Abstract

The actions of the nicotinic noncompetitive antagonist meproadifen on both the acetylcholine (ACh) receptor-ion channel complex and electrically excitable membrane were examined in frog sciatic-nerve sartorius muscle preparations. Meproadifen (10-25 microM) blocked the nerve-evoked twitch without affecting the directly evoked twitch, the threshold, overshoot, amplitude, rate of rise, or falling phase of the directly elicited action potential in muscle. This suggests that this agent, at the concentrations that affect the nicotinic receptor, had negligible effect on the excitable membrane. In addition, the drug did not affect either the quantal content or quantal size of the end-plate potential. Meproadifen caused a voltage- and time-dependent decrease in the peak amplitude of the end-plate current (EPC) without significantly shortening the time constant of EPC decay. The voltage- and time-dependent effects of meproadifen were more pronounced at more negative potentials, as evidenced by hysteresis loops and nonlinearity in the current-voltage relationship of the EPC. Both hysteresis and nonlinearity in the current-voltage relationship of the EPC were eliminated when brief conditioning pulses were used for stepwise changes of membrane potentials. The decay time constant of the EPC in the presence of meproadifen remained an exponential function of time. Meproadifen blocked iontophoretically elicited EPCs but did not affect single-channel lifetime, conductance, or the decay time constant of the miniature EPC. Thus, the blockade was more marked on iontophoretically elicited EPCs than on miniature EPCs. Meproadifen also caused desensitization of both the junctional and extrajunctional ACh receptors, but, more important, meproadifen accelerated steady-state desensitization by several-fold (compared with the agonist). The marked depression of peak EPC amplitude and miniature EPC, its high affinity for the binding sites in the presence of the agonist, and acceleration of agonist-induced desensitization suggest that meproadifen interacts with the ACh-bound but nonconducting state of the ACh receptor-ion channel complex. Therefore, it appears that meproadifen interacts with the closed ionic channel of the ACh receptor in its resting and activated but nonconducting states, and only slightly affects the open conformation of the ionic channel.