Hypertrophic cardiomyopathy characterised by beta-adrenoceptor density, relative amount of beta-adrenoceptor subtypes and adenylate cyclase activity

QT interval instability and QRS interval dispersion in healthy cats and cats with a hypertrophic cardiomyopathy phenotype

OBJECTIVES

Hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats. Electrocardiographic (ECG) analysis can help with the diagnosis of HCM and also in the investigation of the secondary consequences of the disease. This study investigated ECG markers of QT interval variability (total instability [TI], short-term instability [STI], long-term instability [LTI], QT variance [QTv]), mean QT interval (QTa) and QT interval corrected for heart rate (QTac), as well as the duration (QRSd) and dispersion (QRSv) of the QRS interval in healthy cats and in those with HCM.

METHODS

Data were collected from 63 domestic cats: 40 in the control group and 23 in the HCM group. Fifty consecutive QT intervals were recorded for all cats and then QTa, QTac, QTv, TI, LTI and STI were calculated. QRSd and QRSv were also obtained for all animals. A Mann-Whitney U-test was used for group comparison. Receiver operating characteristic curves were plotted to evaluate the sensitivity and specificity of all markers for HCM. Logistic regression analysis was performed to assess the risks of cats having HCM, based on the studied indexes.

RESULTS

QTa (P <0.01), QTac (P <0.01), QRSd (P <0.01) and STI (P = 0.02) were higher in the HCM group. QTa >158.8 ms, QTac >27.4 ms and QRSd >0.045 s had an accuracy of 77.4%, 68.2% and 80.9%, respectively, in detecting HCM. Logistic regression showed that cats with QTa >158 ms, QTac >27.4 ms and QRSd >0.045 s had a 1.58-, 1,23- and 6.5-fold higher risk, respectively, of developing HCM.

CONCLUSIONS AND RELEVANCE

Cats with HCM had higher ventricular instability as assessed by STI and showed a prolongation of the QT and QRS intervals via the QTa, QTac and QRSd markers. These markers show potential as ancillary screening tools for identifying the presence of HCM.

Diagnostic and prognostic utility of surface electrocardiography in cats with left ventricular hypertrophy

OBJECTIVES

To assess the ability of electrocardiography (ECG) to predict left ventricular hypertrophy (LVH) in the cat and to investigate the prognostic value of selected ECG variables in cats with LVH.

ANIMALS

Fifty-seven privately owned cats: 22 clinically healthy cats and 35 cats with LVH.

MATERIAL AND METHODS

This is a clinical cohort study. Echocardiographic diagnosis and surface ECG were available. Electrocardiography analysis included rhythm diagnosis and specific electrocardiographic measurements. In cats with LVH, cause of death and outcome data were recorded and analyzed using Kaplan-Meier curves.

RESULTS

The presence of arrhythmia had sensitivity and specificity of 31% and 100%, respectively, for identifying LVH. Among ECG measurements, duration of QT interval (QT) and QT interval corrected for heart rate (QTc) was statistically different between healthy cats and cats with LVH (p = 0.007). Overall, the most accurate cutoffs to identify LVH were QT > 170 ms (sensitivity and specificity 48.3% and 91%, respectively) and QTc > 188 ms (sensitivity and specificity 62% and 77%, respectively). In healthy cats, the highest QT and QTc values were 180 ms and 200 ms, respectively. Mean survival time was 58 days and indeterminable for cats with QT > 180 ms and QT ≤ 180 ms, respectively (p = 0.042) and 125 days and indeterminable for cats with QTc > 200 ms and QTc ≤ 200 ms, respectively (p = 0.017).

CONCLUSIONS

Arrhythmias as well as prolonged QT and QTc are useful ECG parameters in identifying LVH and predicting survival in affected cats.

Therapeutic potential of c-Myc inhibition in the treatment of hypertrophic cardiomyopathy

Investigating the pathophysiological importance of the molecular and mechanical development of cardiomyopathy is critical to find new and broader means of protection against this disease that is increasing in prevalence and impact. The current available treatment options for cardiomyopathy mainly focus on treating symptoms and strive to make the patient more comfortable while preventing progression of disease and sudden death. The proto-oncogene c-Myc (Myc) has been shown to be increased in many different types of heart disease, including hypertrophic cardiomyopathy, before any signs of the disease are present. As the mechanisms of action and multiple pathways of dependent actions of Myc are being dissected by many research groups, inhibition of Myc is becoming an attractive paradigm for prevention and treatment of cardiomyopathy and heart failure. Elucidating the role Myc plays in the development, propagation and perpetuation of cardiomyopathy and heart failure will one day translate into potential therapeutics for cardiomyopathy.

Changes of beta-adrenergic signaling in compensated human cardiac hypertrophy depend on the underlying disease

In human heart failure, desensitization of the beta-adrenergic signal transduction has been reported to be one of the main pathophysiological alterations. However, data on the beta-adrenergic system in human compensated cardiac hypertrophy are very limited. Therefore, we studied the myocardial beta-adrenergic signaling in patients suffering from hypertrophic obstructive cardiomyopathy (HOCM, n = 9) or from aortic valve stenosis (AoSt, n = 8). beta-Adrenoceptor density determined by [(125)I]iodocyanopindolol binding was reduced in HOCM and AoSt compared with nonhypertrophied, nonfailing myocardium (NF) of seven organ donors. In HOCM the protein expression of stimulatory G protein alpha-subunit (G(s)alpha) measured by immunoblotting was unchanged, whereas the inhibitory G protein alpha-subunit (Galpha(i-2)) was increased. In contrast, in AoSt, Galpha(i-2) protein was unchanged, but G(s)alpha protein was increased. Adenylyl cyclase stimulation by isoproterenol was reduced in HOCM but not in AoSt. Plasma catecholamine levels were normal in all patients. In conclusion, both forms of hypertrophy are associated with beta-adrenoceptor downregulation but with different changes at the G protein level that occur before symptomatic heart failure due to progressive dilatation of the left ventricle develops and are not due to elevated plasma catecholamine levels.

Overexpression of transforming growth factor-beta1 and insulin-like growth factor-I in patients with idiopathic hypertrophic cardiomyopathy

BACKGROUND

Idiopathic hypertrophic cardiomyopathy (HCM) is characterized by regional myocardial hypertrophy. To investigate involvement of growth factors on myocardial hypertrophy in HCM patients, we evaluated gene expression and cellular localization of transforming growth factor-beta1 (TGF-beta1), insulin-like growth factors (IGF-I and IGF-II), and platelet-derived growth factor-B (PDGF-B) in ventricular biopsies obtained from patients with HCM (n=8), aortic stenosis (AS) (n=8), or stable angina (SA) (n=8) and from explanted hearts with ischemic cardiomyopathy (TM) (n=7).

METHODS AND RESULTS

Levels of TGF-beta1, IGF-I, IGF-II, and PDGF-B transcripts were quantified with the use of multiplex RT-PCR. Glyceraldehyde 3-phosphate dehydrogenase was used as an internal standard. Antibodies against TGF-beta and IGF-I were used to localize their peptides within the myocardium. Antisense and sense (control) cRNA probes of TGF-beta1 and IGF-I, labeled with digoxigenin, were used to localize the growth factor transcripts by in situ hybridization. mRNA levels (densitometric ratio of growth factor/glyceraldehyde-3-phosphate dehydrogenase) of TGF-beta1 and IGF-I in HCM (0.75+/-0.05 and 0.85+/-0.15, respectively; mean+/-1 SEM) were significantly (P<.01 for all groups) elevated in comparison with non-HCM myocardium (AS: 0.38+/-0.07, 0.29+/-0.06; SA: 0.32+/-0.04, 0.18+/-0.05; TM: 0.25+/-0.03, 0.15+/-0.03). mRNA levels of TGF-beta1 and IGF-I in the hypertrophic AS myocardium were greater (P=.02, P=.05) than those in the explanted myocardium (TM). Immunohistochemical and in situ hybridization studies showed increased expression of TGF-beta1 and IGF-I in the HCM cardiomyocytes.

CONCLUSIONS

Gene expression of TGF-beta1 and IGF-I was enhanced in idiopathic hypertrophic cardiomyopathy and may be associated with its development.

Cardiac beta-adrenoceptors and adenylyl cyclase activity in human left ventricular hypertrophy due to pressure overload

The effect of left ventricular hypertrophy (LVH) due to chronic pressure overload on right atrial (RA) and left ventricular (LV) myocardial beta-adrenergic receptor (beta-AR) density and subtypes, adenylyl cyclase (AC) activity and ADP-pertussis toxin ribosylated proteins was investigated in humans with LVH due to aortic stenosis and in patients without LVH undergoing heart surgery for mitral stenosis or coronary artery disease taken as controls. Both groups presented normal systolic function or plasma catecholamine levels. In LVH and controls, beta-AR density was similar in RA (62 +/- 6 vs 77 +/- 12 fmol.mg-1 protein) and LV (39 +/- 7 vs 32 +/- 2 fmol.mg-1 protein). In LVH, beta 1-AR percentage was < than in controls in LV (35 +/- 11 vs 73 +/- 5%, P < 0.05) but not in RA (79 +/- 5 vs 73 +/- 8%). Basal AC activity in RA (19 +/- 4 vs 21 +/- 6 pmol.mg-1 protein) and LV (22 +/- 5 vs 27 +/- 3 pmol.mg-1 protein) was similar in LVH and in controls. Isoprenaline-induced stimulation of AC in RA was similar in LVH and in controls (51 +/- 18 vs 36 +/- 18%) but < in LV of LVH (7 +/- 6 vs 45 +/- 6%, P < 0.05). In the presence of ICI-118,551 (a beta 2-adrenoceptor antagonist), isoprenaline failed to induce any increase in cAMP in LVH. The quantification of ADP-pertussis toxin ribosylated proteins indicated a lower concentration of substrates in LV myocardial membranes from LVH. These data indicate that in LVH due to pressure overload, there is a down-regulation of beta 1-AR and an increase in beta 2-AR density. This is associated with alterations of the transmembrane signalling marked by a decreased capacity of isoprenaline to stimulate AC and an impaired expression of Gi proteins.

Changes in cardiac beta-adrenoceptors in human heart diseases: relationship to the degree of heart failure and further evidence for etiology-related regulation of beta 1 and beta 2 subtypes

Radioligand binding studies were performed to investigate total beta-adrenoceptor density (Bmax) and beta 1 and beta 2 subtype distribution in left ventricular biopsies obtained from 8 prospective transplant donors serving as controls and from 143 patients with different degrees of heart failure (NYHA class II to IV) undergoing aortic or mitral valve surgery due to aortic or mitral stenosis, aortic or mitral regurgitation, as well as combined aortic or mitral valve lesions (stenosis and regurgitation). In 13 other patients, heart failure was due to hypertrophic obstructive cardiomyopathy (N = 6, NYHA III), tetralogy of Fallot (N = 4, NYHA III), or Becker's muscular dystrophy (N = 3, NYHA IV). Bmax was assessed by (-)-(125I)-iodocyanopindolol used as radioligand. Competition experiments with the highly selective beta 1-adrenoceptor antagonist CGP 20712A were performed for determination of beta 1- and beta 2-adrenoceptor subtypes. In biopsies taken from transplant donors, the Bmax was found to be 70.1 +/- 5.8 fmol/mg protein. In all groups investigated the extent of total beta-adrenoceptor downregulation was related to the degree of heart failure. The decrease in Bmax was found to be about 20% (NYHA II), 45% (NYHA III), and 60% (NYHA IV) when compared with controls. There was no significant difference in the reduction of total beta-adrenoceptor density between isolated aortic or mitral valve diseases and combined valve lesions. Independent of the degree of heart failure, selective downregulation of the beta 1 subtype was found in patients with isolated or combined aortic valve diseases, hypertrophic obstructive cardiomyopathy, and Becker's muscular dystrophy.(ABSTRACT TRUNCATED AT 250 WORDS)

Diffuse reduction of myocardial beta-adrenoceptors in hypertrophic cardiomyopathy: a study with positron emission tomography

OBJECTIVES

This study was conducted to determine the myocardial beta-adrenoceptor density as a marker of sympathetic function in patients with hypertrophic cardiomyopathy and normal control subjects.

BACKGROUND

Although some cases of hypertrophic cardiomyopathy are familial with an autosomal dominant pattern of inheritance, there remains a substantial proportion of cases in which neither a family history nor genetic abnormalities can be demonstrated. Additional abnormalities, both genetic and acquired, may be important in the phenotypic expression of this condition. Clinical features of the disease and metabolic studies suggest an increased activity of the sympathetic nervous system.

METHODS

Eleven patients with hypertrophic cardiomyopathy, none of whom had previously received beta-blocking drugs, and eight normal control subjects underwent positron emission tomography to evaluate regional left ventricular beta-adrenoceptor density and myocardial blood flow using carbon-11-labeled CGP 12177 and oxygen-15-labeled water as tracers. Plasma catecholamines were also measured.

RESULTS

Mean (+/- SD) myocardial beta-adrenoceptor density was significantly less in the hypertrophic cardiomyopathy group than in the control group (7.70 +/- 1.86 vs. 11.50 +/- 2.18 pmol/g tissue, p < 0.001). Myocardial blood flow was similar in both groups (0.91 +/- 0.22 vs. 0.91 +/- 0.21 ml/min per g, p = NS). The distribution of beta-adrenoceptor density was uniform throughout the left ventricle in both groups. In the hypertrophic cardiomyopathy group, there was no correlation between regional wall thickness and myocardial beta-adrenoceptor density. There were no significant differences in either plasma norepinephrine or epinephrine concentrations between the two groups.

CONCLUSIONS

There is a diffuse reduction in myocardial beta-adrenoceptor density in patients with hypertrophic cardiomyopathy in the absence of significantly elevated circulating catecholamine concentrations. This most likely reflects downregulation of myocardial beta-adrenoceptors secondary to increased myocardial concentrations of norepinephrine and is consistent with the hypothesis that cardiac sympathetic drive is increased in this condition.

QT-interval abnormalities in hypertrophic cardiomyopathy

To examine whether QTc and QTc dispersion across the leads of a surface electrocardiogram (ECG) are different in patients with hypertrophic cardiomyopathy (HCM) compared with normal subjects, we measured QT and calculated QTc in all 12 leads of a surface ECG in 24 patients with HCM and in 20 age- and sex-matched normal control subjects. Maximal QTc was prolonged in HCM patients (465 +/- 24 ms) compared with controls (410 +/- 20 ms) (p < 0.001). QTc dispersion defined as the difference of maximum-minimum QTc was also greater in HCM patients (71 +/- 21 ms) compared with normals (35 +/- 11 ms) (p < 0.001). A correlation was found between the degree of left ventricular hypertrophy expressed by the maximal wall thickness and maximal QTc (r = 0.48, p < 0.02). However, QTc dispersion did not correlate with maximal wall thickness. Thus, patients with HCM show a prolonged QTc (> 440 ms) and increased QTc dispersion compared with normal subjects. In addition, the degree of left ventricular hypertrophy correlates with maximal QTc. The presence of a prolonged QT with increased regional dispersion may be associated with the occurrence of serious ventricular arrhythmia and sudden death in HCM.

Amiodarone reduces QT dispersion in patients with hypertrophic cardiomyopathy

To compare QT interlead variability (dispersion) in patients who receive a class III antiarrhythmic with those not on antiarrhythmic therapy, we measured QT in all 12 leads of a standard ECG in 24 patients with hypertrophic cardiomyopathy, 12 (50%) of whom were on amiodarone monotherapy and 12 (50%) who were not on amiodarone or other cardioactive medication which could affect QT. Age, functional class, chamber dimension or the degree of left ventricular hypertrophy expressed by maximal wall thickness (21 +/- 5 vs 20 +/- 4 mm; p = NS) was not different between the amiodarone and the non-amiodarone group. Maximal corrected QT (QTc) was greater in patients receiving (488 +/- 25 ms) compared to those not receiving amiodarone (451 +/- 23 ms) (p less than 0.001). However, QTc dispersion defined as the difference of maximum minus minimum QTc was decreased in the amiodarone (48 +/- 10 ms) compared to the non-amiodarone group (78 +/- 17 ms) (p less than 0.001). We conclude that in patients with hypertrophic cardiomyopathy, amiodarone prolongs QTc but reduces QTc dispersion. These results agree with expected changes in ventricular recovery time in patients who receive Class III antiarrhythmic agents and provide further support to the theory that QTc dispersion reflects regional differences in ventricular recovery time.

Pathogenesis of hypertrophic cardiomyopathy: another viewpoint

A genetically determined error in the handling of catecholamines by the developing heart has been speculated to cause hypertrophic cardiomyopathy. A critical appraisal of the published literature reveals that there is little actual experimental or clinical evidence favouring the role of catecholamines in the pathogenesis of hypertrophic cardiomyopathy. Other factors seem to be more important. An abnormality of myocardial growth, either induced by excessive growth promoting substance, or a genetically determined abnormality in myocardial responses to normal growth factors during life (but not in the fetal handling of catecholamines) is likely to be responsible for hypertrophic cardiomyopathy.

Mapping a gene for familial hypertrophic cardiomyopathy to chromosome 14q1

To identify the chromosomal location of a gene responsible for familial hypertrophic cardiomyopathy, we used clinical and molecular genetic techniques to evaluate the members of a large kindred. Twenty surviving and 24 deceased family members had hypertrophic cardiomyopathy; 58 surviving members were unaffected. Genetic-linkage analyses were performed with polymorphic DNA loci dispersed throughout the entire genome, to identify a locus that was inherited with hypertrophic cardiomyopathy in family members. The significance of the linkage detected between the disease locus and polymorphic loci was assessed by calculating a lod score (the logarithm of the probability of observing coinheritance of two loci, assuming that they are genetically linked, divided by the probability of detecting coinheritance if they are unlinked). A DNA locus (D14S26), previously mapped to chromosome 14 and of unknown function, was found to be coinherited with the disease in this family. No instances of recombination were observed between the locus for familial hypertrophic cardiomyopathy and D14S26, yielding a lod score of +9.37 (theta = 0). These data indicate that in this kindred, the odds are greater than 2,000,000,000:1 that the gene responsible for familial hypertrophic cardiomyopathy is located on chromosome 14 (band q1).

The challenge of cardiomyopathy

The combined clinical and pathophysiologic characteristics and diagnostic features as well as current concepts of pathogenesis, therapy and prevention of the principal forms of cardiomyopathy are reviewed. These include hypertrophic cardiomyopathy, dilated cardiomyopathy, restrictive cardiomyopathy and specific cardiac muscle disease. Emphasis is placed on recent developments and unresolved questions requiring application of newer techniques of molecular biology and genetics and adult myocyte culturing.

Cardiac norepinephrine kinetics in hypertrophic cardiomyopathy

We examined the uptake and release of norepinephrine in the cardiac circulation and other regional vascular beds in 11 patients with hypertrophic cardiomyopathy (HCM) and in 10 control subjects during simultaneous infusion of tracer-labeled norepinephrine and isoproterenol. Cardiac neuronal uptake of norepinephrine was assessed by comparing regional removal of tracer-labeled norepinephrine with that of tracer-labeled isoproterenol (which is not a substrate for neuronal uptake) and by the relation between production of dihydroxyphenylglycol (DHPG), an exclusively intraneuronal metabolite of norepinephrine, and regional spillover of norepinephrine. Cardiac extraction of norepinephrine averaged 59 +/- 17% in the patients with HCM, significantly less than in the control subjects (79 +/- 13%, p less than 0.05), whereas cardiac extraction of isoproterenol was similar in the two groups (13 +/- 23% versus 13 +/- 14%), indicating that neuronal uptake of norepinephrine was decreased in the patients with HCM. The cardiac arteriovenous difference in norepinephrine was significantly larger in the patients with HCM than in the control subjects (73 +/- 77 versus 13 +/- 50 pg/ml, p less than 0.05), as was the product of the arteriovenous difference in norepinephrine and coronary blood flow (7.3 +/- 7.3 versus 0.8 +/- 3.0 ng/min, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-antagonist receptors in the atrial tissue of patients with hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy is characterized by a nondilated, hypertrophied left ventricle in the absence of any overt cause. A possible role of adrenergic innervation or of cellular calcium regulation is suggested by the presence of hyperdynamic left ventricular function and by the clinical and symptomatic improvement seen in patients treated with beta-receptor antagonists or calcium antagonists. Therefore, we measured the density of calcium-antagonist receptors and beta-adrenoceptors in the atrial myocardium of 16 patients with hypertrophic cardiomyopathy and 19 patients with various other cardiac disorders. For comparison, we also measured the number of voltage-sensitive sodium channels. Calcium-antagonist binding sites, measured as the amount of dihydropyridine bound to atrial tissue, were increased by 33 percent in patients with hypertrophic cardiomyopathy (mean [+/- SD], 397 +/- 104 fmol per milligram of protein in patients with hypertrophic cardiomyopathy, as compared with 299 +/- 108 in patients with other cardiac disorders; P less than 0.01). The densities of saxitoxin-binding sites on voltage-sensitive sodium channels and beta-adrenoceptors were the same in the two groups, although the density of beta-adrenoceptors was higher in atrial samples from patients receiving beta-receptor antagonists (165 +/- 86 fmol per milligram of protein [patients receiving beta-blockers] vs. 85 +/- 60 [patients not receiving beta-blockers]; P less than 0.04). The increase in the number of calcium-antagonist receptors in hypertrophic cardiomyopathy suggests that abnormal calcium fluxes through voltage-sensitive calcium channels may play a pathophysiologic part in the disease.

The effectiveness of inotropic agents in isolated cardiac preparations from the human heart

This review analyses the present knowledge on differences in human and animal heart preparations in respect to positive inotropic stimulation. The pharmacological effects of new positive inotropic drugs usually are evaluated in cardiac preparations from healthy and young animals. Human myocardium, especially from patients with heart failure has, however, distinctly different properties in respect to the positive inotropic effectiveness of several agents. This fact prohibits the extrapolation of results of experiments in laboratory animals to the diseased human heart. The partial ineffectiveness of several established positive inotropic substances in papillary muscles from failing human hearts indicates a membrane defect not present in healthy animal myocardium.

Relative selectivity of different beta-adrenoceptor antagonists for human heart beta 1- and beta 2-receptor subtypes assayed by a radioligand binding technique

The affinity constants of inhibition (Ki values) for both beta 1- and beta 2-receptor subtypes were determined for four different beta-adrenoceptor antagonists by a radioligand binding technique in a human myocardial membrane preparation. The radioligand was the high affinity antagonist [125I]-(-)-iodocyanopindolol (ICYP), and the drugs tested were atenolol, metoprolol, ICI 141,292 and ICI 118,551. Different concentrations of the drugs at test were allowed to compete with a constant concentration of ICYP for the specific binding sites (beta-receptors). Ki values for beta 1- and beta 2-receptors for each beta-adrenoceptor antagonist were developed from these data by computer calculations. Atenolol and metoprolol were found to differ slightly regarding potency (absolute Ki values) and to be practically equal regarding relative selectivity (approx. 40; i.e. ratio between high and low Ki values), while ICI 141,292 was found to have slightly higher relative selectivity (approx. 60) and much higher potency. All these drugs exhibited highest affinity for the beta 1-receptor population. In contrast, ICI 118,551 exhibited a very high relative selectivity (approx. 300) with highest affinity for the beta 2-receptor subtype. The method represents a good supplement to physiological and clinical examinations of selectivity of beta-blockers, and offers several advantages regarding simplicity, specificity and accuracy.

Beta 2-adrenoceptor-mediated positive inotropic effect of adrenaline in human ventricular myocardium. Quantitative discrepancies with binding and adenylate cyclase stimulation

Experiments were designed to unravel the relative contribution of beta 1- and beta 2-adrenoceptors to the positive inotropic effects of adrenaline and noradrenaline in isolated tissues of left ventricular myocardium of man. We also analyzed relationships between the fractions of human left ventricular beta 1- and beta 2-adrenoceptors, estimated from binding assays, and stimulation of adenylate cyclase and contractile force by adrenaline and noradrenaline. Selective blockade of beta 2-adrenoceptors by erythro-(+/-)-(alpha-methyl-indan-4-yloxy)-3-isopropylaminobuta n-2-ol (ICI 118,551) attenuated the increase of contractile force caused by adrenaline but not by noradrenaline, suggesting some involvement of beta 2-adrenoceptors. Selective blockade of beta 2-adrenoceptors without affecting beta 1-adrenoceptors still enabled both adrenaline and noradrenaline to cause maximum possible increases of contractile force through beta 1-adrenoceptors. A direct involvement of beta 2-adrenoceptors became manifest by selectively antagonizing beta 1-adrenoceptors by 1-[2[3-carbamoyl-4-hydroxy)phenoxy)ethylamino]- 3-[4(1-methyl-4-trifluoromethyl-2-imidazolyl)phenoxy]-2-propanol (CGP 20712 A) without affecting beta 2-adrenoceptor. beta 2-adrenoceptors can mediate half of the maximum increase of contractile force elicited by low concentrations of adrenaline and also contribute to the increase of contractile force caused by high concentrations of noradrenaline. beta-adrenoceptors were labelled in membrane particles with 3H-(-)-bupranolol in the absence (beta 1 & beta 2) and presence of 500 nmol/l CGP 20712 A (beta 2). 71% of the beta-adrenoceptors were beta 1 and 29% beta 2. Binding inhibition experiments with CGP 20712 A and ICI 118,551 yielded 74% beta 1 and 26% beta 2.(ABSTRACT TRUNCATED AT 250 WORDS)

Relative potencies of various beta-adrenoceptor antagonists (BAA) at the level of the human myocardial beta-adrenoceptor-adenylate cyclase (AC) complex. Is intrinsic sympathomimetic activity (ISA) due to AC activation?

Nine different beta-adrenoceptor antagonists (BAA), five with intrinsic sympathomimetic activity (ISA), were examined for their ability to inhibit isoproterenol-stimulated adenylate cyclase (AC) activity and specific 125I-cyanopindolol (CYP) binding in crude membrane particles from human myocardium. The BAA's were: propranolol, pindolol, timolol, alprenolol, metoprolol, atenolol, prenalterol, ICI 141.292 'Visacor', and ICI 118.587 'Corwin'. Whether BAAs with strong ISA were able to stimulate AC activity by themselves were examined in separate experiments and compared to the AC stimulation by full agonists. All the BAAs caused a concentration dependent, and at high doses apparently complete, inhibition of both isoproterenol-stimulated AC activity and 125I-CYP binding. Both assays made possible a 'potency-ranking' of the different BAAs (pindolol greater than or equal to propranolol and timolol greater than ICI 142.292 and alprenolol greater than ICI 118.587, prenalterol and metoprolol greater than atenolol). Corrected IC50-values, derived from inhibition curves with both techniques, show that receptor binding and inhibition of receptor function follow each other closely. Prenalterol caused a very weak AC activation (5.4% of maximum), whereas the 'ISA-blockers', pindolol, ICI 141.292, and ICI 118.587 were unable to stimulate AC activity at concentrations which completely displaced 125I-CYP binding. In comparison, norepinephrine stimulated AC activity to the same level as isoproterenol (three to four times basal activity) and the beta 2-selective agonist terbutaline caused some 50% of maximal AC stimulation. This raises the question whether ISA is due to AC activation. The effect upon AC activation and 125I-CYP binding of drugs with beta-selectivity shows that both beta 1- and beta 2-receptors are coupled to the AC.