ISSN:
1573-4919
Keywords:
amphibian myocardial relaxation
;
α and β-adrenergic stimulation
;
Ca2+ myofibrillar sensitivity
;
protein phosphorylation
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
,
Medicine
Notes:
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 $$( + \dot T)$$ and a positive lusitropic effect characterized by a decrease in both the ratio $$ + \dot T/ - \dot T (23 \pm 5\% )$$ (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 [y 32P] 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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00926171
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