Development of a radioiodinated ligand for characterising alpha 1-adrenoceptors

Two alpha-adrenoceptor antagonists, phentolamine and 2-(beta-(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 alpha-adrenoceptors in guinea pig brain whereas the 125I-BE 2254 derivative binds rapidly, reversibly and with high affinity to these receptors with a Kd of 230 pM. At low concentrations of 125I-BE 2254 (less than 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 alpha 1 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 alpha-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 micrograms 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.