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Delineation of three pharmacological subtypes of α 2 ‐adrenoceptor in the rat kidney
Author(s) -
Uhlén Staffan,
Wikberg Jarl E.S.
Publication year - 1991
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1991.tb12485.x
Subject(s) - affinities , prazosin , chemistry , arc (geometry) , competition (biology) , oxymetazoline , stereochemistry , adrenergic receptor , receptor , biology , antagonist , ecology , biochemistry , mathematics , geometry
1 Simultaneous computer modelling of plain and ARC 239‐ and guanoxabenz‐masked [ 3 H]‐RX821002 saturation curves, plain ARC 239 and guanoxabenz competition curves as well as ARC 239‐masked guanoxabenz competition curves revealed that the drugs bound to three α 2 ‐adrenoceptor subtypes in the rat kidney with grossly differing selectivities. These α 2 ‐adrenoceptor subtypes were termed α 2A , α 2B1 and α 2B2 . The order of affinities for [ 3 H]‐RX821002 for the adrenoceptor sites was α 2A > α 2B1 > α 2B2 , the K d s being 0.62 ± 0.05, 2.52 ± 0.11 and 6.74 ± 1.21 n m , respectively. The order of affinities for ARC 239 was α 2B1 > α 2B2 ≫ α 2A with K d s 4.78 ± 1.04, 28.8 ± 4.1 and 1460 ± 270 n m , respectively. For guanoxabenz the order of affinities was α 2A > α 2B1 ≫ α 2B2 with K d s 99.7 ± 15.1, 508 ± 135 and 25,400 ± 2400 n m , respectively. 2 Binding constants for 14 compounds for the three rat kidney α 2 ‐adrenoceptor subtypes were determined by the simultaneous computer modelling of plain and ARC 239‐ and guanoxabenz‐masked drug competition curves, plain ARC 239 and guanoxabenz competition curves as well as ARC 239‐masked guanoxabenz competition curves. Of the 14 compounds tested, oxymetazoline and guanfacine were found to bind with low affinities to both of the α 2B1 ‐ and α 2B2 ‐adrenoceptors but with high affinity to the α 2A ‐adrenoceptor. Prazosin instead bound with high affinities to the α 2B1 ‐ and α 2B2 ‐adrenoceptors but with low affinity to the α 2A ‐adrenoceptor. By contrast, guanoxabenz and ARC 239 delineated clearly between all the three α 2 ‐adrenoceptor subtypes. Notably the affinities of guanoxabenz for α 2B1 ‐ and α 2B2 ‐adrenoceptors differed 72 fold and for α 2A ‐ and α 2B2 ‐adrenoceptors 380 fold. The selectivities of a number of other drugs were less marked but their K d s were consistent with all three sites being α 2 ‐adrenoceptors. 3 (−)‐Adrenaline and (−)‐noradrenaline showed dissimilar order of affinities for the three α 2 ‐adrenoceptors. For (−)‐adrenaline the order of affinities was α 2B1 ≥ α 2A > α 2B2 and for (−)‐noradrenaline α 2B2 ≥ α 2B1 > α 2A . All three α 2 ‐adrenoceptors showed the expected stereoselective binding for adrenaline enantiomers, the (+)‐form being 7–10 fold less potent than the (−)‐form. 4 [ 3 H]‐yohimbine was also used as radioligand. The data with this ligand were fully compatible with the [ 3 H]‐RX821002 data. However, [ 3 H]‐yohimbine appeared to label only α 2B1 ‐ and α 2B2 ‐adrenoceptors presumably because it had too low an affinity for α 2A ‐adrenoceptors. 5 We conclude that three pharmacological subtypes of α 2 ‐adrenoceptors are labelled by [ 3 H]‐RX821002 in the rat kidney. Guanoxabenz and ARC 239 may be used in competition studies to delineate between these three α 2 ‐adrenoceptor subtypes. Moreoever, we here present a method allowing the determination of binding constants for an arbitrary drug to the three α 2 ‐adrenoceptor subtypes.