β-Adrenergic Receptor Blockade in Chronic Heart Failure

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Abstract
The medical treatment of chronic heart failure has undergone a remarkable transition in the past 10 years. The approach has changed from a short-term hemodynamic/pharmacological paradigm to a more long-term, reparative strategy that aims to favorably alter the biological properties of the failing heart.1 This is dramatically illustrated by the recent success in treating mild-to-moderate chronic heart failure with β-adrenergic blocking agents. This review describes how a treatment that began as a contraindication1 2 3 became an established treatment of chronic heart failure. The failing human heart is adrenergically activated,4 5 6 which helps to maintain cardiac performance over the short term by increasing contractility and heart rate. In contrast, in the resting state there is no adrenergic support of normally functioning human left ventricles.6 Multiple lines of evidence7 8 9 indicate that it is the increase in cardiac adrenergic drive rather than an increase in circulating norepinephrine that is both initially supportive and then ultimately damaging to the failing human heart. As shown in Table 1⇓, there are 3 adrenergic receptors (β1, β2, and α1) in human cardiac myocytes coupled to a positive inotropic response and cell growth.10 11 12 β-Adrenergic receptors are coupled via the “stimulatory” G protein Gs to the effector enzyme adenylyl cyclase, which converts the substrate MgATP to cAMP. cAMP is a positively inotropic and chronotropic second messenger and is strongly growth promoting. In nonfailing human left or right ventricles, the β1/β2 ratio is 70 to 80/30 to 20, but in failing human ventricles, 35% to 40% of the total number of β-receptors are β2 because of selective downregulation in the β1 subtype.10 11 α1 Receptors are coupled via a different G protein (G …