Acetylcholine (ACh) activates in the synaptic membrane of skeletal muscle an inward current composed of many elementary currents1,2. High resolution current measurements in adult frog muscle have shown that the elementary current is a pulse-like event of unit amplitude, indicating that ACh opens ion channels which have only two conductance states, fully open or closed3. We now present evidence for a third conductance state. In the membrane of uninnervated embryonic rat muscle we observe that ACh activates two independent classes of currents of different amplitude and average duration, apparently arising from two populations of ACh receptor (AChR) channels. The currents from both classes show, at low incidence, transitions between a main level and a sublevel of lower amplitude. From this we conclude that AChR channels in embryonic muscle adopt, in addition to a "main" conductance state, a "substate" of lower conductance.
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