β³-Adrenergic Receptor Subtype Signaling In Senescent Heart: Nitric Oxide Intoxication or “Endogenous”β Blockade for Protection?

COMPLEX biochemical, pharmacologic, and physiologic changes occur with advancing age in the myocardium. These changes may be subtle and virtually undetectable under resting steady state conditions, but become prominent under stressful stimulation. In this issue of Anesthesiology, Birenbaum et al.  1provide evidence for impaired inotropic response (approximately 30%) to β-adrenergic stimulation in aged versus  young rat hearts. Apart from β¹-adreneric receptor (AR) down-regulation, they further demonstrate that increased formation of nitric oxide deriving from neuronal nitric oxide synthase (nNOS) via  the activated β³-AR subtype signaling pathway is responsible for the reduced inotropy in senescent hearts.

Since the late 1980s, it is known that three β-AR subtypes, namely the β¹-, β²-, and β³-AR, participate in the regulation of cardiovascular function. Expression of β³-ARs in human hearts was first reported in 1996 by Gauthier et al.  2The β³-AR shares only approximately 50% homology with β^1/2-ARs and is particular in certain aspects (table 1). First, in contrast to β^1/2-ARs (monoexonic genes), the gene encoding this receptor on chromosome 8 contains two exons and one intron enabling alternative splicing and formation of two different receptor isoforms with different pharmacologic properties.3Second, the β³-AR lacks phosphorylation sites for G protein–coupled receptor kinase and protein kinase A in the cytoplasmic C-terminus tail, and thus is resistant to catecholamine-induced desensitization.4Third, besides the strong lusitropic effects, the majority of studies suggest inhibition of cardiac contractility after β³-AR stimulation. Hence, in the presence of labetalol or prazosin plus nadolol, i.e. , α- plus β^1/2-AR blockade, norepinephrine becomes cardiodepressive.5In cardiomyocytes (fig. 1), β³-ARs activate, through a pertussis toxin–sensitive Gⁱprotein–coupled mechanism, endothelial nitric oxide synthase (eNOS or NOS3), located between sarcolemmal and T-tubular caveolae, and nNOS (or NOS1), located in the sarcoplasmic reticulum (or sarcolemma in the failing heart), and exert nitric oxide–mediated negative inotropic effects via  the guanylyl cyclase–cyclic guanosine monophosphate pathway on L-type Ca²⁺channels and on myofilaments.5–8Nitric oxide further inhibits contractility by S-nitrosylation of key proteins of the respiratory chain,9energy metabolism,10contractile apparatus, and Ca²⁺handling, and it potentiates the inhibitory effect of the Gi protein on protein kinase A by activating phosphodiesterase II,11thereby reducing the second messenger cyclic adenosine monophosphate of the Gs-coupled β^1/2-ARs. A last feature of the β³-AR is its low expression in healthy myocardium with a preference in left ventricular tissue, while its expression is reported to increase by two to three times in various pathologic conditions including sepsis,12diabetes,13and heart failure14and, as shown in the current study by Birenbaum et al. ,1during the aging process.

In perioperative medicine, the sympathetic nervous system plays an important role in the pathogenesis of cardiovascular complications.15Therefore, gaining control over the adrenergic activity represents a core task in anesthetic practice.16,17On the other side, sustained activation of the sympathetic nervous system with enhanced β-AR signaling plays an important role in cardiovascular aging by promoting inflammation, oxidation and nitrosylation of key proteins, and cell death.18Consistent with the free radical theory of aging, previous studies showed increased formation of reactive oxygen and nitrogen species in aged myocardium, mainly generated by mitochondria and nitric oxide synthase (NOS) isoforms.19Nitric oxide is produced by nearly all cell types in the heart, and is synthesized from l-arginine by the catalytic reaction of the three different highly compartmentalized isoforms of nitric oxide synthase (neuronal or type 1 NOS [nNOS or NOS1], inducible or type 2 NOS [iNOS or NOS2], and endothelial or type 3 NOS [eNOS or NOS3]).20In the aged heart, iNOS and nNOS, mainly found in cardiomyocytes and autonomic nerves, are up-regulated, whereas eNOS, primarily expressed in endothelial cells (but also myocytes), is down-regulated.21,22In the adult heart, nitric oxide regulates cardiac biology by affecting energy metabolism, substrate utilization, apoptosis, hypertrophy, regeneration, and preconditioning.23However, its biologic effects are often “double-edged,” with a variety of opposing effects depending on the dose and site of action within the cell. Li et al.  24demonstrated the deleterious effects of nitric oxide derived from up-regulated iNOS in isolated perfused aged versus  young rat hearts. These authors found increased nitric oxide and peroxynitrite formation deriving from up-regulated iNOS, which aggravated postischemic cardiac dysfunction and enlarged infarct size under β-AR stimulation with isoproterenol in aged versus  young hearts. nNOS expression was also increased in aged hearts in that study, but its contribution to isoproterenol-induced postischemic damage was not investigated. Nonetheless, this study provides evidence that a phenotypic change of NOS isoforms in aged hearts may predispose to increased myocardial damage under sympathetic tone. As can be speculated from the results by Birenbaum et al. ,1the observed detrimental effect in the study of Li et al.  24may be well explained by upregulation of β³-ARs. Augmented cardiac nitric oxide formation by β³-AR signaling was also reported to be deleterious in sepsis, where the release of endogenous inflammatory mediators such as tumor necrosis factor α and interleukin 1β dramatically decrease the responsiveness of cardiomyocytes to catecholamines.12Interestingly, this refractoriness to catecholamines is absent in septic β³-AR knockout mice.12In contrast to the toxic effects of β³-AR signaling in certain conditions, in healthy adult hearts, β³-AR signaling is necessary to counteract the proarrhythmogenic, and otherwise unchained chronotropic, dromotropic, and inotropic effects of β^1/2-ARs.8This phylogenetically highly conserved and protective negative feedback loop against catecholamine toxicity is important for a balanced and fine-tuned contractility.8Generally speaking, β³-AR signaling acts as a countervailing “brake” against adrenergic overstimulation comparable with an “endogenous”β blockade. This concept is further supported by the fact that β³-ARs maintain cardiac sympathovagal balance by reinforcing vagal tone. Beyond these benefits, but in contrast to pure β blockade, nitric oxide from β³-AR signaling further exerts coronary and peripheral vasodilatation, and may have “pleiotropic” effects on cardiac stem cell biology25and the prevention of arteriosclerosis.26Indeed, the “pharmacologic profile” of β³-AR signaling strongly reminds of the one observed with the novel third-generation β-blocker nebivolol, which promotes nitric oxide formation.27Research in the field of heart failure also supports the idea that β³-AR signaling is beneficial rather than detrimental. In the failing heart, the β³-AR response is preserved even under sustained activation of the adrenergic system and thus antagonizes catecholamine toxicity. However, this does not exclude that adverse effects do occur from β³-AR–mediated negative inotropy, particularly during the progression of heart failure. Interestingly, β³-AR knockout mice show increased SERCA2a expression and phospholamban phosphorylation resulting in enhanced SERCA2a activity.28Hence, one could speculate that β³-AR blockade could play a salutary role in the therapy of heart failure, at least at later stages of the disease. Together, the biologic consequences of enhanced β³-AR signaling range from “nitric oxide intoxication” on one side to protection comparable to some sort of “physiologic” or “endogenous”β-blockade on the other side. Although the study by Birenbaum et al.  1shows undesirable short-term effects of enhanced β³-AR signaling on inotropy in aged hearts, the biologic (long-term) consequences of this pathway remain elusive.

A number of important questions need to be addressed in future studies before we can translate the findings of Birenbaum et al.  to the clinical arena. First, based on genome-wide transcriptional profiling, the molecular mechanisms involved in reduced β-adrenergic responsiveness in the aged heart seem to be more complex and not just confined to a single signaling pathway. Accordingly, Dobson et al.  29reported up-regulation of 19 transcripts involved in age-related antiadrenergic activity, including adenosine A¹, muscarinic M³, and nicotinic β³acetylcholine receptors, and many more. Second, female tissue was reported to express less β³-ARs,30and nitric oxide is differentially regulated in male and female hearts, as evidenced in nNOS knockout mice, where males develop more pronounced cardiac remodeling than females.31Therefore, sex-based differences may be of relevance in β³-AR and nitric oxide signaling, specifically in aged hearts. Third, although the rat and human β³-AR share 79% identity, there are pronounced heterogeneous pharmacologic profiles of β³-AR agonists/antagonists between different species. Therefore, caution must be applied in extrapolating data obtained from rat hearts to human myocardium. Finally, β³-ARs are also expressed in vessels. Namely, β³-ARs are expressed on the endothelium of coronary microarteries,32but their role in ischemia–reperfusion is unknown, and it is unclear, to date, whether vascular β³-ARs are similarly up-regulated during the aging process.

Modulation of β³-AR signaling in the diseased heart in general and in the senescent heart in particular opens new therapeutic approaches. However, much research is required to fully understand the complexity of β-AR biology with its contrasting influences. New compounds with β³-AR agonistic and/or antagonistic properties could help to better modulate and control the adrenergic system in the perioperative period. For example, in coronary artery bypass graft surgery patients, there is β^1/2-AR but not β³-AR dysfunction after cardiopulmonary bypass, and β³-AR stimulation could increase graft flow, decrease oxygen consumption, and thus protect the heart. However, these effects must be carefully balanced against the negative inotropy of β³-AR signaling. Interestingly, the single nucleotide polymorphism of the β³-AR at codon 64 from tryptophan to arginine was found to be associated with insulin resistance, diabetes, obesity, and hypertension.8Hence, patient studies investigating the impact of β-AR genomics on cardiac biology and clinical outcome might help to discern the therapeutic potential of β³-AR signaling in the context of other important adrenergic polymorphisms33in human heart disease. Clearly, the findings by Birenbaum et al.  1are important and will stimulate future research aiming at improving the cardiovascular management of elderly surgical at-risk patients. In this challenging endeavor, however, we need to expand our understanding of complex β-AR signaling beyond the platitude that “it is not by the gray of the hair that one knows the age of the heart” (Robert Bulwer-Lytton).

*Department of Anesthesiology and Pain Medicine, University of Alberta, and Perioperative Translational Medicine, Mazankowski Alberta Heart Institute, Edmonton, Canada. michael.zaugg@ualberta.ca. †Institute of Pharmacology, University of Zurich, Switzerland.