Triphenylmethylphosphonium blocks the nicotinic acetylcholine receptor noncompetitively

Triphenylmethylphosphonium (TPMP) blocked the nerve-elicited twitch tension of frog sartorius muscles by 50% at a concentration of about 20 microM. This neuromuscular blockade by TPMP, which originated at the level of the nicotinic receptor, was due in part to the ability of the drug to block the quiescent receptor by degrees that depended on the TPMP concentration and on membrane potential. In addition, the blockade was markedly enhanced when the receptor was activated by acetylcholine. Finally, TPMP shortened the lifetimes of ACh-activated ion channels. These findings were interpreted as follows. Under resting conditions, TPMP shifted the equilibrium of the receptor channel complex toward the desensitized state. TPMP united with the activated ion channel to shorten channel lifetime and to deepen the blockade. High concentrations (greater than or equal to 50 microM) of TPMP altered muscle action potentials and often increased, by about 30-fold, the frequency of miniature endplate potentials.