Rat Spinal Cord α₂‐Adrenoceptors are of the α_2A‐Subtype: Comparison with α_2A‐ and α_2B‐Adrenoceptors in Rat Spleen, Cerebral Cortex and Kidney Using ³H‐RX821002 Ligand Binding

Abstract

Binding of the alpha 2-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 alpha 2A- or alpha 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, Kd's of the drugs being compatible with the notion that these sites corresponded to alpha 2A- and alpha 2B-adrenoceptors. Moreover, rauwolscine and yohimbine were found to be about 14 and 9-fold selective for alpha 2B-adrenoceptors in the kidney. In all other tissues studied drug competition curves were uniphasic and computer modelled into one site fits, drug Kd's being well correlated to those for the alpha 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 alpha 2-adrenoceptor for agonists. It is concluded that 3H-RX821002 is useful to label both alpha 2A- and alpha 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 alpha 2-adrenoceptors of the alpha 2A-type.

[3H]‐MK 912 binding delineates two α2‐adrenoceptor subtypes in rat CNS one of which is identical with the cloned pA2d α2‐adrenoceptor

“…We have previously shown that radioligand binding with [3HJ-RX821002 can clearly delineate between M2A-and a2B-adrenoceptors in the rat kidney (Uhlen & Wikberg, 1991b). In another study we have shown that [3H]-RX821002 can label an X2A-type of adrenoceptor in the cerebral cortex and spinal cord of the rat (Uhlen & Wikberg, 1991a). We now present data that a recently introduced radioligand, [3H]-MK 912 (L-657,743) (Pettibone et al, 1989), besides labelling the z2A-adrenoceptor can delineate one additional receptor subtype belonging to the a2-adrenoceptor family in the cerebral cortex and spinal cord of the rat.…”

“…Membranes from cerebral cortex and spinal cord were prepared from Sprague-Dawley rats, as described previously (Uhlen & Wikberg, 1991a). The final pellets were diluted to protein concentrations.…”

ICA-17043, a novel Gardos channel blocker, prevents sickled red blood cell dehydration in vitro and in vivo in SAD mice

“…60 Finally, the cellular effects of CLT observed in the SAD mouse have been predictive of the red cell changes seen in patients with sickle cell disease. 24,32,60 Treatment with ICA-17043 produced a large and sustained inhibition of RBC Gardos channel activity in SAD mice. Associated with the Gardos channel block was an increase in red cell K ϩ content, an increase in Hct, a decrease in MCHC, and a decrease in red cell densities.…”

Antisense technology reveals the alpha2A adrenoceptor to be the subtype mediating the hypnotic response to the highly selective agonist, dexmedetomidine, in the locus coeruleus of the rat.

“…Pharmacological studies have defined four subtypes named ␣ 2A , ␣ 2B , ␣ 2C , and ␣ 2D , with the ␣ 2D representing a species homologue of ␣ 2A (the term ␣ 2A will be used to designate the ␣ 2A/D subtype). The three ␣ 2 adrenergic receptor subtypes are nonhomogeneously distributed in the central nervous system (7)(8)(9)(10)(11)(12)(13)(14)(15) with the ␣ 2A and ␣ 2C subtypes predominating while the ␣ 2B subtype is sparsely represented and only in the diencephalon (16). In this study, using antisense oligodeoxynucleotide (ODN) technology (17-21), we establish that the ␣ 2A is the subtype mediating the hypnotic response to dexmedetomidine in the LC.…”