Decamethonium both opens and blocks endplate channels.

Abstract

Miniature endplate currents, endplate current fluctuations (membrane noise) and voltage-jump current relaxations were studied in voltage-clamped frog muscle fibers during decamethonium action. All 3 types of experiments revealed 2 kinetic processes controlling the opening of endplate channels, one that reflects agonist action and another that reflects local anesthetic-like blocking activity. The kinetic constants for these 2 steps were evaluated from measurements of the fast and slow time constants as a function of decamethonium concentration. At -130 mV membrane potential and 13.degree. C, the mean open time of decamethonium-activated channels is 2.8 ms. The forward and backward rate constants for channel blocking are 1.7 .times. 107 M-1 s-1 and 103 s-1. The voltage dependencies of the channel lifetime and of the blocking equilibrium are similar to those seen with pure agonists and local anesthetics, respectively.