Chloride conductance and P(i) transport are separate functions induced by the expression of NaPi-1 in Xenopus oocytes

Abstract

Expression of the protein NaPi-1 in Xenopus oocytes has previously been shown to induce an outwardly rectifying Cl- conductance (G(Cl)), organic anion transport and Na+-dependent P(i)-uptake. In the present study we investigated the relation between the NaPi-1 induced G(Gl) and P(i)-induced currents and transport. NaPi-1 expression induced P(i)-transport, which was not different at 1-20 ng/oocyte NaPi-1 cRNA injection and was already maximal at 1-2 days after cRNA injection. In contrast, G(Cl) was augmented at increased amounts of cRNA injection (1-20 ng/oocyte) and over a five day expression period. Subsequently all experiments were performed on oocytes injected with 20 ng/oocytes cRNA. P(i)-induced currents (Ip) could be observed in NaPi-1 expressing oocytes at high concentrations of P(i) (≤ 1 mM P(i)). The amplitudes of Ip correlated well with G(Cl). Ip was blocked by the Cl- channel blocker NPPB, partially Na+-deDendent and completely abolished in Cl- free solution. In contrast, P(i)-transport in NaPi-1 expressing oocytes was not NPPB sensitive, stronger depending on extracellular Na+ and weakly affected by Cl- substitution. Endogenous P(i)-uptake in water-injected oocytes amounted in all experiments to 30-50% of the Na+-dependent P(i)-transport observed in NaPi-1 expressing oocytes. The properties of the endogenous P(i)-uptake system (K(m) for P(i) > 1 mM; partial Na+- and Cl-dependence; lack of NPPB block) were similar to the NaPi-1 induced P(i)-uptake, but no Ip could be recorded at P(i)-concentrations ≤3 mM. In summary, the present data suggest that Ip does not reflect charge transfer related to P(i)-uptake, but a P(i)-mediated modulation of G(Cl). Show more