Beta-agonist-induced constitutive beta(2)-adrenergic receptor activity in bovine tracheal smooth muscle.
According to the two state receptor model, the beta(2)-adrenergic receptor (beta(2)-AR) isomerizes between an inactive state and a constitutively active state, which couples to the stimulatory G-protein in the absence of agonist. In bovine tracheal smooth muscle (BTSM), we investigated the effect of short and long term beta(2)-AR activation by fenoterol on constitutive receptor activity. Preincubation of the BTSM strips for 5 min, 30 min and 18 h with 10 microM fenoterol, followed by extensive washout (3 h, 37 degrees C), 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 (P:<0.05 all). At all time points, the EC(50) values to KCl were significantly reduced as well. Preincubation of BTSM with 0.1, 1.0 and 10 microM fenoterol during 18 h caused a concentration-dependent decrease of the 40 mM KCl response to 70+/-5, 47+/-12 and 43+/-9% of control, respectively (P:<0.05 all). The reduced KCl contractions were reversed in the presence of 1 microM timolol. Moreover, the sensitivity to KCl in the presence of timolol was enhanced after fenoterol incubation. Inverse agonism was also found for other beta-blockers, with a rank order of efficacy of pindolol >/=timolol=propranolol>alprenolol>/=sotalol>labetalol. At 25 mM KCl-induced tone, the contraction induced by cumulative timolol administration was competitively antagonized by the less efficacious inverse agonist labetalol, indicating that the fenoterol-induced effects cannot be explained by residual beta-agonist binding. In conclusion, fenoterol treatment of BTSM causes a time- and concentration-dependent development of constitutive beta(2)-AR activity, which can be reversed by various inverse agonists. The beta-agonist-induced changes could represent a novel regulation mechanism of beta(2)-AR activity.
de Vries
B
B
Department of Molecular Pharmacology, University Centre for Pharmacy, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands.
Meurs
H
H
Roffel
A F
AF
Elzinga
C R
CR
Hoiting
B H
BH
de Vries
M M
MM
Zaagsma
J
J
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000818:Animals; D002417:Cattle; D004305:Dose-Response Relationship, Drug; D005280:Fenoterol / Q000494:pharmacology; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects* / Q000502:physiology; D011189:Potassium Chloride / Q000494:pharmacology; D018343:Receptors, Adrenergic, beta-2 / Q000187:drug effects* / Q000502:physiology; D013999:Timolol / Q000494:pharmacology; D014132:Trachea / Q000187:drug effects* / Q000502:physiology
D000318:Adrenergic beta-Agonists; D000319:Adrenergic beta-Antagonists; D018343:Receptors, Adrenergic, beta-2; D005280:Fenoterol; D011189:Potassium Chloride; D013999:Timolol