The masking role of the sodium pump and chloride ions on the effect of carbachol in- and outside the endplate region of rat diaphragm muscle.

The effect of carbachol (10(-3)M) on the membrane potential of rat diaphragm muscle fibres near and remote from the endplate has been investigated by means of a microelectrode technique. In the endplate region a rather slow depolarization was observed followed by spontaneous repolarization. Outside the endplate region similar, but smaller, responses could be measured. In a muscle part separated from the endplate zone, carbachol had no depolarizing effect. Inhibition of the sodium pump by ouabain (10(-4)M) or potassium-free medium not changing the membrane potential itself, allowed carbachol to exert a depolarizing effect on this endplate-free membrane, while in endplate-free regions continuous with the endplate-zone the depolarizations already observed were enhanced. In low chloride solutions carbachol also had a depolarizing effect on endplate-free muscle membrane. It is concluded from these experiments that a direct effect of cholinergic drugs on endplate-free muscle membrane of rat diaphragm is counteracted by the sodium pump in combination with passive chloride movements. The results are discussed in the context of known receptor densities and chloride effects.