Β-adrenoceptor Desensitization Research Articles

Abstract MP122: Effects of Cardiac Beta3-Adrenoceptors on the Inhibition of G Protein-coupled Receptor Kinase 2 via S-nitrosylation

G Protein-Coupled Receptor Kinase 2 (GRK2) is a culprit in the loss of cardiac contractile function in heart failure due to β-Adrenoceptor (AR) desensitization after its upregulation. Indeed, its inhibition has been demonstrated to improve cardiac function and increased GRK2 in the heart leads to larger injury after an ischemic insult. Nitric oxide (NO) via S-nitrosothiol (SNO) at residue Cys340 is a reported endogenous inhibitor of GRK2 activity. ß3ARs, on the other hand, are known to be resistant to desensitization by GRK2 and they are upregulated in cardiac pathologies. Activation of ß3ARs can be cardioprotective via NO signalling. Thus, in the present study, we aimed to investigate the interaction between ß3ARs and GRK2 through NO signaling to determine if ß3AR cardioprotection can occur via NO-mediated GRK2 inhibition. We used wild type C57BL/6 mice (WT), global β3AR knockout (KO) mice and GRK2-C340S knockin mice, which harbor a point mutation that changes Cys340 with a serine, meaning all endogenous GRK2 globally cannot be inhibited via NO-mediated S-nitrosylation. We exposed WT, GRK2-C340s KI and β3AR KO mice to ischemia/reperfusion (I/R) injury (40 min ischemia followed by 24 hrs reperfusion). We found that WT mice had significantly diminished cardiac function evaluated by echocardiography and Millar Catheterization, and this was rescued by treating these mice with CL316,243 (a selective β3AR agonist) at the time of reperfusion. On the other hand, GRK2-C340S KI mice did worse after I/R injury compared to WT mice and CL316,243 did not rescue this dysfunction as it did in WT mice. As expected, β3AR KO mice had worsened cardiac function compared to WT mice and CL316,243 had no functional benefit. Infarct size measurements after I/R revealed that β3AR KO mice had larger infarcts than WT mice supporting β3ARs as being protective. Indeed, CL316,243 induced robust cardioprotection in WT mice, reducing infarct size. GRK2-C340S mice had larger infarcts than WT mice and CL316,243 failed to offer any cardioprotection. Thus, β3AR-mediated cardioprotection clearly involves inhibition of GRK2 as part of its therapeutic mechanism and this appears to involve NO-mediated S-nitrosylation.

Catecholamine-Induced Cardiomyopathy in Pheochromocytoma: How to Manage a Rare Complication in a Rare Disease?

Pheochromocytomas and paragangliomas (PHEOs) are rare neuroendocrine tumors. Clinical manifestations include different cardiovascular signs and symptoms, which are related to excessive secretion of catecholamines. Catecholamine-induced cardiomyopathy in PHEO (CICMPP) is a rare but dreaded complication of PHEO. Once patient is diagnosed with this condition, the prognosis is worse and a surgical risk is much higher than expected. This article focuses on how catecholamines affect the heart and the pathophysiologic mechanism of CICMPP. The cardiovascular responses to catecholamine depend mostly on which catecholamine is released as well as the amount of catecholamine that is released. The acute release of norepinephrine and epinephrine from PHEO increases heart rate, systemic vascular resistance, myocardial contractility, and reduces venous compliance. The excessive adrenergic stimulation by catecholamine results in severe vasoconstriction and coronary vasospasm, myocardial ischemia, and subsequently damage, and necrosis. Chronically elevated catecholamine levels lead to significant desensitization of cardiac β-adrenoceptors. The increased levels of the enzyme β-adrenoceptors kinase (βARK) in the heart seems to mediate these biochemical and physiological changes that are consistently correlated with attenuated responsiveness to catecholamine stimulation. Through these mechanisms different types of cardiomyopathy (CMP) can be formed. This review discusses extensively the 3 types of cardiomyopathies that can be present in a PHEO patient. It also provides the clinical presentation and diagnostic and therapeutic algorithm in managing patients with CICMPP.

Phosphodiesterase PDE2 activity, increased by isoprenaline, does not reduce β-adrenoceptor-mediated chronotropic and inotropic effects in rat heart.

Myocardial PDE2 activity increases in terminal human heart failure and after isoprenaline infusion in rat heart. PDE2 inhibitors do not potentiate the murine sinoatrial tachycardia produced by noradrenaline. We investigated whether isoprenaline infusion induces PDE2 to decrease the chronotropic and inotropic effects of catecholamines in rat heart. Sprague-Dawley rats were infused with isoprenaline (2.4mgkg-1day-1) for 3days. We used spontaneously beating right atria, paced right ventricular strips and left ventricular papillary muscles. The effects of the PDE2 inhibitors EHNA (10μM) and Bay 60-7550 (0.1-1μM) were investigated on the cardiostimulation produced by noradrenaline (ICI118551 50nM present to block β2-adrenoceptors) and adrenaline (CGP20712A 300nM present to block β1-adrenoceptors). Hydrolysis of cAMP by PDE2 was measured by radioenzyme assay. Bay 60-7550 but not EHNA increased sinoatrial beating. A stable tachycardia elicited by noradrenaline (10nM) or adrenaline (1μM) was not increased by the PDE2 inhibitors. Isoprenaline infusion increased the hydrolytic PDE2 activity threefold in left ventricle, reduced the chronotropic and inotropic effects and potency of noradrenaline and abolished the effects of adrenaline. The potency of the catecholamines was not increased by the PDE2 inhibitors. Neither EHNA nor Bay 60-7550 potentiated the effects of the catecholamines. Rat PDE2 decreased basal sinoatrial beating but did not reduce the sinoatrial tachycardia or increases of ventricular force mediated through β1- and β2-adrenoceptors. The β-adrenoceptor desensitization induced by the isoprenaline infusion was not reversed by the PDE2 inhibitors despite the increased hydrolysis of cAMP by PDE2.

Blood pressure variability provokes vascular β-adrenoceptor desensitization in rats

Spontaneous variation in blood pressure is defined as ‘blood pressure variability’ (BPV). Sinoaortic denervation (SAD) is characterized by BPV without sustained hypertension. In the present study, we investigated whether BPV could be related to vascular β-adrenoceptor desensitization in rats. Three days after surgery (SAD and control), aortic rings were placed in an organ chamber and the relaxation stimulated by β-adrenoceptor agonists, isoprenaline, terbutaline, BRL37344 and cyanopindolol was verified. The participation of intracellular nucleotides signaling pathways was also verified using forskolin, sodium nitroprusside and acetylcholine to induce relaxation. The effects of BPV on the increase in endothelial cytosolic Ca2+ concentration stimulated by the β2-adrenoceptor agonist was examined by confocal microscopy. In addition, the vascular expression of the β2-adrenoceptor was also examined by immunohistochemistry. The results show that isoprenaline and terbutaline-induced relaxation was lower in the aortas of rats with BPV. Relaxation responses to other vasorelaxant compounds were similar in both groups of rats. Histological analysis revealed a lower level of β2-adrenoceptor and confocal microscopy showed minor cytosolic Ca2+ concentration in endothelial cells stimulated by the β2-adrenoceptor agonist in rats with BPV. In conclusion, BPV leads to desensitization of the β2-adrenoceptor, which could contribute to worse β-adrenoceptor agonist-induced relaxation in isolated aortas.

Rosiglitazone is a superior bronchodilator compared to chloroquine and β-adrenoceptor agonists in mouse lung slices

BackgroundCurrent therapy for relieving bronchoconstriction may be ineffective in severe asthma, particularly in the small airways. The aim of this study was to further characterise responses to the recently identified novel bronchodilators rosiglitazone (RGZ) and chloroquine (CQ) under conditions where β-adrenoceptor agonist efficacy was limited or impaired in mouse small airways within lung slices.MethodsRelaxation to RGZ and CQ was assessed following submaximal methacholine (MCh) pre-contraction, in slices treated overnight with either RGZ, CQ or albuterol (ALB) (to induce β-adrenoceptor desensitization), and in slices treated with caffeine/ryanodine in which contraction is associated with increases in Ca2+ sensitivity in the absence of contractile agonist-induced Ca2+ oscillations. Furthermore, the effects of RGZ, CQ, ALB and isoproterenol (ISO) on the initiation and development of methacholine-induced contraction were also compared.ResultsRGZ and CQ, but not ALB or ISO, elicited complete relaxation with increasing MCh pre-contraction and maintained their potency and efficacy following β-adrenoceptor desensitization. RGZ, CQ and ALB maintained efficacy following overnight incubation with RGZ or CQ. Relaxation responses to all dilators were generally maintained but delayed after caffeine/ryanodine. Pre-treatment with RGZ, but not CQ, ALB or ISO, reduced MCh potency.ConclusionsThis study demonstrates the superior effectiveness of RGZ in comparison to CQ and β-adrenoceptor agonists as a dilator of mouse small airways. Further investigation of the mechanisms underlying the relatively greater efficacy of RGZ under these conditions are warranted and should be extended to include studies in human asthmatic airways.

Role of Sphingosine-1-Phosphate in β-adrenoceptor Desensitization via Ca2+ Sensitization in Airway Smooth Muscle

The correlation between inflammatory cells and airway smooth muscle plays fundamental roles in the pathophysiology of asthma. This study was designed to determine whether pre-exposure of airway smooth muscle to sphingosine-1-phosphate (S1P), which is released from mast cells by allergic reactions, causes a deterioration of β-adrenoceptor function. Isometric tension and the ratio of fluorescence intensities at 340 and 380 nm (F(340)/F(380)), an indicator of intracellular Ca2+ levels, were simultaneously measured using fura-2 loaded guinea-pig tracheal tissues. Intracellular cAMP levels were also measured. Pre-exposure to S1P caused a reduction in the inhibitory effects of 0.3μM isoprenaline, a β-adrenoceptor agonist, and 10μM forskolin, a direct activator of adenylyl cyclase, against 1μM methacholine-induced contraction in concentration- and time- dependent manners. In contrast, the values of F(340)/F(380) were not augmented under this experimental condition. After incubation with S1P in the presence of 0.001-1μM Y-27632, a Rho-kinase inhibitor, the reduced responsiveness to forskolin induced by S1P was reversed in a concentration-dependent manner. Moreover, pre-treatment with pertussis toxin (PTX), an inhibitor of G(i), suppressed the loss of forskolin-induced relaxation induced by S1P. Pre-exposure to S1P markedly inhibited the augmentation of cAMP accumulation induced by forskolin. However, addition of Y-27632 and pre-exposure to PTX returned forsokin-induced cAMP accumulation to the control level. Pre-exposure to S1P causes heterologus desensitization of β-adrenoceptors by increasing the sensitivity of airway smooth muscle to intracellular Ca2+. Ca2+ sensitization regulated by G(i) and Rho-kinase is involved in this phenomenon.

Desensitization of beta-adrenoceptors following repeated injections of 2-substituted 4-phenylquinolines.

The chronic effects of five 2-substituted 4-phenylquinoline derivatives on the sensitivity of the noradrenergic cyclic AMP-generating system in the rat brain cortex have been determined, and compared with those of the typical and atypical antidepressants imipramine and trazodone, respectively. Acute treatment (single i.p. dose of 20 mg kg-1) and sub-chronic treatment (20 mg kg-1 daily for 10 days) induced no significant desensitization of the beta-adrenoceptors. However, chronic treatment (20 mg kg-1 daily for 3 weeks) significantly decreased the isoprenaline-induced increase in cyclic AMP, suggesting desensitization. This effect, coupled with previous findings, points to a potential role of these compounds as antidepressants.

Role of β-adrenoceptors, cAMP phosphodiesterase and external Ca2+ on polyamine-induced relaxation in isolated bovine tracheal strips

Polyamines relax several smooth muscles and elicit cardiotonic effects in the rat heart via interactions with β-adrenoceptors. The aim of this work was to establish whether β2-adrenoceptors were involved in polyamine-relaxation of bovine tracheal strips. Endogenous polyamines displaced the specific radioligand, [ H]dihydroalprenolol, but spermine was the most potent. The polyamines elicited an acute transient relaxation, which was independent of β-adrenoceptor activation, followed by a maintained component, which was shown to be dependent on β-adrenoceptor activation because it was antagonized and reversed by propranolol. Polyamines did not alter salbutamol-induced acute relaxation. Polyamines modified the salbutamol-induced long-term effect on airway tone, which was shown by a partial reversal of β-adrenoceptor desensitization. This process was delayed by α-difluoromethylornithine, but spermine increased the latency and time of reversal and decreased receptor desensitization. Putrescine prolonged the time-constant without changes in the desensitization. Spermine, but not putrescine, might block Ca2+ channels, because it relaxed KCl- or electrical stimulated-contractions, which are related to Ca2+ influx, and the inhibition of cAMP phosphodiesterase activity. These differences might explain the functional differences observed between putrescine and spermine. Therefore, polyamines may modulate airway smooth muscle tone and interfere with the mechanism of receptor desensitization via several mechanisms involving β2-adrenoceptors, Ca2+ influx and cAMP phosphodiesterase.

Role of Ca 2+ mobilization in desensitization of β-adrenoceptors by platelet-derived growth factor in airway smooth muscle

Platelet-derived growth factor (PDGF), which is released from eosinophils and fibroblasts, may be implicated in the pathophysiology of bronchial asthma. To examine the involvement of airway inflammation in β-adrenergic desensitization, the present study was designed to determine whether pre-exposure to PDGF deteriorates β-adrenoceptor function in airway smooth muscle. We focused on Ca 2+ signaling as an intracellular mechanism involved in this phenomenon. Isometric tension and F 340/F 380 (an indicator of intracellular Ca 2+ concentration) induced by isoprenaline and other cAMP-related agents were simultaneously measured before and after exposure to PDGF in fura-2-loaded guinea-pig tracheal smooth muscle. Indomethacin was applied throughout the experiments to abolish prostaglandin synthesis by PDGF. After exposure of the tissues to 10 ng/ml PDGF for 15 min, the effects of isoprenaline, a β-adrenoceptor agonist, and forskolin, a direct inhibitor of adenylyl cyclase, against methacholine-induced contraction were markedly reduced with increasing F 340/F 380. However, in the presence of verapamil, an inhibitor of voltage-dependent Ca 2+ channels, the reduced responsiveness to isoprenaline and forskolin induced by pre-exposure to PDGF was reversed with reducing F 340/F 380. Reduced responsiveness to isoprenaline by PDGF was also not observed in the presence of Ca 2+-free solution. The inhibitory effects of db-cAMP, an analogue of cAMP, and theophylline, a nonselective inhibitor of phosphodiesterase, were not attenuated by PDGF. In conclusion, pre-exposure to PDGF causes impairment of the β-adrenoceptors/adenylyl cyclase processes in airway smooth muscle that is independent of cyclooxygenase synthesis by PDGF. Ca 2+ mobilization by Ca 2+ influx through voltage-dependent Ca 2+ channels is involved in this heterologous desensitization of β-adrenoceptors.

Modulation of β-adrenoceptor signaling in the hearts of 4-wk simulated weightlessness rats

The modulation of beta-adrenoceptor signaling in the hearts of hindlimb unweighting (HU) simulated weightlessness rats has not been reported. In the present study, we adopted the rat tail suspension for 4 wk to simulate weightlessness; then the effects of simulated microgravity on beta-adrenoceptor signaling were studied. Mean arterial blood pressure (ABP), left ventricular pressure (LVP), systolic function (+dP/dtmax), and diastolic function (-dP/dtmax) were monitored in the course of the in vivo experiment. Single rat ventricular myocyte was obtained by the enzymatic dissociation method. Hemodynamics, myocyte contraction, and cAMP production in response to beta-adrenoceptor stimulation with isoproterenol or adenylyl cyclase stimulation with forskolin were measured, and Gs protein was also determined. Compared with the control group, no significant changes were found in heart weight, body weight and ABP, while LVP and +/-dP/dtmax were significantly reduced. The ABP decrease, LVP increase, and +/-dP/dtmax in response to isoproterenol administration were significantly attenuated in the HU group. The effects of isoproterenol on electrically induced single-cell contraction and cAMP production in myocytes of ventricles in the HU rats were significantly attenuated. The biologically active isoform, Gsalpha (45 kDa) in the heart, was unchanged. Both the increased electrically induced contraction and cAMP production in response to forskolin were also significantly attenuated in the simulated weightlessness rats. Above results indicated that impaired function of adenylyl cyclase causes beta-adrenoceptor desensitization, which may be partly responsible for the depression of cardiac function.

The Failing Cardiomyocyte

This article presents an integrated account of the subcellular alterations underlying each of the main observed defects in function of the ventricular cardiomyocytes from failing human heart: poor contraction, slow relaxation, β-adrenoceptor desensitization, and tendency to arrhythmia. The key role of the β-adrenoceptor system is highlighted: loss of tonic support from cyclic AMP-stimulated pathways contributes to the contraction and relaxation defects observed, but it is residual β-adrenoceptor function that exacerbates arrhythmias. Most of the alterations are “acquired” during the process of adaptation of the heart to cell loss, and are common to various etiologies of heart failure. Information gained from studies on genetic causes of cardiomyocyte malfunction, however, has been instructive in the interpretation of the functional effects of subcellular alterations.

Effect of Interleukin-1β and Tumor Necrosis Factor-α on the Expression of G-Proteins in CD4+ T-Cells of Atopic Asthmatic Subjects

Chronic use of β2-agonists and increased production of inflammatory mediators during the late allergic reaction after the antigen challenge result in the desensitization of β-adrenoceptors in the airways with an accompanying rise in nonspecific airway hyperresponsiveness. Several proinflammatory cytokines, including interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), play a significant role in orchestrating and perpetuating the inflammatory response and induce the decreased response to bronchodilators in vitro. However, the underlying mechanisms are unknown. In this study, we examined the effect of two cytokines, IL-1β and TNF-α, on the expression of guanine nucleotide binding regulatory proteins (G-proteins), Gsα and Giα-3, by Western blotting in the CD4+ cells of nonatopic nonasthmatic (NANA), atopic nonasthmatic (ANA), and atopic asthmatic (AA) subjects. In the purified CD4+ cells, the basal expression of Gsα was higher in the ANA group, and significantly lower in the AA group as compared to the NANA group. The basal expression of Giα-3 was significantly greater (3–15 fold) than Gsα, with no significant difference between any of the three groups. Both cytokines IL-1β and TNF-α significantly decreased the expression of Gsα in the CD4+ cells of the NANA and ANA groups, with no effect in the AA group. However, these cytokines increased the expression of Giα-3, proteins in the AA group, but had no effect in the CD4+ cells of the NANA and ANA groups. These data suggest that a decreased response to β2-agonists in the late allergic response in allergic asthmatic subjects could be due to the release of inflammatory cytokines, which induce a decrease in the stimulatory G-proteins and an increase in the inhibitory G-proteins.

Cardiac β-Adrenoceptor Desensitization Due to Increased β-Adrenoceptor Kinase Activity in Chronic Uremia

ABSTRACT. Patients with chronic renal failure develop an autonomic dysfunction with impaired baroreflex control and attenuated cardiovascular β-adrenoceptor response to noradrenaline. In rats that underwent 5/6-nephrectomy (SNX), cardiac β-adrenoceptor responsiveness was reduced as well. Therefore, the aim of this study was to further investigate the mechanism underlying cardiac β-adrenoceptor desensitization in SNX rats. For this purpose, right and left ventricular β-adrenoceptor density, activity of the G-protein–coupled receptor kinase, and activity and density of the neuronal noradrenaline transporter (uptake1) were assessed in SNX rats. Seven weeks after SNX, rats had developed left heart hypertrophy. Plasma creatinine, urea, and noradrenaline levels were significantly increased; left and right ventricular noradrenaline content was significantly decreased when compared with sham-operated control rats. In these SNX rats, left, but not right, ventricular β-adrenoceptor density was significantly reduced, and membrane-associated G-protein–coupled receptor kinase activity was significantly increased compared with sham-operated rats. Although right and left ventricular activity of uptake1was unchanged, the neuronal noradrenaline transporter density was significantly reduced in both ventricles of SNXversussham-operated rats. An increase in left ventricular G-protein–coupled receptor kinase activity, possibly triggered by enhanced cardiac noradrenaline release, might be responsible for the decrease in left ventricular β-adrenoceptor responsiveness in SNX rats.

Impact of off-pump coronary artery surgery on myocardial performance and β-adrenoceptor function

Objective: To determine the hemodynamic changes during beating heart revascularization of the left anterior descending artery, the circumflex artery, and the right coronary artery as well as cardiovascular β-adrenoceptor function before and after off-pump coronary artery bypass surgery. Design: Prospective study. Setting: University department of cardiothoracic anesthesia. Participants: Twenty patients scheduled for off-pump coronary artery bypass surgery using the Octopus 2 stabilizer system. Interventions: Isoproterenol, 4 μg, was administered intravenously after induction of anesthesia and again after surgery to monitor cardiac β-receptor function. Measurements and Main Results: The hemodynamic responses to isoproterenol and cardiovascular variables were monitored before, during, and after immobilization of the target coronary artery with catheters in the radial and pulmonary arteries. During surgery on the left anterior descending artery (n = 23), stroke volume and cardiac index decreased 17 mL (21%) and 400 mL (17%). During revascularization of the circumflex artery (n = 9), stroke volume and cardiac index decreased 19 mL (28%) and 300 mL (17%). During surgery on the posterior aspect of the heart (n = 13), stroke volume and cardiac index decreased 22 mL (29%) and 400 mL (17%). All the cardiovascular variables had returned to baseline values 5 minutes after releasing the heart. The hemodynamic responses to isoproterenol were equal before and after surgery. Conclusion: This study provides evidence that the hemodynamic changes associated with off-pump surgery on the 3 major coronary arteries are similar and of short duration. No desensitization of cardiovascular β-adrenoceptors was found. This finding is in contrast to the deterioration in β-adrenoceptor function seen after surgery with cardiopulmonary bypass. Copyright © 2001 by W.B. Saunders Company

Vascular α 1D-Adrenoceptor Function is Maintained During Congestive Heart Failure After Myocardial Infarction in the Rat

Background During congestive heart failure, desensitization of β-adrenoceptors is related to a lower adrenergic responsiveness in the heart; little is known about α 1-adrenoceptors in the vasculature under this condition. We evaluated α 1D-adrenoceptor response in aorta and carotid arteries in a model of congestive heart failure (CHF) post-myocardial infarction. Methods Noradrenaline-elicited contraction was determined in endothelium-denuded arterial rings from young (10-week-old) Wistar rats in the absence and presence of the α 1D-adrenoceptor antagonist BMY 7378 (8-(2-(4-(2-methoxyphenyl)-1-piperazinyl) ethyl)-8-azaspiro(4,5)decane-7,9-dione dihydrochloride) in sham-operated rats and in rats that developed CHF 4 weeks or 7 months after myocardial infarction. Results In the thoracic aorta, BMY 7378 displaced noradrenaline effect to the right with pA 2 values of: sham, 8.58 ± 0.12; CHF, 8.36 ± 0.13, and sham, 8.56 ± 0.10; CHF, 7.99 ± 0.13 at 4 weeks and 7 months after myocardial infarction, respectively. While in carotid arteries, the pA 2 values were: sham, 8.43 ± 0.19; CHF, 8.81 ± 0.19, and sham, 8.35 ± 0.18; CHF, 8.29 ± 0.08 at 4 weeks and 7 months after myocardial infarction, respectively. When adult (7-month-old) rats were subjected to myocardial infarction, CHF was not installed and pA 2 values were similar and high in both sham and infarcted rats. Conclusions These results indicate that α 1D-adrenoceptors remained as the main receptors involved in contraction in aorta and carotid arteries, irrespective of CHF duration.

Ontogeny of Cardiacβ-Adrenoceptor Desensitization Mechanisms: Agonist Treatment Enhances Receptor/G-Protein Transduction Rather than Eliciting Uncoupling

In the fetus and neonate,β-adrenoceptor stimulation fails to produce physiological desensitization. The current study explores the mechanisms underlying the response pattern in neonatal rats. Homologous cardiacβ-adrenergic desensitization caused by isoproterenol treatmentin vivowas demonstrable in adult rats by the immediate (2 h) and specific loss of the ability of isoproterenol, but not glucagon, to stimulate adenylyl cyclasein vitro. Homologous desensitization was absent when the same treatment was given to neonates. By 12 h post-treatment, the adults showed heterologous desensitization (loss of the response to glucagon), an effect which was once again absent in the immature rats. The absence of desensitization in neonates did not reflect a deficiency in the activity or subcellular distribution ofβARK1, the enzyme that initiates the phosphorylation and consequent desensitization ofβ-adrenoceptors. On the other hand, neonates showed relatively poor receptor-Gstransduction as assessed by the GTP-induced shift in receptor ligand binding. Repeated isoproterenol treatment of adult rats led to uncoupling of receptor-G-protein transduction but the same treatment in neonatesenhancedtransduction. Furthermore, neonatal sympathectomy with 6-OHDA interfered with the ontogenetic rise inβ-adrenoceptor-Gsinteractions. These results indicate that the maintenance of agonist responses in the face of neonatal adrenergic stimulation does not reflect simply an absence of the ability to elicit homologous or heterologous desensitization but rather represents an active regulatory mechanism in which neural input exerts a positive trophic role at the level of G-protein function.

An in vivo model of beta-adrenoceptor desensitization

Chronic use of β 2-agonists and increased production of inflammatory mediators during the late allergic reaction after antigen challenge results in the desensitization of β-adrenoceptors in the airways and the accompanying rise in nonspecific airway hyperresponsiveness. In this study, we established an in vivo model of β 2-adrenoceptor desensitization in guinea pig airways by administration of IL-1β intratracheally or chronic albuterol by inhalation. In the establishment of β-adrenoceptor desensitization in response to both β-agonist or inflammatory mediator, baseline pulmonary function responses were established to methacholine and isoproterenol-induced relaxation of methacholine bronchoconstriction. This was followed by the administration of IL-1β (500 IU/d intratracheally for 2 days) or chronic albuterol (0.1 g/L by aerosol for 1 min three times a day for 10 days). After administration, the methacholine and isoproterenol–methacholine response was once again evaluated. Intratracheal administration of IL-1β or chronic administration of albuterol significantly decreased ( p < 0.05) the protective effect of isoproterenol on methacholine-induced bronchoconstriction, eliciting β-adrenoceptor desensitization in vivo. The in vivo model will be very useful in monitoring the effect of other potential drugs on β-adrenoceptor function in the airways.

Relationship between Desensitization and Downregulation of β-Adrenoceptors in Cardiac Tissues after Prolonged In Vivo Infusion of T-0509, a β1-Adrenoceptor Agonist

To examine the contribution of β-adrenoceptor (βAR) downregulation to desensitization of βARs by chronic administration of a βAR agonist, we compared the adenylyl cyclase (AC) activities in two kinds of cardiac ventricular membranes with decreased available βARs: one was derived from rats infused with a selective β1AR agonist, T-0509 [( — )-(R)-1-(3,4-dihydroxyphenyl)-2-[(3,4-dimethoxyphenethyl)-amino]ethanol hydrochloride], in vivo (40 μg/kg/hr, s.C. for 6 days); and the other was obtained from treatment of control membranes with an irreversible βAR antagonist, bromoacetyl alprenolol methane (BAAM). T-0509 infusion decreased the densities of βARs and β2ARs by 26% and 32%, respectively, and reduced the maximal isoproterenol-stimulated AC activity by 53%. The amount of GSα and Giα proteins in the membranes was not significantly changed by T-0509 infusion. To make preparations that mimic the T-0509-induced downregulation, we treated the control membranes with 100 nM BAAM in vitro. The BAAM treatment decreased the Bmax value of [125I]iodocyanopindolol for βARs and β2ARs by 29% and 36%, respectively, whereas it reduced the maximal effect of isoproterenol on AC activity only by 37%. These results suggest that downregulation of βARs cannot fully account for the desensitization by chronic treatment of T-0509 and that other mechanism(s) can play a significant role in the loss of responsiveness.

β-Adrenoceptor desensitization despite cardiac denervation in human failing transplanted myocardium

The aim of this study, using right ventricular endomyocardial biopsies, was to determine whether adenylyl cyclase activity is decreased — despite cardiac denervation — in human failing transplanted myocardium. β-Adrenoceptor-stimulated adenylyl cyclase activity in membranes of moderately failing transplanted hearts was significantly decreased by 49% in comparison to non-failing transplanted hearts. Receptor-independent adenylyl cyclase stimulation (forskolin + GTP, forskolin + Mn 2+) and the level of inhibitory G protein α-subunits were not significantly different in membranes of failing vs. non-failing transplanted myocardium. The results indicate that similar to failing non-transplanted myocardium the responsiveness to β-adrenoceptor agonists is also decreased in failing transplanted myocardium.

Role of prostaglandin E 2 in alterations of the β-adrenergic system from rat eclamptic uterus

The inotropic effect of isoproterenol, as well as the β-adrenoceptor population, was measured in pregnant uterine tissue from female spontaneous hypertensive rats (SHR) (control group: C) and female SHR that were grafted with skin from Holtzman male rats (eclamptic group: E). The K d value of the concentration response curve of isoproterenol was higher for uteri from E rats than C rats. This phenomenon was not accompanied by a modification in the expression of β-adrenoceptors. Inhibition of the synthesis of prostaglandins prevented the hyporeactivity to isoproterenol during eclampsia. Moreover, uteri from E rats generated and released greater amounts of prostaglandin E 2 (PGE 2) than uteri from C rats, even in the presence or absence of isoproterenol. In addition, whereas isoproterenol administered alone increased basal cyclic AMP (cAMP) production from C uteri, PGE 2 administered alone enhanced cAMP production in E uterine tissue. These results suggest that the decrease in β-adrenergic response to the agonist in E rats is ascribed to PGE 2 production. The abnormal reactivity to the β-agonist could be associated with a heterologous desensitization of uterine β-adrenoceptors exerted by PGE 2 overload in uteri from E rats. These results bear directly on the regulation of uterine motility during pregnancy, since an impaired response to β-adrenergic innervation could lead to increased uterine motility, impairing the maintenance of pregnancy.