title: Effects of beta 2-agonist- and dexamethasone-treatment on relaxation and regulation of beta-adrenoceptors in human bronchi and lung tissue.
abstract: 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.
authors.affiliation: 1. Medizinischen Klinik und Poliklinik, Technische Universität, München, Germany.
authors.forename: R W
authors.initials: RW
authors.lastname: Hauck
authors.forename: M
authors.initials: M
authors.lastname: Harth
authors.forename: C
authors.initials: C
authors.lastname: Schulz
authors.forename: H
authors.initials: H
authors.lastname: Präuer
authors.forename: M
authors.initials: M
authors.lastname: Böhm
authors.forename: A
authors.initials: A
authors.lastname: Schömig
mesh_terms: D058666:Adrenergic beta-2 Receptor Agonists; D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000328:Adult; D000368:Aged; D000628:Aminophylline / Q000494:pharmacology; D001980:Bronchi / Q000187:drug effects* / Q000502:physiology; D003907:Dexamethasone / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D008168:Lung / Q000187:drug effects* / Q000502:physiology; D008297:Male; D008875:Middle Aged; D009126:Muscle Relaxation / Q000187:drug effects; D011943:Receptors, Adrenergic, beta / Q000187:drug effects*; D013726:Terbutaline / Q000494:pharmacology
chemical_list: D058666:Adrenergic beta-2 Receptor Agonists; D000318:Adrenergic beta-Agonists; D011943:Receptors, Adrenergic, beta; D000628:Aminophylline; D003907:Dexamethasone; D013726:Terbutaline