AFLUID November 46/5

Murer, Heini, Ian Forster, Nati Hernando, Georg Lambert, Martin Traebert, and Jürg Biber. Posttranscriptional regulation of the proximal tubule NaPi-II transporter in response to PTH and dietary Pi. Am. J. Physiol. 277 (Renal Physiol. 46): F676–F684, 1999.—The rate of proximal tubular reabsorption of phosphate (Pi) is a major determinant of Pi homeostasis. Deviations of the extracellular concentration of Pi are corrected by many factors that control the activity of Na-Pi cotransport across the apical membrane. In this review, we describe the regulation of proximal tubule Pi reabsorption via one particular Na-Pi cotransporter (the type IIa cotransporter) by parathyroid hormone (PTH) and dietary phosphate intake. Available data indicate that both factors determine the net amount of type IIa protein residing in the apical membrane. The resulting change in transport capacity is a function of both the rate of cotransporter insertion and internalization. The latter process is most likely regulated by PTH and dietary Pi and is considered irreversible since internalized type IIa Na-Pi cotransporters are subsequently routed to the lysosomes for degradation.