Lusitropic effects of α- and β-adrenergic stimulation in amphibian heart

Abstract

The effects of β and α-adrenergic stimulation in amphibian superfused hearts and ventricular strips were studied. Superfusion with 3×10-8 M isoproterenol produced a positive inotropic effect, as detected by a 92±24% increase in the maximal rate of contraction {Mathematical expression} and a positive lusitropic effect characterized by a decrease in both the ratio {Mathematical expression} (23±5%) and the half relaxation time (t1/2) (19±4%). The mechanical behavior induced by the β-agonist was associated with an increase in the intracellular cAMP levels from control values of 173±19 to 329±28 nmol/mg wet tissue. Hearts superfused with32P in the presence of isoproterenol showed a significant increase in Tn 1 phosphorylation (from 151±13 to 240±44 pmol32P/mg MF protein) without consistent changes in phosphorylation of C-protein. In sarcoplasmic reticulum membrane vesicles, no phospholamban phosphorylation was detected either by β-adrenergic stimulation of superfused hearts or when phosphorylation conditions were optimized by direct treatment of the vesicles with cAMP-dependent protein kinase (PKA) and [y32P] ATP. The effect of α-adrenergic stimulation on ventricular strips was studied at 30 and 22°C. At 30°C, the effects of 10-5 to 10-4M phenylephrine on myocardial contraction and relaxation were diminished to non significant levels by addition of propranolol. At 22°C, blockage with propranolol left a remanent positive inotropic effect (10% of the total effect of phenylephrine) and changed the phenylephrine-induced positive lusitropic effect into a negative lusitropic action. These propranolol-resistant effects were abolished by prazosin. Our results suggest that in amphibian heart, both the inotropic and lusitropic responses to catecholamines are mainly due to a β-adrenergic stimulation which predominates over the α-adrenergic response. Phospholamban phosphorylation seems not to be involved in mediating the positive lusitropic effect of β-adrenergic agents whereas phosphorylation of troponin 1 may play a critical role. © 1994 Kluwer Academic Publishers.