Characterization and expression of a novel Na+-inorganic phosphate transporter at the liver plasma membrane of the rat

Abstract

Background: Phosphate transport across the plasma membrane of the intestine and kidney occur by an Na+-dependent process. The mechanism by which phosphate enters the hepatocytes across the basolateral membrane is not known. Therefore, our study was designed to investigate whether the plasma membranes of the liver possess a specialized transport system for translocation of phosphate into the hepatocyte. Methods: Liver plasma membrane vesicles and expression of liver poly(A)+ RNA into Xenopus laevis oocytes was used. Results: Phosphate was driven into the intravesicular space as depicted by the equation y = 493.96x + 0.001, r2 = 0.96. Inwardly directed Na+ and pH gradients stimulated phosphate uptake, with a Vmax of 0.58 ± 0.03 and 0.22 ± 0.03 nmol/mg protein/10 s, at pH 6.1 and 7.4, respectively (P < 0.05). Km values were 0.39 ± 0.07 and 0.25 ± 0.1 mmol/L, respectively. To confirm the presence of a phosphate carrier, the liver Na+ phosphate transporter was expressed in X. laevis oocytes. The size-selected messenger RNA encoding for the phosphate transporter was 1.6 kilobases (kb) with kinetic parameters of Vmax 6.75 ± 0.9 pmol/mg protein/5 min and Km of 0.29 ± 0.1 mmol/L, compared with a Vmax of 0.41 ± 0.03 pmol/mg protein/5 min and a Km of 0.085 ± 0.022 mmol/L for water injection. Colonic poly(A)+ RNA did not stimulate phosphate uptake in the oocytes. Conclusion: The combined studies of plasma membrane vesicles and the expression system into X. laevis oocytes confirm the presence of an Na+-phosphate transporter at the liver plasma membranes.