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Non classical, multiple‐site interaction of [ 3 H]‐prazosin with the α 1 ‐adrenoceptor of intact BC 3 H 1 cells
Author(s) -
Sladeczek Fritz,
Kirk Christopher J.,
Bockaert Joël,
Schmidt Bernard H.
Publication year - 1989
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.1476-5381.1989.tb12567.x
Subject(s) - prazosin , agonist , chemistry , dissociation constant , binding site , receptor , inositol phosphate , catecholamine , inositol , stereochemistry , endocrinology , antagonist , biochemistry , biology
1 In intact BC 3 H 1 cells the EC 50 of noradrenaline (NA) for the inositol phosphate response measured at 37°C (EC 50 = 193 n m ) was much lower than its apparent dissociation constant ( K i 37°c = 83.211 μ m ) determined at this temperature by [ 3 H]‐prazosin binding. 2 After pretreatment of the cells with NA at 37°C for 45 min, the time used in binding assays at this temperature, this difference between EC 50 and K i 37°c did not decrease significantly. An agonist‐induced reduction in α 1 ‐adrenoceptor affinity can therefore not explain the very high K i 37°C value. 3 NA pretreatment at 37°C decreased the number of [ 3 H]‐prazosin binding sites (assessed by whole cell binding at 2°C) by only 49%; not by 100%, the value expected if agonist‐induced receptor internalization were the origin of the very low K i 37°C . 4 The EC 50 of NA for the inositol phosphate response in the presence of 156 pM [ 3 H]‐prazosin was 1.841 μ m but the IC 50 of NA for the inhibition of [ 3 H]‐prazosin binding (126 p m ) was 316 μ m . As there is no α 1 ‐adrenoceptor reserve in these cells we propose that at 37°C [ 3 H]‐prazosin interacts, not only with the catecholamine recognition site (site 1) of the receptor, but also reacts weakly with another site from which it cannot be directly displaced by catecholamine‐like substances (site 2).