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Three distinct binding sites for [ 3 H]‐prazosin in the rat cerebral cortex
Author(s) -
Oshita Masafumi,
Kigoshi Shigeru,
Muramatsu Ikunobu
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.tb12533.x
Subject(s) - prazosin , yohimbine , phentolamine , chemistry , binding site , stereochemistry , receptor , biochemistry , antagonist
1 The putative α 1 ‐adrenoceptor subtypes of rat cerebral cortex membranes were characterized in binding experiments with [ 3 H]‐prazosin. 2 Specific binding of [ 3 H]‐prazosin was saturable between 20–5000 p m . Scatchard plots of the binding data were non‐linear, indicating the presence of two distinct affinity sites for prazosin (pK D, high = 10.18, R high = 308 fmol mg −1 protein; pK D, low = 8.96, R low = 221 fmol mg −1 protein). 3 In the membranes pretreated with chlorethylclonidine (CEC) two affinity sites for prazosin were also observed: the affinities were similar to those without CEC pretreatment, but the maximum numbers of binding sites were reduced by CEC pretreatment to 23 and 62% for prazosin‐high (R high ) and low affinity sites (R low ), respectively. 4 The prazosin‐high affinity sites were further subdivided into two subclasses by WB4101(2‐(2,6‐dimethoxyphenoxyethyl)aminomethyl‐1,4‐benzodioxane) and phentolamine; the low affinity sites for WB4101 and phentolamine were more potently inactivated by CEC as compared with the high affinity sites. On the other hand, prazosin, HV723 (α‐ethyl‐3,4,5‐trimethoxy‐α‐(3‐((2‐(2‐methoxyphenoxy)ethyl)‐amino)‐propyl)benzeneacetonitrile fumarate) and yohimbine inhibited [ 3 H]‐prazosin binding to prazosin‐high affinity sites monophasically. 5 In addition to the high affinity sites, the prazosin‐low affinity sites were labelled at high concentrations of [ 3 H]‐prazosin. Thus, prazosin and WB4101 showed shallow displacement curves. On the other hand, HV723 and yohimbine did not discriminate between prazosin‐high and low affinity sites. 6 Two distinct α 1 ‐adrenoceptor subclassifications have been recently proposed (α 1A , α 1B subtypes and α 1H , α 1L , α 1N subtypes). According to the criteria defined with competitive antagonists in both the subclassifications, the present results indicate that the α 1 ‐adrenoceptors of rat cerebral cortex consist of three different subtypes, presumably α 1A , α 1B and α 1L , and suggest that the α 1A and α 1B subtypes are identified as a single affinity site for prazosin (α 1H ). The results also indicate that care must be taken in the use of CEC for α 1 ‐adrenoceptor subclassification because of its low selectivity.