Summary
The α2A-adrenoceptors in rat spleen, kidney, spinal cord and cerebral cortex were studied using [3H]-RX821002 radioligand binding. In the spleen, spinal cord and cerebral cortex, the ligand bound to saturable sites with a K d of about 1 nmol/l and capacities of 134, 240 and 290 fmol/mg protein, respectively. Computer modelling competition curves for 39 drugs, including those for α2A-, α2B- or α2C-adrenoceptor selective drugs, indicated that the sites labelled by [3H]-RX821002 in the spleen consisted of a single population of α2A-adrenoceptors. However, the competition curves for guanoxabenz were definitely biphasic and resolved into two site fits, indicating that guanoxabenz was binding to both high affinity (K d = 35 nmol/1) and low affinity (K d = 8900 nmol/1) α2A-adrenoceptor sites in the proportions 57% and 43%, respectively. The K d Sfor a number of α2-adrenoceptor subtype selective drugs, measured in competition with [3H]-RX821002 in cerebral cortex and spinal cord, were highly correlated with those obtained in the spleen indicating their α2A-adrenoceptor nature. However, by contrast to the results with the spleen, the guanoxabenz competition curves for the spinal cord and cerebral cortex were monophasic and resolved only into one site fits, the K d of guanoxabenz being about 4000 nmol/l for both tissues. Drug K d Sfor kidney α2A-adrenoceptors were also determined using [3H]-RX821002. For nearly all drugs tested, the K d Swere highly correlated with those found for the α2A-adrenoceptors in the other rat tissues. However, for guanoxabenz, the data indicated that it competed with [3H]-RX821002 at a single α2A-adrenoceptor site with a K d of 39 nmol/1. When the rat α2A-adrenoceptor gene RG20 was transiently expressed in COS-7 cells and its ligand binding properties probed using [3H]-RX821002, the drug K d Sobtained were also highly correlated with those found for the α2A-adrenoceptors in the spleen, cerebral cortex, spinal cord and kidney of the rat. For the RG20 encoded receptor, the guanoxabenz competition curves were steep and monophasic and modelled best into one site fits, with the Kd of guanoxabenz being 5200 nmol/1.
It is suggested that guanoxabenz can differentiate between two forms of α2A-adrenoceptors in the rat: α2A1 and α2A2. The α2A1-form is present in the spleen and kidney where it shows a high apparent affinity for guanoxabenz. The α2A2-form shows a low apparent affinity for guanoxabenz and is present in the spleen, cerebal cortex and spinal cord. The α2A2-form of the rat α2-adrenoceptor appears to be encoded by the RG20 gene. The α2A, and α2A2-adrenoceptor forms do not represent high and low affinity receptor forms for agonists because assays included EDTA, Gpp(NH)p and Na+, which eliminated the high affinity receptors for agonists.
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Uhlén, S., Xial, Y., Chhajlanil, V. et al. Evidence for the existence of two forms of α2A-adrenoceptors in the rat. Naunyn-Schmiedeberg's Arch Pharmacol 347, 280–288 (1993). https://doi.org/10.1007/BF00167446
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DOI: https://doi.org/10.1007/BF00167446