COMPARISON OF THE BINDING ACTIVITIES OF SOME DRUGS ON ALPHA(2A), ALPHA(2B) AND ALPHA(2C)-ADRENOCEPTORS AND NONADRENERGIC IMIDAZOLINE SITES IN THE GUINEA-PIG

Simultaneous computer modelling of control and guanfacine-masked [H-3]-MK 912 saturation curves as well as guanfacine competition curves revealed that both alpha(2A)- and alpha(2C)-adrenoceptor subtypes were present in the guinea pig cerebral cortex. The K-d value of [H-3]-MK 912 determined for the alpha(2A)-subtype was 403 pM and for the alpha(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 alpha(2A)- and alpha(2C)-adrenoceptor, respectively. In the guinea pig kidney [H-3]-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 alpha(2B)-adrenoceptor. Binding constants of 22 compounds for the three guinea pig alpha(2)-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 [H-3]-MK 912 as labelled ligand. Of the tested drugs guanfacine and BRL 44408 were found to be clearly alpha(2A)- selective. Spiroxatrine, yohimbine, rauwolscine and WB 4101, as well as [H-3]-MK 912 itself, were found to be alpha(2C)-selective. The most selective compounds for alpha(2B)-adrenoceptors, when compared to alpha(2A)-adrenoceptors, were ARC 239 and prazosin. In the guinea pig kidney [H-3]-p-aminoclonidine bound to alpha(2)-adrenoceptors as well as to non-adrenergic imidazoline sites. The alpha(2)-adrenoceptors could be completely blocked using 10 mu M (-)-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 K(d)s of 310 and 47,000 nM, respectively. Competition curves of 16 compounds for the non-adrenergic [H-3]-p-aminoclonidine sites were shallow and resolved into two-site fits. For the high affinity [H-3]-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 [H-3]-p-aminoclonidine site. These data indicated that the [H-3]-p-aminoclonidine binding sites of the guinea pig kidney are grossly different from the [H-3]-idazoxan binding I-2-receptors previously demonstrated also to be present in the guinea pig kidney.