Beta-adrenergic and muscarinic cholinergic receptor densities in the human sinoatrial node: identification of a high beta 2-adrenergic receptor density.

The objective of this study was to measure autonomic receptor densities in the human sinoatrial node and adjacent atrial myocardium to gain further insights into autonomic regulation of sinoatrial node function in the human heart. Sinoatrial nodes (n = 9) were acquired from human donors. Quantitative light microscopic autoradiography of radioligand binding sites in tissue sections was used to compare beta-adrenergic and muscarinic cholinergic receptor densities within specific tissue compartments of the sinoatrial node and adjacent myocardium. Total beta-adrenergic receptors were measured with the nonsubtype selective radioligand [125I]iodocyanopindolol. beta 2-Adrenergic receptors were determined by measuring the amount of radioactivity bound to sections incubated with radioligand in the presence of the highly beta 1-selective antagonist CGP-20712A. Specific autoradiographic grain densities were normalized to myocyte area/unit tissue area. Myocytes in the sinoatrial node occupied 47.7% +/- 0.1% of the total tissue area compared with 92.8% +/- 0.1% in myocardium (P < 0.001). Total specific beta-adrenergic receptor density per unit myocyte area was 3.5 +/- 0.9 times greater in the sinoatrial node than in myocardium (P < 0.001). The relative densities of beta 1-(4.2, P < 0.002), beta 2-(2.6, P < 0.002), and muscarinic (3.3, P < 0.001) receptors were significantly greater in the sinoatrial node than in the atrium. Thus, total beta-adrenergic and muscarinic cholinergic receptor densities are > 3-fold higher in the sinoatrial node than adjacent atrial myocardium, reflecting their specialized roles in regulating cardiac rate and rhythm. The beta 1-subtype is predominant in both regions. The beta 2-subtype, however, is > 2.5-fold more abundant in the sinoatrial node than in atrial myocardium. The relatively high beta 2-receptor density in the human sinoatrial node is consistent with physiologic studies that implicate this receptor in regulating cardiac chronotropism.

Relative densities of muscarinic cholinergic and beta-adrenergic receptors in the canine sinoatrial node and their relation to sites of pacemaker activity.

The site of earliest extracellular electrical activation in the sinoatrial node (SAN) is known to shift in response to autonomic stimuli, but the mechanisms underlying this phenomenon and the determinants of the location of dominant pacemaker activity have not been elucidated. The present study was designed to characterize the spatial distribution of muscarinic cholinergic and beta-adrenergic receptors in the canine SAN and to determine whether a consistent relationship exists between autonomic receptor densities and the site of dominant pacemaker activity. We used quantitative light-microscopic autoradiography of radioligand binding sites to characterize the spatial distribution of muscarinic cholinergic and beta-adrenergic receptor subtypes in tissue sections containing the SAN and adjacent right atrial muscle from 18 canine hearts. Muscarinic receptor density was 5.4 times greater in SAN cells than in atrial myocytes (P < .01). Total beta-adrenergic receptor density was more than 3 times greater in SAN cells than in atrial myocytes (P < .0001), due entirely to the significantly greater number of beta 1-adrenergic receptors in the SAN. The region of dominant pacemaker activity, localized in 4 hearts with in vitro mapping, consistently exhibited greater densities of muscarinic and beta 1-adrenergic receptors than other SAN regions. Muscarinic receptor density in the dominant pacemaker region was 18 +/- 2% and 29 +/- 7% higher than in adjacent superior and inferior regions, respectively. beta 1-Receptor density in the dominant site was 53 +/- 5% and 26 +/- 4% higher than in adjacent superior and inferior SAN regions, respectively. Thus, the SAN is richly endowed with both muscarinic cholinergic and beta 1-adrenergic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmural heterogeneity of norepinephrine uptake in failing human hearts.

OBJECTIVES: The aim of this study was to determine whether regional heterogeneity in myocardial sympathetic neural function measured by the uptake of norepinephrine could account for the spatial heterogeneity of beta-adrenergic receptor down-regulation that occurs in the failing human heart. BACKGROUND: Myocardial beta-adrenergic receptor density and function are diminished in patients with chronic heart failure. Down-regulation occurs predominantly in the subendocardium, suggesting that local rather than systemic alterations in sympathetic neural function may be responsible. Although some studies have implicated hypofunction of cardiac sympathetic nerves with defective norepinephrine uptake, others suggest increased cardiac sympathetic nerve activity with unimpaired uptake. METHODS: We measured norepinephrine uptake by incubating transmural slices of the left ventricle from 19 patients who had chronic heart failure and three nonfailing control hearts with [3H]norepinephrine with or without desipramine, a neuronal uptake blocker. The density of uptake sites was measured in subepicardial and subendocardial myocyte regions with light microscopic autoradiography. RESULTS: Although the amount of [3H]norepinephrine uptake varied considerably in failing ventricles, uptake was directly proportional (r = 0.46, p < 0.05) to beta 1-adrenergic receptor density measured in additional slices with radioligand binding assays. In addition, marked transmural heterogeneity in [3H] norepinephrine uptake was consistently observed in failing ventricles. Uptake in subendocardial myocyte regions was significantly less than in subepicardial regions (mean [ +/- SD] subepicardial/subendocardial uptake ratio 4.7 +/- 4.8, p < 0.01). The extent of transmural heterogeneity in norepinephrine uptake was similar in patients with idiopathic and ischemic cardiomyopathy. In contrast, nonfailing hearts exhibited more uniform transmural [3H]norepinephrine uptake (subepicardial/subendocardial uptake ratio 1.8 +/- 1.2, p = NS). CONCLUSIONS: Specific [3H]norepinephrine accumulation is approximately fivefold lower in subendocardial regions of failing left ventricles than in subepicardial regions. These findings support the hypothesis that a subendocardial defect in norepinephrine uptake may chronically elevate local interstitial catecholamine levels and thereby down-regulate beta-adrenergic receptors in a spatially heterogeneous distribution.

Heterogeneous transmural distribution of beta-adrenergic receptor subtypes in failing human hearts.

BACKGROUND: Downregulation of myocardial beta-adrenergic receptor density does not occur in a spatially uniform distribution in patients with congestive heart failure. Rather, it results primarily from loss of receptors in the subendocardium. In patients with dilated cardiomyopathy, beta 1-receptors have been found to be downregulated selectively. These observations suggest that considerable transmural heterogeneity in the distribution of beta-adrenergic receptor subtypes exists in the failing human heart. The present study was designed to test this hypothesis. METHODS AND RESULTS: We used quantitative autoradiography of radioligand binding sites to measure the distribution of beta-adrenergic receptor subtypes in transmural sections of left ventricular myocardium obtained from cardiac transplant patients with ischemic (n = 13) and idiopathic dilated (n = 12) cardiomyopathy and from 4 subjects with no history of cognitive heart failure. Analysis of radioligand binding isotherms revealed a significant reduction in total beta-adrenergic receptor density in hearts of patients with ischemic and idiopathic cardiomyopathy (20.3 +/- 1.9 and 18.2 +/- 2.0 fmol/mg protein, respectively, versus 40.0 +/- 11.4 in control subjects; P < .01 for both). Loss of the beta 1-subtype accounted for 86% of the total reduction in beta-receptor density in failing hearts. Despite the significant decreases in overall tissue receptor content, the densities of total beta-receptors and beta-receptor subtypes in subepicardial myocytes were equivalent in failing and control hearts. However, in contrast to control hearts, in which the transmural distribution of total and beta 1-receptors was uniform (endocardial: epicardial receptor density ratios, 0.97 +/- 0.14 and 1.0 +/- 0.2, respectively), hearts of patients with ischemic and idiopathic dilated cardiomyopathy had significantly lower total beta-receptor and beta 1-receptor densities in the subendocardium (ratios, 0.66 +/- 0.06 and 0.46 +/- 0.09 for total and beta 1-receptors, respectively, in ischemic cardiomyopathy and 0.60 +/- 0.08 and 0.52 +/- 0.11 in dilated cardiomyopathy; P < .001 for all values compared with a ratio of 1). Thus, beta 1: beta 2 receptor density ratios were markedly decreased in the subendocardium of ischemic and idiopathic dilated left ventricles compared with control hearts. CONCLUSIONS: A significant transmural gradient in the density of myocardial beta 1-adrenergic receptors exists in the hearts of patients with ischemic and dilated cardiomyopathy, resulting in a markedly altered beta 1: beta 2 receptor density ratio in the subendocardium.

Distinct patterns of connexin expression in canine Purkinje fibers and ventricular muscle.

Electrical conduction is more rapid in Purkinje fibers than in ventricular muscle, which are distinct cardiac tissues that have different active and passive electrophysiological properties. We have recently demonstrated that canine myocardium contains multiple gap junction proteins or connexins that form channels with unique electrophysiological properties. To determine whether differences in connexin expression may account, in part, for the characteristic conduction properties of Purkinje fibers and ventricular muscle, we assessed the amounts of mRNA for two connexins, Cx40 and Cx43, in these tissues obtained from canine hearts by Northern blot analysis and in situ hybridization. We also characterized the distribution and relative abundance of these two connexins in gap junctions with immunocytochemistry. A significantly greater amount of Cx40 mRNA was observed in Purkinje fibers compared with ventricular muscle, a difference that was at least threefold according to quantitative in situ hybridization (p < 0.001) and densitometric analysis of Northern blots. Purkinje fibers also demonstrated greater immunostaining intensity when incubated with anti-Cx40 antibodies than did ventricular muscle. In contrast, Cx43 mRNA and protein appeared to be abundant in both tissues. Quantitative in situ hybridization demonstrated a modest but not statistically significant increase in Cx43 mRNA in Purkinje fibers compared with ventricular myocardium. These results indicate that Purkinje fibers and ventricular muscle express distinct patterns of connexins. This tissue-specific pattern of connexin expression could contribute to differences in the conduction properties of Purkinje fibers and ventricular muscle.

Antiarrhythmic drug therapy for congestive heart failure with focus on moricizine.

Many patients who have serious ventricular arrhythmia requiring antiarrhythmic drug therapy have congestive heart failure (CHF). However, the pharmacokinetic and pharmacodynamic properties of the antiarrhythmic drugs are altered in the presence of CHF. It has been reported that some adverse effects, primarily aggravation of arrhythmia and CHF occur more frequently in patients with a history of left ventricular (LV) dysfunction. Moreover, antiarrhythmic drugs are less effective in patients with a history of CHF and a reduced LV ejection fraction (LVEF). Moricizine, a new antiarrhythmic drug, has been undergoing clinical trials for over 13 years in the United States. The data base involving 1,072 patients was analyzed to establish the effect of this agent in patients with CHF. The presence of CHF does not alter the absorption, half-life and clearance of moricizine. The incidence of CHF exacerbation definitely related to moricizine was low (2%) and occurred primarily in patients with a history of CHF. Aggravation of arrhythmia and conduction abnormalities also occurred more often in patients with prior CHF. However, the incidence of all other adverse effects involving other organ systems was the same in patients with and without CHF and was also unrelated to the baseline LVEF. The effect of moricizine for suppressing spontaneously occurring ventricular ectopy was also similar in patients with and without CHF and was independent of LVEF. However, the drug is less effective in preventing sustained ventricular arrhythmia in patients with CHF.