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Voltage Dependence of the Apparent Affinity for External Na(+) of the Backward-Running Sodium Pump
The steady-state voltage and [Na(+)](o) dependence of the electrogenic sodium pump was investigated in voltage-clamped internally dialyzed giant axons of the squid, Loligo pealei, under conditions that promote the backward-running mode (K(+)-free seawater; ATP- and Na(+)-free internal solution conta...
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Formato: | Texto |
Lenguaje: | English |
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The Rockefeller University Press
2001
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Materias: | |
Acceso en línea: | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2217255/ https://www.ncbi.nlm.nih.gov/pubmed/11279252 |
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author | De Weer, Paul Gadsby, David C. Rakowski, R.F. |
author_facet | De Weer, Paul Gadsby, David C. Rakowski, R.F. |
author_sort | De Weer, Paul |
collection | PubMed |
description | The steady-state voltage and [Na(+)](o) dependence of the electrogenic sodium pump was investigated in voltage-clamped internally dialyzed giant axons of the squid, Loligo pealei, under conditions that promote the backward-running mode (K(+)-free seawater; ATP- and Na(+)-free internal solution containing ADP and orthophosphate). The ratio of pump-mediated (42)K(+) efflux to reverse pump current, I(pump) (both defined by sensitivity to dihydrodigitoxigenin, H(2)DTG), scaled by Faraday's constant, was −1.5 ± 0.4 (n = 5; expected ratio for 2 K(+)/3 Na(+) stoichiometry is −2.0). Steady-state reverse pump current-voltage (I(pump)-V) relationships were obtained either from the shifts in holding current after repeated exposures of an axon clamped at various V(m) to H(2)DTG or from the difference between membrane I-V relationships obtained by imposing V(m) staircases in the presence or absence of H(2)DTG. With the second method, we also investigated the influence of [Na(+)](o) (up to 800 mM, for which hypertonic solutions were used) on the steady-state reverse I(pump)-V relationship. The reverse I(pump)-V relationship is sigmoid, I(pump) saturating at large negative V(m), and each doubling of [Na(+)](o) causes a fixed (29 mV) rightward parallel shift along the voltage axis of this Boltzmann partition function (apparent valence z = 0.80). These characteristics mirror those of steady-state (22)Na(+) efflux during electroneutral Na(+)/Na(+) exchange, and follow without additional postulates from the same simple high field access channel model (Gadsby, D.C., R.F. Rakowski, and P. De Weer, 1993. Science. 260:100–103). This model predicts valence z = nλ, where n (1.33 ± 0.05) is the Hill coefficient of Na binding, and λ (0.61 ± 0.03) is the fraction of the membrane electric field traversed by Na ions reaching their binding site. More elaborate alternative models can accommodate all the steady-state features of the reverse pumping and electroneutral Na(+)/Na(+) exchange modes only with additional assumptions that render them less likely. |
format | Text |
id | pubmed-2217255 |
institution | National Center for Biotechnology Information |
language | English |
publishDate | 2001 |
publisher | The Rockefeller University Press |
record_format | MEDLINE/PubMed |
spelling | pubmed-22172552008-04-22 Voltage Dependence of the Apparent Affinity for External Na(+) of the Backward-Running Sodium Pump De Weer, Paul Gadsby, David C. Rakowski, R.F. J Gen Physiol Original Article The steady-state voltage and [Na(+)](o) dependence of the electrogenic sodium pump was investigated in voltage-clamped internally dialyzed giant axons of the squid, Loligo pealei, under conditions that promote the backward-running mode (K(+)-free seawater; ATP- and Na(+)-free internal solution containing ADP and orthophosphate). The ratio of pump-mediated (42)K(+) efflux to reverse pump current, I(pump) (both defined by sensitivity to dihydrodigitoxigenin, H(2)DTG), scaled by Faraday's constant, was −1.5 ± 0.4 (n = 5; expected ratio for 2 K(+)/3 Na(+) stoichiometry is −2.0). Steady-state reverse pump current-voltage (I(pump)-V) relationships were obtained either from the shifts in holding current after repeated exposures of an axon clamped at various V(m) to H(2)DTG or from the difference between membrane I-V relationships obtained by imposing V(m) staircases in the presence or absence of H(2)DTG. With the second method, we also investigated the influence of [Na(+)](o) (up to 800 mM, for which hypertonic solutions were used) on the steady-state reverse I(pump)-V relationship. The reverse I(pump)-V relationship is sigmoid, I(pump) saturating at large negative V(m), and each doubling of [Na(+)](o) causes a fixed (29 mV) rightward parallel shift along the voltage axis of this Boltzmann partition function (apparent valence z = 0.80). These characteristics mirror those of steady-state (22)Na(+) efflux during electroneutral Na(+)/Na(+) exchange, and follow without additional postulates from the same simple high field access channel model (Gadsby, D.C., R.F. Rakowski, and P. De Weer, 1993. Science. 260:100–103). This model predicts valence z = nλ, where n (1.33 ± 0.05) is the Hill coefficient of Na binding, and λ (0.61 ± 0.03) is the fraction of the membrane electric field traversed by Na ions reaching their binding site. More elaborate alternative models can accommodate all the steady-state features of the reverse pumping and electroneutral Na(+)/Na(+) exchange modes only with additional assumptions that render them less likely. The Rockefeller University Press 2001-04-01 /pmc/articles/PMC2217255/ /pubmed/11279252 Text en © 2001 The Rockefeller University Press This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.rupress.org/terms). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 4.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/4.0/). |
spellingShingle | Original Article De Weer, Paul Gadsby, David C. Rakowski, R.F. Voltage Dependence of the Apparent Affinity for External Na(+) of the Backward-Running Sodium Pump |
title | Voltage Dependence of the Apparent Affinity for External Na(+) of the Backward-Running Sodium Pump |
title_full | Voltage Dependence of the Apparent Affinity for External Na(+) of the Backward-Running Sodium Pump |
title_fullStr | Voltage Dependence of the Apparent Affinity for External Na(+) of the Backward-Running Sodium Pump |
title_full_unstemmed | Voltage Dependence of the Apparent Affinity for External Na(+) of the Backward-Running Sodium Pump |
title_short | Voltage Dependence of the Apparent Affinity for External Na(+) of the Backward-Running Sodium Pump |
title_sort | voltage dependence of the apparent affinity for external na(+) of the backward-running sodium pump |
topic | Original Article |
url | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2217255/ https://www.ncbi.nlm.nih.gov/pubmed/11279252 |
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