Efflux of inorganic phosphate from rat hepatocytes: lack of effect of insulin

Expression of the mammalian Xenotropic Polytropic Virus Receptor 1 (XPR1) in tobacco leaves leads to phosphate export

Phosphate homeostasis in multicellular eukaryotes depends on both phosphate influx and efflux. The mammalian Xenotropic Polytropic Virus Receptor 1 (XPR1) shares homology to the Arabidopsis PHO1, a phosphate exporter expressed in roots. However, phosphate export activity of XPR1 has not yet been demonstrated in a heterologous system. Here, wedemonstrate that transient expression in tobacco leaves of XPR1-GFP leads to specific phosphate export. Like PHO1-GFP, XPR1-GFP is localized predominantly to the endomembrane system in tobacco cells. These results show that tobacco leaves are a good heterologous system to study the transport activity of members of the PHO1/XPR1 family.

Uptake of phosphate by rat hepatocytes in primary culture: a sodium-dependent system that is stimulated by insulin

Primary cultures of rat hepatocytes take up phosphate by a saturable Na(+)-dependent process. Thus the plasma membrane possesses an N(+)-Pi cotransporter of the type described for many cell types, e.g., kidney proximal tubular cells and enterocytes. Coupling to Na+ overcomes the barrier to anion entry represented by the membrane potential. At 0.12 mM Pi, the effect of Na+ is cooperative with a Hill coefficient of 1.7 suggesting two sodium sites per molecule of carrier. At 37 degrees C, the Km (for Pi) and Vmax for the sodium-dependent fraction of Pi uptake are approx. 1 mM and 0.35 nmol Pi/min per mg cell protein, respectively. Insulin stimulates Vmax four-fold with no significant effect on Km. Pi uptake in the absence of sodium is not affected by insulin. The stimulation by insulin could be of metabolic significance. Glucose phosphorylation at the expense of ATP is raised in liver following insulin stimulation, and thus, initially there may be an increased demand for Pi for oxidative phosphorylation until new steady-state conditions of hexose phosphate concentrations and of ATP turnover become established.