ESTIMATION OF THE DISSOCIATION CONSTANT OF d-TUBOCURARINE AND THE RECEPTOR FOR ENDOGENOUS ACETYLCHOLINE

¹ Department of Physiology and Pharmacology, College of Veterinary Medicine, University of Georgia, Athens, Georgia

The equilibrium dissociation constant for d-tubocurarine (d-TC) and the receptor for endogenous acetylcholine (transmitter) in the magnesium-paralyzed frog sartorius nervemuscle preparation was determined by a quantal content dose ratio method in which amplitudes of end-plate potentials recorded in the presence of d-TC were matched with those recorded in its absence. Concentration of transmitter (quantal content) was measured by application of the quantum theory and was raised in the presence of d-TC 1) by facilitation during a train of repetitively evoked end-plate potentials, 2) by increasing frequency of repetitive stimulation or 3) by increasing Ca⁺⁺ concentration of the bathing solution. The mean equilibrium dissociation constant (K_d-TC) for d-TC as determined with the quantal content dose ratio method was 3.47 ± 0.23 x 10^-7 M. The similarity of this value for K_d-TC to those derived from results obtained in other studies utilizing bath application of depolarizing agents suggests that d-TC antagonizes endogenously and exogenously applied agonists to a similar extent.