| Abstract: | The binding properties of a newly developed, highly selective beta 1-adrenoceptor antagonist radioligand, (-)3H]bisoprolol (EMD 33512) were investigated in rabbit lung membranes containing a mixture of 80% beta 1-and 20% beta 2-adrenoceptors. The binding of (-)3H]bisoprolol at 25 degrees C was saturable, of high affinity (KD = 4.7 +/- 0.6 nM, N = 4), rapid and readily reversible. The maximal number of (-)3H]bisoprolol binding sites (244 +/- 31 fmol bound/mg protein, N = 4), however, was only 80% of the number of sites labelled by the non-selective beta-adrenoceptor radioligand (-)125I]iodocyanopindolol (299 +/- 36 fmol bound/mg protein, N = 4). beta-Adrenoceptor antagonists (non-selective: propranolol, alprenolol; beta 1-selective: metoprolol, practolol, bisoprolol; beta 2-selective: ICI 118,551) inhibited (-)3H]bisoprolol binding with monophasic displacement curves and pseudo-Hill coefficients of 1.0 indicating that in rabbit lung membranes (-)3H]bisoprolol labels a homogeneous class of beta-adrenoceptors. Agonists inhibited binding with an order of potency: (-)-isoprenaline greater than (-)-noradrenaline = (-)-adrenaline, which is a typical one for beta 1-adrenoceptors. It is concluded that in rabbit lung membranes (-)3H]bisoprolol selectively labels beta 1-adrenoceptors. (-)3H]Bisoprolol therefore seems to be a suitable ligand for direct determination of the properties of beta 1-adrenoceptors in those tissues where both beta-adrenoceptor subtypes coexist. |
