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Carvedilol is an adrenoceptor antagonist which modulates the activity not only of beta 1 and beta 2 but also of alpha 1 adrenergic receptors present on the cell surface membrane of the human cardiac myocyte. In the heart, carvedilol has approximately 7 times higher potency for beta 1 and beta 2 adrenoceptors, but in the doses 50-100 mg . day-1 used in clinical practice, it is essentially non-selective. In human myocardial preparations and in cultured heart cells, carvedilol has no intrinsic sympathomimetic activity but is able to identify high affinity agonist-binding receptors whose pharmacological signature is reduction in binding by incubation with guanine nucleotides (guanine nucleotide-modulatable binding). This property is more prominent for the human beta 2 than for the beta 1 adrenoceptor. The property of gaunine nucleotide-modulatable binding for carvedilol and structurally related bucindolol correlates with their ability to directly down-regulate beta 1-like receptors present in cultured chick myocytes, and with a lack of reversal of down-regulation of cardiac beta-receptors in patients with heart failure. Carvedilol does not exhibit high levels of inverse agonist activity, which may contribute to its good tolerability in subjects with heart failure. These data indicate that carvedilol produces a high degree of adrenergic receptor blockade in the failing human heart, and does not re-sensitize the beta-receptor pathway to stimulation by adrenergic agonists.

Background: Aging decreases cardiac beta-adrenergic responsiveness in model systems and in humans in vivo. The purpose of this study was to comprehensively evaluate the age-related changes in the beta-receptor-G protein-adenylyl cyclase complex in nonfailing human hearts., Methods and Results: Twenty-six nonfailing explanted human hearts aged 1 to 71 years were obtained from organ donors and subjected to pharmacological investigation of beta-adrenergic neuroeffector systems. When the population was subdivided into the 13 youngest and 13 oldest subjects, total beta-receptor density assessed by maximum [125I]ICYP binding (beta max) was reduced in older hearts by 37% in left ventricles and 31% in right ventricles (both P < .05), and the downregulation was confined to the beta 1 subtype (r = .78 left ventricle beta 1 density versus donor age). Older donor hearts exhibited a 3- to 4-fold rightward shift of ICYP-isoproterenol (ISO) competition curves and demonstrated 43% fewer receptors in a high-affinity agonist binding state (P < .05). Older hearts exhibited decreased adenylyl cyclase stimulation by ISO, by zinterol (beta 2-agonist), and by the G protein-sensitive probes forskolin, Gpp(NH)p, and NaF. In contrast, there was no change in response to manganese, a specific activator of the adenylyl cyclase catalytic subunit. Toxin-catalyzed ADP ribosylation in membranes prepared from older versus younger hearts revealed a 29% to 30% reduction (P < .05) with cholera toxin (Gs) but no difference with pertussis toxin (Gi). The systolic contractile response of isolated right ventricular trabeculae to ISO was decreased by 46%, with a 10-fold increase in ISO EC50 in older relative to younger donor hearts., Conclusions: There is a profound decrease in cardiac beta-adrenergic responsiveness with aging. This occurs by multiple mechanisms including downregulation and decreased agonist binding of beta 1-receptors, uncoupling of beta 2-receptors, and abnormal G protein-mediated signal transduction.

In order to investigate the general cause of beta-adrenergic receptor neuroeffector abnormalities in the failing human heart, we measured ventricular myocardial adrenergic receptors, adrenergic neurotransmitters, and beta-adrenergic receptor-effector responses in nonfailing and failing hearts taken from nonfailing organ donors, subjects with endstage biventricular failure due to idiopathic dilated cardiomyopathy (IDC), and subjects with primary pulmonary hypertension (PPH) who exhibited isolated right ventricular failure. Relative to nonfailing PPH left ventricles, failing PPH right ventricles exhibited (a) markedly decreased beta 1-adrenergic receptor density, (b) marked depletion of tissue norepinephrine and neuropeptide Y, (c) decreased adenylate cyclase stimulation in response to the beta agonists isoproterenol and zinterol, and (d) decreased adenylate cyclase stimulation in response to Gpp(NH)p and forskolin. These abnormalities were directionally similar to, but generally more pronounced than, corresponding findings in failing IDC right ventricles, whereas values for these parameters in nonfailing left ventricles of PPH subjects were similar to values in the nonfailing left ventricles of organ donors. Additionally, relative to paired nonfailing PPH left ventricles and nonfailing right ventricles from organ donors, failing right ventricles from PPH subjects exhibited decreased adenylate cyclase stimulation by MnCl2. These data indicate that: (a) Adrenergic neuroeffector abnormalities present in the failing human heart are due to local mechanisms; systemic processes do not produce beta-adrenergic neuroeffector abnormalities. (b) Pressure-overloaded failing right ventricles of PPH subjects exhibit decreased activity of the catalytic subunit of adenylate cyclase, an abnormality not previously described in the failing human heart.

We evaluated the effects of the diterpene compound forskolin in human myocardial adenylate cyclase preparations, isolated trabeculae and papillary muscles derived from failing human hearts, and acutely instrumented dogs. Forskolin was a potent, powerful activator of human myocardial adenylate cyclase and produced maximal effects that were 4.82 (normally functioning left ventricle) and 6.13 (failing left ventricle) fold greater than isoproterenol. In contrast to isoproterenol, forskolin retained full activity in membrane preparations derived from failing hearts. In cyclase preparations, forskolin demonstrated unique substrate and Mg2+ kinetic properties that could be distinguished from hormone receptor-coupled agonists or fluoride ion. The adenylate cyclase stimulatory effect of forskolin was synergistic with isoproterenol, apparently due to the location of forskolin activation being beyond the level of hormone receptor-agonist in the receptor-cyclase complex. Forskolin was a potent positive inotrope in failing human myocardium, producing a stimulation of contraction that was similar to isoproterenol. Finally, in open chest dogs forskolin was a positive inotropic agent that reduced preload and afterload. We conclude that forskolin belongs to a class of agents that may have therapeutic potential in the treatment of congestive heart failure.

We developed methods for identifying beta-adrenergic receptors in human right ventricular endomyocardial biopsy tissue with the radioligand (-)[125I]iodocyanopindolol (ICYP). Specific ICYP binding in a crude, high-yield membrane preparation derived from endomyocardial biopsy tissue was high (specificity greater than 90%), of high affinity (KD around 20 pM), saturable and stereospecific for the (-) vs the (+) isomer of isoproterenol. Subjects with mild-moderate and severe biventricular dysfunction had respective decreases in beta-adrenergic receptor density of 38.2% and 57.7% when normalization methods were averaged, with no significant differences in ICYP dissociation constant. A subgroup of subjects was subdivided by left ventricular ejection fraction (LVEF) into those with mild cardiac dysfunction (LVEF less than 0.50 greater than 0.40) and severe heart failure (LVEF less than 0.20) and given graded sequential infusions of dobutamine and calcium gluconate. Those with severe cardiac dysfunction had marked impairment of the dobutamine dP/dt and stroke work index response, whereas these responses to calcium did not differ in the two groups. These data indicate that in the intact human heart endomyocardial biopsy may be used for direct analysis of beta-adrenergic receptors, heart failure-associated myocardial beta-adrenergic down-regulation begins with mild-moderate ventricular dysfunction, reduction in myocardial beta-receptor density is related to degree of heart failure, and beta-receptor down-regulation is associated with pharmacologically specific impairment of the beta-agonist-mediated contractile response.