The beta-adrenergic receptor in human lymphocytes: subclassification by the use of a new radio-ligand, (+/-)-125 Iodocyanopindolol

(±)?¹²⁵Iodocyanopindolol (ICYP), a new radio-ligand with high affinity and specificity to β-adrenoceptors was used to identify and characterized β-adrenergic receptors in human lymphocytes. Binding of ICYP was saturable with 1.56 ± 0.2 fmol ICYP specifically bound/10⁶ cells at maximal occupancy of the sites and of high affinity (K_D=57 ± 7.1pM, N=4. In contrast to ¹²⁵Iodohydroxybenzylpindolol ICYP-binding was not affected by phentolamine (up to 10^?4M) or serotin (up to 10^?5M). Analysis of inhibition of ICYP-binding via a pseudo-Scatchard-plot (“Hofstee-plot”) by β_1-selective (practocol, metaprolol) and β_2-selective (IPS 339, zinterol) adrenergic drugs resulted in linear plots suggesting the existence of a homogeneous population of β-adrenergic receptorsin human lymphocytes. From the resulting K_D-values for practolol (16.8 μM), metoprolol (4.11 μM), zinterol (0.08 μM) and IPS 339 (0.002 μM) is concluded that the β-adrenergic receptor present in human lymphocytes is of the β₂-subtype. According to its low non-specific binding and its high specificity to β-adrenergic receptors ICYP appears to be an ideal ligand for long-term studies on the regulation of β-adrenergic receptors of human lymphocytes.