Comparison of the Binding Activities of Some Drugs on α_2A, α_2B and α_2C‐Adrenoceptors and Non‐Adrenergic Imidazoline Sites in the Guinea Pig

Abstract: Simultaneous computer modelling of control and guanfacine‐masked [³H]‐MK 912 saturation curves as well as guanfacine competition curves revealed that both α_2A‐ and α_2C‐adrenoceptor subtypes were present in the guinea pig cerebral cortex. The K_d value of [₃H]‐MK 912 determined for the α_2A‐subtype was 403 pM and for the α_2C‐subtype 79.8 pM; the receptor sites showing capacities 172 and 19.5 fmol/mg protein, respectively. The K_ds of guanfacine were 20 and 880 nM for the α_2A‐ and α_2C‐adrenoceptor, respectively. In the guinea pig kidney [₃H]‐MK 912 bound to a single saturable site with K_d 8.34 nM and capacity 285 fmol/mg protein, the site showing pharmacological properties like an α_2B‐adrenoceptor. Binding constants of 22 compounds for the three guinea pig α₂‐adrenoceptor subtypes were determined by computer modelling competition curves using for the cerebral cortex a “3‐curve assay”, for the kidney an “1‐curve assay”, and using [³H]‐MK 912 as labelled ligand. Of the tested drugs guanfacine and BRL 44408 were found to be clearly α_2A‐selective. Spiroxatrine, yohimbine, rauwolscine and WB 4101, as well as [₃H]‐MK 912 itself, were found to be α_2C‐selective. The most selective compounds for α_2B‐adrenoceptors, when compared to α_2A‐adrenoceptors, were ARC 239 and prazosin. In the guinea pig kidney [₃H]‐p‐aminoclonidine bound to α₂‐adrenoceptors as well as to non‐adrenergic imidazoline sites. The α₂‐adrenoceptors could be completely blocked using 10 μM (‐)‐adrenaline without the non‐adrenergic sites being affected. During these conditions the analysis of combined saturation and competition studies using labelled and unlabeled p‐aminoclonidine with computer modelling revealed that the ligand labelled two different sites with K_ds of 310 and 47,000 nM, respectively. Competition curves of 16 compounds for the non‐adrenergic [₃H]‐p‐aminoclonidine sites were shallow and resolved into two‐site fits. For the high affinity [₃H]‐p‐aminoclonidine site the highest affinities were shown by 1‐medetomidine, UK‐14,304, guanabenz and detomidine; the K_ds of these drugs ranging 26–72 nM. All drugs tested showed low but varying affinities for the low affinity [₃H]‐p‐aminoclonidine site. These data indicated that the [³H]‐p‐aminoclonidine binding sites of the guinea pig kidney are grossly different from the [₃H]‐idazoxan binding I₂‐receptors previously demonstrated also to be present in the guinea pig kidney. 1995 Nordic Pharmacological Society