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© Copyright 2004 The British Pharmacological Society

003P University of Bath
Summer Meeting July 2004

Selectivity of neutral β-adrenoceptor antagonists for the two sites of the human β1-adrenoceptor

Jillian G Baker. Institute of Cell Signalling, University of Nottingham, Queen’s Medical Centre, Nottingham NG7 2UH, UK

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Baker JG

The human β1 receptor is currently thought to have two agonist activation sites or conformations. Recent studies have shown that several β-adrenoceptor antagonists inhibit agonist responses elicited via the catecholamine site with higher affinity that those of the secondary site (Konkar et al., 2000; Baker et al., 2003). Several clinically used antagonists have been found to have agonist activity and this may by activation of the catecholamine site, the secondary site or both (Baker et al., 2003). The aim of this study was to determine the affinity of several clinically-used neutral β-adrenoceptor antagonists, for these two sites.

CHO-K1 cells stably expressing both the human β1-adrenoceptor (at 79fmol/mg protein) and a luciferase reporter gene under the transcriptional control of six cAMP response elements were used. CRE-luciferase production 5 hours after the addition of agonist was measured as previously described (Baker et al., 2003).

Isoprenaline stimulated an increase in CRE-luciferase that was 6.5 ± 0.3 fold over basal (log EC₅₀ = -7.99 ± 0.07; n=25). The endogenous agonists adrenaline (log EC₅₀ -6.75 ± 0.04; n=22) and noradrenaline (log EC₅₀ -7.22 ± 0.05; n=21) and CGP 12177 (log EC₅₀ -7.43 ± 0.02; n=24) stimulated responses that were respectively 100.2 ± 1.0; 99.9 ± 1.5 and 34.9 ± 1.2 % of the maximum response to isoprenaline in the same experiments. These agonist responses were antagonised by β-adrenoceptor antagonists to yield log K_D values as shown in the Table.

antagonist	isoprenaline	n	adrenaline	n	CGP 12177	n
timolol	-8.65 ± 0.23	5	-8.49 ± 0.01	4	-6.30 ± 0.04	5
bupranolol	-8.53 ± 0.16	5	-8.64 ± 0.15	4	-7.33 ± 0.04	5
betaxolol	-8.30 ± 0.13	5	-8.24 ± 0.14	4	-5.74 ± 0.04	4
propanolol	-8.27 ± 0.18	5	-8.36 ± 0.09	4	-6.34 ± 0.09	5
bisoprolol	-8.06 ± 0.16	5	-8.13 ± 0.16	4	-5.53 ± 0.06	5
nadolol	-7.59 ± 0.13	5	-7.33 ± 0.09	4	-5.95 ± 0.04	5
metoprolol	-7.53 ± 0.10	5	-7.34 ± 0.11	4	-5.24 ± 0.05	5
ICI 118551	-6.80 ± 0.27	4	-6.61 ± 0.08	4	-5.87 ± 0.06	4
atenolol	-6.73 ± 0.18	5	-6.84 ± 0.09	4	-3.82 ± 0.04	5
sotalol	-5.76 ± 0.07	5	-5.71 ± 0.11	4	-3.66 ± 0.09	5

Table. Log K _D values for a range of β-adrenoceptor antagonists for antagonism of responses to each of the four agonists. Values are expressed as mean ± S.E.M. for n separate experiments.

This study shows that the affinity of many clinically-used β-adrenoceptor antagonists is higher for the catecholamine site than the secondary site of the β1-adrenoceptor. However, they differ in the extent to which they discriminate between the two sites. Furthermore, the endogenous agonists adrenaline and noradrenaline behave in a very similar manner to isoprenaline.

Baker J.G. et al., (2003). Mol. Pharmacol.63, 1312-1321.
Konkar A.A. et al., (2000) J. Pharmacol. Exp. Ther.294, 923-932.