CELLULAR MOLECULAR CONTROL OF RENAL NA/P-I-COTRANSPORT/

A type II Na/P-i-cotransporter located in the brush border membrane is the rate limiting and physiologically regulated step in proximal tubu lar phosphate (P-i) reabsorption. In states of altered P-i-reabsorptio n [for example, in response to parathyroid hormone (PTH) and to altere d dietary intake of P-i or as a consequence of genetic abnormabilities ], brush border expression of the type II Na/P-i-cotransporter is acco rdingly modified. PTH Initiates regulatory cascade leading to membrane retrieval, followed by lysosomal degradation of this transporter; rec overy from inhibition requires its de novo synthesis. P-i-deprivation leads to an increased brush border expression of transporters that doe s not appear to require de novo synthesis in the short term. P-i-overl oad leads to membrane retrieval and degradation of transporters. Final ly, in animals with genetically altered P-i-handling (Hyp; Gy) the bru sh border membrane expression of the type II Na/P-i-cotransporter is a lso reduced, suggesting that a genetically altered protein (such as PE X in Hyp) controls the expression of this transporter.