Ion permeation through single channels activated by acetylcholine in denervated toad sartorius skeletal muscle fibers: Effects of alkali cations

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Abstract
The gigaohm seal technique was used to study ion permeation through acetylcholine-activated channels in cell-attached patches of the extrajunctional membrane of chronically denervated, enzyme-treated cells from the sartorius muscle of the toadBufo marinus. The most frequently occurring channel type (>95% of channel openings), provisionally classified as ‘extrajunctional,’ had a chord conductance of approximately 25 pS under normal conditions (−70 mV, 11°C, Normal Toad Ringer's). The less frequently observed channel type (P K>P Na>P Li—a “proportional” selectivity sequence. This was different from the single channel conductance sequence which was found to beγ K>γ Cs>γ Na>γ Li implying that ions do not move independently through the channel. The relative binding constant sequence for the channel sites was found to be a “polarizability” sequence, i.e.,K Li>K Cs>K Na>K K There was an inverse relationship for the cations examined. Under conditions when the single-channel conductance was relatively high, the conductance at depolarized potentials was lower than that predicted by both electrodiffusion and rate theory models, suggesting that there was a rate-limiting access step for ions, from the intracellular compartment into the channel.