Distinct beta-adrenergic receptor subtype signaling in the heart and their pathophysiological relevance

In the heart, stimulation of beta-adrenergic receptors (betaAR) serves as the most powerful means to increase cardiac contractility and relaxation in response to stress or a "fight-or-flight" situation. However, sustained beta-adrenergic stimulation promotes pathological cardiac remodeling such as myocyte hypertrophy, apoptosis and necrosis, thus contributing to the pathogenesis of chronic heart failure. Over the past decade, compelling evidence has demonstrated that coexisting cardiac betaAR subtypes, mainly beta(1)AR and beta (2)AR, activate markedly different signaling cascades. As a result, acute beta(1)AR stimulation activates the G(s) -adenylyl cyclase-cAMP-PKA signaling that can broadcast throughout the cell, whereas beta(2)AR-evoked cAMP signaling is spatially and functionally compartmentalized, due to concurrent G(i) activation. Chronic stimulation of beta(1)AR and beta(2)AR elicits opposing effects on the fate of cardiomyocytes: beta(1)AR induces hypertrophy and apoptosis; but beta(2)AR promotes cell survival. The cardiac protective effect of beta(2)AR is mediated by a signaling pathway sequentially involving G(i), G(betagamma), PI3K and Akt. Unexpectedly, beta(1)AR-induced myocyte hypertrophy and apoptosis are independent of the classic cAMP/PKA pathway, but require activation of Ca(2+)/calmodulin-dependent kinase II (CaMK II). The outcomes of cardiac-specific transgenic overexpression of either beta AR subtype in mice have reinforced the fundamentally different functional roles of these betaAR subtypes in governing cardiac remodeling and performance. These new insights regarding betaAR subtype stimulation not only provide clues as to cellular and molecular mechanisms underlying the beneficial effects of beta AR blockers in patients with chronic heart failure, but also delineate rationale for combining selective beta(1)AR blockade with moderate beta(2)AR activation as a potential novel therapy for the treatment of chronic heart failure.