The Voltage Dependence of a Cloned Mammalian Renal Type II Na+/Pi Cotransporter (NaPi-2)

Author:

Forster Ian1,Hernando Nati1,Biber Jürg1,Murer Heini1

Affiliation:

1. From the Physiologisches Institut, Universität Zürich, CH-8057 Zürich, Switzerland

Abstract

The voltage dependence of the rat renal type II Na+/Pi cotransporter (NaPi-2) was investigated by expressing NaPi-2 in Xenopus laevis oocytes and applying the two-electrode voltage clamp. In the steady state, superfusion with inorganic phosphate (Pi) induced inward currents (Ip) in the presence of 96 mM Na+ over the potential range −140 ≤ V ≤ +40 mV. With Pi as the variable substrate, the apparent affinity constant (KmPi) was strongly dependent on Na+, increasing sixfold for a twofold reduction in external Na+. KmPi increased with depolarizing voltage and was more sensitive to voltage at reduced Na+. The Hill coefficient was close to unity and the predicted maximum Ip (Ipmax) was 40% smaller at 50 mM Na+. With Na+ as the variable substrate, KmNa was weakly dependent on both Pi and voltage, the Hill coefficient was close to 3 and Ipmax was independent of Pi at −50 mV. The competitive inhibitor phosphonoformic acid suppressed the steady state holding current in a Na+-dependent manner, indicating the existence of uncoupled Na+ slippage. Voltage steps induced pre–steady state relaxations typical for Na+-coupled cotransporters. NaPi-2-dependent relaxations were quantitated by a single, voltage-dependent exponential. At 96 mM Na+, a Boltzmann function was fit to the steady state charge distribution (Q-V) to give a midpoint voltage (V0.5) in the range −20 to −50 mV and an apparent valency of ∼0.5 e−. V0.5 became more negative as Na+ was reduced. Pi suppressed relaxations in a dose-dependent manner, but had little effect on their voltage dependence. Reducing external pH shifted V0.5 to depolarizing potentials and suppressed relaxations in the absence of Na+, suggesting that protons interact with the unloaded carrier. These findings were incorporated into an ordered kinetic model whereby Na+ is the first and last substrate to bind, and the observed voltage dependence arises from the unloaded carrier and first Na+ binding step.

Publisher

Rockefeller University Press

Subject

Physiology

Reference43 articles.

1. Charge movement in the membrane of striated muscle;Adrian;Annu Rev Biophys Bioeng,1978

2. Effect of pH on phosphate transport in rat renal brush border vesicles;Amstutz;Am J Physiol,1985

3. Na+ binding to the Na+-glucose cotransporter is potential dependent;Bennett;Am J Physiol,1992

4. The molecular mechanism and potential dependence of the Na+/glucose cotransporter;Bennett;Biophys J,1996

5. Electrogenicity of phosphate transport by renal brush-border membranes;Béliveau;Biochem J,1988

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