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Interaction of pinaverium (a quaternary ammonium compound) with 1,4‐dihydropyridine binding sites in rat ileum smooth muscle
Author(s) -
Feron Olivier,
Wibo Maurice,
Christen MarieOdile,
Godfraind Théophile
Publication year - 1992
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.1476-5381.1992.tb14279.x
Subject(s) - isradipine , chemistry , dihydropyridine , binding site , dissociation constant , calcium , pharmacology , biophysics , biochemistry , biology , receptor , organic chemistry
1 The interaction of pinaverium bromide, a quaternary ammonium compound, with binding sites for (L‐type) calcium channel blockers was investigated in rat ileum smooth muscle. 2 Pinaverium inhibited [ 3 H]‐(+)‐PN200–110 ([ 3 H]‐(+)‐isradipine) specific binding to tissue homogenates incompletely ( K i 0.38 μ m ; maximal inhibition 80%). In contrast, binding to single cell preparations (obtained by collagenase treatment) and to saponin‐treated homogenates was completely inhibited. These data are compatible with the view that, in untreated homogenates, 20% of [ 3 H]‐(+)‐isradipine binding sites are not accessible to pinaverium because it is associated with sealed inside‐out vesicles. 3 Pinaverium bromide increased the apparent K D of [ 3 H]‐(+)‐isradipine binding to saponin‐treated homogenates but did not significantly affect the B max value. Moreover, the dissociation rate constant of [ 3 H]‐(+)‐isradipine binding was not changed by pinaverium. These data suggest that pinaverium interacts with the dihydropyridine binding site in a competitive manner. However, in contrast to uncharged dihydropyridine calcium antagonists, pinaverium inhibited, rather than stimulated, [ 3 H]‐diltiazem binding to rat brain membranes (at 30–37°C). 4 Although B max values of [ 3 H]‐(+)‐isradipine were similar in homogenates prepared from tissue and cells (collagenase‐treated), the K D value was significantly higher in cell homogenates (166 vs 95 p m ). Similarly, the K i value of pinaverium was higher in cell preparations than in tissue homogenates (0.77 vs 0.38 μ m ). Thus, collagenase can significantly modify the dihydropyridine recognition site. 5 The competitive interaction of pinaverium, a permanently charged drug, with [ 3 H]‐(+)‐isradipine bound to intact cells and its absence of interaction with [ 3 H]‐(+)‐isradipine bound to sealed inside‐out vesicles imply that the dihydropyridine receptor lies near the external surface of the plasma membrane.