Rat Spinal Cord α2-Adrenoceptors are of the α2A-Subtype: Comparison with α2A- and α2B-Adrenoceptors in Rat Spleen,Cerebral Cortex and Kidney Using 3H-RX821002 Ligand Binding

Abstract: Binding of the α₂-adrenoceptor antagonist radioligand ³H-RX821002 was investigated in membranes from rat spinal cord, spleen, cerebral cortex and kidney. The ligand was found to bind to saturable binding sites with apparent uniform affinities within each tissue. Seven compounds, some of which have previously been reported to be selective for either α_2A- or α_2B-adrenoceptors, were used in competition with ³H-RX821002. By using computer modelling, competition curves generated for three of these compounds (ARC 239, prazosin and oxymetazoline) could be resolved into two site fits in the kidney, K_d's of the drugs being compatible with the notion that these sites corresponded to α_2A- and α_2B-adrenoceptors. Moreover, rauwolscine and yohimbine were found to be about 14 and 9-fold selective for α_2B-adrenoceptors in the kidney. In all other tissues studied drug competition curves were uniphasic and computer modelled into one site fits, drug K_d's being well correlated to those for the α_2A-adrenoceptor. In rat spinal cord 26 further drugs, which showed wide variation in structure, were evaluated in competition with ³H-RX821002. Of these compounds, competion curves of the agonists UK-14,304, (—) and (+) adrenaline were modelled into two site fits whereas those of the remaining compounds could be modelled only into one site fits. Since the high affinity site for UK-14,304, (—) and (+) adrenaline was eliminated when EDTA, Gpp(NH)p and 140 mM NaCl was present in the assay the heterogeniety observed in spinal cord was considered to be due to formation of high and low affinity conformations of the α₂-adrenoceptor for agonists. It is concluded that ³H-RX821002 is useful to label both α_2A- and α_2B-adrenoceptors in the rat. Moreover, the binding sites labelled by ³H-RX821002 in the spinal cord appear to consist of a single population of α₂-adrenoceptors of the α_2A-type.