Bi‐affinity α₁‐adrenoceptor binding in normal rat brain in vivo

The specific binding in rat brain in vivo of [¹²⁵I]HEAT ([¹²⁵I]iodo‐2‐[β;‐(4‐hydroxyphenyl)ethylamino methyl)], a selective α₁‐adrenoceptor ligand, was analyzed by a method designed to distinguish sites with different affinities. The data indicate at least two sites with different affinities for the α₁‐adrenoreceptor in normal rat brain in vivo: A high‐affinity site with K_d (half‐saturation constant) of 3.6 ± 0.7 nM (AV ± SD), and a low‐affinity site with K_d of 668 ± 552 nM. The density (B_max) of the high‐affinity site in nine brain regions—auditory, visual, sensorimotor (four layers) and frontal cortex and lateral thalamic and medial geniculate nuclei—varied from 2.2 ± 0.8 to 14.6 ± 0.6 pmole/g, while the low‐affinity range was 149 ± 44 to 577 ± 30 pmole/g. The results also revealed a very dynamic relationship between the two sites regulated by the concentration of the ligand ranging from 80% preponderance of the high‐affinity sites at low ligand concentrations to more than 95% preponderance of the low‐affinity sites at high ligand concentrations.