Abstract

Earlier studies of acetyicholine (ACh) and choline depolarizing effects on frog sartorius membrane are extended to a wider concentration range and to include carbamyicholine. Differences between motor end-plate-free regions and motor end-plate membrane are examined. On end-plate-free membrane, ACh depolarization stabilizes in 30 minutes and increases with increasing ACh concentration. Washing with Ringer's solution reverses it. Motor end-plate membrane depolarizes very rapidly and repolarizes within 10 minutes. Choline in similar doses produces a transitory depolarization away from end-plates, with a rise time of 20 minutes and a 30-minute recovery. The motor end-plate membrane response to choline is similar to its response to ACh. Carbamylcholine depolarization away from end-plates is similar to choline, but it occurs at lower concentrations. Carbamylcholine depolarizes the end-plate similarly to ACh. Two different ACh-sensitive systems are proposed to explain these membrane responses, a motor end-plate ACh receptor and a conductance control system occurring throughout the cell membrane. The state of the conductance control system determines membrane potential. It reacts slowly with quaternary ammonium ions at high concentrations. The ACh receptorat the end-plate is much more sensitive to quaternary ammonium ions. It is coupled to the conductance control system and can trigger a change of state in it to produce the end-plate potential.