AT amphibian neuromuscular junctions, the binding of acetylcholine to postsynaptic receptors opens endplate channels. An endplate current (e.p.c.) is generated as sodium and potassium ions flow through these channels according to their electrochemical gradients1. In normal conditions the e.p.c. is a net inward current of predominantly sodium ions. It has been shown that the lifetime of endplate channels is affected by temperature2,3 and membrane potential3, depends on the nature of the cholinergic agonist4,5 and can be influenced by various chemicals and drugs 6,7. But, it is not known what normally determines the lifetime and conductance of an open channel. We report here that the nature of the permeating cations that generate the e.p.c. affects the lifetime and conductance of endplate channels. These observations indicate that these cations influence the mechanisms that determine channel behaviour.
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