Relationship between beta-adrenoceptors and calcium channels in human ventricular myocardium

Modulation of the inwardly rectifying K+ channel in isolated human atrial myocytes by alpha 1-adrenergic stimulation

We have examined the alpha 1-adrenergic modulation of the inwardly-rectifying K+ channel (IK1) in isolated human atrial myocytes using the patch clamp technique. alpha 1-Adrenergic agonist methoxamine produced action potential prolongation and a depolarization of the resting membrane potential. Under whole-cell voltage-clamp conditions, bath application of methoxamine can inhibit macroscopic IK1. The methoxamine-induced inhibition was reversible and concentration dependent, with the concentration for half-maximal inhibition being 18 microM. The methoxamine-induced inhibition of IK1 was prevented by bath application of alpha 1-adrenergic blocker prazosin. The current was similarly inhibited by phorbol ester (PMA), an activator of protein kinase C (PKC). In contrast, methoxamine failed to inhibit the current in the presence of a specific PKC inhibitor H-9, suggesting that PKC is involved in the methoxamine-induced inhibition of IK1. In single channel recording from cell-attached patches, bath-applied methoxamine could suppress IK1 channels by decreasing the frequency and duration of bursting without affecting unitary amplitude. Direct application of purified PKC to excised inside-out patches inhibited channel activity similar to methoxamine in cell-attached patches. The PKC selective inhibitor, PKC19-36, prevented the PKC-induced inhibition of the channel. We conclude that human atrial IK1 can be inhibited by alpha 1-adrenergic stimulation via PKC-dependent pathways.

Left ventricular diastolic function: physiology, methods of assessment, and clinical significance

Diastole, that portion of the cardiac cycle that begins with isovolumic relaxation and ends with mitral valve closure, results in ventricular filling and involves both active (energy-dependent) and passive processes. The interactions between active processes (myocardial relaxation) that primarily influence early ventricular filling and passive processes, such as loading conditions, myocardial compliance, and valvular disease, are complex. Clinical methods to assess ventricular filling include cardiac catheterization, radionuclide angiography, and echocardiography. Any measurements of diastolic function must be made with an understanding of the determinants of ventricular filling and the limitations of the diagnostic test. Many cardiac disorders are characterized by elevated pulmonary venous pressures in the face of normal systolic ventricular function, which suggests a primary abnormality of diastolic function. Abnormalities in diastolic function have been observed in coronary artery disease, congestive heart failure (with and without systolic dysfunction), hypertrophic cardiomyopathy, hypertension, and in healthy elderly subjects. Identification of these abnormalities may be useful clinically, particularly in patients with symptoms of heart failure and normal systolic function. Data are not available to determine the optimal therapy for such patients, although evidence suggests that calcium channel blockers, beta blockers, and agents that reverse myocardial hypertrophy may be useful. This review briefly summarizes the physiology of diastole, the methods of clinical assessment of diastolic function, and the role of diastolic function in cardiovascular disease.

Hypertrophic cardiomyopathy: a desensitized cardiac beta-adrenergic system in the presence of normal plasma catecholamine concentrations

Only few data are available concerning the biochemical and functional state of the beta-adrenergic system in hypertrophied human myocardium. The present study was to investigate the myocardial beta-adrenergic signal transduction system in hypertrophic obstructive cardiomyopathy (HOCM). Thin myocardial strips were prepared from surgically excised, septal myocardium from 7 patients with HOCM and their force of contraction was measured in vitro. The positive inotropic effects of calcium and dihydro-ouabain, both acting independently of beta-adrenoceptors and cAMP, were similar in these preparations to those, previously published, seen with nonfailing myocardium. In contrast, the beta-adrenoceptor agonist isoprenaline and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) had reduced positive inotropic effects. Their EC50-values were about 10 fold higher than the respective EC50-values published for nonfailing myocardium. The positive inotropic potencies of isoprenaline and IBMX were reduced in HOCM by as much as they were in the additionally investigated myocardium from 6 patients with severe mitral regurgitation (MR, NYHA III). In order to clarify whether the functional alterations are related to changes in the beta-adrenoceptors, beta-adrenoceptor density and beta 1: beta 2-adrenoceptor subtype distribution were determined in the same myocardium using 125I-Iodocyanopindolol saturation binding. Myocardial beta-adrenoceptor density was reduced to 68% in HOCM and to 56% in MR compared to nonfailing myocardium controls (NF: 64.8 +/- 6.5 fmol/mg protein). In HOCM, this reduction was due to a selective down regulation of beta 1-adrenoceptors (24.9 +/- 3.7 fmol/mg protein vs NF: 46.4 +/- 6.8 fmol/mg protein, P < 0.05), whereas beta 2-adrenoceptor density was unchanged (19.0 +/- 1.9 fmol/mg protein vs NF: 18.4 +/- 3.3 fmol/mg protein, n.s.). In MR both beta-adrenoceptor subtypes were reduced (beta 1: 26.9 +/- 1.4 fmol/mg protein, beta 2: 9.6 +/- 1.7 fmol/mg protein; both P < 0.05 vs NF). Electrochemically determined plasma catecholamine levels were elevated in MR. However, plasma catecholamine levels were normal or slightly below normal in HOCM. In summary, myocardial beta-adrenoceptors are downregulated and their function is impaired in HOCM. This desensitization is not caused by a negative feedback regulation due to increased plasma catecholamines. The present results show that the desensitizations of the beta-adrenergic system associated with HOCM has characteristics that indicate a major deviation in its development from that of the beta-adrenergic desensitization previously described to occur in congestive heart failure.

Uniformity of calcium channel number and isometric contraction in human right and left ventricular myocardium

We compared contractile performance in trabeculae carneae (n = 25) from non-failing right and left ventricles (n = 25) of brain dead organ donors without known cardiovascular disease and measured connective tissue content in trabeculae carneae from both non-failing and failing human hearts. Peak twitch force and time-course of contraction were not different between muscles taken from right or left ventricles. Peak twitch force was 13.9 +/- 3 vs. 13.7 +/- 2.7 mN/mm2 for right and left ventricular trabeculae carneae, respectively in 2.5 mM [Ca2+]0 at a 0.33 Hz stimulation frequency. Time to peak tension (405 +/- 21 vs. 405 +/- 12 ms), time to 50% relaxation from peak contractile response (277 +/- 21 vs. 278 +/- 14.6 ms) and time to 80% relaxation (428 +/- 29 vs. 433 +/- 22) were not different between right and left ventricular trabeculae carneae. Calcium channel number determined by [3H]PN200-100 dihydropyridine-radioligand binding assay was also not different (56.2 +/- 6.5 fmol/mg protein vs. 58.6 +/- 8.4 fmol/mg protein for right and left heart preparations, respectively). However, in myocardium obtained from ischemic hearts the left ventricle showed a reduced number of calcium channels compared to the right ventricle (55.3 +/- 3.8 vs. 36.6 +/- 3.9 fmol/mg protein for right and left ventricle, respectively p = 0.027). No differences were noted in the number of DHP receptor binding sites between right and left ventricular myocardium from patients with idiopathic dilated cardiomyopathy (51.4 +/- 7.6 fmol/mg protein vs. 61.8 +/- 6.5 fmol/mg protein respectively). Our data indicate that calcium channel number is similar for non-failing left and right human ventricle. Contractile response to changes in [Ca2+]0 and frequency were similar for trabeculae carneae from the left and right ventricles of non-failing human hearts. Studies involving calcium channel activation or inhibition in ischemic human myocardium, where there may be differences in calcium channel number and/or function are warranted. Whether changes in calcium channel number have biological consequences on contractile function remains to be determined. Importantly, careful studies of calcium channel function under in vivo conditions are warranted.

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.

Contribution of both alpha- and beta-adrenoceptors to the inotropic effects of catecholamines in the rabbit heart

The functional role of alpha-adrenoceptors was investigated in different parts of the rabbit heart. Phenylephrine (PE) caused a marked increase in force of contraction (Fc) and a prolongation of the action potential (AP) in preparations from the left atrium and the right ventricle. The response was less pronounced in the right atrium and in the left ventricle, whereas APs of spontaneously beating sinoatrial preparations remained completely unchanged. Phentolamine as well as the diesters phorbol 12,13 dibutyrate (PDBu) or 12-O-tetradecanoyl-phorbol-13-acetate (TPA) eliminated the effects of PE. The contribution of alpha-adrenoceptors to the effects of adrenaline (Adr) and noradrenaline (NA) on Fc was determined in preparations from the right ventricle. Phentolamine and the phorbol diesters reduced the effects of Adr and NA by about 30 to 60%; the remaining response was abolished by propranolol. It can be derived from our experiments that, in some parts of the rabbit heart, a considerable amount of the effects of Adr and NA is due to the stimulation of alpha-adrenoceptors. The present findings therefore support the view that, in the rabbit heart, the maximally effective drive of the heart requires the stimulation of both alpha- and beta-adrenoceptors. The inhibitory effects of phorbol diesters on the alpha-adrenoceptor-mediated response indicate that the activation of protein kinase C (PKC) specifically uncouples alpha-adrenoceptors from the effector system, whereas the response to beta-adrenoceptor stimulation remains unchanged.

[3H]PN200-110 and [3H]glibenclamide binding in normal and cardiomyopathic hamsters

1. We examined the binding of the Ca2+ channel ligand [3H]PN200-110 and the ATP-sensitive K+ channel ligand [3H]glibenclamide to brain and heart from cardiomyopathic hamsters and compared them to controls. 2. We found that [3H]PN200-110 binding site density was elevated in the heart, but not in the brain, of 30- and 180-day old cardiomyopathic hamsters when compared to controls. 3. [3H]Glibenclamide binding site density was greatly reduced in the heart of 180-day old cardiomyopathic animals compared with all other groups. 4. Quantitative autoradiography revealed that [3H]glibenclamide binding was elevated in several brain areas of 30-day old cardiomyopathic hamsters relative to controls. 5. It is concluded that alterations in both Ca2+ and K+ channels exist in the cardiomyopathic hamster.

Expression of dihydropyridine receptor (Ca2+ channel) and calsequestrin genes in the myocardium of patients with end-stage heart failure

Cytoplasmic free calcium ions (Ca2+) play a central role in excitation-contraction coupling of cardiac muscle. Abnormal Ca2+ handling has been implicated in systolic and diastolic dysfunction in patients with end-stage heart failure. The current study tests the hypothesis that expression of genes encoding proteins regulating myocardial Ca2+ homeostasis is altered in human heart failure. We analyzed RNA isolated from the left ventricular (LV) myocardium of 30 cardiac transplant recipients with end-stage heart failure (HF) and five organ donors (normal control), using cDNA probes specific for the cardiac dihydropyridine (DHP) receptor (the alpha 1 subunit of the DHP-sensitive Ca2+ channel) and cardiac calsequestrin of sarcoplasmic reticulum (SR). In addition, abundance of DHP binding sites was assessed by ligand binding techniques (n = 6 each for the patients and normal controls). There was no difference in the level of cardiac calsequestrin mRNA between the HF patients and normal controls. In contrast, the level of mRNA encoding the DHP receptor was decreased by 47% (P less than 0.001) in the LV myocardium from the patients with HF compared to the normal controls. The number of DHP binding sites was decreased by 35-48%. As reported previously, expression of the SR Ca(2+)-ATPase mRNA was also diminished by 50% (P less than 0.001) in the HF group. These data suggest that expression of the genes encoding the cardiac DHP receptor and SR Ca(2+)-ATPase is reduced in the LV myocardium from patients with HF. Altered expression of these genes may be related to abnormal Ca2+ handling in the failing myocardium, contributing to LV systolic and diastolic dysfunction in patients with end-stage heart failure.

Electrophysiologic and inotropic effects of alpha-adrenoceptor stimulation in human isolated atrial heart muscle

The effects of alpha-adrenoceptor stimulation on force of contraction were investigated in human atrial heart muscle and compared with those of beta-adrenoceptor stimulation. The maximal positive inotropic effect produced by stimulation of alpha-adrenoceptors with phenylephrine (in the presence of atenolol 10 mumol/l) was significantly smaller than that seen in response to beta-adrenoceptor stimulation with isoprenaline. The maximal effect of phenylephrine (25% of the maximal effect of isoprenaline) required far higher concentrations (1 mmol/l) than isoprenaline (100 nmol/l); the EC50 values amounted to 33.1 mumol/l and 3.3 nmol/l, respectively. In the presence of the alpha-adrenoceptor blocking agent phentolamine (1 mumol/l), the concentration-response curve of phenylephrine was displaced to higher concentrations of the agonist; under these conditions, the EC50 value amounted to 52.5 mumol/l. The effects of the catecholamines noradrenaline and adrenaline on force of contraction remained unchanged in the presence of phentolamine (1 mumol/l) or prazosin (1 mumol/l). The positive inotropic effect of phenylephrine (1 mmol/l) was associated with a slight decrease in action potential duration; the effects on action potential were completely blocked in the presence of phentolamine (1 mumol/l). These findings support the view that selective stimulation of alpha-adrenoceptors may mediate a small but detectable positive inotropic effect in human atrial tissue under in vitro conditions. The requirement of high concentrations of alpha-adrenoceptor agonists and the lack of effects of the endogenous catecholamines adrenaline and noradrenaline on alpha-adrenoceptors (in concentrations which fully elicit the beta-adrenoceptors-mediated response) do not provide a basis for a functional role of alpha-adrenoceptor-mediated effects under in vivo conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of effect of chronic calcium antagonist treatment on beta 1- and beta 2-adrenoceptors in right atria from patients with or without heart failure

1. We studied the effects of chronic calcium antagonist (calcium entry blocker, CEB; nifedipine, verapamil, diltiazem) treatment on beta-adrenoceptor density (assessed by (-)-[125I]-iodocyanopindolol [ICYP] binding) and subtype distribution in right atria from 65 patients without apparent heart failure undergoing elective coronary artery bypass grafting (CAD-patients) and from 13 patients with moderate heart failure (NYHA class III to class III-IV) undergoing mitral valve replacement (MVD-patients). 2. In CAD-patients atrial beta-adrenoceptor density was 79.3 +/- 7.9 fmol ICYP bound mg-1 protein (n = 18), the beta 1:beta 2-adrenoceptor ratio 69:31%. Chronic CEB-treatment did not affect either atrial beta-adrenoceptor density or beta 1:beta 2-adrenoceptor ratio. 3. In contrast, in CAD-patients chronically treated with beta 1-adrenoceptor antagonists (atenolol, bisoprolol, metoprolol) and CEB, atrial beta-adrenoceptor density was significantly increased (108.6 +/- 10.5 fmol ICYP bound mg-1 protein, n = 21); this increase was due to a selective increase in beta 1-adrenoceptors. 4. In MVD-patients atrial beta-adrenoceptor density (55.5 +/- 8.7 fmol ICYP bound mg-1 protein, n = 7) was significantly lower (P less than 0.05) than in CAD-patients; beta 1:beta 2-adrenoceptor ratio, however, was not changed (67:33%). Chronic CEB-treatment of MVD-patients did not prevent the decrease in atrial beta-adrenoceptors. 5. We conclude that chronic CEB-treatment does not affect human right atrial beta-adrenoceptor density, either in patients without apparent heart failure or in patients with moderate heart failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor systems in the non-failing human heart

Catecholamines acting through beta 1- and beta 2-adrenoceptors cause positive inotropic and chronotropic effects in the human heart. In recent years, however, evidence has accumulated that in the human heart also other receptor systems can affect heart rate and/or contractility. Positive inotropic effects can be mediated by receptor systems acting through accumulation of intracellular cAMP (Gs-protein coupled receptors such as 5-HT4-like, histamine H2, and vasoactive intestinal peptide) or by receptor systems acting independent of cAMP possibly through the phospholipase C/diacylglycerol/inositol-1,4,5-trisphosphate pathway (such as alpha 1-adrenergic, angiotensin II, and endothelin). In the non-failing human heart, however, activation of all these receptor systems induces only submaximal positive inotropic effects when compared with those caused by beta-adrenoceptor stimulation, indicating that in humans the cardiac beta-adrenoceptor-Gs-protein-adenylate cyclase pathway is the most powerful mechanism to increase heart rate and contractility. On the other hand, at least three receptor systems acting through inhibition of cAMP formation (Gi-protein coupled receptors) exist in the human heart: muscarinic M2-, adenosine A1-, and somatostatin-receptors. Activation of M2- and A1-receptors causes negative inotropic effects in the non-failing human heart: in atria activation of both receptors causes decreases in basal as well as in isoprenaline-stimulated force of contraction, but in ventricles only isoprenaline-stimulated force of contraction is depressed.

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.

Cardiac inotropic responses to calcium and forskolin are not altered by prolonged isoproterenol infusion

Effects of prolonged isoproterenol infusion upon the density of cardiac calcium channels, calcium-mediated contractile responses, and the ability of forskolin to enhance tension development and cyclic AMP accumulation were studied in ventricular muscle preparations from Sprague-Dawley rats. Isoproterenol infusion (400 micrograms/kg per h s.c., 4 days) significantly decreased calcium channel density (Bmax) in cardiac microsomal membranes as quantified by a 32% decrease in specific [3H]nitrendipine binding sites; binding affinity (KD) was unchanged. A 57% decrease of beta-adrenoceptors confirmed homologous down regulation. To examine functional effects of decreased [3H]nitrendipine binding sites, responses to calcium, BAY K8644 and nifedipine were determined in isolated right ventricular strips. Significant decreases in basal developed tension were observed in muscles from isoproterenol-infused rats. However, concentration-dependent increases in contractility in response to CaCl2 or BAY K8644 were comparable, and the negative inotropic effect of nifedipine was unchanged. Whereas isoproterenol infusion was associated with significantly decreased basal cardiac cyclic AMP concentrations, exposure of ventricular strips from either vehicle- or isoproterenol-infused rats to 10 microM forskolin resulted in comparable increases in cyclic AMP and in developed tension. Cumulative, submaximal concentrations of forskolin also produced similar increases in contractility with maximum responses in ventricular strips from vehicle-infused animals attained at 4.4 microM forskolin. Higher concentrations resulted in automaticity. By contrast, ventricle from isoproterenol-infused animals responded to 14.4 microM forskolin with maximal increases in force of contraction.

Different negative inotropic activity of Ca2(+)-antagonists in human myocardial tissue

To evaluate the negative inotropic effect of various Ca2(+)-antagonists in human myocardium without additional influences of preload, afterload, or frequency, we examined their effects on isometric force of contraction in isolated human papillary muscle strips and in auricular trabeculae. The 1,4-dihydropyridines isradipine, nitrendipine, and nifedipine, the phenylalkylamine verapamil, and the benzothiazepine diltiazem exerted concentration-dependent negative inotropic effects. The potency of the investigated Ca2(+)-antagonists was identical in papillary muscle strips of patients with only moderate clinical signs of heart failure undergoing mitral valve replacement-operation (NYHA II-III) and in terminally failing (heart transplantation, NYHA IV) human hearts. The IC50 values were lower in auricular trabeculae than in papillary muscle strips. The difference was significant for nifedipine, nitrendipine, and verapamil. The restorative effects of external Ca2+ after pretreatment with Ca2(+)-antagonists were significantly less strong after pretreatment with 1,4-dihydropyridine than with non-dihydropyridines in papillary muscle strips. It is concluded that 1,4-dihydropyridines and verapamil and diltiazem did differently influence Ca2(+)-mediated increase in force of contraction. Moreover, a relation between the therapeutically active free plasma concentration in vivo and the negative inotropic potency in vitro can be found. This relation follows a rank order of potency for negative inotropism (isradipine less than or equal to nitrendipine less than diltiazem less than nifedipine less than verapamil) and might have clinical relevance in the treatment of patients with compromised cardiac function.

Effects of calcium antagonists on beta-receptors of cultured cardiac myocytes isolated from neonatal rat ventricle

The effects of calcium antagonists (verapamil, diltiazem, and nicardipine) on beta-adrenergic receptors of cultured cardiac myocytes isolated from neonatal rat ventricle were studied with the hydrophilic ligand [3H]CGP-12177, which identifies cell surface-bound beta-receptors. The three calcium antagonists suppressed spontaneous beating of the myocytes, increased the number of beta-receptors, but did not alter the affinity (Kd). These effects were dose and time dependent. Verapamil (10(-6) M) increased the beta-receptor density by about 13% after 6 hours of incubation, and this increase in density reached a plateau of about 45% after 24 hours of incubation. beta-Receptor density increased by 15% with 5 x 10(-7) M and by 37% with 10(-6) M verapamil. The increased beta-receptors appeared to retain their normal function, as assessed by the increased spontaneous beating of the myocytes in response to applied isoproterenol. The increase in beta-receptors was abolished by colchicine but not by cycloheximide. When the calcium ion concentration of the medium was lowered to 0.1 mM, no significant change occurred in the density of beta-receptors compared with that in 1.8-mM Ca2+ medium. The results suggest that calcium antagonists increase beta-receptors by accelerating recycling by microtubules but not by decreasing the inward calcium current. Such effects of calcium antagonists may be clinically important and promise insight into the mechanism of the withdrawal phenomenon of calcium antagonists.

Function of myocardial alpha-adrenoceptors

In addition to beta-adrenoceptors (beta ARs), cardiac myocytes of animals and man possess alpha 1ARs, but not alpha 2ARs. Norepinephrine and epinephrine have a higher affinity for myocardial alpha 1ARs than for beta ARs. Unlike beta AR stimulation, myocardial alpha 1AR stimulation does not increase the slow inward current. The alpha 1AR-mediated positive inotropic effect seen in isolated heart preparations appears to involve increased Ca sensitivity of myofibrils and production of inositol triphosphate (IP3) and diacylglycerol (DAG), but the functions of IP3 and DAG are not clear. Myocardial alpha 1AR stimulation reduces rate of isolated atria and Purkinje fibers and lengthens refractory period and action potential duration. Hypoxia increases alpha 1AR density in cardiomyocytes. alpha 1AR-mediated arrhythmias occur in isolated Purkinje fibers during hypoxia, following infarction, and in the presence of Ba2+ or high Ca2+. In animals, coronary artery occlusion and/or reperfusion increase myocardial alpha 1AR density and responsiveness, and alpha AR blocking drugs attenuate arrhythmias. However, an antiarrhythmic effect of alpha AR blocking drugs mediated by action on coronary vascular alpha ARs cannot be excluded. Presently available drugs do not differentiate between myocardial and vascular alpha ARs and thus affect the coronary and systemic circulations and, indirectly, the heart. Additional myocardial alpha 1AR-mediated effects include production of cardiac hypertrophy, stimulation of glucose uptake and phosphofructokinase and cyclic AMP phosphodiesterase activity, and release of atrial natriuretic peptide.

Effect of calcium and beta-adrenoceptor antagonists on beta-adrenoceptor density and Gs alpha expression in human atria

In 21 patients undergoing open heart surgery the effects of the beta-adrenoceptor antagonists propranolol and sotalol and the calcium antagonist nifedipine (alone or in combination) on right atrial beta-adrenoceptor density and G protein expression were studied. The non-selective beta-adrenoceptor antagonists increased both right atrial beta 1- and beta 2-adrenoceptor density; this increase was not affected by nifedipine. There was no correlation between beta-adrenoceptor density and expression of the G protein subunit Gs alpha suggesting independent regulation of the gene expression for G proteins and their coupled receptors.

Is there a third heart beta-adrenoceptor?

A series of partial agonists with high affinity for myocardial beta 1- and beta 2-adrenoceptors cause stimulant effects in heart that are resistant to blockade of beta 1- and beta 2-adrenoceptors. The concentrations of partial agonist that cause stimulant effects greatly exceed those that cause blockade. Alberto Kaumann suggests that such non-conventional partial agonists, often analogues of pindolol, may act through a third heart beta-adrenoceptor, which resembles the beta 3-adrenoceptor of white adipocytes and smooth muscle of airways and ileum.

Alterations in calcium antagonist receptors and sodium-calcium exchange in cardiomyopathic hamster tissues

The Syrian cardiomyopathic (CM) hamster (BIO 14.6) develops a progressive cardiomyopathy characterized by cellular necrosis, hypertrophy, and, eventually, cardiac dilatation and congestive heart failure. Several lines of evidence implicate cellular calcium overload as an important etiologic factor. We previously reported an increased number of receptors for calcium antagonist drugs, which block voltage-dependent calcium channels, in heart, skeletal muscle, and brain tissue of these hamsters in the early necrotic stage of the disease. To better characterize the pathophysiological significance of this abnormality we evaluated calcium antagonist receptor binding and Na+-Ca2+ exchange in CM and control hamsters at different stages of disease as documented by quantitative histopathologic assessment. In CM hamsters as young as 10 days, an age previously thought to be before the onset of disease, we identified cardiac myocyte hypertrophy, a twofold increase in calcium antagonist receptor binding in heart and brain, and a 50% increase in skeletal muscle. Overt histological lesions were present in skeletal muscle at 25 days and in heart between 28-30 days. The size of cardiac lesions increased over time and changed from necrotic foci with cellular infiltration to fibrotic or calcified lesions by 360 days. Myocardial cellular hypertrophy persisted through the late stages of the disease (360 days), but increased calcium antagonist binding was present in heart only to 6 months of age, in skeletal muscle to 90 days, and in brain to 30 days. Na+-Ca2+ exchange in heart was normal until 15 days and then increased by 400% at 30 days suggesting that this augmentation might be a secondary response to the earlier increase in calcium antagonist receptors. At 360 days cardiac Na+-Ca2+ exchange was decreased by 50%, likely reflecting progressive cardiac damage. The increase in calcium antagonist receptors in CM animals as young as 10 days supports the hypothesis that abnormalities in voltage-dependent calcium channels play a role in the pathophysiology of CM hamsters.

Homologous regulation of voltage-dependent calcium channels by 1,4-dihydropyridines

Chronic treatment of PC 12 cells with the 1,4-dihydropyridine Ca2+ channel antagonist nifedipine [5 x 10-8M/5 days] and the activator S Bay K 8644 [5 x 10-7 M/5 days] resulted in up- and down-regulation of 1,4-dihydropyridine binding site density by 29 and 24%, respectively, without change in affinity. These changes in binding site density represent functional changes as indicated by the corresponding changes in K+ depolarization-induced 45Ca2+ uptake and in whole cell currents carried by Ba2+ ions. This homologous regulation of voltage-dependent Ca2+ channels [VDCC] by potent and specific ligands parallels that observed for other classes of membrane receptors.

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.

Time course and extent of alpha 1-adrenoceptor density changes in rat heart after beta-adrenoceptor blockade

1. It has been suggested that impaired beta-adrenoceptor stimulation is a condition under which the functional role of cardiac alpha 1-adrenoceptors is enhanced. We therefore investigated the extent and time course of changes in alpha 1-adrenoceptor characteristics after chronic treatment with the beta-adrenoceptor blocker propranolol in rat heart. For comparison beta-adrenoceptors were also studied. The mechanism of the changes in adrenoceptor density was investigated with cycloheximide, an inhibitor of protein synthesis. The functional significance of an increased alpha 1-adrenoceptor density was tested by measuring isometric force of contraction in the presence of phenylephrine or isoprenaline in right ventricular papillary muscles. 2. Rats were treated with propranolol (9.9 mg kg-1 daily) or 0.9% NaCl, applied with osmotic minipumps for 1, 2, 3 or 7 days. Propranolol treatment resulted in a maximally 28% increase of alpha 1-adrenoceptor density after 3 days (NaCl 95.9 +/- 3.5 vs. propranolol 123.0 +/- 1.6 fmol mg-1 protein, n = 6, P less than 0.01). This up regulation reached significant levels after 2 days of treatment and was reversible after cessation of treatment within two days. KD-values were the same for NaCl- and propranolol-treated rats. Changes of Bmax and KD in beta-adrenoceptor binding assays did not reach significant levels. 3. Cycloheximide (1.5 mg kg-1 i.p. daily for 3 days) inhibited the propranolol-induced increase in Bmax of alpha 1-adrenoceptors completely. In addition, cycloheximide also decreased the density of alpha 1- and beta-adrenoceptors also under control conditions. 4. pD2-values for the positive inotropic effect of phenylephrine and isoprenaline in isolated electrically driven papillary muscle were similar in NaCl- and propranolol-treated rats (phenylephrine: 5.41 + 0.11 vs. 5.41 + 0.19, n = 7; isoprenaline: 6.31 + 0.18 vs. 6.65 + 0.19, n = 7). The observed increase in alpha-adrenoceptor density in healthy rat heart may therefore not be high enough to enhance the phenylephrine-induced increase in force of contraction. 5. In conclusion, time course and effects of cycloheximide indicate that the increase in B,,,, of myocardial alpha 1-adrenoceptors is due to de novo synthesis of receptors. However, at least for the rat heart model, a functional significance of this increase could not be demonstrated.

Relations between beta-adrenoceptor occupancy and increases of contractile force and adenylate cyclase activity induced by catecholamines in human ventricular myocardium. Acute desensitization and comparison with feline ventricle

The function of beta-adrenoceptors was investigated in ventricular myocardium obtained from patients undergoing open heart surgery. 1. Dopamine increased contractile force up to 1/2 and 1/4 of the maximum increase caused by (-)-noradrenaline or (-)-adrenaline in right and left ventricular preparations, respectively. 2. beta-Adrenoceptors were labelled with 3H-(-)-bupranolol. For 3/4 of the receptors (beta 1) the affinity of (-)-noradrenaline was 20 times higher than for the remaining 1/4 (beta 2). (-)-Adrenaline and dopamine appeared to be non-selective for beta 1 and beta 2. 3. Dopamine was able to stimulate the adenylate cyclase only up to 1/3 of the maximum stimulation caused by (-)-noradrenaline and (-)-adrenaline. 4. Increases in contractile force by (-)-noradrenaline were closely associated with small increases of cyclase activity through beta 1-adrenoceptors, consistent with a common link. 5. The experiments on human myocardium were compared with similar experiments on feline myocardium. Feline ventricle exhibited a 20- to 30-fold higher sensitivity to catecholamines as activators of contractile force than did human ventricle. However, the binding affinities for catecholamines were similar in cat and man. 6. A 3 h exposure of human and feline ventricular myocardium to (-)-isoprenaline caused desensitization by uncoupling beta-adrenoceptors from the adenylate cyclase. Desensitization reduced the maximum contractile response to (-)-isoprenaline in human but not in feline ventricle. 7. The more efficient activation of contractile force by (-)-noradrenaline in cat, compared to man, appears to be related to a 2-fold higher density of beta 1-adrenoceptors, a 6-fold higher production of cyclic AMP per beta 1-adrenoceptor and possibly to a more effective use of cyclic AMP for contraction.

Calcium channel binding characteristics in the human heart

The number, affinity, pharmacologic specificity and regional distribution of calcium channel binding sites in human hearts obtained at autopsy and open heart surgery were characterized using the radioligand [3H]nitrendipine. Scatchard analyses of saturation data from 6 autopsy hearts revealed a homogeneous distribution of high affinity binding sites (affinity-1 [KD] = 0.44 +/- 0.06, 0.52 +/- 0.07, 0.32 +/- 0.02, 0.30 +/- 0.03, and 0.45 +/- 0.01 nM; binding capacity [Bmax] = 30 +/- 4, 27 +/- 6, 25 +/- 7, 33 +/- 3, and 28 +/- 4 fmol/mg protein in right atrium, right ventricle, left atrium, left ventricle and ventricular septum, respectively). In ligand competition experiments, nifedipine and nitrendipine completely displaced binding with partial displacement by verapamil and 35% enhancement of binding by 10(-5) M diltiazem at 37 degrees. Analyses of right atrial appendages obtained at open heart surgery from 5 coronary artery bypass patients provided similar results (KD = 0.2 +/- 0.03 nM, Bmax = 42 +/- 2 fmol/mg protein). In addition, no significant differences in KD or Bmax were found in 3 hamster hearts assayed at the time of death or up to 18 hours postmortem at either 4 or 25 degrees. In contrast, there was a significant increase in Bmax (110 fmol/mg protein) with no change in KD (0.3 nM) in a myomectomy specimen from a patient with obstructive hypertrophic cardiomyopathy compared with either autopsy or surgical specimens. These studies illustrate the feasibility and potential advantages of studying calcium channels directly in human hearts.

Adrenoceptor-mediated changes of action potential and force of contraction in human isolated ventricular heart muscle

1. The effects of alpha-adrenoceptor stimulation on the action potential and force of contraction were investigated in human isolated ventricular heart muscle and compared with those of beta-adrenoceptor stimulation. 2. The maximal stimulation by isoprenaline of beta-adrenoceptors produced large changes in the force of contraction, which were accompanied by moderate increases in the height of the action potential. The maximal inotropic effect produced by stimulation of alpha-adrenoceptors with phenylephrine, in the presence of propranolol (1 mumol 1(-1)) was much smaller (about 10% of that seen in response to beta-adrenoceptor stimulation), and no significant changes of the action potential configuration were observed. 3. The effects of noradrenaline and adrenaline on the force of contraction were not affected by prazosin. 4. It is concluded that the adrenoceptor-mediated changes of the force of contraction (in the presence of either noradrenaline or adrenaline) in the human ventricle are due virtually exclusively to the stimulation of beta-adrenoceptors.

Effect of thyroid status on beta-adrenoceptors and calcium channels in rat cardiac and vascular tissue

To determine the influence of thyroid hormone on beta-adrenoceptors and Ca2+ channels, rats were treated with thyroxine (75 micrograms/100 g sc daily for 5 days) or propylthiouracil (0.05% in drinking water for 30 days). beta-Adrenoceptor density in ventricular tissue, measured by [125I]iodocyanopindolol binding, was significantly increased and decreased respectively, following thyroxine or propylthiouracil treatment to 124.7 +/- 7.11 fmol/mg protein and 71.98 +/- 5.37 fmol/mg protein from euthyroid (control) levels of 93.7 +/- 4.58 fmol/mg protein. Ca2+ channel density, measured by [3H]nitrendipine binding, was altered in the opposite direction; it was significantly decreased and increased to 324 +/- 24 fmol/mg protein and 691 +/- 31 fmol/mg protein from 562 +/- 35 fmol/mg protein after thyroxine or propylthiouracil treatment, respectively. No changes in affinity of either ligand were observed. Responses of isolated papillary muscles from propylthiouracil-treated animals accorded with changes seen in the binding studies. The geometric mean EC50 of isoproterenol increased from 9.5 x 10(-9) mol/l to 5.5 x 10(-8) mol/l, and the EC50 for calcium decreased from 3.16 x 10(-3) mol/l to 1.36 x 10(-3) mol/l; moreover, the responsiveness to the Ca2+ channel activator Bay K 8644 was increased. The corresponding responses in thyroxine-treated animals could not be examined because of prominent arrhythmic activity. As with papillary muscles the sensitivity of left atria to isoproterenol was decreased after treatment with propylthiouracil, with geometric mean EC50 values increasing from 3.21 x 10(-9) mol/l to 89.4 x 10(-9) mol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation by chronic drug administration of neuronal and cardiac calcium channel, beta-adrenoceptor and muscarinic receptor levels

Chronic administration of atropine (40-100 mg/kg, 23 days) produced a 29-33% increase in muscarinic receptors, measured by [3H]quinuclidinyl benzilate binding, in rat brain. Diisopropyl phosphorofluoridate (0.9 mg/kg, 14 days) produced a 35% decrease in muscarinic receptors. Propranolol administration (800 micrograms/kg/hr, 10 days) increased beta-adrenoceptors, measured by [3H]dihydroalprenolol binding, by 69 and 50% in brain and heart respectively. Isoproterenol administration (800 micrograms/kg/hr, 10 days) produced a 50% reduction in cardiac beta-adrenoceptors but did not alter brain receptors. These drug treatments were without effect on binding of the Ca2+ channel ligands, [3H]nimodipine and [3H]nitrendipine, to brain or heart respectively. However, chronic administration of nifedipine for 20 days (36 and 360 micrograms/kg/hr) did produce down-regulation of both cardiac and neuronal Ca2+ channels and a similar down-regulation of beta-adrenoceptors. Co-regulation of Ca2+ channels and neurotransmitter receptors may occur but may not be an automatic consequence of either receptor or channel regulation.