Article

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

DOI:http://www.ncbi.nlm.nih.gov/pubmed/17342377
Source: OAI

ABSTRACT 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 (Km(pi)) of 10 +/- 1 microM: at -60 mV. Unlike for rat NaPi-IIa, (Km(pi)) increased with membrane hyperpolarization, as reported for human NaPi-IIa, flounder NaPi-IIb and zebrafish NaPi-IIb2. The apparent affinity constant for Na(+) (Km(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 <or= V <or= 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.

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Keywords

apparent affinity constant
 
charge distribution midpoint potential
 
curvilinear steady-state cotransport characteristic
 
divalent Pi
 
external Na(+)
 
flounder NaPi-IIb
 
immobilized pre-steady-state charge movements
 
membrane hyperpolarization
 
mouse NaPi-IIb
 
mouse small intestine
 
moveable charge
 
NaPi-IIb
 
Pi dose dependence
 
pre-steady-state charge movement
 
Pre-steady-state currents
 
steady state
 
steady-state Pi-induced current
 
strong dependence
 
voltage dependence
 
zebrafish NaPi-IIb2