Molecular Pharmacology, Vol 12, 82-91, Copyright © 1976 by the American Society for Pharmacology and Experimental Therapeutics

Kinetics of Interaction of Batrachotoxin and Tetrodotoxin on Rat Diaphragm Muscle

¹ Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore, Maryland 21201

The interaction between batrachotoxin (BTX) and tetrodotoxin (TTX) and local anesthetics (procaine and lidocaine) was examined on rat diaphragm muscle at 37°. At low concentrations TTX shifted the curve for BTX-induced membrane depolarization to the right, and at higher concentrations it depressed the maximal membrane depolarization induced by BTX. A plot of log (dose ratio - 1) vs. log TTX concentration gave a straight line with a slope of 1.31. The estimated dissociation constant K_B of TTX was 30.2 nM. Both procaine and lidocaine inhibited BTX-induced membrane depolarization when applied either before or during exposure to BTX; a form of noncompetitive antagonism was disclosed. Although TTX reversed the depolarization induced by BTX, the persistent and irreversible action of BTX became apparent after TTX had been washed from the bath. When procaine and lidocaine were applied in a similar manner the repolarization of the muscle membrane was incomplete; upon washing in drug-free physiological solution, the membrane potential and frequency of spontaneous miniature end plate potentials returned toward control values. During the protective action of procaine and lidocaine, sodium activation associated with an action potential was only slightly reduced. The results suggest that TTX and BTX react with different sites along the same sodium channel, but that procaine and lidocaine probably interfere with the binding of BTX to its receptor site.

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ACKNOWLEDGMENTS We thank Ms. Mabel A. Zelle for expert technical and secretarial assistance, and Dr. Sharad Deshpande for helpful criticism of the manuscript. We also thank Drs. John W. Daly and Bernhard Witkop for their generous supply of batrachotoxin and for their helpful criticism of the manuscript.