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[ 3 H]‐CLONIDINE BINDING TO α‐ADRENOCEPTORS IN MEMBRANES PREPARED FROM REGIONS OF GUINEA‐PIG KIDNEY: ALTERATION BY MONOVALENT AND DIVALENT CATIONS
Author(s) -
SUMMERS R.J.
Publication year - 1980
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.1476-5381.1980.tb10909.x
Subject(s) - divalent , chemistry , membrane , renal cortex , binding site , sodium , dissociation constant , magnesium , calcium , kidney , inorganic chemistry , stereochemistry , receptor , endocrinology , biochemistry , biology , organic chemistry
1 [ 3 H]‐clonidine binds reversibly to membranes prepared from regions of guinea‐pig kidney. 2 Higher levels of binding were obtained in the membranes prepared from renal cortex (2.15 ± 0.27 pmol/g wet wt.) than renal medulla (0.53 ± 0.07 pmol/g wet wt.) or papilla (0.14 ± 0.06 pmol/g wet wt.; n = 4). 3 Scatchard analysis performed by addition of unlabelled clonidine (1 to 30 pmol) gave figures for the dissociation constant ( K d ) for the binding of [ 3 H]‐clonidine to renal cortical membranes of 9.0 ± 0.8 n m and B max of 21.6 ± 1.7 pmol/g wet wt. ( n = 4). Hill plots of these data gave gradients close to unity, indicating a lack of co‐operative site interactions. 4 The monovalent cations, sodium and potassium, and the divalent cation, calcium, produced concentration‐dependent decreases in [ 3 H]‐clonidine binding to membranes prepared from renal cortex, the EC 50 s being respectively 25 m m , 37 m m and 23 m m . 5 At low concentrations the divalent cations, magnesium (1 m m ) and manganese (0.1 m m ), produced enhancement of [ 3 H]‐clonidine binding. At higher concentrations (> 10 m m ) both divalent cations inhibited binding. 6 Scatchard analysis of [ 3 H]‐clonidine binding performed in the presence of sodium (100 m m ), magnesium (1 m m ) or manganese (0.1 m m ) revealed that the alterations in binding are primarily due to changes in apparent affinity rather than a change in the number of binding sites. Sodium (100 m m ) produced a change in the K d from 7.0 ± 0.4 n m ( n = 8) to 42.3 ± 27.5 n m ( n = 3), whereas magnesium (1 m m ) decreased the K d to 6.0 ± 0.9 n m and manganese (0.1 m m ) to 4.0 ± 1.0 n m ( n = 3). 7 The results indicate that [ 3 H]‐clonidine labels a binding site that has properties resembling an α 2 ‐adrenoceptor, located in the renal cortex. The changes produced by the addition of monovalent and divalent cations are entirely due to changes in the apparent affinity of [ 3 H]‐clonidine binding.