Identical binding properties of (+/-)- and (-)-125Iodocyanopindolol to beta 2-adrenoceptors in intact human lymphocytes.

In intact human lymphocytes the properties of (+/-)- and (-)-125iodocyanopindolol (ICYP) binding to beta 2-adrenoceptors were investigated. Binding of both ligands was in concentrations ranging from 10-200 pM saturable resulting in identical Bmax-values (about 550-650 specific ICYP binding sites/cell); the KD-value of (-)-ICYP (14.8 +/- 0.1 pM, N = 3), however, was about two times lower than that of (+/-)-ICYP (26.5 +/- 2.5 pM, N = 3). Irrespective of the ligand used alprenolol, betaxolol, ICI 118,551 and isoprenaline inhibited binding with identical KI-values. Binding was stereospecific, since the (+)-isomers of alprenolol and isoprenaline were about 50 times less potent in inhibiting binding than their respective (-)-isomers. In concentrations greater than 250 pM both ligands labeled a second class of binding sites in a non-saturable manner. These low affinity sites showed no stereospecificity; they may be related to unspecific uptake of the ligands into the cell. In kinetic experiments biphasic dissociation reactions were obtained for (+/-)- as well as (-)-ICYP; for both ligands the ratio fast/slow dissociating component was 40/60. It is concluded, that at low receptor concentrations (2.5-5 pmoles/l in the present study) the contribution of the (+)-isomer of ICYP to binding of the racemic ligand can be neglected. Thus, at low receptor concentrations both (+/-) and (-)-ICYP exhibit identical binding properties to beta 2-adrenoceptors in intact human lymphocytes.