Cardiac dilatation and pump dysfunction without intrinsic myocardial systolic failure following chronic β-adrenoreceptor activation

Abstract

There is no direct evidence to indicate that pump dysfunction in a dilated chamber reflects the impact of chamber dilatation rather than the degree of intrinsic systolic failure resulting from myocardial damage. In the present study, we explored the relative roles of intrinsic myocardial systolic dysfunction and chamber dilatation as mediators of left ventricular (LV) pump dysfunction. Administration of isoproterenol, a β-adrenoreceptor agonist, for 3 mo to rats (0.1 mg·kg−1·day−1) resulted in LV pump dysfunction as evidenced by a reduced LV endocardial fractional shortening (echocardiography) and a decrease in the slope of the LV systolic pressure-volume relation (isolated heart preparations). Although chronic β-adrenoreceptor activation induced cardiomyocyte damage (deoxynucleotidyl transferase-mediated dUTP nick-end labeling) as well as β1- and β2-adrenoreceptor inotropic downregulation (attenuated contractile responses to dobutamine and salbutamol), these changes failed to translate into alterations in intrinsic myocardial contractility. Indeed, LV midwall fractional shortening (echocardiography) and the slope of the LV systolic stress-strain relation (isolated heart preparations) were unchanged. A normal intrinsic myocardial systolic function, despite the presence of cardiomyocyte damage and β-adrenoreceptor inotropic downregulation, was ascribed to marked increases in myocardial norepinephrine release, to upregulation of α-adrenoreceptor-mediated contractile effects as determined by phenylephrine responsiveness, and to compensatory LV hypertrophy. LV pump failure was attributed to LV dilatation, as evidenced by increased LV internal dimensions (echocardiography), and a right shift and increased volume intercept of the LV diastolic pressure-volume relation. In conclusion, chronic sympathetic stimulation, despite reducing β-adrenoreceptor-mediated inotropic responses and promoting myocyte apoptosis, may nevertheless induce pump dysfunction primarily through LV dilatation, rather than intrinsic myocardial systolic failure.