Normal phosphate transport in cells from the S-2 and S-3 segments of Hyp-mouse proximal renal tubules

An intrinsic phosphate (Pi) transport defect in the proximal tubule (PT) presumably underlies X-linked hypophosphatemic rickets. We recently reported normal Pi transport in the S-1 segment of the Hyp mouse PT. Whether Pi wasting results from an abnormality in the S-2 or S, segment remains unknown. Thus, we compared Pi transport in S-2 and S-3 immortalized cells from transgenic (simian virus 40) normal and Hyp mice. These cells display biochemical features of PT cells, including alkaline phosphatase- and hormone-stimulated cAMP activity as well as gluconeogenesis. Moreover, kinetic studies in S-2 cells reveal a similar K-m [0.26 +/- 0.03 (+/-SEM) vs. 0.22 +/- 0.03 mM] and maximum velocity (V-max; 5.5 +/- 0.66 vs. 5.9 +/- 0.72 nmol/mg . 5 min) in normal and Hyp mice, respectively. K-m and V-max were also similar in cells from the S-3 segment; however, the V-max values in S-3 cells in normal and Hyp mice (2.8 +/- 0.45 and 3.0 +/- 0.56 nmol/mg . 5 min) were reduced in both animal models compared to those in S-2 cells (P < 0.001), whereas the K-m values in S-3 cells from normal and Hyp mice (0.10 +/- 0.02 and O.11 +/- 0.04 mM) were increased relative to those in S-2 cells (P < 0.001). These data indicate that Pi transport throughout the PT of Hyp mice is intrinsically normal. Such observations exclude the presence of a nascent defect in renal Pi transport in the kidneys of Hyp mice and support the hypothesis that a humoral abnormality underlies X-linked hypophosphatemic rickets.