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We identified abnormalities in the vascular beta-adrenergic receptor (beta-AR) signaling pathway in heart failure after myocardial infarction (MI). To examine these abnormalities, we measured beta-AR-mediated hemodynamics, vascular reactivity, and the vascular beta-AR molecular signaling components in rats with heart failure after MI. Six weeks after MI, these rats had an increased left ventricular (LV) end-diastolic pressure, decreased LV systolic pressure, and decreased rate of LV pressure change (dP/dt). LV dP/dt responses to isoproterenol were shifted downward, although the responses for systemic vascular resistance were shifted upward in heart failure rats (P < 0.05). Isoproterenol- and IBMX-induced vasorelaxations were blunted in heart failure rats (P < 0.05) with no change in the forskolin-mediated vasorelaxation. These changes were associated with the following alterations in beta-AR signaling (P < 0.05): decreases in beta-AR density (aorta: 58.7 +/- 6.0 vs. 35.7 +/- 1.9 fmol/mg membrane protein; carotid: 29.6 +/- 5.6 vs. 18.0 +/- 3.9 fmol/mg membrane protein, n = 5), increases in G protein-coupled receptor kinase activity levels (relative phosphorimage counts of 191 +/- 39 vs. 259 +/- 26 in the aorta and 115 +/- 30 vs. 202 +/- 7 in the carotid artery, n = 5), and decreases in cGMP and cAMP in the carotid artery (0.85 +/- 0.10 vs. 0.31 +/- 0.06 pmol/mg protein and 2.3 +/- 0.3 vs. 1.2 +/- 0.1 pmol/mg protein, n = 5) with no change in Galpha(s) or Galpha(i )in the aorta. Thus in heart failure there are abnormalities in the vascular beta-AR system that are similar to those seen in the myocardium. This suggests a common neurohormonal mechanism and raises the possibility that treatment in heart failure focused on the myocardium may also affect the vasculature.

A mouse model of hypertrophic cardiomyopathy (HCM) was created by expression of a cardiac alpha-myosin transgene including the R(403)Q mutation and a deletion of a segment of the actin-binding domain. HCM mice show early histopathology and hypertrophy, with progressive hypertrophy in females and ventricular dilation in older males. To test the hypothesis that dilated cardiomyopathy (DCM) is part of the pathological spectrum of HCM, we studied chamber morphology, exercise tolerance, hemodynamics, isolated heart function, adrenergic sensitivity, and embryonic gene expression in 8- to 11-mo-old male transgenic animals. Significantly impaired exercise tolerance and both systolic and diastolic dysfunction were seen in vivo. Contraction and relaxation parameters of isolated hearts were also decreased, and lusitropic responsiveness to the beta-adrenergic agonist isoproterenol was modestly reduced. Myocardial levels of the G protein-coupled beta-adrenergic receptor kinase 1 (beta-ARK1) were increased by more than twofold over controls, and total beta-ARK1 activity was also significantly elevated. Induction of fetal gene expression was also observed in transgenic hearts. We conclude that transgenic male animals have undergone cardiac decompensation resulting in a DCM phenotype. This supports the idea that HCM and DCM may be part of a pathological continuum rather than independent diseases.

We studied alterations in the beta-adrenergic receptor (beta-AR) system of rabbit hearts during the development of heart failure (HF) after myocardial infarction (MI) to determine whether the molecular beta-AR abnormalities associated with human HF exist in this animal model. Rabbit HF was established 3 wk after left circumflex coronary artery (LCX) ligation by in vivo physiological measurements, and molecular beta-AR signaling was examined in tissue and cultured ventricular myocytes. We found that there was a significant global reduction in beta-AR density by approximately 50% in both ventricles of MI animals compared with sham-operated control animals and that functional beta-AR coupling was significantly reduced. Importantly, as found in human HF, myocardial protein levels and activity of the beta-AR kinase (beta-ARK1) and Galphai were found to be significantly elevated in MI rabbits, suggesting that these molecules are contributing to myocardial dysfunction. Thus the myocardial beta-AR system of this rabbit model of HF shares important biochemical characteristics with human HF and therefore is an ideal laboratory model to investigate novel therapeutic targets for the treatment of HF.

Transgenic mice were generated with cardiac-specific overexpression of the G protein-coupled receptor kinase 3 (GRK3) to explore the in vivo role of this GRK in cardiac function. GRK3 is expressed in the heart along with the beta-adrenergic receptor kinase (beta-ARK1) and GRK5. We have previously demonstrated that myocardial-targeted overexpression in transgenic mice of beta-ARK1 (Koch, W.J., H. A. Rockman, P. Samama, R. A. Hamilton, R. A. Bond, C. A. Milano, and R. J. Lefkowitz. Science 268: 1350-1353, 1995) or GRK5 (Rockman, H.A., D.-J. Choi, N. U. Rahman, S. A. Akhter, R. J. Lefkowitz, and W. J. Koch. Proc. Natl. Acad. Sci. USA 93: 9954-9959, 1996) results in significant attenuation of beta-adrenergic signaling and in vivo cardiac function and selective desensitization of angiotensin (ANG) II-mediated cardiac responses. Surprisingly, myocardial overexpression of GRK3 resulted in normal biochemical signaling through beta-adrenergic receptors (beta-ARs), and in vivo hemodynamic function in response to a beta-AR agonist was indistinguishable from that in nontransgenic controls. Furthermore, in vivo signaling and functional responses to ANG II were unaltered. However, myocardial thrombin signaling, as assessed by p42/p44 mitogen-activated protein (MAP) kinase activation, was significantly attenuated in GRK3 transgenic mouse hearts, indicating a distinct in vivo substrate specificity for GRK3.