A beta-adrenergic partial agonist (befunolol) discriminates two different affinity sites.

Interactions of beta-adrenergic partial agonists with the beta-adrenoceptor were studied in isolated guinea-pig taenia caecum. The competitive inhibition curve for specific binding of a high concentration (50 nM) of [3H]-befunolol by befunolol showed a biphasic shape, although the curve for specific binding of a low concentration (1 nM) was monophasic. All the competitive inhibition curves for specific binding of [3H]-befunolol (1 nM and 50 nM) by isoprenaline and propranolol showed monophasic shapes. These results suggest that befunolol may be able to discriminate two different binding sites of the beta-adrenoceptor: the high affinity site and the low affinity site.