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Electrogenic Kinetics of a Mammalian Intestinal Type IIb Na+/Pi Cotransporter

Forster, Ian ; Virkki, Leila ; Bossi, Elena ; Murer, Heini ; Biber, Jürg

In: The Journal of Membrane Biology, 2006, vol. 212, no. 3, p. 177-190

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    Titre
    • Electrogenic Kinetics of a Mammalian Intestinal Type IIb Na+/Pi Cotransporter
    Auteur
    • Forster, Ian. Institute of Physiology and Center for Integrative Human Physiology, University of Zurich, Zurich, Switzerland
    • Virkki, Leila. Institute of Physiology and Center for Integrative Human Physiology, University of Zurich, Zurich, Switzerland
    • Bossi, Elena. Department of Structural and Functional Biology, University of Insubria, Varese, Italy
    • Murer, Heini. Institute of Physiology and Center for Integrative Human Physiology, University of Zurich, Zurich, Switzerland
    • Biber, Jürg. Institute of Physiology and Center for Integrative Human Physiology, University of Zurich, Zurich, Switzerland
    Type de document
    • Postprint
    Langue
    • Anglais
    Publié dans
    • The Journal of Membrane Biology, 2006, vol. 212, no. 3, p. 177-190. Springer-Verlag; www.springer-ny.com
    Autre version électronique
    Identifiant OAI-PMH
    • oai:doc.rero.ch:311391
    Summary
    The kinetics of a type IIb Na+-coupled inorganic phosphate (Pi) cotransporter (NaPi-IIb) cloned from mouse small intestine were studied using the two-electrode voltage clamp applied to Xenopus oocytes. In the steady state, mouse NaPi-IIb showed a curvilinear I-V relationship, with rate-limiting behavior only for depolarizing potentials. The Pi dose dependence was Michaelian, with an apparent affinity constant for Pi ( $ {K_{\rm m}}^{\rm P_i} $ ) of 10±1μM at −60 mV. Unlike for rat NaPi-IIa, $ {K_{\rm m}}^{\rm P_i} $ increased with membrane hyperpolarization, as reported for human NaPi-IIa, flounder NaPi-IIb and zebrafish NaPi-IIb2. The apparent affinity constant for Na+ ( $ {K_{\rm m}}^{\rm Na} $ ) was 23±1 mM at −60 mV, and the Na+ activation was cooperative with a Hill coefficient of approximately 2. Pre-steady-state currents were documented in the absence of Pi and showed a strong dependence on external Na+. The hyperpolarizing shift of the charge distribution midpoint potential was 65 mV/log[Na]. Approximately half the moveable charge was attributable to the empty carrier. A comparison of the voltage dependence of steady-state Pi-induced current and pre-steady-state charge movement indicated that for −120 mV≤V≤0 mV the voltage dependence of the empty carrier was the main determinant of the curvilinear steady-state cotransport characteristic. External protons partially inhibited NaPi-IIb steady-state activity, independent of the titration of mono- and divalent Pi, and immobilized pre-steady-state charge movements associated with the first Na+ binding step