Interaction of pinaverium (a quaternary ammonium compound) with 1,4‐dihydropyridine binding sites in rat ileum smooth muscle

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 [3H]‐(+)‐PN200–110 ([3H]‐(+)‐isradipine) specific binding to tissue homogenates incompletely (Ki 0.38 μm; 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 [3H]‐(+)‐isradipine binding sites are not accessible to pinaverium because it is associated with sealed inside‐out vesicles. 3 Pinaverium bromide increased the apparent KD of [3H]‐(+)‐isradipine binding to saponin‐treated homogenates but did not significantly affect the Bmax value. Moreover, the dissociation rate constant of [3H]‐(+)‐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, [3H]‐diltiazem binding to rat brain membranes (at 30–37°C). 4 Although Bmax values of [3H]‐(+)‐isradipine were similar in homogenates prepared from tissue and cells (collagenase‐treated), the KD value was significantly higher in cell homogenates (166 vs 95 pm). Similarly, the Ki value of pinaverium was higher in cell preparations than in tissue homogenates (0.77 vs 0.38 μm). Thus, collagenase can significantly modify the dihydropyridine recognition site. 5 The competitive interaction of pinaverium, a permanently charged drug, with [3H]‐(+)‐isradipine bound to intact cells and its absence of interaction with [3H]‐(+)‐isradipine bound to sealed inside‐out vesicles imply that the dihydropyridine receptor lies near the external surface of the plasma membrane.