TY - JOUR T1 - Binding of Perhydro-histrionicotoxin to the Postsynaptic Membrane of Skeletal Muscle in Relation to Its Blockade of Acetylcholine-Induced Depolarization JF - Molecular Pharmacology JO - Mol Pharmacol SP - 1 LP - 14 VL - 13 IS - 1 AU - JO. O. DOLLY AU - E. X. ALBUQUERQUE AU - J. M. SARVEY AU - B. MALLICK AU - E. A. BARNARD Y1 - 1977/01/01 UR - http://molpharm.aspetjournals.org/content/13/1/1.abstract N2 - Perhydro-histrionicotoxin (H12-HTX), a potent neuromuscular blocking agent, was investigated as a potential probe for conductance systems associated with the acetylcholine receptor. In rat muscles, H12-HTX blocked the end plate potential and also the extrajunctional acetylcholine sensitivity, with half-maximal effects seen at about 8.4 µM and 2.4 µM, respectively. The retardation of α-bungarotoxin binding to denervated muscle membranes by increasing concentrations of H12-HTX showed a noncompetitive type of behavior, with the half-maximal effect at concentrations of H12-HTX much higher than needed to block the acetylcholine response. The binding of H12-HTX, as detected by this protection from α-bungarotoxin, was essentially the same in the membrane and in the pure, soluble receptor. H12-HTX at sufficient concentrations could block 90-100% of the bungarotoxin binding sites; the same was true for d-tubocurarine, confirming that these are the receptor sites, in both innervated and denervated muscle preparations. Measurements of the affinity of d-tubocurarine, by blockade of electro-physiological response, gave values for the apparent dissociation constant in rat extensor digitorum longus muscle (innervated) of 0.039 µM, and in rat soleus (denervated) of 0.8 µM, at 23°. The results of these studies support an earlier proposal that H12-HTX blocks neuromuscular transmission by acting at a site other than the acetylcholine recognition site of the receptor. ER -