Non-competitive antagonism of β2-agonist-mediated cyclic AMP accumulation by ICI 118551 in BC3H1 cells endogenously expressing constitutively active β2-adrenoceptors

1 Constitutive activity of the beta(2)-adrenoceptor, which is sensitive to inhibition by an inverse agonist such as ICI 118551, has been readily demonstrated in recombinant systems expressing constitutively-active mutant receptors or over-expressing the wild-type beta(2)-adrenoceptor. Here we demonstrate the presence of constitutive beta(2)-adrenoceptor activity in BC3H1 cells which endogenously express this receptor. 2 In BC3H1 cells, only ICI 118551 behaved as an inverse agonist at beta(2)-adrenoceptors, while propranolol, ICI 118551, atenolol and, to a lesser extent, alprenolol exhibited inverse agonism in CHO-beta(2)4 cells transfected with cDNA for the human beta(2)-adrenoceptor (310 fmol.mg protein(-1)). The level of expression of beta(2)-adrenoceptors in BC3H1 cells was not high (78 fmol.mg protein(-1)) and the efficiency of receptor-effector coupling in this cell line was much lower than in the recombinant CHO-beta(2)4 cells (as judged by the partial agonist nature of both salbutamol and clenbuterol). 3 ICI 118551 (log K-D -9.73+/-0.07) and propranolol (log K-D-9.25+/-0.12) both behaved as conventional competitive antagonists of isoprenaline-stimulated cyclic AMP accumulation in high expressing CHO-beta(2)4 cells. In contrast, ICI 118551 appeared to act as a non-competitive antagonist in BC3H1 cells and in low expressing CHO-beta(2)6 cells (50 fmol.mg protein(-1)). 4 This non-competitive effect of ICI 118551 in BC3H1 cells was also observed when either salbutamol was used as agonist, or the incubation period with isoprenaline was extended to 30 min. 5 The possibility that these effects of ICI 118551 are due to an interaction with different affinity states (R, R* and R') of the receptor is discussed.