Development of a radioiodinated ligand for characterising α1-adrenoceptors

Two α-adrenoceptor antagonists, phentolamine and 2-(β-(4-hydroxyphenyl)-ethylaminomethyl)-tetralone (BE 2254) which are phenolic derivatives were radioiodinated after chloramine-T oxidation of Na125I and the labelled material isolated by chromatography. 125I-Phentolamine does not bind selectively to α-adrenoceptors in guinea pig brain whereas the 125I-BE 2254 derivative binds rapidly, reversibly and with high affinity to these receptors with a K/sub d/ of 230 pM. At low concentrations of 125I-BE 2254 (< 100 pM) approx. 90% of the bound radioligand is specifically bound and under these conditions drug displacement studies show that the ligand binds predominantly to the α1 subclass of adrenoceptors. Binding measurements to kidney and smooth muscle membrane preparations indicate that 125I-BE 2254 may also be a useful tool in the study of α-adrenoceptors in peripheral tissues. The high specific activity of 125I-BE 2254 permits the use of minimal quantities of membrane material for receptor assay and ligand displacement measurements, e.g. 250 μg per assay tube, and this provides a significant advantage over the use of existing radioligands such as 3H-prazosin which requires approx. 40 times as much tissue