Comparison of the Binding Activities of Some Drugs on α2A, α2B and α2C‐Adrenoceptors and Non‐Adrenergic Imidazoline Sites in the Guinea Pig

Simultaneous computer modelling of control and guanfacine-masked [ 3 H]-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 [ 3 H]-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 d s of guanfacine were 20 and 880 nM for the α 2A - and α 2C -adrenoceptor, respectively. In the guinea pig kidney [ 3 H]-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 α 2 -adrenoceptor subtypes were determined by com puter modelling competition curves using for the cerebral cortex a «3-curve assay», for the kidney an «1-curve assay», and using [ 3 H]-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 [ 3 H]-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 [ 3 H]-p-aminoclonidine bound to α 2 -adrenoceptors as well as to non-adrenergic imidazoline sites. The α 2 -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 unlabelled p-aminoclonidine with computer modelling revealed that the ligand labelled two different sites with was of 310 and 47,000 nM, respectively. Competition curves of 16 compounds for the non-adrenergic [ 3 H]-p-aminoclonidine sites were shallow and resolved into two-site fits. For the high affinity [ 3 H]-p-aminoclonidine site the highest affinities were shown by 1-medetomidine, UK-14,304, guanabenz and detomidine; the K d s of these drugs ranging 26-72 nM. All drugs tested showed low but varying affinities for the low affinity [ 3 H]-p-aminoclonidine site. These data indicated that the [ 3 H]-p-aminoclonidine binding sites of the guinea pig kidney are grossly different from the [ 3 H]-idazoxan binding I 2 -receptors previously demonstrated also to be present in the guinea pig kidney