The actions of tubocurarine at the frog neuromuscular junction.

Abstract

The action of tubocurarine on voltage-clamped frog muscle end-plates was re-examined by equilibrium dose-ratio measurements, current fluctuation measurements and voltage-jump relaxation measurements. The equilibrium measurements can be interpreted as implying that tubocurarine has a competitive blocking action, with a Kd of 0.34 .mu.M, which is not dependent on membrane potential, and an additional voltage-dependent blocking action. In the presence of tubocurarine 2 kinetic components can be seen. The faster one is similar to, but rather faster than, the normal ion channel closing rate. The other is much slower (1-3 s), and, in relaxation experiments it is in the opposite direction to the fast relaxation. A combination of competitive block (or block of shut channels), with a strongly voltage-dependent block of open ion channels by tubocurarine is a possible mechanism of action of the drug. Estimates of the rate constants for channel blocking (and their voltage dependence) are derived. From these estimates the Kd for the binding of tubocurarine to open channels appears to be .apprx. 0.12 .mu.M at -70 mV and 0.02 .mu.M at -120 mV. Several potential sources of error in the experiments and in their interpretation are discussed. Particularly problems associated with diffusion in the small volume of the synaptic cleft, i.e., changes in cleft concentration consequent on changes in binding, and ionophoretic flux of antagonist and agaonist into the synaptic cleft.