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J Am Coll Cardiol, 2004; 44:2016-2018, doi:10.1016/j.jacc.2004.09.001 © 2004 by the American College of Cardiology Foundation |
,*
Division of Cardiology
Departments of Pharmacology and Medicine, UCSD Medical Center, San Diego, California
* Reprint requests and correspondence: Dr. Paul A. Insel, University of California-San Diego, Department of Pharmacology, 9500 Gilman Drive, LaJolla, California 92093 (Email: pinsel{at}ucsd.edu).
Norepinephrine and epinephrine, the major catecholamines of the sympathoadrenal system, alter function via AR, of which three major types (
1, 2, and ß), each with three subtypes (1a,b,d, 2a,b,c, and ß1,2,3) are found in the human genome (5). Studies initiated two decades ago showed that downregulation (decreased number) of ß1-AR and uncoupling (decreased functionality) of ß2-AR contribute to the pathophysiology and progression of HF (6). Such changes presumably result from the enhanced levels of catecholamines, especially NE, that occur with worsening HF (1–3). An important question is whether all patients (and their receptors) are equally susceptible to such changes.
Most AR subtypes show genetic variations that include deletions and coding and non-coding single nucleotide polymorphisms (SNPs) (5,7). High-efficiency sequencing techniques have made SNP discovery feasible and analysis of large clinical datasets for "SNP hits" fashionable. When such studies are "positive," they provide a priori evidence that the SNP is detrimental even in the absence of supporting mechanistic data. Several recent reviews summarize the results of such studies and of in vitro experiments with AR variants; thus far the assay for variants has not reached the stage of clinical utility (5,7–9).
The AR variants of particular relevance to cardiologists are located both pre- and post-synaptically. They influence the amount of NE present in the synaptic cleft and the regulation of cardiac myocyte responses to NE (5,7–9). The variants also can alter the desensitization/downregulation of ARs that occurs in response to agonist exposure. Because genetic variants in AR may influence cardiac NE levels and responses to the neurotransmitter, they may predispose to HF and affect clinical outcomes.
This information establishes the context for the accompanying article by Kaye et al. (10). That study, although limited by relatively small sample size (n = 60) and confounding variables (e.g., the use of carvedilol and amiodarone by 37% and 23% of patients, respectively), is important because it furthers understanding as to how differences in AR genes may influence the development and progression of HF. The authors used approaches that are not widely available to assess key aspects of sympathetic nerve function in HF: systemic and cardiac NE clearance and spillover in Australian-Caucasian subjects with known left ventricular dysfunction who were referred for right heart catheterization. The authors correlated these measurements with AR genotypes: 2c-AR Del322-325, ß1-Ser49Gly and -Arg389Gly; ß2-Arg16Gly and -Gln27Gly.
The authors' negative findings are the most interesting and relevant: common variants of the three ARs did not correlate with heart rate, mean arterial pressure, pulmonary capillary wedge pressure, or cardiac output, nor with systemic or cardiac NE spillover, thus implying that these variants have little effect on post-synaptic responses to and pre-synaptic control of NE release in patients with established HF.
Because of the limited number of subjects, this study was underpowered to detect small differences, and thus negative findings (perhaps falsely negative) would be expected. It is therefore intriguing that the authors obtained a positive finding: Heart rate was higher for a given level of NE stimulation in the 25% of subjects who were ß2-AR Arg16 homozygotes.
The actions of cardiac ß2-AR are complex. Normally, the ratio of ß1- to ß2-AR density is about 70:30, but in HF this ratio decreases to nearly 1:1 because of the decreased expression of ß1-AR (6). The ß2-AR are expressed pre-synaptically (on sympathetic nerve terminals where they enhance NE release) and post-synaptically on cardiomyocytes (stimulating inotropy and chronotropy), cardiac fibroblasts, and vascular cells (11). Among ß2-AR variants, Arg16 is less common and more resistant to agonist-promoted downregulation than Gly16, though these variants need to be considered as part of receptor haplotypes, i.e., together with other variants in linkage disequilibrium (5,12). The findings by Kaye et al. (10) regarding the ß2-AR variants and heart rate may relate to the relative lack of receptor downregulation on myocytes (or nodal tissue) in patients who are homozygous for ß2-AR Arg16, though a larger study powered to assess receptor haplotypes is needed to strengthen this conclusion.
When assessed in vitro, the 2c-AR deletion variant shows decreased agonist-promoted signaling, leading to the prediction of greater NE release and resultant post-synaptic stimulation, as 2c-AR receptors inhibit sympathetic neuronal release of NE (13). Because Kaye et al. found that this deletion variant did not correlate with cardiac NE level or response, their data do not support the model proposed in a study of HF patients with the 2c-AR deletion variant (14). As such, the results also appear to disagree with indirect measurements of NE release (estimated by quantifying the uptake of MIBG, an analog of NE that is transported into sympathetic neurons through the uptake-1 transporter) in which subjects with at least one copy of the 2c-AR deletion variant had greater cardiac MIBG uptake than did those lacking this variant (15). The current data, demonstrating unaltered NE spillover in patients with the 2c-AR deletion variant, show no necessity for a compensatory increase in uptake-1 transporter activity, as hypothesized for such patients.
Aside from differences in subject ethnicity, another explanation for the discordant results between this and the previous studies that assessed synaptic mechanisms is that Kaye et al. may have had problems with genotyping 2c-AR deletion variants. Only 
13% of their subjects were homozygous for the variant and none were heterozygous, frequencies not expected for genetic equilibrium and inconsistent with the roughly equal number of homozygotes and heterozygotes originally described (13). In this regard, the genotyping of the 2c-AR deletion is difficult as it occurs in a GC-richregion of the gene and the published restriction enzyme digestions result in multiple bands of similar molecular weight (16,17).
The findings for the ß1-AR variants are a further negative result of interest. Such variants have been associated with altered receptor responses and desensitization, in some, but not all, studies (8,9). Earlier reports showed discordant or negative results in responses to beta-blocker therapy; several recent studies have indicated that Arg389 may be a good predictor of beta-blocker response both in HF and hypertension (18–21). When viewed from this perspective, it is relevant that Kaye et al. (10) found that the ß1 variants showed no variation in the heart rate response when normalized (or not) to the NE level. A possible explanation for this negative result is that patients studied in the current report had substantial downregulation of ß1-AR as a consequence of the severity of HF along with a high rate of use of antiadrenergic medications.
In summary, Kaye et al. describe useful approaches to assess cardiac effects of genetic variants of AR. Especially when employed together, the NE spillover technique and the measurement of hemodynamic responses provide important insights regarding the effect of AR variants on cardiovascular physiology at both pre-synaptic and post-synaptic sites. Heart failure subjects, such as those studied here, have elevated plasma NE levels secondary to left ventricular dysfunction and accompanying AR desensitization/downregulation and multiple compensatory mechanisms. The authors found an association of heart rate with a ß2-AR variant but failed to find statistically significant relationships with several other AR variants, including 2c- or ß1-AR. Future studies that assess the role of AR variants in cardiac physiology should ideally include a larger sample size, subjects without left ventricular dysfunction and not taking antiadrenergic drugs, and obtain information regarding receptor haplotypes. Such studies may allow definitive conclusions regarding AR genetic variants in the modulation of cardiac NE spillover and cardiac physiology, and ultimately, their role in risk for and progression of HF.
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2c-adrenergic receptor confers impaired coupling to multiple effectors J Biol Chem 2000;275:23059-23064.2c-adrenergic receptor J Nucl Cardiol 2003;10:583-589.[CrossRef][Medline]
2c allelic variants and personality factors Am J Med Genet 2002;114:649-651.[CrossRef][Medline]
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