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

Abstract

The kinetics of a type IIb Na⁺-coupled inorganic phosphate (P_i) 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 P_i dose dependence was Michaelian, with an apparent affinity constant for P_i (${K_{\rm m}}^{\rm P_i}$) of 10 ± 1 μM 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 P_i 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 P_i-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 P_i, and immobilized pre-steady-state charge movements associated with the first Na⁺ binding step.