results indicate that a Na+/H+ exchanger-induced intracellular alkalosis, possibly mediated by PKC activation, may fully account for the negative lusitropism of alpha 1-adrenoceptor stimulation.
authors.affiliation: Centro de Investigaciones Cardiovasculares, Universidad Nacional de La Plata, Argentina.
authors.forename: M
authors.initials: M
authors.lastname: Vila-Petroff
authors.forename: G N
authors.initials: GN
authors.lastname: Pérez
authors.forename: B
authors.initials: B
authors.lastname: Alvarez
authors.forename: H E
authors.initials: HE
authors.lastname: Cingolani
authors.forename: A
authors.initials: A
authors.lastname: Mattiazzi
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D000889:Anti-Arrhythmia Agents / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002415:Cats; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007425:Intracellular Membranes / Q000378:metabolism; D009200:Myocardial Contraction / Q000502:physiology*; D010210:Papillary Muscles / Q000502:physiology*; D010656:Phenylephrine / Q000494:pharmacology; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000502:physiology; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*
chemical_list: D000889:Anti-Arrhythmia Agents; D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D000584:Amiloride; D011493:Protein Kinase C; D002118:Calcium; C039614:ethylisopropylamiloride