RELATIONSHIP BETWEEN THE (−)-[3H]-DIHYDROALPRENOLOL BINDING TO β-ADRENOCEPTORS AND TRANSMEMBRANE 86Rb EFFLUX OF THE BC3H1 NONFUSING MUSCLE CELL LINE

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Abstract
1 We have studied the binding properties of the β-adrenoceptor antagonist [3H]-dihydroalprenolol ([3H]-DHA) on a membrane preparation of the non-fusing muscle cells BC3H1. 2 [3H]-DHA appears to bind to two classes of sites. The first site has a high affinity (KD = 0.53 nm) and a low capacity (Bmax = 58 fmol/mg of protein). The second site has a low affinity (KD =110 nm) and a high capacity (Bmax = 1100 fmol/mg of protein). 3 The pharmacological properties of the high affinity low capacity site correspond to the known properties of the β2-adrenoceptors since the agonists inhibit [3H]-DHA binding following the series isoprenaline > adrenaline > noradrenaline > phenylephrine and the antagonists following the series alprenolol ≃ propranolol > butoxamine > practolol > phentolamine. 4 The binding properties of the β-adrenoceptors were correlated with the effect of β-adrenoceptor agonists and antagonists on 86Rb efflux rate from BC3H1 cells. 5 There is very good correlation between the dissociation constants obtained by inhibition of [3H]-DHA binding by the antagonists alprenolol and propranolol, and the inhibition constants calculated from their antagonism of the 86Rb efflux rate stimulation by adrenaline. The ratio of the dissociation constants obtained by inhibition of [3H]-DHA binding by agonists and their EC50, calculated from 86Rb efflux curves, is higher than 1. This high KD/EC50 ratio indicates a high coupling efficiency between receptor occupancy by agonists and the biological effect measured.