Studies in vivo and in vitro of terbutaline-induced beta-adrenoceptor desensitization in healthy subjects

Potential mechanisms to explain how LABAs and PDE4 inhibitors enhance the clinical efficacy of glucocorticoids in inflammatory lung diseases

Inhaled glucocorticoids acting via the glucocorticoid receptor are a mainstay treatment option for individuals with asthma. There is a consensus that the remedial actions of inhaled glucocorticoids are due to their ability to suppress inflammation by modulating gene expression. While inhaled glucocorticoids are generally effective in asthma, there are subjects with moderate-to-severe disease in whom inhaled glucocorticoids fail to provide adequate control. For these individuals, asthma guidelines recommend that a long-acting β₂-adrenoceptor agonist (LABA) be administered concurrently with an inhaled glucocorticoid. This so-called “combination therapy” is often effective and clinically superior to the inhaled glucocorticoid alone, irrespective of dose. LABAs, and another class of drug known as phosphodiesterase 4 (PDE4) inhibitors, may also enhance the efficacy of inhaled glucocorticoids in chronic obstructive pulmonary disease (COPD). In both conditions, these drugs are believed to work by elevating the concentration of cyclic adenosine-3',5'-monophosphate (cAMP) in target cells and tissues. Despite the success of inhaled glucocorticoid/LABA combination therapy, it remains unclear how an increase in cAMP enhances the clinical efficacy of an inhaled glucocorticoid. In this report, we provide a state-of-the-art appraisal, including unresolved and controversial issues, of how cAMP-elevating drugs and inhaled glucocorticoids interact at a molecular level to deliver enhanced anti-inflammatory benefit over inhaled glucocorticoid monotherapy. We also speculate on ways to further exploit this desirable interaction. Critical discussion of how these two drug classes regulate gene transcription, often in a synergistic manner, is a particular focus. Indeed, because interplay between glucocorticoid receptor and cAMP signaling pathways may contribute to the superiority of inhaled glucocorticoid/LABA combination therapy, understanding this interaction may provide a logical framework to rationally design these multicomponent therapeutics that was not previously possible.

Cyclic nucleotide-based therapeutics for chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) defines a group of chronic inflammatory disorders of the airways that are characterised by a progressive and largely irreversible decline in expiratory airflow. Drugs used to treat COPD through actions mediated by cyclic AMP (cAMP) are restricted to long-acting and short-acting β2-adrenoceptor agonists and, in a subset of patients with chronic bronchitis, a phosphodiesterase 4 inhibitor, roflumilast. These agents relax airway smooth muscle and suppress inflammation. At the molecular level, these effects in the airways are mediated by two cAMP effectors, cAMP-dependent protein kinase and exchange proteins activated by cAMP. The pharmacology of newer agents, acting through these systems, is discussed here with an emphasis on their potential to interact and increase therapeutic effectiveness.

Effects of therapeutic doses of albuterol on beta2-adrenergic receptor density and metabolic changes

Beta2-agonist drugs at inhaled supratherapeutic doses or when given orally or parenterally alter peripheral lymphocyte beta2-adrenoceptor density (betaAR) and have demonstrable metabolic effects. However, it is not known whether these changes occur at therapeutic inhaled doses. We therefore studied the effects of therapeutic doses of inhaled albuterol in five asthmatic subjects (mean age 23.0+/-2.4 years) and six normal subjects (mean age 28.3+/-3.3 years). Subjects were studied in a randomized, double-blind protocol in which each subject received either inhaled albuterol (270 microg four times daily) for 2 weeks followed by placebo or vice versa in two sequential 2-week periods separated by a 2-week washout period. In the asthmatics, baseline FEV1 increased significantly (p < 0.05) after 2 weeks of inhaled albuterol treatment compared to the initial visit and after 2 weeks of placebo (mean FEV1: 3.2 L+/-0.7 L, 2.9 L+/-0.5 L, and 3.0 L+/-0. 7 L, respectively). Baseline peripheral lymphocyte betaAR was not significantly different (p > 0.05) between the asthmatic (mean: 757+/-176) and normal subjects (mean: 732+/-251). However, in neither group was there any significant change (p > 0.05) in betaAR or plasma potassium, insulin, or glucose, either acutely or after 2 weeks of albuterol therapy. The present study confirms that there is no difference in peripheral lymphocyte betaAR between asthmatic and normal subjects and also shows that at therapeutic doses of inhaled albuterol, there are no significant changes in betaAR or metabolic effects.

The beta-adrenoceptor

The human beta-adrenoceptor is a member of the seven-transmembrane family of receptors, encoded by a gene on chromosome 5. beta-Adrenoceptors have been classified into beta1, beta2, and beta3 subgroups, with beta2-receptors being widely distributed in the respiratory tract, particularly in airway smooth muscle. Intracellular signaling following beta2-adrenoceptor activation is largely affected through a trimeric Gs protein coupled to adenylate cyclase. Cyclic AMP (cAMP) induces airway relaxation through phosphorylation of muscle regulatory proteins and attenuation of cellular Ca2+ concentrations. Alternative cAMP-independent pathways involving activation of membrane maxi-K+ channels and coupling through Gi to the MAP kinase system have also been described. Site-directed mutagenesis has identified Asp 113 and Ser 204/207 within the third and fourth membrane domains as the active site of the beta2-receptor, critical for beta2-agonist binding and activity. beta2-Agonists have been characterized as those that directly activate the receptor (albuterol), those that are taken up into a membrane depot (formoterol), and those that interact with a receptor-specific auxiliary binding site (salmeterol). These differences in mechanism of action are reflected in the kinetics of airway smooth muscle relaxation and bronchodilation in patients with asthma. beta-Adrenoceptor desensitization associated with beta2-agonist activation is a consequence of phosphorylation by beta-ARK and uncoupling of the receptor from Gs following beta-arrestin binding, of internalization and recycling of the receptor through processes of sequestration and resensitization and downregulation, modulated by an effect on receptor gene expression. The degree of receptor desensitization appears to differ, depending on the cell or tissue type, and is reflected in the different profiles of clinical tolerance to chronic beta2-agonist therapy. A number of polymorphisms of the beta2-receptor have been described that appear to alter the behavior of the receptor following agonist exposure. These include Arg-Gly 16, Glu-Gln 27, and Thr-lle 164. The Gly 16 receptor downregulates to a greater extent and is associated with increased airway hyperreactivity, nocturnal symptoms, and more severe asthma. The Glu 27 form appears to protect against downregulation and is associated with less reactive airways. An individual can be homozygous or heterozygous for given polymorphisms, and large populations will have to be studied to determine their importance to the asthma phenotype.

Effect of long-term beta2-agonist dosing on human cardiac beta-adrenoceptor expression in vivo: comparison with changes in lung and mononuclear leukocyte beta-receptors

BACKGROUND

Tachyphylaxis to the cardiac effects of beta-adrenoceptor stimulation after long-term beta2-agonist administration is well recognized, but the influence on global cardiac beta-adrenoceptor density has not been previously investigated in vivo. Positron emission tomography (PET) has made possible the noninvasive quantification of regional receptor density. This study assesses the effect of long-term beta2-agonist dosing on cardiac beta-adrenoceptors.

METHODS AND RESULTS

Beta-adrenoceptors in the hearts of 29 healthy male subjects aged 35 +/- 8 years were imaged and quantified in vivo by means of PET and compared with the receptor density in the same subjects' lung tissue. Mononuclear leukocyte (MNL) beta-receptor density was determined in vitro by means of a radioligand binding assay. Beta-receptor density was 8.41 +/- 2.03 pmol/gm tissue in heart, 10.81 +/- 1.91 pmol/gm tissue in lung, and 38.0 +/- 17.5 fmol/mg protein on MNLs. There was a weak relationship between cardiac and pulmonary beta-receptor densities (r = 0.45, p < 0.02) but not between cardiac and MNL receptor density. In seven subjects, the measurements were repeated after 2 weeks of albuterol treatment (4 mg orally twice daily and 200 microg inhaled four times daily in the first week, with doubling of the dose during the second week). After the albuterol treatment, beta-receptor density fell on average by 19% (p < 0.05) in the heart compared with 22% (p < 0.05) in the lung and 42% (p < 0.05) in MNLs. Correlations were found between the percentage changes in receptor density in heart and lung (r = 0.98, p < 0.001) and in heart and MNLs (r = 0.99, p < 0.002).

CONCLUSIONS

Two weeks of high-dose albuterol results in equivalent downregulation of beta-receptors in vivo, both in the lung and in the heart.

Effects of beta 2-agonist- and dexamethasone-treatment on relaxation and regulation of beta-adrenoceptors in human bronchi and lung tissue

1. Long-term treatment with beta 2-adrenoceptor agonists can lead to a decreased therapeutic efficacy of bronchodilatation in patients with obstructive pulmonary disease. In order to examine whether or not this is due to beta-adrenoceptor desensitization, human bronchial muscle relaxation was studied in isolated bronchial rings after pretreatment with beta 2-adrenoceptor agonists. Additionally, the influence of pretreatment with dexamethasone on desensitization was studied. 2. The effect of beta 2-agonist incubation alone and after coincubation with dexamethasone on density and affinity of beta-adrenoceptors was investigated by radioligand binding experiments. 3. In human isolated bronchi, isoprenaline induces a time- and concentration-dependent beta-adrenoceptor desensitization as judged from maximal reduction in potency by a factor of 7 and reduction of 73 +/- 4% in efficacy of isoprenaline to relax human bronchial smooth muscle. 4. After an incubation period of 60 min with 100 mumol l-1 terbutaline, a significant decline in its relaxing efficacy (81 +/- 8%) and potency (by a factor 5.5) occurred. 5. Incubation with 30 mumol l-1 isoprenaline for 60 min did not impair the maximal effect of a subsequent aminophylline response but led to an increase in potency (factor 4.4). 6. Coincubation of dexamethasone with isoprenaline (120 min; 30 mumol l-1) preserved the effect of isoprenaline on relaxation (129 +/- 15%). 7. In radioligand binding experiments, pretreatment of lung tissue for 60 min with isoprenaline (30 mumol l-1) resulted in a decrease in beta-adrenoceptor binding sites (Bmax) to 64 +/- 1.6% (P < 0.05), while the antagonist affinity (KD) for [3H]-CGP-12177 remained unchanged. 8. In contrast, radioligand binding studies on lung tissue pretreated with either dexamethasone (30 mumol l-1) or isoprenaline (30 mumol l-1) plus dexamethasone (30 mumol l-1) for 120 min did not lead to a significant change of Bmax (160 +/- 22.1% vs 142.3 +/- 28.7%) or KD (5.0 nmol l-1 vs 3.5 nmol l-1) compared to the controls. 9. In conclusion, pretreatment of human bronchi with beta-adrenoceptor agonists leads to functional desensitization and, in lung tissue, to down-regulation of beta-adrenoceptors. This effect can be counteracted by additional administration of dexamethasone. Our model of desensitization has proved useful for the identification of mechanisms of beta-adrenoceptor desensitization and could be relevant for the evaluation of therapeutic strategies to counteract undesirable effects of long-term beta-adrenoceptor stimulation.

Pharmacodynamic modelling of the drug-induced downregulation of a beta 2-adrenoceptor mediated response and lack of restoration of receptor function after a single high dose of prednisone

Changes in beta 2-adrenoceptor function by chronic dosing of beta 2-mimetics and the possible influence of a single dose of prednisone have been studied as changes over time in the concentration-effect relationship of the beta 2-adrenoceptor agonist terbutaline. Hypokalaemia was used as the specific beta 2-adrenoceptor mediated effect. 8 healthy volunteers were given subcutaneous terbutaline 0.01 mg.kg-1 BW on 3 occasions over a 10-day experimental protocol: 1 Control experiment on Day 1; 2 After 7 days of oral terbutaline 5 mg t.i.d. (Day 8); and 3 After 8 days on oral terbutaline and 12 h after prednisone 100 mg orally (Day 10). The time course of the terbutaline concentrations and hypokalaemia was related using a pharmacokinetic-pharmacodynamic model. A sigmoid and a threshold Emax model were used to relate drug concentrations to effects. The oral terbutaline treatment caused a 35% increase in the distribution volume of SC terbutaline. After one week on oral terbutaline the concentration-effect relationship was shifted to the right and was steeper, with a higher EC50 of terbutaline and higher values for the apparent threshold concentration. These observations are compatible with a decrease in receptor numbers after 7 days of terbutaline in a system characterised by the presence of spare receptors. The data after prednisone pretreatment showed an apparent decline in the baseline plasma potassium concentrations that could be included in the Emax model. There was no change in the concentration-effect relationship 12 hours after prednisone.

Asthma control during and after cessation of regular beta 2-agonist treatment

It has been suggested that regular treatment with high doses of beta 2-agonists might result in poorer control of asthma and increased bronchial responsiveness. We have examined change in FEV1 (delta FEV1), bronchial reactivity, peak expiratory flow (PEF), and symptoms during and after 3 wk of regular treatment with a relatively low dose of albuterol and broxaterol, a new beta 2-agonist. Eleven subjects 18 to 50 yr of age with mild asthma inhaled albuterol (200 micrograms), broxaterol (400 micrograms), or placebo three times a day for 3 wk with a 2- to 4-wk run-in/washout period between treatments. Ipratropium bromide was allowed for symptomatic relief. The PD20 (dose of histamine causing a 20% fall in FEV1) was measured before and 11, 35, and 59 h after cessation of treatment and a bronchodilator dose-response study before and 83 h after cessation of treatment. Change from baseline after albuterol and broxaterol are compared with change after placebo. Diurnal change in PEF (amplitude % mean) increased during treatment with albuterol by 6.5% (95% CI, 1.7-12.3; p < 0.02) mainly because of a fall in morning PEF. Cessation of treatment with both beta 2-agonists was associated with a fall in FEV1 and PD20 compared with placebo.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of terbutaline on basal thermogenesis of human skeletal muscle and Na-K pump after 1 week of oral use--a placebo controlled comparison with propranolol

1. A double-blind placebo-controlled study was conducted on the effects of oral terbutaline (beta 2-adrenoceptor agonist) and propranolol (beta 1 beta 2-adrenoceptor antagonist) on basal heat production of skeletal muscle, measured ex vivo by direct microcalorimetry. Terbutaline slow-release 7.5 mg, propranolol 80 mg, and matching placebo were randomly administered twice daily for 1 week to 15 healthy males, using a cross-over design. 2. Resting heat production in biopsied vastus lateralis was lowered by median 27% (P < 0.01) after terbutaline medication as compared with placebo. The cause of this hypometabolism at the cellular level is obscure but may possibly be explained by desensitization of beta 2-receptors. 3. Propranolol decreased the metabolic rate by 17% (P > 0.3); this might imply that the sympathetic nervous system is playing only a minor role in the regulation of basal metabolic rate in muscle, or that up-regulation of beta-receptors had influenced the decline. 4. The muscle utilized about 6% of its total energy for the Na-K pump as assessed after inhibition by ouabain. 5. Serum potassium was significantly lowered by terbutaline and slightly increased by propranolol with no relationship between changes in extracellular levels and muscle content of potassium under resting conditions. Energy values for the Na-K pump in muscle after 1 week of terbutaline or propranolol medication were similar to placebo. The results are not consistent with the hypothesis that decreased serum potassium during continuous beta 2-adrenoceptor agonist treatment is due to a chronically activated Na-K pump, at least not in resting muscle.

Lymphocyte beta 2-adrenoceptors and adenosine 3':5'-cyclic monophosphate during and after normal pregnancy

1. The beta 2-sympathomimetics, used to inhibit preterm labour, bind predominantly to beta 2-adrenoceptors, activating adenylate cyclase to form adenosine 3':5'-cyclic monophosphate (cyclic AMP), a messenger substance which inhibits the enzyme cascade triggering smooth muscle contraction. beta 2-Adrenoceptor density and cyclic AMP formation can be used as markers of beta 2-adrenergic effect. 2. The present study addresses the influence of pregnancy on the beta-adrenoceptor system. beta 2-Adrenoceptor density and cyclic AMP concentrations (basal and evoked by isoprenaline) in circulating lymphocytes were determined at three points in gestation (16, 29 and 37 weeks) and 9 weeks post partum in 22 normal pregnancies. (-)-[125Iodo]-cyanopindolol was used as the ligand to identify a homogeneous population of beta 2-adrenoceptors on lymphocytes. B- and T-cell fractions were estimated from the same samples. 3. beta 2-Adrenoceptor density decreased significantly during gestation until week 37 (P < 0.01), then increased post partum (P < 0.005). Cyclic AMP concentrations (basal and evoked by isoprenaline) were significantly lower after 16 weeks of gestation than post partum (P < 0.05). 4. The results, which cannot be explained in terms of a shift in the lymphocyte (B- and T-cell) ratio, indicate that beta-adrenoceptor density and function are reduced in normal pregnancy and only return to normal post partum. These findings may be of significance in devising future tocolytic therapy with beta 2-adrenoceptor agonists.

Beta-receptor responsiveness after desipramine treatment

To examine whether a tricyclic antidepressant affects the functional response to a beta-receptor agonist in man, the response of heart rate, blood pressure, and plasma cAMP to isoproterenol was measured in 14 normal controls taking 75 mg desipramine daily. Desipramine significantly increased the bolus dose of isoproterenol needed to increase heart rate by 25 bpm at 14-30 days but not at 3-8 days. During infusions of isoproterenol, the increase in systolic blood pressure was blunted at both 3-8 days and 14-30 days, while the decrease in diastolic blood pressure was unaffected. Blood pressure findings were not affected by preadministration of bethanechol. In ten controls, isoproterenol infusions increased plasma cAMP, but this was unaffected by desipramine treatment. These findings suggest a decrease in the functional response of beta 1, but not beta 2, receptors after treatment with desipramine.

Up-regulation of beta-adrenoceptors on circulating mononuclear cells after reduction of central sympathetic outflow by clonidine in normal subjects

Short term regulation of beta-adrenoceptors in peripheral blood mononuclear cells after sympathetic activation has been previously documented in normal individuals but changes after a central reduction in sympathetic activity are not known. We have studied beta-adrenoceptor number and affinity on peripheral blood mononuclear cells in normal subjects, before and after intravenous clonidine, an alpha 2-adrenoceptor agonist which lowers blood pressure predominantly by reducing central nervous system sympathetic outflow. After clonidine there was a decrease in plasma levels of noradrenaline and adrenaline, and an increase in growth hormone. There was up-regulation of beta-adrenergic receptors on peripheral blood mononuclear cells 30 and 60 min after clonidine which was related to the fall in blood pressure, noradrenaline and adrenaline levels and to the increase in growth hormone levels. The affinity of the receptors was decreased. Return to baseline values was observed after 2 h. Intracellular production of cAMP after isoproterenol stimulation demonstrated that the up-regulation was not functional. Our studies indicate short term up-regulation of beta-adrenoceptors in peripheral blood mononuclear cells after clonidine. These observations after a reduction in sympathetic activity may be of importance if they mirror the pattern of redistribution of adrenoceptors, which are present in a wide range of tissues.

Influence of beta-adrenergic receptor function during terbutaline treatment on allergen sensitivity and bronchodilator response to terbutaline in asthmatic subjects

Nine asthmatic patients with an allergy to birch or timothy underwent bronchial allergen provocations on three different trial days, with intervals of 2 to 5 wk. Two weeks prior to one of the provocations, no medication was allowed. Before the other two provocations the patients had been on continuous treatment with oral terbutaline (7.5-mg slow-release pill bid) for 2 wk, which was discontinued 12 or 48 h before the allergen provocation. After allergen challenges, terbutaline was inhaled in increasing doses (0.5 mg, 1.0 mg, and 2.0 mg), and pulmonary function was measured after each dose. Before each allergen provocation, blood samples were drawn for measurements of catecholamine and terbutaline concentrations and for in vitro measurements of beta-adrenergic receptor function on lymphocytes (isoproterenol-induced accumulation of cyclic AMP). Beta-adrenergic receptor function on blood lymphocytes was impaired after the two treatment periods, compared with the drug-free period, and was significantly more depressed at 12 h than 48 h after dosing. The bronchial responsiveness to allergen, defined as PC20PEF (median values), was 1,700 biologic units (BU) after the period of no treatment and 220 BU and 445 BU at 12 and 48 h after discontinuation of the terbutaline treatment (p less than 0.1 after 48 h). Five of the nine patients exhibited increased bronchial responsiveness 48 h after treatment, compared to results without treatment. The responsiveness was similar on all occasions in three patients. The bronchodilator response to inhaled terbutaline after allergen-induced bronchoconstriction was attenuated (p less than 0.01) at both 12 and 48 h after terbutaline, compared to results without treatment, indicating desensitization also of the bronchial beta-adrenergic receptors. We conclude that the early bronchial responsiveness to allergen is increased following a period of continuous treatment with a beta-adrenergic receptor agonist in some asthmatic patients and that the capability of a beta-agonist to reverse allergen-induced bronchoconstriction is attenuated after beta-agonist treatment.

Beta-adrenoceptors in human alveolar macrophages isolated by elutriation

1. beta-adrenoceptors on human alveolar macrophages obtained by bronchoalveolar lavage (BAL) from healthy smoking volunteers (n = 26) were characterized by studying cyclic AMP (cAMP) accumulation in intact macrophages evoked by adrenaline or isoprenaline, with or without appropriate antagonists and by radioligand binding to macrophage membranes, using [125I]-iodopindolol (125IPIN) as beta-adrenoceptor ligand. 2. In a second study, cAMP responses of alveolar macrophages to isoprenaline and PGE1 and of peripheral blood lymphocytes to isoprenaline were compared in smoking and non-smoking healthy volunteers (n = 9 + 9), as our initial studies were performed in smokers, due to their higher cell yield. 3. BAL yielded 47 +/- 23 x 10(6) cells in smokers and 12 +/- 6 x 10(6) cells in non-smokers with a recovery of 82 +/- 8% in the elutriation step (means +/- s.d.). The cell preparation consisted of 99.2 +/- 0.8% macrophages and their viability (trypan blue exclusion) was 97.5 +/- 5.2%. 4. Isoprenaline or adrenaline increased cAMP accumulation approximately 40-fold with or without the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX, 10(-4) M), which enhanced basal and stimulated cAMP accumulation approximately five-fold. Peak responses were seen after 2 min. EC50s for isoprenaline and adrenaline were 3-5 x 10(-7) M. Phentolamine did not alter responses to adrenaline, indicating absence of inhibitory alpha 2-adrenoceptors. Propranolol inhibited isoprenaline induced cAMP accumulation stereoselectively; pD2-values were 8.2 for (-)-propranolol, 5.6 for atenolol and 7.5 for ICI 118,551, suggesting a predominance of beta 2-adrenoceptors. 5. Specific 125IPIN binding to macrophage membranes was rapid and saturable. Non-specific binding was determined in the presence of 1 microM (-)-propranolol. KD values were 71 +/- 7 pM and the density of specific binding sites was 36 +/- 3 fmol mg-1 protein (three experiments on a membrane pool from 10 subjects; r values for Scatchard analyses = 0.98 +/- 0.01). Similar values were obtained when 200 microM isoprenaline (+ GTP) was used to assess non-specific binding. Competition experiments again showed stereoselectivity for propranolol and a predominance of beta 2-adrenoceptors, as judged by the displacement of specific 125IPIN binding by atenolol and ICI 118,551. 6. Macrophages from smokers responded with less marked cAMP accumulation upon stimulation with isoprenaline or PGE1 than did cells from non-smokers (difference approximately 30%; P less than 0.05 for both agonists) in the presence of IBMX. Thus macrophages from smokers may produce less cAMP due to post-receptor changes in responsiveness.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of beta-adrenoceptors in the guinea pig left ventricle and skeletal muscle following chronic agonist treatment

Pretreatment of guinea pigs with adrenaline, isoprenaline or terbutaline for 7 days significantly reduced the Bmax for the radioligand [125I]cyanopindolol (ICYP) in the gastrocnemius muscle (beta 2-adrenoceptors). Pretreatment of guinea pigs with terbutaline reduced the responsiveness of gastrocnemius muscle slices adenylate cyclase to isoprenaline (10(-4) M). In the left ventricle (predominantly beta 1-adrenoceptors) pretreatment of guinea pigs with isoprenaline or adrenaline for 7 days did not alter the Bmax for ICYP. The responsiveness of adenylate cyclase to isoprenaline (10(-4) M) in left ventricle slices was significantly reduced following isoprenaline pretreatment of the guinea pigs. Thus desensitisation of beta-adrenoceptors in left ventricle and skeletal muscle developed following chronic agonist pretreatment. Reduction of beta 2-adrenoceptors in the skeletal muscle could be responsible for the desensitisation of adenylate cyclase. In the left ventricle the receptors were resistant to agonist induced down-regulation. It is proposed that other mechanisms which are tissue- and species-specific independent of the receptor subtype can be responsible for agonist-induced desensitisation in the left ventricle of the guinea pig in vivo.

Effects of sequential doses of parenteral terbutaline on plasma levels of potassium and related cardiopulmonary responses

beta-agonist bronchodilator drugs are known to cause hypokalemia; nevertheless, successive parenteral doses are recommended for the treatment of status asthmaticus. The metabolic and cardiopulmonary effects of sequential, subcutaneous doses of terbutaline were studied in eight stable adult asthmatic patients. Each subject received terbutaline, 0.25 mg subcutaneously, followed 30 min later either by a second subcutaneous dose of terbutaline (sequential) or normal saline (single) in a randomized, double-blind, crossover fashion. Maximum reductions in plasma potassium from baseline were significantly greater (p less than 0.05) following sequential treatment compared to single treatment (-1.1 versus -0.6 mEq/L, respectively). Improvements in forced expiratory volume in 1 s were temporally related to changes in serum potassium and were significantly greater (p less than 0.01) following sequential terbutaline treatment. Prolongation of the QTc interval occurred following both treatment regimens. The change in QTc was statistically significant only following sequential treatment, increasing from 377 +/- 21 to 441 +/- 39 ms (p less than 0.05). Sequential doses of parenterally administered terbutaline result in clinically significant reductions in plasma potassium that are temporally related to changes in pulmonary function and associated with important electrocardiographic alterations.

Altered adrenoceptor responsiveness during adrenaline infusion but not during mental stress: differences between receptor subtypes and tissues

1. Effects of 3 h infusions of adrenaline (0.4 nmol kg-1 min-1) or placebo and of mental stress evoked by a colour word test (CWT) on adrenergic receptor function were investigated in healthy men. Responses of heart rate, blood pressure, plasma catecholamines, plasma cyclic AMP and plasma free fatty acids (FFA) were evaluated during infusions and CWT. In vitro beta 2-adrenoceptor numbers [( 125I]-HYP binding) and function (isoprenaline induced cyclic AMP accumulation) were studied on lymphocytes in all experiments. alpha 2-adrenoceptor binding [( 3H]-yohimbine and adrenaline) to intact platelets was evaluated in the infusion experiments only. 2. Placebo infusion evoked no major alterations of any parameter. 3. Adrenaline infusion raised venous plasma adrenaline levels to 4-5 nmol l-1, increased heart rate by 14 +/- 3 beats min-1 and plasma cyclic AMP by 17 +/- 3 nmol l-1, and decreased diastolic blood pressure by 15 +/- 5 mm Hg. These responses persisted throughout the infusion. Plasma FFA levels, on the other hand, increased at 30 min of infusion (from 236 +/- 44 to 717 +/- 92 mumol l-1) and returned to basal levels after 3 h of infusion. 4. In vitro, lymphocytes showed increased beta 2-responsiveness after 30 min of adrenaline infusion (delta cyclic AMP increased from 1.86 +/- 0.24 to 3.06 +/- 0.58 pmol/10(6) cells), but a decreased response (0.47 +/- 0.10 pmol/10(6) cells) after 3 h of infusion. [125I]-HYP binding to lymphocyte membranes showed a three-fold increase of Bmax at 30 min of adrenaline infusion followed by a return to basal values after 3 h of infusion. [125I]-HYP binding reflected the functional responsiveness of the lymphocytes in vitro poorly. alpha 2-adrenoceptors on platelets were not altered with regard to Bmax or Kd for [3H]-yohimbine binding or Ki for adrenaline displacement of [3H]-yohimbine binding. 5. CWT evoked marked circulatory changes, a four-fold increase in plasma adrenaline and a 60% increase in beta 2-adrenoceptor binding sites without changes in functional responsiveness of the lymphocytes. 6. We conclude that exposure to high physiological levels of adrenaline in vivo alters lymphocyte beta-adrenoceptor responsiveness in a biphasic manner, with an early increase followed by a later decrease, but that most beta-adrenoceptor mediated responses to adrenaline in vivo remain intact. Lymphocyte alterations may reflect recruitment of cells into the circulation during sympathoadrenal stimulation. Platelet alpha 2-adrenoceptors are apparently not easily subjected to agonist induced dynamic receptor regulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Beta-adrenergic receptor responsiveness to isoprenaline in humans: concentration-effect, as compared with dose-effect evaluation and influence of autonomic reflexes

1. Different techniques of assessing beta-adrenoceptor sensitivity in vivo, by use of i.v. infusions or bolus injections of isoprenaline (ISO), were compared in healthy volunteers. The importance of autonomic reflexes for responses to ISO was evaluated by studying the influence of 'autonomic blockade' by atropine and clonidine, which antagonize muscarinic effects and reduce sympathetic activity, respectively. Estimates of in vivo responsiveness to ISO were compared with parameters reflecting beta 2-adrenoceptor function in vitro in lymphocytes. 2. Heart rate responses to infused ISO were not significantly altered by 'autonomic blockade' when evaluated from concentration-effect curves. When related to the infused dose of ISO, however, sensitivity was artefactually increased (P less than 0.05), as the plasma concentrations of ISO were 40% higher after atropine and clonidine. Heart rate responses to bolus injections of ISO were attenuated (P less than 0.05) by 'autonomic blockade', suggesting that facilitatory reflexes contribute to these non-steady state responses. Intersubject variations in heart rate responsiveness to ISO were greater than the intrasubject variability caused by counterregulatory reflexes. 3. 'Autonomic blockade' lowered venous plasma noradrenaline at rest. The noradrenaline response to ISO infusion was attenuated and the diastolic blood pressure response enhanced, indicating that a counterregulatory vasoconstrictor reflex normally is activated by ISO-induced vasodilatation. The plasma cyclic AMP response to ISO, on the other hand, was unaffected by atropine and clonidine and reflects beta 2-adrenoceptor responsiveness in vivo. 4. In vitro data for beta-adrenoceptor binding sites (Bmax;[125I]-IHYP binding) and cyclic AMP responses to ISO in lymphocytes correlated with DBP and noradrenaline responses to infused ISO. No correlations were found between in vitro data and heart rate, plasma cyclic AMP or plasma glycerol responses to infused ISO in vivo. 5. During prolonged ISO infusions (in six other healthy subjects) physiological responses reached greater than 90% of their steady state level after 8 min, but no definite steady state level could be defined for the plasma concentration of ISO during 40 min of infusion. 6. The ISO infusion test showed a good reproducibility, especially when repeated on the same day. Evaluation of plasma concentration-effect relationships increase the precision of the ISO infusion test as confounding inter- and intra-individual variations in ISO concentrations (as caused by e.g. autonomic blockade) will be taken into account.(ABSTRACT TRUNCATED AT 400 WORDS)

Human tissue adrenergic receptors are not predictive of responses to epinephrine in vivo

To test the hypotheses that adrenergic receptor and adenylate cyclase characteristics of easily accessible circulating cells reflect those of relatively inaccessible extravascular catecholamine target tissues in a subtype-specific fashion and that these characteristics predict responses to catecholamines in vivo, we studied 22 normal humans. Adrenergic receptors and their linked adenylate cyclase systems were measured in mononuclear leukocytes (MNL; beta 2), platelets (alpha 2), skeletal muscle membranes (beta 2), and fat cells (B1 and alpha 2) and compared with the responses to stepped, intravenous epinephrine infusions in vivo. MNL beta 2-adrenergic receptor densities (but not antagonist affinities) were correlated (r = 0.627; P less than 0.01) with skeletal muscle beta 2-adrenergic densities. However, other adrenergic receptor characteristics and basal and maximally stimulated adenosine 3',5'-cyclic monophosphate (cAMP) contents of MNL and all adrenergic receptor characteristics and cAMP contents of platelets were unrelated to the corresponding measurements in skeletal muscle and fat. Furthermore, there were no consistent relationships between tissue adrenergic receptor-adenylate cyclase characteristics and the chronotropic, diastolic depressor, lipolytic, ketogenic, glycemic, or glycogenolytic-glycolytic responses to epinephrine in vivo. Thus the data support the hypothesis that adrenergic receptor densities on circulating cells reflect those of extravascular target tissues in a subtype-specific fashion. On the other hand, the data do not support the hypothesis that physiological interindividual variation of adrenergic receptor characteristics is of sufficient magnitude to alter sensitivity to epinephrine in vivo.

Increased fat and skeletal muscle beta-adrenergic receptors but unaltered metabolic and hemodynamic sensitivity to epinephrine in vivo in experimental human thyrotoxicosis

Based largely on evidence of increased target tissue beta-adrenergic receptor densities and responsiveness in animal and, to a lesser extent, human tissues, it is often assumed that thyroid hormone excess results in increased sensitivity to catecholamines in vivo, thus explaining several clinical manifestations of thyrotoxicosis. To test the hypothesis that thyrotoxicosis results in increased target tissue beta-adrenergic receptor densities and correspondingly increased metabolic and hemodynamic sensitivity to epinephrine in vivo, we measured these in 10 normal humans before and after administration of triiodothyronine (100 micrograms daily) for 10 d. Thyrotoxicosis increased beta-adrenergic receptor densities in fat (approximately 60%) and skeletal muscle (approximately 30%). Despite increments in beta-adrenergic receptor densities in these and probably other target tissues, metabolic and hemodynamic sensitivity to epinephrine in vivo was unaltered. An apparently adaptive increase in insulin secretion plausibly explains normal glycemic, glycogenolytic/glycolytic, lipolytic, and ketogenic sensitivity to epinephrine in the thyrotoxic state. In view of this striking homeostatic efficiency of the intact individual, the finding of altered adrenergic receptors, even in relevant target tissues, should not be extrapolated to altered sensitivity to catecholamines in vivo in the absence of direct testing of that hypothesis. With respect to the clinical issue, these data suggest that increased sensitivity to catecholamines does not explain clinical manifestations of thyrotoxicosis in humans.

Lymphocyte basal cyclic AMP production predicts blood pressure

Cyclic AMP (cAMP) serves as the second messenger for a variety of receptor systems. While much attention has been placed on the importance of receptor-stimulated levels of cAMP accumulation, little attention has been given to the potential importance of the unstimulated basal levels. We measured both stimulated and unstimulated cAMP accumulation, as well as receptor number, in lymphocytes in 15 subjects and found relatively strong associations between basal cAMP accumulation and systolic (p = .004) and diastolic (p less than .001) blood pressure. Blood pressure was unrelated to either receptor number or adrenergically-stimulated levels of cAMP. These results underscore not only the complexity of the role of cAMP in the regulation of blood pressure but also the usefulness of peripheral blood cells as models of more central physiological regulation.

Beta-adrenoceptors and human skeletal muscle characterisation of receptor subtype and effect of age

1. Rectus abdominis muscle biopsies were obtained from 28 patients undergoing abdominal surgery. In membranes prepared from these biopsies beta-adrenoceptor binding was examined. The apparent affinity (KD) and the density (Bmax) of the receptors for the radioligand (-)-[125I]cyanopindolol were 28.5 +/- 2.7 (pM) and 25.9 +/- 2.1 (fmol mg-1 protein) (mean +/- s.e. mean) respectively. In forceps biopsies from vastus lateralis muscle from four healthy volunteers the values for KD and Bmax were 22.5 +/- 4.4 (pM) and 16.4 +/- 2.2 (fmol mg-1 protein). The binding characteristics for the radioligand were similar in the biopsies from the two muscle sites. 2. Inhibition of the radioligand binding by the selective beta 2-adrenoceptor antagonist ICI 118551 (KI = 117 +/- 45 nM) and selective beta 1-adrenoceptor antagonist metoprolol (KI = 15229 +/- 5046 nM) suggests the dominance of beta 2-adrenoceptor subtype in human skeletal muscle. 3. There were no significant differences in the skeletal muscle beta-adrenoceptor densities or affinities between the young and older patients.

Regulation of beta-adrenoceptors by catecholamines in the rabbit skeletal muscle

(-)-[125I]Cyanopindolol (ICYP) was used to characterise beta-adrenoceptors in the gastrocnemius muscle of the rabbit. The binding of ICYP was saturable. The KI value for the selective beta 2-adrenoceptor antagonist, ICI 118551, was 0.16 +/- 0.01 nM (mean +/- S.E.M) and for the selective beta 1-adrenoceptor antagonist, metoprolol, was 300.1 +/- 33.3 nM. Hofstee plots for both antagonists were linear indicating the presence of homogeneous beta 2-adrenoceptors in the skeletal muscle of the rabbit. In gastrocnemius muscles obtained from rabbits chronically pretreated in vivo with vehicle (0.1% ascorbic acid), adrenaline (40-50 nmol.kg-1.h-1) or noradrenaline (80-100 nmol.kg-1.h-1) via osmotic mini-pumps implanted for 10 days, the Bmax values were 44 +/- 3, 25 +/- 4 (P less than 0.05) and 41 +/- 7 (fmol.mg-1 protein) and KD values were 7.3 +/- 0.9, 6.3 +/- 0.8 and 9.3 +/- 2.5 (pM), respectively. Thus pretreatment with the circulating hormone adrenaline but not noradrenaline down-regulated the number of skeletal muscle beta 2-adrenoceptors but did not influence the affinity.

Beta-adrenoceptor desensitization in guinea-pig isolated trachea

Exposure to (-)-isoprenaline (25 microM, 1 h) caused a stereoselective, time and concentration-related decrease in smooth muscle beta 2-adrenoceptor function in guinea-pig trachea. Furthermore, tracheal relaxant responsiveness to the beta-adrenoceptor agonists (+/-)-fenoterol and (-)-noradrenaline was reduced, while that to theophylline and nitroprusside was unaffected. Responsiveness to forskolin was marginally but significantly reduced. Indomethacin, a cyclooxygenase inhibitor and mepacrine, an inhibitor of phospholipid turnover, had no significant effect on the extent of isoprenaline-induced desensitization. Conversely, cortisol (25 microM) significantly reduced desensitization and enhanced the rate of spontaneous recovery of responsiveness to isoprenaline. Desensitization was not accompanied by a reduction in the density of beta-adrenoceptors in the trachea, as assessed by binding and light microscopic autoradiography using [125I]iodocyanopindolol [( 125I]CYP). Thus, desensitization was probably caused primarily by beta-adrenoceptor/adenyl cyclase uncoupling. This model may be useful in investigations of the effect of glucocorticoids on the beta-adrenoceptor dysfunction recognized in severe asthma.

Platelet function in classic migraine during attack-free periods

It has been reported that migraine sufferers have an enhanced platelet activity, as well as an enhanced thromboembolic risk. In this study, 12 subjects with classic migraine were compared with 12 age- and sex-matched controls. The following parameters were tested: platelet aggregation; platelet content of ADP, ATP and cyclic-AMP (cAMP); plasma levels of cAMP, 6-keto-PGF1 alpha and thromboxane B2, serum levels of TxB2 and lipids. ADP- and adrenaline-induced platelet aggregation, platelet content of ADP, ATP and cAMP did not differ significantly between the groups. Platelets from migraine subjects, however, showed a decreased beta-adrenoceptor response to stimulation with isoprenaline when determined as platelet cAMP production in vitro. The prostacyclin metabolite 6-keto-PGF 1 alpha was significantly decreased in the migraine group. No difference were observed in TxB2. Plasma lipids were similar, except that the migraine cases had higher levels of LDL-cholesterol. These data do not confirm a general platelet malfunction in migraine patients, but suggest a decreased functional capacity of platelet beta-adrenoceptors and an altered metabolism of prostacyclin and LDL-cholesterol.

Rebound increase in bronchial responsiveness after treatment with inhaled terbutaline

To investigate whether cessation of regular beta-agonist treatment results in an increase in bronchial responsiveness, two double-blind, randomised crossover studies were done. Subjects with mild asthma were investigated to determine the course of change in bronchial responsiveness, measured as the provocative dose (PD20) of histamine that caused a 20% fall in forced expiratory volume in 1 s after short-term and longer term treatment with an inhaled beta-agonist. In the first study in 8 subjects, 500 and 2000 micrograms terbutaline thrice in 1 day protected against histamine-induced bronchoconstriction, and the increase in PD20 compared with placebo remained high throughout the day and overnight. In the second study 8 subjects received placebo or terbutaline 750 micrograms thrice daily for 14 days. The protection afforded by terbutaline against histamine-induced bronchoconstriction on day 14 was less than that on day 1 by 40% in the morning and 82% in the afternoon. On day 15 PD20 was lower after stopping terbutaline than placebo, with a maximum difference of 1.5 (95% CI 0.6-2.5) doubling-doses of histamine 23 h after the end of treatment. Thus treatment with terbutaline for 1 day did not result in any rebound increase in bronchial responsiveness. Treatment for 2 weeks impaired the ability of terbutaline to protect against histamine-induced bronchoconstriction, and was followed by a rebound increase in bronchial responsiveness after cessation of treatment.

Effect of prednisolone and ketotifen on beta 2-adrenoceptors in asthmatic patients receiving beta 2-bronchodilators

In 13 patients with bronchial asthma, who were on beta 2-adrenergic bronchodilator therapy, the effects of prednisolone and ketotifen on lymphocyte beta 2-adrenoceptor density and -responsiveness were investigated. The mean lymphocyte beta 2-adrenoceptor density and -responsiveness was significantly lower than in healthy controls, presumably due to the long-term beta 2-adrenergic bronchodilator treatment. Both prednisolone 100 mg i.v. and ketotifen 1 mg b.d.p.o. for 6 days rapidly improved lymphocyte beta 2-adrenoceptor function. 16 h after prednisolone and about 6 days after the first dose of ketotifen lymphocyte beta 2-adrenoceptor density and -responsiveness had risen to values within the range in normal volunteers. The improvement of lymphocyte beta 2-adrenoceptor function was accompanied by a significant increase in peak expiratory flow rate before and after inhalation of salbutamol. It is concluded that prednisolone and ketotifen may act beneficially on the recovery of beta 2-adrenoceptor responsiveness to beta 2-adrenergic bronchodilators in tolerant asthmatic patients.