Dietary phosphate and parathyroid hormone alter the expression of the calcium-sensing receptor (CaR) and the Na+-dependent P-i transporter [NaPi-2) in the rat proximal tubule

Dietary phosphate (P-i) intake and parathyroid hormone (PTH) are essential regulators of proximal tubular (PT) P-i reabsorption; both factors are asso ciated with adaptive changes in PT apical brush border membrane (BBM) Na/P- i-cotransport activity and specific transporter protein (NaPi-2) content. U rinary P-i excretion is also inversely correlated with luminal Ca2+ concent ration ([Ca2+]) both in a PTH-dependent and -independent fashion. A cell-su rface, Ca2+(/polyvalent cation)-sensing receptor (CaR) has been localized t o the PT BBM with unknown function. To investigate whether PTH and/or dieta ry P-i intake could affect the distribution or the expression of the CaR, w e evaluated their effects on rat kidney CaR and the NaPi-2 expression by We stern blot analysis and immunofluorescence microscopy. A chronic high-P-i ( 1.2%) versus low-P-i (0.1%) diet and acute PTH (1-34) infusion significantl y reduced the PT BBM expression of both NaPi-2 and CaR proteins. CaR-specif ic immunoreactivity in nephron segments other than the PT was not affected by PTH or P-i intake. These results suggest that reduced renal PT CaR expre ssion by a high-P-i diet and by increased circulating PTH levels could cont ribute to the local control of PT handling of Ca2+ and P-i.