The Opossum Kidney Cell Type IIa Na/PiCotransporter Is a Phosphoprotein

AbstractBackground/Aim:Parathyroid hormone (PTH)-dependent inhibition of proximal tubular Pireabsorption is mediated by protein kinase A and/or C and is associated with reduced border membrane expression of the type IIa Na/Picotransporter. The aim of this study was to analyze phosphorylation of the type IIa cotransporter protein. Methods:Opossum kidney cells were used as a ‘proximal tubular’ cell model. Protein phosphorylation was determined by immunoprecipitation of the type IIa Na/Picotransporter, followed by autoradiography. The transporter protein content was evaluated by Western blotting and transport activity by tracer Piuptake. Results:Under control conditions (no PTH) the transporter was phosphorylated; upon treatment with PTH, a decrease in phosphorylation was observed. A protein phosphatase inhibitor (okadaic acid) was unable to prevent PTH-induced Na/Picotransporter inhibition but reduced transporter degradation. Conclusion:The type IIa Na/Picotransporter is a phosphoprotein, but alterations in its phosphorylation seem not to be involved in Pitransport inhibition.Copyright © 2001 S. Karger AG, Basel