Identification of a homogeneous class of beta 2-adrenoceptors in human platelets by (-)-125I-iodopindolol binding.

The highly specific beta-adrenoceptor radioligand (-)-125I-iodopindolol (IPIN) was used to label in membranes from human platelets beta-adrenoceptors. Binding of IPIN was at 25 degrees C saturable (Bmax = 7.4 +/- 2.5 fmoles bound/mg protein, N = 10) of high affinity (KD-value = 34.8 +/- 3.3 pM, N = 10), rapid and reversible. beta-Adrenoceptor antagonists inhibited binding of IPIN with monophasic displacement curves and pseudo Hill coefficients (nH) not significantly different from 1.0. The beta 2-selective antagonist ICI 118,551 was much more potent in inhibiting IPIN binding than the beta 1-selective antagonist bisoprolol. IPIN binding was stereospecific as indicated by the 100-times greater potencies of (-)-alprenolol and (-)-isoprenaline in inhibiting binding than their (+)-isomers. beta-Adrenoceptor agonists inhibited binding with slightly shallow displacement curves and nH-values of 0.85-0.88; the order of potency was: isoprenaline greater than adrenaline greater than noradrenaline. These results demonstrate that IPIN labels (in human platelet membranes with high affinity) a homogeneous population of beta 2-adrenoceptors with low capacity.