β‐Agonist‐induced constitutive β₂‐adrenergic receptor activity in bovine tracheal smooth muscle

Abstract

1 According to the two state receptor model, the b 2 -adrenergic receptor (b 2 -AR) isomerizes between an inactive state and a constitutively active state, which couples to the stimulatory Gprotein in the absence of agonist. In bovine tracheal smooth muscle (BTSM), we investigated the e ect of short and long term b 2 -AR activation by fenoterol on constitutive receptor activity. 2 Preincubation of the BTSM strips for 5 min, 30 min and 18 h with 10 mM fenoterol, followed by extensive washout (3 h, 378C), caused a rapid and time-dependent inhibition of KCl-induced contraction, reaching 68+10, 51+6 and 46+4% of control, respectively, at 40 mM KCl (P50.05 all). At all time points, the EC 50 values to KCl were signi®cantly reduced as well. 3 Preincubation of BTSM with 0.1, 1.0 and 10 mM fenoterol during 18 h caused a concentrationdependent decrease of the 40 mM KCl response to 70+5, 47+12 and 43+9% of control, respectively (P50.05 all). 4 The reduced KCl contractions were reversed in the presence of 1 mM timolol. Moreover, the sensitivity to KCl in the presence of timolol was enhanced after fenoterol incubation. Inverse agonism was also found for other b-blockers, with a rank order of e cacy of pindolol 5timolol=propranolol4alprenolol5sotalol4labetalol. 5 At 25 mM KCl-induced tone, the contraction induced by cumulative timolol administration was competitively antagonized by the less e cacious inverse agonist labetalol, indicating that the fenoterol-induced e ects cannot be explained by residual b-agonist binding. 6 In conclusion, fenoterol treatment of BTSM causes a time-and concentration-dependent development of constitutive b 2 -AR activity, which can be reversed by various inverse agonists. The b-agonist-induced changes could represent a novel regulation mechanism of b 2 -AR activity. British Journal of Pharmacology (2000) 131, 915 ± 920

Terbutaline causes immobilization of single β2-adrenergic receptor-ligand complexes in the plasma membrane of living A549 cells as revealed by single-molecule microscopy

“…Receptor immobilization due to interactions with the cytoskeletal network or binding to intracellular proteins can be expected from a constitutively active receptor population, which is involved in the internalization process independent of a strong agonistic stimulation. 22 Additionally, single β 2 AR-Alexa-NA complexes were observed diffusing within restricted domains, which corroborates the localization of the receptor within microdomains like lipid rafts. 23,24 Also, sterical hindrance of free diffusion due to the close proximity of the cytoskeleton to the plasma membrane is possible.…”

A Systematic Review of Inverse Agonism at Adrenoceptor Subtypes

“…The group of Zaagsma and Meurs from Groningen, The Netherlands, used bovine tracheal smooth muscle strips and found that an 18 h treatment with fenoterol reduced airway contractility; when various antagonists were added after the fenoterol treatment (all dosed to achieve 98–99% receptor occupancy), they restored airway contractility with a rank order of efficacy of pindolol ≥ timolol = propranolol > alprenolol ≥ sotalol > labetalol [ 124 ]. Of note, these effects do not necessarily imply IA as the underlying mechanism, particularly as the observed rank order does not match that observed for the extent of IA (see above).…”

Pharmacodynamic study of interaction of aqueous leaf extract of psidium guajava linn. (myrtaceae) with receptor systems using isolated tissue preparations

“…Several pathways are known to involve in the relaxation of isolated trachea including β2-agonism, H 2 -antagonism, NO induced relaxation and prostonoids-induced relaxation [21][22][23][24] . The exact cause of relaxant effect of β2-agonists in carbachol-contracted rat tracheal muscle is not yet clear.…”