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

Constitutive activity of the β2‐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 β2‐adrenoceptor. Here we demonstrate the presence of constitutive β2‐adrenoceptor activity in BC3H1 cells which endogenously express this receptor. In BC3H1 cells, only ICI 118551 behaved as an inverse agonist at β2‐adrenoceptors, while propranolol, ICI 118551, atenolol and, to a lesser extent, alprenolol exhibited inverse agonism in CHO‐β24 cells transfected with cDNA for the human β2‐adrenoceptor (310 fmol.mg protein−1). The level of expression of β2‐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‐β24 cells (as judged by the partial agonist nature of both salbutamol and clenbuterol). ICI 118551 (log KD−9.73±0.07) and propranolol (log KD−9.25±0.12) both behaved as conventional competitive antagonists of isoprenaline‐stimulated cyclic AMP accumulation in high expressing CHO‐β24 cells. In contrast, ICI 118551 appeared to act as a non‐competitive antagonist in BC3H1 cells and in low expressing CHO‐β26 cells (50 fmol.mg protein−1). 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. 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.