Ion permeation through single channels activated by acetylcholine in denervated toad sartorius skeletal muscle fibers: Effects of alkali cations |
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Authors: | Nino Quartararo Peter H Barry Peter W Gage |
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Institution: | (1) School of Physiology and Pharmacology, University of New South Wales, Sydney, Australia;(2) Present address: Department of Physiology, The John Curtin School of Medical Research, The Australian National University, 2601 Canberra, A.C.T. |
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Abstract: | Summary 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 (<5% of channel openings), classified as a junctional type, had a conductance of 35 pS under the same conditions, and a similar null potential. In many patches, a small percentage (usually <2%) of openings of the extrajunctional channel displayed a lower conductance state. The shape of theI–V curves obtained for the extrajunctional channel dependend on the predominant extracellular cation. For Cs and K, theI–V curves were essentially linear over the voltage range +50 to –150 mV across the patch, suggesting that the potential independent component of the energy profile within the channel was symmetrical. For Li, theI–V curve was very nonlinear, displaying a significant sublinearity at hyperpolarized potentials. Both an electrodiffusion and a symmetrical uniform four-barrier, three-site rate-theory model provided reasonable fits to the data, whereas symmetrical two-barrier, single-site rate-theory models did not. For the alkali cations examined, the relative permeability sequence wasP
Cs>P
K>P
Na>P
Li—a proportional selectivity sequence. This was different from the single channel conductance sequence which was found to be
K>
Cs>
Na>
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. |
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Keywords: | ions ion permeation ion selectivity channels single channels ACh channels alkali cations gigaohm seal technique patch clamp skeletal muscle electrodiffusion rate theory |
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