| Popis: |
The potent dihydropyrazole insecticide RH 3421 (methyl 1-( N -(α,α,α,-trifluoro- p -tolyl)carbamoyl-3-(4-chlorophenyl)-4-methyl-2-pyrazolin-4-ylcarboxylate), which has previously been shown to block voltage-gated and neurotoxin-activated sodium currents through sodium channels in insect nerves and insect and mammalian synaptic vesicles, inhibited the binding of [ 3 H]batrachotoxinin A 20-α-benzoate (BTX-B) to the alkaloid activator recognition site (Site 2) of voltage-sensitive sodium channels in mouse brain preparations. The presence of veratridine or a high concentration of BTX-B reduced the potency of RH 3421 as an inhibitor of BTX-B binding, but a concentration of RH 3421 close to its I 50 value did not alter the inhibitory potency of veratridine. Equilibrium saturation studies showed that RH 3421 at 0.2 μ M acted as a noncompetitive inhibitor of BTX-B binding, decreasing the apparent density of BTX-B binding sites in mouse brain vesicles by approximately 40% without affecting the affinity of mouse brain sodium channels for this ligand. Kinetic experiments showed that RH 3421 at 0.2 μ M had no effect on the rates of association or dissociation of BTX-B, but RH 3421 at 10 μ M increased the dissociation rate approximately threefold. Preincubation of vesicles with RH 3421 followed by repeated washing and resuspension produced a progressive recovery of specific BTX-B binding, thereby demonstrating that the binding of RH 3421 to sodium channels was slowly reversible. These results show that RH 3421 binds to a site on the voltage-sensitive sodium channel that is allosterically coupled to Site 2, the activator recognition site, and acts as a noncompetitive inhibitor of the binding of Site 2 ligands. The dihydropyrazole recognition site exhibits properties similar to the binding site(s) for local anesthetics, class I anticonvulsants, and class I antiarrhythmics on the sodium channel. |
