Abstract
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 (pKi 9.54 +/- 0.04, nH 1.01) for the [3H]-(+/-)-PN 200-110 binding site in rat cerebral cortex membranes. In the presence of 5 mM Ca2+ this affinity was reduced (pKi 8.82 +/- 0.01, nH 1.05) whilst the affinity of nitrendipine was unaffected by this concentration of Ca2+. 3. Saturation binding experiments in rat cerebral cortex carried out in the absence and presence of SR 33557 (0.1-1.0 nM) indicated an apparently competitive interaction at the dihydropyridine site, in that SR 33557 reduced the KD of [3H]-(+/-)-PN 200-110 binding without any effect on Bmax. In kinetic experiments, the rate of dissociation of [3H]-(+/-)-PN 200-110 from rat cerebral cortex was unchanged in the presence of SR 33557 (5 nM). 4. D-cis-diltiazem fully reversed the inhibition [3H]-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 [3H]-SR 33557 (0.01-0.8 nM) labelled a single class of binding sites in rat cerebral cortex membranes with high affinity (KD 0.12 +/- 0.01, Bmax 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 [3H]-SR 33557 and [3H]-(+/-)-PN 200-110 sites. The [3H]-SR 33557 site was sensitive to the inhibitory effect of divalent cations. The affinity of Cd2+ was 0.026 +/- 0.015 mM and the rank order of affinity was Cd2+ >Ca2+ >Mn2+ >Mg2+ >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.