Premium
Identification of a novel calcium antagonist binding site in rat brain by SR 33557
Author(s) -
Kenny Barry A.,
Fraser Stuart,
Speeding Michael
Publication year - 1993
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.1476-5381.1993.tb13445.x
Subject(s) - nitrendipine , binding site , chemistry , calcium , cerebral cortex , antagonist , biophysics , endocrinology , medicine , stereochemistry , receptor , biochemistry , biology , organic chemistry
1 In K + ‐depolarized taenia preparations from guinea‐pig caecum SR 33557 was a potent antagonist of Ca 2+ ‐induced contractions and antagonized the effect of the calcium channel activator Bay K 8644. 2 SR 33557 displayed high affinity (pK i 9.54 ± 0.04, n H 1.01) for the [ 3 H]‐(±)‐PN 200‐110 binding site in rat cerebral cortex membranes. In the presence of 5 m m Ca 2+ this affinity was reduced (pK i 8.82 ± 0.01, n H 1.05) whilst the affinity of nitrendipine was unaffected by this concentration of Ca 2+ . 3 Saturation binding experiments in rat cerebral cortex carried out in the absence and presence of SR 33557 (0.1–1.0 n m ) indicated an apparently competitive interaction at the dihydropyridine site, in that SR 33557 reduced the K D of [ 3 H]‐(±)‐PN 200‐110 binding without any effect on B max . In kinetic experiments, the rate of dissociation of [ 3 H]‐(±)‐PN 200‐110 from rat cerebral cortex was unchanged in the presence of SR 33557 (5 n m ). 4 d ‐cis‐diltiazem fully reversed the inhibition [ 3 H]‐nitrendipine binding to rat cerebral cortex produced by SR 33557 indicating the site of action of SR 33557 to be distinct from the dihydropyridine (DHP) binding site. 5 Saturation analysis indicated that [ 3 H]‐SR 33557 (0.01–0.8 n m ) labelled a single class of binding sites in rat cerebral cortex membranes with high affinity ( K D 0.12 ± 0.01, B max 222 ± 20 fmol mg −1 protein), although kinetic data indicated the existence of negative cooperativity between the binding sites. 6 In competition studies, a variety of different calcium antagonists displayed similar affinity for [ 3 H]‐SR 33557 and [ 3 H]‐(±)‐PN 200‐110 sites. The [ 3 H]‐SR 33557 site was sensitive to the inhibitory effect of divalent cations. The affinity of Cd 2+ was 0.026 ± 0.015 m m and the rank order of affinity was Cd 2+ > Ca 2+ > Mn 2+ > Mg 2+ > Na + . 7 We propose that SR 33557 labels a distinct site in rat cerebral cortex. The coupling between the SR 33557 and DHP site appears to be very close, resulting in apparently competitive interactions in some experimental protocols but can be revealed as negatively allosteric in other circumstances.