Pressure dependence of sodium gating currents in the squid giant axon |
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Authors: | F Conti I Inoue F Kukita W Stühmer |
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Institution: | (1) Istituto di Cibernetica e Biofisica, Consiglio Nazionale delle Richerche, I-16032 Camogli, Italy;(2) Present address: Institute for Marine Biology, University of Tokushima, Naruto, Japan;(3) Present address: National Institute for Physiological Sciences, Okazaki, Japan;(4) Present address: Max-Planck-Institut für Biophysikalische Chemie, D-3400 Göttingen, Federal Republic of Germany |
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Abstract: | Asymmetric displacement currents, Ig, were measured in squid axons at different hydrostatic pressures, P, up to 60 MPa. Potassium and sodium currents were abolished by intracellular Cs+ and TEA+, by extracellular Tetrodotoxin (TTX), and by Na+ substitution with Tris+. The time course of Ig became progressively slower with increasing pressure, and the amplitude decreased. With appropriate scaling in time and amplitude, Ig records at any given P could be made to superimpose very well with those obtained at atmospheric pressure. The same scaling factors yielded a good superposition of all records obtained for voltage steps to membrane potentials in the range-30 to +42 mV. The ratio between the amplitude and time factors was larger than unity and increased with P, indicating a progressive decrease (up to 35% at 60 MPa) of the total charge displaced, Q, with no significant change in its voltage dependence. The time-scaling factor increased exponentially with P, as expected if all the steps involved in the opening of a sodium channel, and producing a major charge redistribution, have the same activation volume, V
g
17 cm3/mol. This value is roughly one-half of that characterizing the pressure dependence of sodium current activation, suggesting that some late, rate-limiting step in the opening of sodium channels has a large activation volume without being accompanied by an easily detected charge movement.Part of the decrease of Q with pressure could be attributed to an increase in sodium inactivation. However, we cannot exclude the possibility that there is a reversible reduction in the number of fast activating sodium channels, similar to the phenomenon that has been reported to occur at low temperatures (Matteson and Armstrong 1982). |
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Keywords: | Sodium channel nerve gating currents pressure activation volumes |
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