[Is the presynaptic action of carbocholine related to the activation of the postsynaptic membrane?]

Experiments were carried out to elucidate whether carbacholine (CCh) exerts a direct effect on motor nerve endings. With this purpose in view the frog sartorius muscle was used to correlate changes in the quantum content of end plate potentials (EPP) with the carbacholine-induced depolarization of the postsynaptic membrane. It was disclosed in the first series of experiments on neuromuscular preparations blocked by the decreased Ca2+ concentration in Ringer solution that CCh lowers EPP at a concentration which produces no depolarizing action on the postsynaptic membrane (5 x 10(-7) mol/l). In the second series of experiments on preparations blocked by transversal dissection of muscle fibers, CCh (5 x 10-6) mol/l) also diminished EEP without affecting the membrane potential of the muscle fibers. In the third series of experiments, the presynaptic effect of the third series of experiments, the presynaptic effect of CCh (5 x 10(-6) mol/l) was reversed by tubocurarine (1 X 10(-7) mol/l), with the postsynaptic action of the mimetic remaining unchanged. The data obtained indicate that the decrease in EPP induced by CCh (5 x 10(-7)--5 x 10(-6) mol/l) does not arise from the depolarization of the end plate.