Beta-adrenoceptor Density Research Articles

Chronic opiate-receptor inhibition in experimental congestive heart failure in dogs

Acute administration of opiate-receptor antagonists has previously been shown to improve cardiac output aortic blood pressure, systolic ventricular performance, and the baroreflex function in conscious dogs with right-sided congestive heart failure (RHF). However, whether similar changes occur after chronic opiate-receptor inhibition in congestive heart failure is not known. To determine the chronic effects of opiate-receptor antagonism on RHF, we administered naltrexone (200 mg/day), a long-acting, orally active opiate-receptor blocking agent, to RHF and sham-operated animals for 6 wk. Naltrexone had no effects on resting heart rate, right atrial pressure, aortic pressure, or cardiac output in RHF dogs but increased the first derivative of right and left ventricular pressure with respect to time (dP/dt) at rest and improved the dP/dt response to isoproterenol. The inotropic responses to isoproterenol and forskolin in isolated right ventricular trabeculate muscle also were improved by chronic naltrexone in RHF. Myocardial beta-receptor density was reduced in the failing right ventricle compared with the control (58 +/- 3 vs. 108 +/- 6 fmol/mg protein, P < 0.01) but was unaffected by addition of naltrexone. Finally, naltrexone prevented the decline in baroreflex sensitivity that occurred in RHF (-0.2 +/- 0.5 vs. -6.0 +/- 0.5 ms/mmHg, P < 0.01). These effects of naltrexone did not occur in the shamoperated animals. Chronic opiate-receptor blockade with naltrexone attenuates the development of reduced adrenergic inotropic responsiveness and baroreflex subsensitivity that occur in RHF. Because there was a similar improvement in the forskolin response in the absence of significant alterations in myocardial beta-adrenoceptor density after naltrexone treatment, the improvement in adrenergically mediated inotropic effects probably is mediated via a postreceptor mechanism.

Effect of short- and long-acting beta 2-adrenoceptor agonists on pulmonary beta 2-adrenoceptor expression in human lung.

beta-Adrenoceptor agonists induce the down-regulation of beta 2-adrenoceptors and mRNA expression in animal lung. The down-regulation of beta 2-adrenoceptors may limit the therapeutic efficacy of beta 2-adrenoceptor-mediated bronchodilators in obstructive airways disease. We examined the effects of three selective beta 2-adrenoceptor agonists, salbutamol, salmeterol and formoterol on beta 2-adrenoceptor binding and mRNA levels in human lung in vitro. Human lung was obtained from cardiac transplantation donors. Peripheral lung was chopped and incubated with three selective beta 2-adrenoceptor agonist for 3 h or 24 h at three different concentrations (0.1, 1 and 10 microM). The affinity and density of beta 2-adrenoceptors was determined by [125I]iodocyanopindolol equilibrium binding in a lung membrane preparation in the presence of 0.1 microM CGP 20712 A (1-{2-[(3-carbamoyl-4-hydroxy)phenoxy] ethylamino}-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl) phenoxy]-propan-2-ol), a selective beta 1-adrenoceptor antagonist. Although treatment with salbutamol for 3 h did not change beta 2-adrenoceptor density, both salmeterol and formoterol reduced beta 2-adrenoceptor density, and exposure to each agonist for 24 h reduced beta 2-adrenoceptor density at all concentrations. Treatment with 10 microM salmeterol increased the equilibrium dissociation constant (Kd), and also shifted the competition curves of (-)-isoprenaline to the left, beta 2-Adrenoceptor mRNA, measured by Northern blot analysis using a human beta 2-adrenoceptor cDNA probe, was reduced after exposure to all beta 2-adrenoceptor agonists at 3 h. Our data provide evidence for down-regulation of beta 2-adrenoceptor protein and mRNA after selective beta 2-adrenoceptor agonist treatment in human lung.

Relation between beta-adrenergic receptor density and lymphocyte proliferation associated with acute stress

The relations between acute changes in beta-adrenoreceptor density on lymphocytes and lymphocyte proliferation to a laboratory speech stressor were examined. Male participants either prepared and delivered a challenging speech task or read from a list of words. Participants engaged in the speech task, but not those involved in reading, exhibited significant increases in systolic blood pressure (an index of sympathetic nervous system arousal) and beta-adrenoreceptor density, accompanied by significant decreases in lymphocyte proliferation to pokeweed mitogen (PWM) but not concanavalin A (Con A). Increases in adrenoreceptor density significantly predicted decreases in lymphocyte activity in response to both mitogens. The increases in beta-adrenoreceptor density in the speech task condition occurred within 5 min after the baseline period, when participants were preparing for the speech task, but not yet speaking. These findings suggest that an acute stressor eliciting sympathetic nervous system activity can evoke rapid changes in beta-adrenoreceptor activity and decreases in mitogenic responses.

Elevated blood pressure and enhanced myocardial contractility in mice with severe IGF-1 deficiency.

To circumvent the embryonic lethality of a complete deficiency in insulin-like growth factor 1 (IGF-1), we generated mice homozygous for a site-specific insertional event that created a mutant IGF-1 allele (igf1m). These mice have IGF-1 levels 30% of wild type yet survive to adulthood, thereby allowing physiological analysis of the phenotype. Miniaturized catheterization technology revealed elevated conscious blood pressure in IGF-1(m/m) mice, and measurements of left ventricular contractility were increased. Adenylyl cyclase activity was enhanced in IGF-1(m/m) hearts, without an increase in beta-adrenergic receptor density, suggesting that crosstalk between IGF-1 and beta-adrenergic signaling pathways may mediate the increased contractility. The hypertrophic response of the left ventricular myocardium in response to aortic constriction, however, was preserved in IGF-1(m/m) mice. We conclude that chronic alterations in IGF-1 levels can selectively modulate blood pressure and left ventricular function, while not affecting adaptive myocardial hypertrophy in vivo.

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.

Myocardial sympathetic denervation prevents chamber-specific alteration of beta-adrenergic transmembrane signaling in rabbits with heart failure

The purpose of this study was to assess the effect of myocardial sympathetic denervation on the chamber-specific alteration of beta-adrenergic signaling in left ventricular failure in rabbits. Local abnormalities in sympathetic nerve terminals, including the neuronal reuptake of norepinephrine, are thought to be responsible for the chamber-specific regulation of beta-adrenergic signaling in heart failure. Sixteen rabbits were given 6-hydroxydopamine, 25 mg/kg body weight intravenously on days 1 and 2 and 50 mg/kg intravenously on days 7 and 8. Another 16 rabbits received vehicle. Aortic regurgitation was induced in eight of the 6-hydroxydopamine-treated and eight of the vehicle-treated rabbits on day 14. Another eight of the 6-hydroxydopamine-treated and eight of the vehicle-treated rabbits underwent a sham operation. The hearts were excised for biochemical analysis on day 21. Hemodynamic characteristics on day 21 showed left ventricular failure in both the aortic regurgitation groups. The plasma norepinephrine concentration on day 21 was higher in both the aortic regurgitation groups than in the sham groups. The beta-adrenoceptor densities and isoproterenol plus 5'-guanylylimidodiphosphate-, 5'-guanylylimidodiphosphate- and sodium fluoride-stimulated adenylyl cyclase activities were decreased only in the failing left ventricle of the vehicle-pretreated aortic regurgitation group, but in both ventricles of the 6-hydroxydopamine-pretreated aortic regurgitation group. The basal and forskolin-stimulated adenylyl cyclase activities were similar in both the aortic regurgitation groups and in the sham groups. Sympathetic denervation prevented chamber-specific alterations in beta-adrenergic signaling in acute left ventricular failure. Local loss of sympathetic nerve endings, and especially the defective neuronal norepinephrine reuptake, are likely to be responsible for the chamber-specific alteration of the beta-adrenoceptor-G protein-adenylyl cyclase system in heart failure in rabbits.

Autoradiographic mapping of beta-adrenoceptors in human skin.

A high density of beta 2-adrenoceptors has been found in human skin. Using autoradiographic mapping we investigated the distribution of beta 1- and beta 2-receptors in normal and diseased human skin. Cryostat sections of human skin obtained at biopsy were incubated with [125I]-iodocyanopindolol and nonspecific binding was identified by incubation of adjacent sections with 200 microM (-)-isoproterenol; beta 2-receptors were visualized using CGP 20712A and beta 1-receptors using ICI 118,551 as competing agents. The epidermis was densely labelled with an even distribution throughout all layers. Most of the beta-receptors were of the beta 2-subtype, with practically no beta 1-receptors. beta-Receptors were also localized to eccrine sweat glands, dermal blood vessels, and perivascular inflammatory cells, but there was no labelling of sebaceous glands. Topical glucocorticoids caused an increase in the density of epidermal beta-receptors. We conclude that keratinocytes and eccrine sweat glands express high densities of beta 2-receptors, suggesting that they may have a physiological role in the regulation of these cells.

Alterations of myocardial and vascular adrenergic receptor-mediated responses in Escherichia coli-induced septic shock in the rat

To investigate responsiveness to exogenous catecholamines in rat bacteremic shock by studying both myocardial and vascular functional parameters; to determine in the same study the relationship of these parameters with other relevant biological parameters of the adrenergic pathway, such as myocardial beta-adrenergic receptors and cyclic adenosine monophosphate (cAMP); and to indirectly approach the roles of tumor necrosis factor-alpha (TNF-alpha) and nitric oxide. Experimental, comparative study. Laboratory in a university hospital. Male Sprague-Dawley rats, weighing 270 to 320 g. Intravenous injection of live Escherichia coli DH5 alpha (2 x 10(10) organisms/kg) or saline (0.6 mL) and comparison of the two groups. Mean arterial pressure and heart rate (HR) were recorded, and circulating TNF-alpha concentrations were measured, during the first 3 hrs after E. coli administration. Myocardial and vascular functional parameters were obtained, respectively, from Langendorff-perfused hearts and isolated aortic rings. Adrenergic biochemical parameters (catecholamines, density and affinity of beta-receptors, and isoproterenol-stimulated myocardial cAMP) were determined 3 hrs after E. coli injection. Mean arterial pressure decreased within 5 to 60 mins after bacteria injection and returned to basal levels in the last 2 hrs; HR was unchanged. Serum TNF-alpha concentrations peaked at 120 mins (7333 +/- 672 pg/mL) and were still increased at 3 hrs. Plasma concentrations of epinephrine and norepinephrine were significantly (p < .05) increased. Baseline values for differential left ventricular pressure and coronary flow were significantly (p < .0001, p < .001, respectively) reduced; HR remained unchanged. Isoproterenol induced a similar increase in differential left ventricular pressure and in HR. There was no decrease in the functional myocardial response to adrenergic stimulation. beta-adrenergic receptors were similar in density and in affinity in the two groups. Isoproterenol-stimulated myocardial cAMP was significantly (p < .01) reduced compared with the control group. In aortic rings, bacteria administration significantly (p < .01) shifted the dose-response curve to norepinephrine to the right, both in the presence and absence of endothelium. NG-monomethyl-L-arginine significantly increased the contractions to attain the control level: p < .001 in presence of endothelium; p < .05 in absence of endothelium. In ex vivo experiments, 3 hrs after E. coli injection, vascular responsiveness was sharply decreased. This impaired response was improved by inhibition of nitric oxide. The heart, nevertheless, was still able to modulate its inotropic and chronotropic response to isoproterenol, even though an impaired beta-adrenergic-receptor stimulation of cAMP was already present.

Effects of short-term dietary change from high fat to high carbohydrate diets on the storage and utilization of glycogen and triacylglycerol in untrained rats.

The effects of short-term diet change from high fat (F) to high carbohydrate (C) (or vice versa) on the storage and utilization of glycogen and triacylglycerol (TG) in muscle and liver were studied in untrained rats. Rats were fed on an F or C diet for 28 days. For an additional 3 days, half of the rats in both F and C groups were fed the same diets as before (F-F and C-C) and the other half of the rats were switched to the counterpart diets (F-C and C-F). On the final day of the experiment, half of the rats in each diet group were exercised by swimming for 1.5 h and the other half were rested. Short-term diet change from F to C diets increased, but the change from C to F diets decreased, glycogen stores of soleus and plantaris muscles and liver, resulting in no difference in glycogen stores between F-C and C-C, and between F-F and C-F. The dietary change also had an affect on TG stores of red gastrocnemius muscle and liver-however, muscle TG stores were still higher in F-C than in C-C and C-F, and there were no differences in liver TG stores between F-C and C-F. Exercise decreased muscle glycogen contents markedly in F-C and C-C, whereas, it decreased muscle TG concentrations in F-F and C-F. Liver glycogen depletion was lower in F-C than in other groups. Lipolytic activities of epididymal adipose tissue at rest and postexercise were no differences between F-F and F-C, and were higher in F-C than in C-C and C-F. beta-adrenergic receptor binding was determined with [125I] iodocyanopindolol, and maximal numbers of beta-adrenergic receptor of plasma membrane from perirenal adipose tissue were approximately 170%-200% higher in F-C than in other groups at rest and postexercise. These results suggested that short-term C diet fed rats adapted to F diet enhanced not only glycogen stores of muscle and liver but also did not decrease lipolytic activity of adipose tissue with increased beta-adrenergic receptor density, resulting in the preservation of energy reserves (glycogen and TG) of muscle at rest, and liver glycogen sparing during exercise.

Beta-adrenoceptor desensitization during the development of canine pacing-induced heart failure.

1. The goal of this review is to emphasize four major points regarding the development of catecholamine desensitization in heart failure (HF). 2. Catecholamine desensitization occurs prior to the development of HF (i.e. after 1 day of rapid pacing, physiological responses to beta-adrenoceptor stimulation are depressed by over 50%, yet no evidence of HF is observed for 3-4 weeks of rapid pacing). 3. Multiple mechanisms in the beta-adrenoceptor cascade are involved. In HF there are decreases in beta 1-adrenoceptors, high affinity beta-adrenoceptors, adenylyl cyclase activity and messenger RNA and increases in Gi. 4. Not all mechanisms appear simultaneously (i.e. early decreases occur in high affinity beta-adrenoceptors and adenylyl cyclase; late increases in Gi and decreases in beta-adrenoceptor density evolves). 5. Mechanisms distal to cAMP generation also play a role (i.e. alterations in ryanodine receptor binding and excitation-contraction coupling also occur).

Circulating noradrenaline and beta-adrenergic receptors in children with congestive heart failure.

This study was designed to investigate changes in plasma catecholamine concentrations and the number of beta-adrenoceptors (beta-AR) of circulating lymphocyte in 94 noncyanotic congenital heart patients. In 43 patients with congestive heart failure, beta-AR density was significantly lower (p < 0.001) and plasma noradrenergic levels were significantly higher (p < 0.001) compared with corresponding values in 51 patients without heart failure. A significant negative correlation between lymphocyte beta-AR density and plasma noradrenergic levels was observed (r = -0.61, p < 0.001). The degree of left-to-right shunt and pulmonary pressure was correlated directly with noradrenaline level and inversely with lymphocyte beta-AR density. Both plasma noradrenaline level and lymphocyte beta-AR density return to normal in children with heart failure after surgical repair. Our results support the idea that changes in noradrenaline level and lymphocyte beta-adrenoceptor density occur concurrently with the presence and severity of heart failure in children.

Reduced beta adrenoceptor density in vivo in human lung tumours: a preliminary study with positron emission tomography.

Reduced beta adrenergic receptor density in tumours has been reported in previous in vitro studies. The aim of the present study was to assess whether this occurs in vivo. Pulmonary beta adrenoceptors were imaged and quantified in vivo using positron emission tomography (PET) and the beta antagonist radioligand (S)-[11C]CGP-12177 in five men with lung tumours of mean age 58 years (range 42-68). The histology of the tumours was squamous cell carcinoma in two cases, adenocarcinoma in one, carcinoid tumour in one, and large cell carcinoma in one. The regional blood volume and extravascular tissue density were also measured using PET. Regions of interest were drawn for both non-tumour and tumour lung tissue. The mean (SD) blood volume was 0.142 (0.025) ml/ml in tumour regions and 0.108 (0.010) ml/ml in normal lung regions--a difference of 31%. Mean (SD) extravascular tissue density was 0.653 (0.133) g/ml in tumour regions, substantially higher than in normal lung regions (0.157 (0.021) g/ml). On the contrary, beta receptor density was 5.1 (1.8) pmol/g in tumour regions, lower than the value of 9.9 (1.6) pmol/g found in adjacent normal lung--a difference of 48%. In vivo beta adrenoceptor density is reduced in human lung tumours.

The effects of the menstrual cycle, race, and gender on adrenergic receptors and agonists*

To examine possible effects of race, sex, and the menstrual cycle on adrenergic receptors (beta 2 and alpha 2) and agonists. Sixty-three normotensive black men and women and white men and women were studied twice, approximately 6 weeks apart. Women were studied once during the follicular phase and once during the luteal phase of the menstrual cycle. beta 2-Adrenergic receptors and adenylate cyclase activity were examined on lymphocytes, and alpha 2-adrenergic receptors were examined on platelets. Norepinephrine and epinephrine were determined in plasma. Women showed greater lymphocyte beta 2-receptor sensitivity (isoproterenol-stimulated cyclic adenosine monophosphate; p = 0.009). Women also showed greater postreceptor adenylate cycle activity independent of the beta-receptor (forskolin stimulation; p = 0.006). When these differences were controlled for, the gender-related differences in beta 2-receptor sensitivity were no longer evident. Black women had a reduced beta 2-receptor sensitivity in the luteal phase compared with the follicular phase, whereas white women showed no significant change (p = 0.018). Black subjects had lower lymphocyte beta 2-receptor density (Bmax) values than white subjects (p = 0.047). There were no significant effects on alpha 2-adrenergic receptors. The findings suggest that although there is no generalized effect of the menstrual cycle on adrenergic receptors in white women, such an effect may occur in black women. The findings also suggest that previously reported gender-related differences in beta 2-receptor sensitivity may be due to gender-related differences in postreceptor activity and not the beta 2-receptor per se.

Myocardial beta-adrenoceptor density and plasma catecholamines in syndrome X

Recent research has cast doubt on the ischemic hypothesis of etiology of syndrome X (anginal pain, ischemic-like changes in the stress electrocardiogram, but normal coronary arteriogram). Abnormalities of pain perception have been shown and abnormal sympathetic nervous system activation has also been implicated. The aim of this study was to test the hypothesis that downregulation of myocardial β adrenoceptors is demonstrable in patients with syndrome X. Such downregulation would be consistent with raised myocardial catecholamine concentrations. We performed positron emission tomography with 11C-CGP-12177 to measure β-adrenoceptor density. Plasma catecholamines were sampled simultaneously and assayed using high-performance liquid chromatography. Twenty syndrome X patients (11 female, age 57 ± 9 SD years, range 33 to 69) and 18 matched controls (9 women, age 50 ± 13 years, range 25 to 65; p = NS vs patients) were studied. Myocardial β-adrenoceptor density did not differ between syndrome X patients and controls: 8.0 (1.9) pmol/g for patients versus 8.3 (2.1) pmol/g for controls; p = 0.62. No differences were found between patients and controls for plasma norepinephrine (2.82 [1.07] and 2.76 [1.18] nM, respectively; p = 0.89) or for epinephrine (0.29 [0.14] and 0.30 [0.20] nM, respectively; p = 0.84). In patients with syndrome X, β-adrenoceptor density is normal and, by inference, myocardial catecholamines would also be normal. This weakens the case for a generalized enhancement of sympathetic activation in this disorder, although increased sympathetic reactivity during actual episodes of chest pain remains a possibility.

Age-related regulation of mouse brain cortex adrenoceptors following camphor vapor exposition

The paper deals with the ability of adrenergic receptors (AR) of mouse brain cortex to be differentially regulated in response to single or multiple expositions to camphor vapor. The regulation of alpha- and beta-adrenoceptors has been studied in young and old Balb/c-nu mice. Results confirm the decrease of total beta-adrenoceptor density previously observed in untreated mice with advancing age; in addition, receptor density decreases in both young and old mice after a single exposition to camphor vapor, followed by an adaptation after multiple stimuli. The beta1, subtype is mainly responsible for density decrease in young animals, while both beta1, and beta2 subtypes contribute to the decrease in old mice. On the contrary, beta2 subpopulation gives the major contribution to the adaptive recovery in both young and old mice. alpha-Adrenoceptors also show an age-related decrease in the control group; after a single exposition they show an increased density with the exception of alpha1-subset in the young group. Repeated expositions lead to a rather general adaptive response towards pre-stimuli conditions. The differential behaviour of receptor subtypes in response to camphor vapor exposition can be related to the differential alterations of receptor characteristics observed during aging and also suggests a possible mechanism through which these alterations may occur.

Sympathetic activity in children undergoing balloon valvuloplasty of pulmonary stenosis.

We studied the influence of balloon valvuloplasty on alpha- and beta-adrenoceptor densities, plasma catecholamine, and cAMP levels in children and infants with pulmonary stenosis before and 10 min after balloon dilatation, employing as controls children undergoing transcatheter occlusion of patent ductus arteriosus (PDA) with Qp/Qs ratio < 1.5. In the PDA group, the alpha-adrenoceptor density (Bmax) was 3.75 +/- 0.72 fmol/10(7) cells (n = 15) before occlusion and remained unchanged at 3.35 +/- 0.47 fmol 10 min thereafter. In the pulmonary stenosis patients (n = 31), the receptor density was 59% higher (p < 0.05) before, and decreased to PDA levels 10 min after, the procedure. The control beta-adrenoceptor density was 64.8 +/- 11.0 fmol/10(6) cells before, and 71.2 +/- 13.2 fmol 10 min after, occlusion. In the study group, the density was 23% lower (p < 0.07) and increased to the PDA levels 10 min after the dilatation. Compared with the PDA, pre- and postdilatation plasma norepinephrine levels were not significantly changed; epinephrine was slightly elevated before, but increased by 73% after, dilatation; dopamine was 80% (p < 0.05); and cAMP was 37% higher before, and remained elevated at 70 and 23% above the PDA values after, the procedure. Accordingly, alpha-adrenoceptor density is significantly elevated in children with pulmonary stenosis and decreases significantly immediately after balloon valvuloplasty. On the other hand, beta-adrenoceptor density is attenuated and increases toward normal levels after the procedure. The immediate reversal of the receptor levels after balloon valvuloplasty suggests that this procedure exerts acute effects on the sympathetic functional level in this disease.

Blphasic changes in the density of muscarinic and beta-adrenergic receptors in cardiac atria of rats treated with diisopropylfluorophosphate

Chronic treatment with organophosphate inhibitors of cholmesterases is known to bring about down-regulation of muscarinic acetylcholine receptors in the heart while its effect on the functionally antagonistic β-adrenergic receptors is not known. We describe experiments in which rats were exposed to daily injections of diisopropylflurophosphate (DFP) and the density of muscarinic and β-adrenergic binding sites in their cardiac atria was measured according to the binding of ( 3H)quinuclidinvl benzilate (( 3H)QNB) and (-)-4-(3-tert-butylamino-2-hydroxypropoxy)-(5,7- 3H) benzimidazol-2-one (( 3H)CGP 12177) as subtype non-specific muscarinic and β-adrenergic ligands, respectively. Biphasic course of changes was discovered with both ligands. With the dosage scheme applied, the density of betaadrenoceptors was augmented 24 h after the first dose of DFP and decreased to about one half of control values after 5 days of treatment with DFP. The density of muscarinic receptors was augmented after two days of treatment with DFP and decreased to about one half of control values after 5 days of treatment. Comparatively small changes in the heart rate were observed during the treatment; they reflected changes in the muscarinic and β-adrenergic receptor density. The finding of DFPinduced changes in the binding of ( 3H)CGP 12177 suggests that the mechanisms responsible for the control of the density of muscarinic and β-adrenergic receptors in the heart cells are interconnected but potential roles of other factors involved in in vivo experiments deserve further analysis. The transient increase in the density of muscarinic receptors after two days of DFP treatment appears related to published data on transient stimulation by cholinergic agonists of the transcription of mRNAs for muscarinic receptors.

Pathogenesis of dilated cardiomyopathy and heart failure: insights from cell morphology and biology.

The cause of dilated cardiomyopathy is not yet clear but the recent discovery of a chromosomal aberration as well as the presence of autoantibodies indicate a multicausal origin. Knowledge of pathogenetic mechanisms continues to evolve and includes decreased sarcoplasmic reticulum Ca2+ uptake, reduced beta-receptor density and decreased contractility, the presence of enlarged myocytes showing numerous degenerative alterations, and fibrosis. Defects in titin, a large sarcomeric protein, may be responsible for disturbances of sarcomerogenesis in dilated cardiomyopathy. New animal models are promising but significant progress will also be made using cell culture systems like adult cardiac myocytes.

Chronic treatment with carbachol sensitizes the myocardium to cAMP-induced arrhythmia.

The present study investigated biochemical and functional consequences of chronic activation of the inhibitory Gi alpha-coupled adenylyl cyclase pathway in the heart. Rats (220 to 260 g) were treated with 4-day infusions of the M-cholinoceptor agonist carbachol (9.6 mg/kg per day) or vehicle. An additional group that received the beta-adrenoceptor agonist isoprenaline (2.4 mg/kg per day) served as control. The main finding was that chronic infusion of carbachol led to a marked increase in isoprenaline- or forskolin-induced arrhythmia in electrically driven papillary muscles (in vitro). Compared with control, the potency of isoprenaline and forskolin to induce arrhythmia in cardiac preparations from carbachol-treated rats was increased 36- and 2.2-fold and the efficacy was increased 7.3- and 2.3-fold, respectively. The potency of carbachol to antagonize the isoprenaline- and forskolin-induced arrhythmia was decreased 30-fold. These changes were accompanied by a decrease in left ventricular M-cholinoceptor density by 15% (P < .05) and a decrease in pertussis toxin-sensitive G proteins (Gi alpha) by 26% (P < .05) without a decrease in the corresponding mRNAs. beta-Adrenoceptor density and basal and stimulated adenylyl cyclase activity remained unchanged. In contrast, isoprenaline infusion induced a decrease in arrhythmogenic potency of forskolin (P = NS), which was accompanied by a decrease in beta-adrenoceptor density, an increase in Gi alpha protein and mRNA levels, and a decrease in basal and stimulated adenylyl cyclase activity. Chronic parasympathetic activation sensitizes the myocardium to cAMP-induced arrhythmia. These changes may be due to quantitative alterations in functional Gi alpha.

Cardiac sympathetic neuronal function in left ventricular volume and pressure overload

In heart failure cardiac sympathetic neuronal function and activity appear to be altered. Although these changes are widely accepted, controversy exists concerning the neurohormonal changes occurring in pressure and volume overloaded hearts. The present study in rabbits was performed to assess the effects of mechanical overload on cardiac sympathetic neuronal function and beta-adrenoceptor density, in relation to left ventricular function. In nine rabbits the aortic valve was perforated to induce left ventricular volume overload. Pressure overload was induced by suprarenal banding of the aorta abdominalis (group 1). Five animals were sham operated (group 2). Subanalysis of group 1 was performed for non-failing (n = 5) and failing (n = 4) hearts. Heart failure was defined as any reduction in left ventricular fractional shortening 2 weeks after the second operation compared to baseline. In animals with cardiac overload, left ventricular weight was higher compared with the control animals, 7.99 +/- 1.13 vs. 6.16 +/- 0.86 g (P < 0.02). Left ventricular end diastolic diameter increased from 1.35 +/- 0.16 to 1.57 +/- 0.15 cm (P < 0.005) after surgically induced overload. Left ventricular end systolic diameter and fractional shortening did not change significantly. Myocardial noradrenaline (NA) concentration and beta-adrenoceptor density were significantly lower in group 1 than in group 2, 1005 +/- 393 vs. 1643 +/- 109 ng/g (P < 0.02) and 167 +/- 36 vs. 224 +/- 36 fmol/mg protein (P < 0.03), respectively. Myocardial [123I]-MIBG uptake did not significantly differ between group 1 and 2, 2.1 +/- 0.58 vs. 1.8 +/- 0.44 (%ID/g x kg). A significant positive correlation between myocardial NA concentration and beta-adrenoceptor density was found (r = 0.66, P < 0.02). Myocardial NA concentration was inversely related to left ventricular weight (r =-0.75, P < 0.003). The present data indicate that in a condition of cardiac volume and pressure overload, sympathetic activity is enhanced as shown by myocardial noradrenaline depletion and beta-adrenoceptor downregulation. In contrast, no cardiac neuronal dysfunction is observed, even in the stage of early heart failure.