Β-adrenergic Receptor Density Research Articles

The Effect of Acute, Chronic and Chronic Intermittent Stress on the Central Noradrenergic System

HELLRIEGEL E. T. AND A. P. D'MELLO. The effect of acute, chronic, and chronic intermittent stress on the central noradrenergic system. PHARMACOL BIOCHEM BEHAV 57(1/2) 207–214. 1997.—The objective of this investigation was to examine the immediate and long term effects of acute, chronic and chronic intermittent stress on the central noradrenergic system of rats. Male Sprague–Dawley rats were subjected to one hour of physical immobilization stress either as a single exposure, or as 14 exposures applied either on consecutive days, or randomly over 60 days. Animals were sacrificed immediately, 6 h and 24 h following the last stressor. Levels of norepinephrine (NE) and 3-methoxy-4-hydroxyphenylethyleneglycol sulfate (MHPG-sulfate) were measured in the hypothalamus, hippocampus, cerebral cortex and locus coeruleus region and β-adrenergic receptor (BAR) density was determined in the cortex. Immediately after acute stress, a significant reduction in hypothalamic NE levels and marked increases in MHPG-sulfate levels in all four brain regions were observed. In contrast, immediately after the last stressor of a chronic or chronic intermittent stress regimen, no change in NE concentration was observed while levels of MHPG-sulfate in the four brain regions showed a smaller increase than that observed after an acute stressor. Acute stress induced changes normalized within 6 h while chronic and chronic intermittently stressed animals had altered NE or MHPG-sulfate levels in certain brain regions for up to 6–24 h. Cortical BAR binding parameters remained unchanged after all stress paradigms.

Early locus coeruleus lesions increase the density of β-adrenergic receptors in the main olfactory bulb of rats

Norepinephrine is supplied to both deep and superficial layers of the olfactory bulb through dense projections from the locus coeruleus.16 Beta-adrenergic receptors are located in nearly all bulb laminae, with high-density foci of β-1 and β-2-adrenoceptors present in the glomerular layer.29 Early olfactory experiences that increase norepinephrine levels in the bulb also decrease the density of β-1- and β-2-adrenoceptors, as well as the number of high-density glomerular foci of β-2-receptors.30 Changes in bulb norepinephrine levels, therefore, may affect the density of β-adrenoceptors in the bulb. In the current study, we test this hypothesis by performing unilateral lesions of the locus coeruleus with 6-hydroxydopamine on postnatal day 4, and examining the density of β-1- and β-2-adrenergic receptors in the main olfactory bulb of the rat using125I-labeled iodopindolol receptor autoradiography on postnatal day 19. Locus coeruleus destruction resulted in a statistically significant increase in the density of adrenergic receptors in the ipsilateral bulb compared to the contralateral bulb. Both β-1- and β-2-adrenoceptor subtypes increased in density with this manipulation, although the number of glomerular layer high-density β-2 foci was not significantly different between the two bulbs. These results are consistent with the hypothesis that changes in olfactory bulb norepinephrine can regulate the density of β-adrenergic receptors in the bulb.

Hypoxia- and normoxia-induced reversibility of autonomic control in Andean guinea pig heart.

We herein describe the regulation of cardiac receptors in a typical high-altitude native animal. Heart rate response to isoproterenol (HRIso) (beats.min-1.mg Iso.kg-1) and atropine, the density of beta-adrenergic (beta AR) and muscarinic (M2) receptors, and the ventricular content of norepinephrine (NE) and dopamine (DA) were studied in guinea pigs (Cavia porcellus). Animals native to Lima, Peru (150 m) were studied at sea level (SL) and after 5 wk at 4,300-m altitude (SL-HA). Animals native to Rancas [Pasco, Peru (4,300 m)] were studied at high altitude (HA) and after 5 wk at SL (HA-SL). HA animals had a lower HRIso, maximum number of beta AR binding sites (Bmax), beta AR dissociation constant (Kd), NE, and DA (P < 0.05) and a higher M2 Bmax (P < 0.001) when compared with the SL group. HA-SL showed an increase of the HRIso, beta Ar Kd, and NE (P < 0.05) and a decrease of the M2 Bmax and Kd (P < 0.0001) when compared with the HA group. The present study demonstrates the differential regulation and reversibility of the autonomic control in the guinea pig heart.

Activation of beta-adrenergic receptor kinase during myocardial ischemia.

During myocardial ischemia, a local release of noradrenaline coincides with an increased density of beta-adrenergic receptors. The functional activity of these receptors, however, is mainly determined by their state of phosphorylation. The beta-adrenergic receptor kinase (beta ARK) specifically phosphorylates and thereby inactivates beta-adrenergic receptors after stimulation by receptor agonists, facilitating the binding of the inhibitor protein beta-arrestin to the receptors. beta ARK activation involves a translocation of the enzyme to the membrane. In the present study, we investigated the density and the functional activity of beta-adrenergic receptors, the enzymatic activity of beta ARK in membranes and cytosol, the mRNA levels of beta ARK-1, and the expression of beta-arrestin during stop-flow and low-flow ischemia in the isolated perfused rat heart. After 60 minutes of stop-flow ischemia, beta-adrenergic receptor density was upregulated, but beta-agonist-mediated adenylate cyclase activity was blunted. Simultaneously, beta ARK activity in the particulate fraction was significantly induced. The increase in beta ARK activity was reversible after inhibition of ischemia-evoked noradrenaline release by desipramine. Also, exposure to externally given noradrenaline increased beta ARK activity in the particulate fraction. Cytosolic beta ARK activity remained largely unchanged during stop-flow or low-flow ischemia. The steady state concentration of beta ARK-1 mRNA increased after 20 minutes of stop-flow ischemia and then returned to baseline values after another 20 minutes. Cardiac ischemia did not alter beta-arrestin levels. During myocardial ischemia, an increase in the number of beta-adrenergic receptors is paralleled by increased membrane activity of the receptor kinase beta ARK. This increased membrane activity may contribute to enhanced receptor phosphorylation and inactivation.

Adenylyl cyclase system is affected differently by endurance physical training in heart and adipose tissue

Adaptive changes in the β-adrenergic adenylyl cyclase (AC) system in response to endurance training were studied in heart and adipose tissue. Training was performed by making male Wistar rats run on a motor-driven treadmill. The changes following exercise training were opposite in the two tissues studied. The density of β-adrenergic receptors in left ventricular membranes of trained rats showed a marked decrease. Comparison of AC activities in cardiac membranes prepared from trained and sedentary rats revealed a depressing effect of endurance training on: 1. the β-adrenergic stimulatory pathway and the inhibition of AC via receptor; 2. the G s component and the G s-adenylyl cyclase coupling, as shown by the response of adenylyl cyclase to GppNHp and NaF; and 3. the enzyme catalytic activity in the presence of Mn 2+ or forskolin. The levels of G sα subunits in the left ventricle, as measured in terms of ADP-ribosylated and immunologically reactive proteins, were decreased by endurance exercise, whereas immunodetectable levels of G iα2 increased in the membranes of trained myocardium. In contrast to the diminished sensitivity that characterizes the behavior of the cardiac β-adrenergic-AC system, endurance physical training increased sensitivity of this signal transduction system in adipose tissue. Thus, the density of β-ARs as well as AC activity and the β-adrenergic stimulatory pathway were increased in adipose membranes of trained rats compared with the corresponding sedentary controls. In addition, the levels of G sα subunits were higher in the adipose plasma membranes of trained rats. However, immunodetectable levels of G i1α and G i3α increased with training, whereas the amount of G i2α decreased in membranes of trained rats. In conclusion, the present study shows that chronic exercise is associated with a tissue-specific adaptation of the β-adrenergic AC system.

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.

Effect of idebenone on in vivo serotonin release and serotonergic receptors in young and aged rats

The effect of idebenone on the serotonergic system was evaluated in the aging rat by measuring the kinetic constants of 3H-5HT and 3H-ketanserin binding sites in the cerebral cortex of rats at 3, 15 and 24 months of age following acute and subchronic administration of the drug. Idebenone displayed no in vitro affinity toward any population of serotonin receptors and did not modify their kinetic parameters after a single dose of 100 mg/kg at any age tested. A subchronic treatment with the drug for 21 days at the dose of 30 mg/kg did not induce any relevant change in 3- and 15-month-old rats, whereas it significantly increased the density of both 3H-5HT and 3H-ketanserin binding sites in 24-month-old rats, where a lower number of receptors is detected as a consequence of aging. This effect was rather specific, since under the same experimental conditions no changes were detected in the density of cortical β-adrenergic receptors in aged animals. In microdialysis studies, acute administration with idebenone did not affect 5HT and 5HIAA release at any age. Conversely, the pattern of serotonin metabolism was significantly modified in aged rats following repeated treatment with idebenone and was partially restored to a value similar to the one observed in young animals. These results suggest that idebenone, a putative neuroprotective agent which has been shown to improve brain metabolism in ischemic conditions, might also attenuate age-associated neuronal damage, acting probably on several neurotransmitter systems which undergo selective modification during aging.

Impairment of the Brain β-Adrenergic System during Experimental Endotoxemia

Brain dysfunction is observed clinically in patients suffering from prolonged endotoxic shock. However, the etiology of brain dysfunction during sepsis is not clear. Certain researchers have reported that the decrease in brain catecholamines concentration during septic shock might be etiologically important in brain dysfunction. Therefore, we hypothesized that the β-adrenergic receptor system undergoes a change during septic shock, and plays a role in the pathogenesis of septic encephalopathy. In this study, we examined two models of septic shock in rats, each of which has a different time course for the shock state. Male Wistar rats were divided into four groups: (1) Control—0.9% saline vehicle, (2) Lipopolysaccharide (LPS) i.v.—Escherichia coliendotoxin 1.0 mg/ml i.v. bolus, (3) Sham-operated, and (4) Cecal ligation and puncture (CLP) model. The rats were killed by decapitation at 3, 12, or 24 hr after the treatments, and the brains were removed and subdivided into three areas: the forebrain, cerebellum, and brain stem. In the LPS i.v. group, the brain tissue norepinephrine (NE) concentration had decreased in the forebrain and brain stem and the tissue epinephrine (E) concentration had decreased in the brain stem by 3 hr after treatment. In the CLP group, the brain tissue NE concentration had decreased in the forebrain, cerebellum, and brain stem (P< 0.05), and the tissue E concentration had decreased in the forebrain and brain stem by 24 hr after treatment (P< 0.05). An alteration in β-adrenergic receptor density in the forebrain was observed at 24 hr in the CLP group (control, 237.0 ± 14.0 fmole/mg protein; LPS i.v., 233.2 ± 3.0 fmole/mg protein; sham-operated, 236.0 ± 3.0 fmole/mg protein; CLP, 177.0 ± 4.2 fmole/mg protein). These alterations in transmitter concentrations and β-adrenergic density in the forebrain may be an important factor in septic encephalopathy.

Reduction in lymphoctye β-adreneraic receptor density in infants and children with heart failure secondary to congenital heart disease

To identify changes in catecholamine levels and β-adrenergic receptor density in children with varying degrees of congestive heart failure, we measured plasma norepinephrine (NE), epinephrine, and β-adrenerSic receptor levels in 91 noncyanotic patients using igh-performance liquid chromatography and a radioligand binding assay. Plasma NE levels in 41 patients with heart failure (694 ± 236 pg/ml) were significantly higher than those in 50 patients without it (274 ± 68 pg/ml, p < 0.001). In addition, β-adrenergic receptor density was significantly lower in patients with heart failure (0.81 ± 0.48 fmol/10 6 cells) than in those without it (2.43 ± 1.09 fmol/10 6 cells, p < 0.001), but epinephrine levels were not significantly different between the 2 groups. The receptor reduction in heart failure correlated well with elevated plasma NE levels (r = −0.60, p < 0.001). The degree of left to right shunt flow and pulmonary systolic pressure correlated directly with plasma NE levels and inversely with β-adrenergic receptor density. From the best compromise between sensitivity and specificity, the optimal cutoff point for heart failure was >390 ng/ml for NE and <1.30 fmol/10 6 cells for β-adrenergic receptor density, respectively. A follow-up study in 15 of 30 patients with heart failure after surgery showed a significant decrease in plasma NE and an increase in β-adrenergic receptor density. Changes in plasma NE levels and β-adrenergic receptor density occur concurrently with clinical symptoms of heart failure and may be used as indexes for assessing the presence and severity of heart failure in infants and children.

Role of beta-adrenergic receptor downregulation in the peak exercise response in patients with heart failure due to idiopathic dilated cardiomyopathy

The effect of β-adrenergic receptor downregulation on peak exercise response in patients with heart failure has not been directly investigated. Seventy-two patients with idiopathic dilated cardiomyopathy who had a mean ejection fraction of 23 ± 1% (mean ±SEM) and New York Heart Association class II or III symptoms were investigated. Subjects underwent maximal exercise testing on a bicycle or a treadmill, hemodynamic assessment by right heart catheterization, and measurement of total β-adrenergic receptor density by 125Iiodocyanopindolol binding performed in the right ventricular endomyocardial biopsy tissue and in peripheral lymphocytes. Endomyocardial biopsy β-adrenergic receptor density (Bmax) was markedly decreased (45 ±2 fmol/mg), and significantly lower than lymphocytes Bmax (107 ± 14 fmol/mg; p < 0.05). By univariate analysis, all exercise variables correlated significantly with biopsy tissue Bmax but not with lymphocyte Bmax. Maximal exercise oxygen consumption (VO 2max) yielded the highest correlation with Bmax (r = 0.61, p < 0.001). By stepwise regression analysis, VO 2 max, Δ heart rate × systolic blood pressure, and ejection fraction were all independently related to Bmax. Myocardial β-adrenergic receptor downregulation is likely to be partially responsible for the reduced chronotropic and inotropic responses to peak exercise in patients with mild to moderate symptomatic heart failure due to idiopathic dilated cardiomyopathy.

Long-term treatment with angiotensin I-converting enzyme inhibitors attenuates the loss of cardiac beta-adrenoceptor responses in rats with chronic heart failure.

Cardiac contractile force in response to beta-adrenoceptor agonists and beta-adrenergic receptor density are decreased in failing human hearts. The effects of angiotensin I-converting enzyme (ACE) inhibitor on cardiac responsiveness to beta-adrenergic stimulation in failing hearts are not established. The present study was undertaken to determine whether ACE inhibitor may improve cardiac beta-adrenergic responsiveness in animals with chronic heart failure (CHF). CHF was induced by left coronary artery ligation in rats. Cardiac output and stroke volume indices decreased 12 weeks after the operation. In sham-operated rats, dobutamine and isoprenaline increased cardiac output and stroke volume indices. In contrast, cardiac output and stroke volume responses to dobutamine and isoprenaline were severely blunted in the CHF rat. Cardiac beta 1-adrenergic receptor density was decreased while its dissociation constant (Kd) was not altered in the viable tissue of the left ventricle of the CHF rat, which is consistent with beta-adrenergic receptor downregulation. Cardiac norepinephrine content decreased in the CHF rats. Rats were treated orally with ACE inhibitors, 3 mg/kg trandolapril or 10 mg/kg enalapril once daily, or 5 mg/kg captopril twice daily from the 2nd to the 12th weeks after the operation. Treatment with ACE inhibitors attenuated the reduction in cardiac output and stroke volume indices and improved the inotropic response to dobutamine and isoprenaline and reversed partially the cardiac norepinephrine content in the CHF rat. ACE inhibitor treatment also attenuated the reduction in beta 1-adrenergic receptor density in the viable tissue of the left ventricle of the CHF rat. The results suggest that ACE inhibitor treatment attenuates the blunting of cardiac responses to beta-adrenergic agonists in the CHF rat and that one of the mechanisms underlying this effect is prevention of cardiac beta 1-adrenergic receptor downregulation.

Myocardial β-Adrenergic Receptor Function and High-Energy Phosphates in Brain Death– Related Cardiac Dysfunction

Cardiac failure remains an important problem after heart transplantation and may be associated with events that occur during brain death (BD) before transplantation. In this study, cardiac function is studied after BD, and biochemical evaluation of myocardial high-energy phosphates and the beta-adrenergic receptor system is presented. The hearts of 17 mongrel dogs (23 to 31 kg) were instrumented with flow probes, micromanometers, and ultrasonic dimension transducers to measure ventricular pressure and volume relationships. In a validated canine BD model, systolic right and left ventricular (RV/LV) function was analyzed by load-insensitive measurements during caval occlusion (preload-recruitable stroke work, PRSW). The beta-adrenergic receptor (BAR) density, adenylate cyclase (AC) activity, and myocardial ATP and creatine phosphate (CP) were measured before and 6 to 7 hours after BD. Results are expressed as mean +/- SEM (*P < .05 versus baseline, paired two-tailed Student's t test). Myocardial function deteriorated significantly from baseline PRSW (RV, 22 +/- 1 erg x 10(3); LV, 75 +/- 4 erg x 10(3)) by 37 +/- 10% for the RV (P < .001) and 22 +/- 7% for the LV (P < .001). BAR density increased from 282 +/- 42 to 568 +/- 173 fmol/mg for the RV and from 291 +/- 64 to 353 +/- 56 fmol/mg for the LV. Isoproterenol-stimulated AC activity was also significantly enhanced after BD. ATP and CP, however, remained unchanged after BD compared with baseline values before BD. BD causes significant systolic biventricular dysfunction. The loss of ventricular function after BD was more prominent in the right ventricle and may contribute to early postoperative RV failure in the recipient. These injuries occurred despite BAR system upregulation after BD. Global myocardial ischemia is unlikely, since ATP and CP remained normal before and after BD.

Relative Densities of Muscarinic Cholinergic and β-Adrenergic Receptors in the Canine Sinoatrial Node and Their Relation to Sites of Pacemaker Activity

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

β-Adrenergic responsiveness of adenylate cyclase in human adipocyte plasma membranes in obesity and after massive weight reduction

The aim of this study was to find out how β-adrenergic responsiveness of adipocytes is altered in obesity and by weight loss and to investigate what mechanisms lead to potential alterations in responsiveness. Crude plasma membranes were prepared from adipocytes of massively obese and normal-weight individuals, as well as previously obese patients that had lost an average of 38% of their initial weight after bariatric surgery. Stimulation of adenylate cyclase by isoproterenol varied considerably in fat cell plasma membranes from different individuals. Crude fat cell plasma membranes from obese patients were less responsive to isoproterenol than those from normal-weight subjects, whereas those from postgastroplasty patients were hyperresponsive. The response was correlated negatively with cell size and positively with β-adrenergic receptor density and with the ratio of β-receptors and stimulatory G-proteins (Gs). There was no correlation with Gs content. However, differences in receptor density between small and large cells or normal-weight, obese, and post-bypass patients could not explain the observed differences in responsiveness to isoproterenol between the different groups.

Additive effects of growth hormone and testosterone on lipolysis in adipocytes of hypophysectomized rats.

The effects of growth hormone (GH) and testosterone, alone or in combination, on the regulation of lipolysis in isolated adipocytes from hypophysectomized rats were investigated. Male Sprague-Dawley rats were hypophysectomized at 50 days of age. One week after operation, hormonal replacement therapy with L-thyroxine and hydrocortisone acetate was given to hypophysectomized rats. Groups of rats were treated with GH (1.33 mg/kg, daily), testosterone (10 mg/kg, once) alone or in combination. After one week of hormonal treatment, adipocytes were isolated from the pooled epididymal and perirenal fat pads and glycerol release after isoproterenol stimulation and 125I-cyanopindolol binding was measured. Hypophysectomy caused a marked decrease in basal and isoproterenol-stimulated lipolysis. There was no effect of testosterone treatment alone on lipolysis, but GH treatment resulted in an increase in isoproterenol-induced lipolysis but not to the levels observed in cells from control rats. Testosterone and GH in combination restored the lipolytic response to isoproterenol. Also 125I-cyanopindolol binding was decreased after hypophysectomy. Testosterone treatment alone and GH treatment alone increased the binding, while in combination the treatment had an additive effect. Affinity was not changed, but the effects seemed to be on receptor number, as determined by Scatchard analysis. Forskolin-stimulated cAMP accumulation in adipocytes was markedly reduced after hypophysectomy. Testosterone treatment alone had no effect. GH treatment alone increased forskolin-stimulated cAMP accumulation, although the level was lower than that found in control rats. The combined treatment resulted in a further increase to levels observed in adipocytes from control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin-converting enzyme inhibition and the progression of congestive cardiomyopathy. Effects on left ventricular and myocyte structure and function.

Clinical trials have demonstrated that angiotensin-converting enzyme inhibition (ACEI) improves survival in patients with long-term left ventricular (LV) dysfunction. However, it remained unclear from these clinical reports whether the beneficial effects of ACEI were due to direct improvements in LV myocardial structure and function. Accordingly, the overall objective of the present study was to examine the direct effects of ACEI on both LV and myocyte structure and function in the setting of cardiomyopathic disease. LV and isolated myocyte function and structure were examined in control dogs (n = 6), in dogs after the development of dilated cardiomyopathy caused by rapid ventricular pacing (RVP, 216 beats per minute, 4 weeks, n = 6), and in dogs with RVP and concomitant ACEI (RVP/ACEI, fosinopril 30 mg/kg BID, n = 6). LV ejection fraction fell with RVP compared with control values (35 +/- 3 versus 73 +/- 2%, P < .05) and was higher with RVP/ACEI compared with RVP values (41 +/- 4%, P = .048). LV end-diastolic volume increased with RVP compared with control values (78 +/- 7 versus 101 +/- 7 cm3, P < .05) and was lower with RVP/ACEI (82 +/- 3 cm3, P < .05). Isolated myocyte length increased with RVP (182 +- 1 versus 149 +/- 1 micron), and the velocity of shortening decreased (36 +/- 1 versus 57 +/- 1 micron/s) compared with control values (P < .05). With RVP/ACEI, myocyte length was reduced (169 +/- 1 micron) and velocity of shortening was increased (45 +/- 1 micron/s) compared with RVP values (P < .05). Myocyte velocity of shortening after beta-adrenergic receptor stimulation with 25 nmol/L isoproterenol was reduced with RVP compared with control values (142 +/- 5 versus 193 +/- 8 micron/s, P < .05) and significantly improved with RVP/ACEI (166 +/- 6 micron/s, P < .05). In the RVP group, beta-adrenergic receptor density fell 26%, and cAMP production with beta-adrenergic receptor stimulation was reduced 48% from control values. RVP/ACEI resulted in a normalization of beta-adrenergic receptor density and cAMP production. LV myosin heavy-chain content when normalized to dry weight of myocardium was unchanged with RVP (149 +/- 11 mg per gram dry weight of myocardium [gdwt]) and RVP/ACEI (150 +/- 4 mg/gdwt) compared with control values (165 +/- 4 mg/gdwt). LV collagen content decreased with RVP compared with control values (7.6 +/- 0.4 versus 9.6 +/- 0.8 mg per gram wet weight of myocardium [gwwt], P < .05) but was increased with RVP/ACEI (14.4 +/- 1.3 mg/gwwt, P < .05). Concomitant ACEI with chronic tachycardia reduced LV chamber dilation and improved myocyte contractile function and beta-adrenergic responsiveness. Contributory cellular and extracellular mechanisms for the beneficial effects of ACEI in this model of dilated cardiomyopathy included a normalization of beta-adrenergic receptor function and enhanced myocardial collagen support. The results from this study provide evidence that ACEI during the development of cardiomyopathic disease provided beneficial effects on LV myocyte contractile processes and myocardial structure.

Cardiac plasma membrane physical properties and β-adrenergic receptor function are unaltered in portal-hypertensive rats

We have previously demonstrated that impairment of cardiac contractility in a rat model of cirrhosis may be secondary to altered cardiac plasma membrane physical properties affecting β-adrenergic receptor function. It is unclear whether this is caused by the cirrhosis or by the portal hypertension in this model, so we studied cardiomyocyte plasma membrane physical properties, lipid composition, and β-adrenergic receptor function in a rat model of prehepatic portal hypertension without cirrhosis. Portal hypertension was induced by graded portal vein stenosis, whereas controls had a sham operation. Cardiac sarcolemmal plasma membrane physical properties were studied by fluorescent polarization methods using diphenylhexatriene and a series of anthroyloxy-stearic acids to estimate, respectively, the static and dynamic components of membrane fluidity. β-Adrenergic receptor density and binding affinity were measured using 3H-dihydroalprenolol as a radioligand. Isoproterenol-stimulated adenylyl cyclase activity was measured by radioimmunoassay. Neither the static nor the dynamic components of membrane fluidity were significantly different in portal-hypertensive rats compared with the controls. The membrane cholesterol, phospholipid content, and the cholesterol/phospholipid ratio did not differ between the two groups. β-Adrenergic receptor density and binding affinity in the portal-hypertensive rats (respectively, 1.65 ± 0.06 pmol/mg protein and 6.78 ± 0.80 nmol/L) were not different from controls (1.76 ± 0.17 pmol/mg protein and 7.25 ± 1.48 nmol/L). Isoproterenol-stimulated adenylyl cyclase activity in membranes from portal-hypertensive rats did not significantly differ from controls. We conclude that cardiac plasma membrane physical properties and β-adrenoceptor function in portal hypertensive rats remains unimpaired, suggesting that the cardiomyopathy of cirrhosis is not caused by portal hypertension per se.

β-Adrenergic receptor changes in learned helplessness may depend on stress and test parameters

Behavioral deficits following inescapable stress (learned helplessness) may serve as an animal model of depression. Previous studies using foot-shock stress to induce learned helplessness and a bar-press test for the stress-induced behavioral deficit have found increased β-adrenergic receptor density in the hippocampus of learned helpless rats. We replicated these experiments using a tail-shock stress and the shuttle-box test. In our experiments, rats that developed learned helplessness after inescapable stress did not demonstrate any significant differences in β-adrenergic receptor density or affinity in the frontal cortex, cerebellum, or hippocampus compared to the nonhelpless rats, nor to the tested control rats. These results suggest that β-adrenergic receptor changes in learned helplessness may depend on the specific stress and test procedures used.

Characteristics of the β-Adrenergic Receptor Complex in the Epicardial Border Zone of the 5-Day Infarcted Canine Heart

The effect of isoproterenol on increasing the peak amplitude of the L-type calcium current is reduced in myocytes dispersed from the epicardial border zone (EBZ) of the 5-day infarcted canine heart when compared with control cells from noninfarcted hearts. This suggests that specific alterations in the beta-adrenergic receptor complex develop in this setting. The present study is an examination of individual components of the beta-adrenergic receptor complex with the aim of elucidating the biochemical defect(s) that might be responsible for the diminished beta-adrenergic receptor responsiveness in the myocytes that survive in the infarcted heart. We compared components of the beta-adrenergic receptor signaling pathway in membranes prepared from the EBZ of the 5-day infarcted heart and a remote, noninfarcted region (RZ) of the same ventricle as well as the corresponding regions of noninfarcted ventricles. Defects in multiple components of the beta-adrenergic receptor complex were confined to the EBZ of the 5-day infarcted heart. These include a decrease in beta-adrenergic receptor density; diminished basal, guanine nucleotide-, isoproterenol-, forskolin-, and manganese-dependent adenylyl cyclase activities; an increase in the EC50 for isoproterenol-dependent activation of adenylyl cyclase; a diminished level of the alpha-subunit of the Gs protein. and an elevated level of the alpha-subunit of the Gi protein. Defects in multiple components of the membrane beta-adrenergic receptor complex were identified in the EBZ of the 5-day infarcted canine heart. This constellation of abnormalities would be predicted to impair functional beta-adrenergic responsiveness and contribute to the defect in isoproterenol-dependent stimulation of the L-type calcium current in myocytes isolated from this tissue.

Downregulation of Thyroid Hormone Receptor Subtype mRNA Levels by Amiodarone during Catecholamine Stress in Vitro

Amiodarone, a powerful antiarrhythmic drug, likely exerts its major effect by antagonism of thyroid hormone (T3), probably at the receptor level. T3 is known to regulate β-adrenergic receptor density in the heart but the effects of sympathomimetic drugs on thyroid hormone receptors (T3R) is not known. The aim of this study was to investigate how amiodarone and isoproterenol affect T3R-mRNA in cultured cardiomyocytes. Confluent, isoproterenol pretreated, AT-1 cardiomyocytes were treated with isoproterenol free medium, amiodarone, T3 and amiodarone together with T3 for 48 hours. Solution hybridization for the determination of mRNA for T3R α1, α2, β1 and β2 were performed. In itself isoproterenol upregulated T3R α1, T3R β1, T3R β2 (p<0.05), but did not affect the levels of T3R α2. Amiodarone and T3, respectively, downregulated T3R α1, T3R β1, T3R β2 (p<0.05), but did not affect the levels of T3R α2. Amiodarone and and T3, added together, upregulated T3R α2 and T3R β1 (p<0.05) as compared to amiodarone or T3 alone. There was an antagonistic effect between amiodarone and T3 for the regulation T3R β1. This is the first evidence showing that amiodarone regulates T3R-mRNA concentrations during cathecholamine stress. Isoproterenol regulation of T3R-mRNA levels provides further evidence for the close interaction between the thyroid hormone and the β-adrenergic systems.