The interaction of acetylcholine and d-tubocurarine on the end-plate free muscle membrane of rat diaphragm in low chloride solution.

The interaction of acetylcholine (ACh) and d-tubocurarine (dTC) on the membrane potential of end-plate free parts of rat diaphragm muscle fibres was studied using a conventional microelectrode technique. ACh only caused membrane depolarizations when the external chloride concentration was diminished. Therefore, all experiments were performed in a 40 mM Cl- containing medium. A bell-shaped log-dose response curve was obtained with ACh. Maximal depolarization occurred at 3.10(-4) M. The pD2 value for ACh was calculated to be 4.2. The log-dose response curve was shifted to the right in the presence of dTC; besides, the maximum seemed to be depressed. The pA2 values for dTC appeared to be 4.9. It was concluded from these data that the cholinergic receptors outside the end-plate region have binding properties, which differ considerably from those of end-plate or postdenervational receptors.