Access channel model for the voltage dependence of the forward-running Na+/K+ pump

• Published in: The Journal of general physiology Vol. 103; no. 5; pp. 869 - 894
• Main Authors: SAGAR, A, RAKOWSKI, R. F
• Format: Journal Article
• Language: English
• Published: New York, NY Rockefeller University Press 01.05.1994; The Rockefeller University Press
• Subjects: Anatomy & physiology; Animals; Biochemistry; Biological and medical sciences; Cell Membrane - metabolism; Cell physiology; Female; Fluoresceins; Fundamental and applied biological sciences. Psychology; Membrane and intracellular transports; Membrane Potentials; Models, Biological; Molecular and cellular biology; Oocytes - cytology; Oocytes - metabolism; Potassium - metabolism; Potassium Channels - metabolism; Sodium - metabolism; Sodium-Potassium-Exchanging ATPase - metabolism; Xenopus laevis; Kinetic model; Na; Enzyme; Membrane transport; Electrophysiology; Ion transport; Hydrolases; exchanging ATPase; Ionic current; K; Membrane potential; Voltage clamp method
• ISSN: 0022-1295; 1540-7748
• DOI: 10.1085/jgp.103.5.869

The voltage dependence of steady state current produced by the forward mode of operation of the endogenous electrogenic Na+/K+ pump in Na(+)-loaded Xenopus oocytes has been examined using a two-microelectrode voltage clamp technique. Four experimental cases (in a total of 18 different experimental conditions) were explored: variation of external [Na+] ([Na]o) at saturating (10 mM) external [K+] ([K]o), and activation of pump current by various [K]o at 0, 15, and 120 mM [Na]o (tetramethylammonium replacement). Ionic current through K+ channels was blocked by Ba2+ (5 mM) and tetraethylammonium (20 mM), thereby allowing pump-mediated current to be measured by addition or removal of external K+. Control measurements and corrections were made for pump current run-down and holding current drift. Additional controls were done to estimate the magnitude of the inwardly directed pump-mediated current that was present in K(+)-free solution and the residual K(+)-channel current. A pseudo two-state access channel model is described in the Appendix in which only the pseudo first-order rate coefficients for binding of external Na+ and K+ are assumed to be voltage dependent and all transitions between states in the Na+/K+ pump cycle are assumed to be voltage independent. Any three-state or higher order model with only two oppositely directed voltage-dependent rate coefficients can be reduced to an equivalent pseudo two-state model. The steady state current-voltage (I-V) equations derived from the model for each case were simultaneously fit to the I-V data for all four experimental cases and yielded least-squares estimates of the model parameters. The apparent fractional depth of the external access channel for Na+ is 0.486 +/- 0.010; for K+ it is 0.256 +/- 0.009. The Hill coefficient for Na+ is 2.18 +/- 0.06, and the Hill coefficient for K+ (which is dependent on [Na]o) ranges from 0.581 +/- 0.019 to 1.35 +/- 0.034 for 0 and 120 mM [Na]o, respectively. The model provides a reasonable fit to the data and supports the hypothesis that under conditions of saturating internal [Na+], the principal voltage dependence of the Na+/K+ pump cycle is a consequence of the existence of an external high-field access channel in the pump molecule through which Na+ and K+ ions must pass in order to reach their binding sites.