Agonist-independent alteration in beta-adrenoceptor-G-protein-adenylate cyclase system in an equine model of recurrent airway obstruction

We examined the inhibitory sympathetic P-adrenergic mechanisms in peripheral lung, bronchi and trachea of an equine model of recurrent airway obstruction (RAO), to support the hypothesis that the P-adrenergic receptor dysfunction is not only restricted to cell surface receptor density but rather encompasses a mechanistic defect apart from the receptor, to the intracellular signaling components. The non-asthmatic lung possessed 3.2-fold more P-adrenergic receptors than bronchi (496 +/- 19.4 vs. 155.1 +/- 19.6 fmol/mg protein; P < 0.01) and 6.2-fold higher than in the trachea (79.8 +/- 12.6 fmol/mg protein; P < 0.001) (assessed by radioligand binding assays using (-)-[I-125]- iodocyanopindolol, ICYP) and in all tissues a greater proportion of the beta(2)- than the beta(1)-subtype (75-80%). The receptor density (B-max) in lung parenchyma and bronchial membranes was 33 and 42%, respectively, lower (P < 0.001) in RAO than in control animals, attributable to a decrease in the beta(2)-subtype. This receptor down-regulation was accompanied with an attenuated coupling efficiency of the receptor to the stimulatory G(S)-protein (P < 0.05 vs. control). Concomitantly, activation of adenylate cyclase evoked by isoproterenol was significantly reduced in lung and bronchial membranes of animals with RAO, whereas effects of 10 mu M GTP, 10 mM NaF, 10 mu M forskolin and 10 mM Mn2+ were not altered. There was no difference in beta-adrenergic receptor density, G(S)-protein or adenylate cyclase coupling in the trachea between asthmatic and control animals. In conclusion, in stable asthma the pulmonary P-adrenergic receptor-G(S)-protein-adenylate cyclase system is impaired, thus the pathologic process involves all signaling components, and due to its close similarity, this animal model seems to serve as a suitable model, at least partly, of chronic asthmatic patients. (c) 2005 Elsevier Ltd. All rights reserved.