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.