The effect of tubocurarine competition on the kinetics of agonist action on the nicotinic receptor.

1 The rates at which tubocurarine associates with, and dissociates from, the nicotinic receptor, while exerting its classical competitive effect, are still in doubt. We have investigated this problem by observing the effect of low concentrations of tubocurarine on the re-equilibration rate, following a step change in membrane potential, of the current produced by carbachol in voltage-clamped endplates of frog muscle. 2 It is expected, and observed, that in order to see the effects of competition (as opposed to ion channel block), sufficiently high agonist concentrations must be used so that the relaxation rate becomes faster than that seen at low agonist concentrations. 3 Small concentrations of tubocurarine were found to reduce this relaxation rate, towards a value appropriate to a lower agonist concentration. 4 The results suggest that tubocurarine equilibrates very rapidly with the nicotinic receptor. 5 Some of the possible technical problems of this sort of experiment are discussed. The results are similar to those already published for nicotinic receptors in eel electric tissue.