The opossum kidney cell type IIa Na/P-i cotransporter is a phosphoprotein

Background/Aim: Parathyroid hormone (PTH)-dependent inhibition of proximal tubular P-i reabsorption is mediated by protein kinase A and/or C and is as sociated with reduced border membrane expression of the type IIa Na/P-i cot ransporter. The aim of this study was to analyze phosphorylation of the typ e IIa cotransporter protein. Methods: Opossum kidney cells were used as a ' proximal tubular' cell model. Protein phosphorylation was determined by imm unoprecipitation of the type IIa Na/P-i cotransporter, followed by autoradi ography. The transporter protein content was evaluated by Western blotting and transport activity by tracer P-i uptake. Results: Under control conditi ons (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/P-i cotransporter inhib ition but reduced transporter degradation. Conclusion: The type tla Na/P-i cotransporter is a phosphoprotein, but alterations in its phosphorylation s eem not to be involved in P-i transport inhibition. Copyright (C) 2001 S. K arger AG, Basel.