ENHANCED ALPHA1-ADRENERGIC RESPONSIVENESS IN CARDIOMYOPATHIC HAMSTER CARDIAC MYOCYTES RELATION TO THE EXPRESSION OF PERTUSSIS TOXIN SENSITIVE G-PROTEIN AND ALPHA1-ADRENERGIC RECEPTORS

The pathogenesis of the myopathy occurring in the heart of the cardiomyopathic strain of the Syrian hamster is not well understood but is believed to be associated with abnormal calcium handling by myopathic cells. The purpose of this study was to determine whether the cardiomyopathy occurring in strain BIO 14.6 animals is associated with an enhanced α1-adrenergic receptor-mediated rise in cytosolic calcium, whether a pertussis toxin-sensitive G protein is involved in coupling the α1-adrenergic receptor to changes in intracellular calcium and whether enhanced α1 responsiveness is associated with an increase in the level of expression of the α1-adrenergic receptor or in the pertussis toxin-sensitive G protein or proteins. To test the hypothesis that the cardiomyopathic tate is associated with a greater α1-receptor-mediated rise in cytosolic calcium, we studied the effect of phenylephrine (in the presence of propranolol) on time-averaged cytosolic calcium concentration ([Ca2+](i)) in isolated cardiac myocytes from cardiomyopathic and age-matched control hamsters. Phenylephrine caused a greater increase both in time-averaged [Ca2+](i) (an increase of 48 ± 8% versus 12 ± 3%, p < 0.01) and in contractility (+181 ± 22% versus +35 ± 9%, p < 0.01) in cardiomyopathic than in normal cardiac myocytes. Exposure to pertussis toxin (200 ng/ml for 3 hours) attenuated the α1-adrenergic receptor-mediated increase in contractility and time-averaged [Ca2+](i) in both cardiomyopathic and normal cells. The level of pertussis toxin-sensitive G protein, as determined by pertussis toxin-mediated [32P]ADP-ribosylation, was 1.6-fold higher in cardiomyopathic versus normal hamster hearts. The density of α1-adrenergic receptors, as measured by the antagonist radioligand [3H]prazosin and the affinity of the receptor for agonist and antagonist were similar in myopathic and normal heart membranes. Thus, in cardiac myocytes from hamsters, the α1-adrenergic receptor-mediated effects on [Ca2+](i) and contractility appear to be mediated by a pertussis toxin-sensitive G protein or proteins. In myocytes from cardiomyopathic hamsters, these α1-adrenergic effects were increased in magnitude, as was the level of pertussis toxin-sensitive G protein, but there was no measurable alteration in the density or ligand binding properties of α1-adrenergic receptors.