(Circulation. 2002;105:1876.)
© 2002 American Heart Association, Inc.
Editorial |
From the University of Pittsburgh Medical Center, Pittsburgh, Pa.
Correspondence to Arthur M. Feldman, MD, PhD, Harry S. Tack Professor of Medicine, University of Pittsburgh Medical Center, 200 Lothrop St, S-572 Scaife Hall, Pittsburgh, PA 15213. E-mail feldmanam@msx.upmc.edu
Key Words: Editorials heart failure signal transduction adenylyl cyclase
In the early 1960s, Earl Sutherland and his colleagues performed a series of experiments that led to the understanding that the positive inotropic effects of ß-adrenergic agonists were mediated by the activation of the enzyme adenylyl cyclase (AC) and the subsequent production of cyclic adenosine monophosphate (cAMP).1 By measuring tissue cAMP levels while simultaneously monitoring the mechanical properties of the heart, these investigators were able to show that cAMP levels increased with extraordinary speed, an increase that correlated directly with increased force of contraction. Subsequent studies demonstrated that AC activity is enhanced through a pathway beginning with ligand binding to ß1-adrenergic receptors (ß1ARs) and subsequent activation of stimulatory guanine nucleotidebinding signal transduction proteins (Gs). The resultant production of cAMP mediates improvements in both cardiac inotropy and lusitropy via stimulation of cAMP-dependent protein kinase A and the protein kinase Adependent phosphorylation of key target proteins, including the L-type calcium channel, phospholamban, and troponin I. In the normal heart, activation of the ßARG proteinAC pathway effects enhanced contractility, which is paramount in facilitating the "flight or fight" response. However, in the failing human heart, profound alterations in multiple components of this ßARG proteinAC signal transduction cascade reduce cardiac reserve and contribute to decreased exercise response in patients with heart failure.2 Paradoxically, long-term activation of the neurohormonal pathway accelerates the natural history of heart failure.
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Thus, a therapeutic conundrum has arisen: How can one enhance myocardial contractility while at the same time protecting the heart from the
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