NDLI: Charge Translocation by the Na+/K+ Pump under Na+/Na+ Exchange Conditions: Intracellular Na+ Dependence

Content Provider	PubMed Central
Author	Holmgren, Miguel Rakowski, Robert F.
Copyright Year	2006
Abstract	The effect of intracellular (i) and extracellular (o) Na+ on pre-steady-state transient current associated with Na+/Na+ exchange by the Na+/K+ pump was investigated in the vegetal pole of Xenopus oocytes. Current records in response to 40-ms voltage pulses from −180 to +100 mV in the absence of external Na+ were subtracted from current records obtained under Na+/Na+ exchange conditions. Na+-sensitive transient current and dihydroouabain-sensitive current were equivalent. The quantity of charge moved (Q) and the relaxation rate coefficient (k tot) of the slow component of the \documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation}{\mathrm{Na}}_{{\mathrm{o}}}^{+}\end{equation}\end{document} -sensitive transient current were measured for steps to various voltages (V). The data were analyzed using a four-state kinetic model describing the Na+ binding, occlusion, conformational change, and release steps of the transport cycle. The apparent valence of the Q vs. V relationship was near 1.0 for all experimental conditions. When extracellular Na+ was halved, the midpoint voltage of the charge distribution (V q) shifted −25.3 ± 0.4 mV, which can be accounted for by the presence of an extracellular ion-well having a dielectric distance δ = 0.69 ± 0.01. The effect of changes of \documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation}{\mathrm{Na}}_{{\mathrm{i}}}^{+}\end{equation}\end{document} on \documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation}{\mathrm{Na}}_{{\mathrm{o}}}^{+}\end{equation}\end{document} -sensitive transient current was investigated. The midpoint voltage (V q) of the charge distribution curve was not affected over the \documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation}{\mathrm{Na}}_{{\mathrm{o}}}^{+}\end{equation}\end{document} concentration range 3.13–50 mM. As \documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation}{\mathrm{Na}}_{{\mathrm{i}}}^{+}\end{equation}\end{document} was decreased, the amount of charge measured and its relaxation rate coefficient decreased with an apparent K m of 3.2 ± 0.2 mM. The effects of lowering \documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation}{\mathrm{Na}}_{{\mathrm{i}}}^{+}\end{equation}\end{document} on pre-steady-state transient current can be accounted for by decreasing the charge available to participate in the fast extracellular Na+ release steps, by a slowly equilibrating (phosphorylation/occlusion) step intervening between intracellular Na+ binding and extracellular Na+ release.
Related Links	http://dx.doi.org/10.1529/biophysj.105.072942
Ending Page	1616
Page Count	10
Starting Page	1607
File Format	PDF
ISSN	00063495
e-ISSN	15420086
Journal	Biophysical Journal
Issue Number	5
Volume Number	90
Language	English
Publisher	Biophysical Society
Publisher Date	2006-03-01
Access Restriction	Open
Rights Holder	Biophysical Society
Subject Keyword	Biophysics Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Biophysics

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