Molecular Pharmacology, Vol 13, 1-14, Copyright © 1977 by the American Society for Pharmacology and Experimental Therapeutics

Binding of Perhydro-histrionicotoxin to the Postsynaptic Membrane of Skeletal Muscle in Relation to Its Blockade of Acetylcholine-Induced Depolarization

¹ Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore, Maryland 21201, and Department of Biochemistry, State University of New York at Buffalo, Buffalo, New York 14214

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 -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 -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.

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				E. Albuquerque, A. Eldefrawi, M. Eldefrawi, N. Mansour, and M. Tsai Amantadine: neuromuscular blockade by suppression of ionic conductance of the acetylcholine receptor Science, February 17, 1978; 199(4330): 788 - 790. [Abstract] [PDF]