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[ 3 H]‐GUANFACINE: A RADIOLIGAND THAT SELECTIVELY LABELS HIGH AFFINITY α 2 ‐ADRENOCEPTOR SITES IN HOMOGENATES OF RAT BRAIN
Author(s) -
JARROTT BEVYN,
LOUIS WILLIAM J.,
SUMMERS ROGER J.
Publication year - 1982
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.1476-5381.1982.tb08801.x
Subject(s) - chemistry , prazosin , radioligand , guanfacine , yohimbine , methoxamine , endocrinology , medicine , population , phentolamine , agonist , binding site , clonidine , receptor , antagonist , biology , biochemistry , environmental health
1 [ 3 H]‐guanfacine (N‐amidino‐2‐(2,6‐dichloro 3[ 3 H] phenyl) acetamide hydrochloride; 24.2 Ci/mmol) has been used as a radioligand in homogenates of rat cerebral cortex. 2 Specific binding of [ 3 H]‐guanfacine was linear with respect to tissue concentration (2.5–15 mg/ml), saturable and not markedly affected in the pH range 6.5–8.0. 3 Analysis of the saturation of [ 3 H]‐guanfacine binding using an iterative least squares fitting procedure gave best fits to a single site model. 4 [ 3 H]‐guanfacine binding was of high affinity ( K d 1.71 ± 0.24 n m ; n = 8) to a population of non interacting sites (nH 0.99 ± 0.02; n = 8) with a density of 118.2 ± 8.4 fmol/mg protein ( n = 8). 5 Highest levels of binding were achieved in cerebral cortex followed by thalamus > hypothalamus > medulla/pons > spinal cord > striatum > cerebellum. 6 Binding was stereoselective with regard to the (—)‐isomer of noradrenaline and the order of potency for displacement of [ 3 H]‐guanfacine by agonists was naphazoline > clonidine > (—)‐adrenaline > (—)‐α methylnoradrenaline > (—)‐noradrenaline > (±)‐α‐methylnoradrenaline > (+)‐noradrenaline > methoxamine > (+)‐adrenaline > phenylephrine and by antagonists was phentolamine > dihydroergocryptine > piperoxane > yohimbine > prazosin > labetalol > indoramin suggested binding to α 2 ‐adrenoceptors. 7 The monovalent cations Na + and K + and also guanosine 5′‐triphosphate (GTP) produced concentration‐dependent inhibition whereas the divalent cations Ca 2+ , Mg 2+ , and Mn 2+ first enhanced, then inhibited [ 3 H]‐guanfacine binding. 8 Na + (150 m m ) or GTP (100 μ m ) produced marked reductions and Mn 2+ (5 m m ) marked increases in the number of receptor sites labelled by [ 3 H]‐guanfacine. 9 It is concluded that [ 3 H]‐guanfacine preferentially labels a high affinity state of the α 2 ‐adrenoceptor in homogenates of rat cerebral cortex.