Evidence for a PTH-independent humoral mechanism in post-transplant hypophosphatemia and phosphaturia.
BACKGROUND
Patients undergoing successful kidney transplantation often manifest overt hypophosphatemia associated with exaggerated phosphaturia during the early post-transplant period (2 weeks to 3 months). The mechanism for this phenomenon has not been fully elucidated. We tested the hypothesis that a circulating serum factor [non-parathyroid hormone (non-PTH)], which operates during chronic renal failure (CRF) to maintain phosphate (Pi) homeostasis, can increase fractional excretion of Pi (FE(PO4)) in normal functioning kidney grafts during the early post-transplant period, thereby causing phosphaturia and hypophosphatemia.


METHODS
Five groups of patients were studied: control subjects (group 1, N = 16), "early" (2 weeks to 1 month) post-transplant patients (group 2, N = 22), "late" (9 to 12 months) post-transplant patients (group 3, N = 14), patients with advanced CRF (glomerular filtration rate = 30 to 40 mL/min; group 4, N = 8), and patients who suffered from end-stage renal failure and were treated by chronic hemodialysis (group 5, N = 14). Group 2 manifested significant hypophosphatemia and phosphaturia when compared with groups 1 and 3 (Pi = 0.9 +/- 0.003 mg/dL, FE(PO4) = 68+/- 5%, P < 0.0005 vs. groups 1 and 3). Sera were taken from each of the five subject groups and applied to the proximal tubular opossum kidney (OK) cells. The activity of Na/Pi-type 4 (that is, OK-specific type II transporter) was evaluated by measuring Na(+)-dependent (32)Pi flux. The expression of Na/Pi type II mRNA and the abundance of Na/Pi protein were determined by Northern and Western blot assays, respectively.


RESULTS
When compared with sera from groups 1 and 3, 10% sera taken from groups 2, 4, and 5 (incubated overnight with OK cells) inhibited (32)Pi flux by 25 to 30% (P < 0.0003). Both Na/Pi mRNA and the expression of Na/Pi protein were markedly augmented under the same conditions (P < 0.05 groups 2, 4, and 5 vs. groups 1 and 3). Time-course analysis revealed that the up-regulation of Na/Pi protein by sera from groups 2, 4, and 5 was observed as early as four hours of incubation, whereas augmented abundance of Na/Pi mRNA was only seen after eight hours of incubation. The addition of PTH (1-34) to sera from groups 2, 4, and 5 abolished the augmented expression of NaPi protein. We labeled OK cell surface membrane proteins with N-hydroxysuccinimide bound to biotin (NHS-SS-biotin). Biotinylated transporters incubated with the different sera were precipitated by strepavidin and identified by Western blot analysis. Cells incubated in sera from group 2 showed increased membrane bound transporter when compared with control sera, whereas the intracellular pool of the transporter was comparable between the two groups.


CONCLUSIONS
A non-PTH circulating serum factor (possibly phosphatonin) that increases FE(PO4) during CRF is also responsible for phosphaturia and hypophosphatemia in the early period following successful kidney transplantation. The putative factor inactivates Na/Pi activity along with inhibition of the transporter trafficking from the cell membrane into the cytosol.
Green
J
J
Department of Nephrology, Rambam Medical Center, B. Rappaport Faculty of Medicine, Technion, Israel Institute of Technology, Haifa, Israel. greeny@rambam.health.gov.il
Debby
H
H
Lederer
E
E
Levi
M
M
Zajicek
H K
HK
Bick
T
T
D000328:Adult; D000368:Aged; D000818:Animals; D001692:Biological Transport; D001769:Blood; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D005260:Female; D006801:Humans; D017674:Hypophosphatemia / Q000097:blood / Q000209:etiology* / Q000652:urine; D007676:Kidney Failure, Chronic / Q000097:blood / Q000628:therapy; D016030:Kidney Transplantation; D008297:Male; D008875:Middle Aged; D009893:Opossums; D010761:Phosphorus Radioisotopes; D011183:Postoperative Complications; D012333:RNA, Messenger / Q000378:metabolism; D050608:Sodium-Phosphate Cotransporter Proteins; D050610:Sodium-Phosphate Cotransporter Proteins, Type II; D027981:Symporters; D013935:Thymic Factor, Circulating / Q000032:analysis
D002352:Carrier Proteins; D010761:Phosphorus Radioisotopes; D012333:RNA, Messenger; D050608:Sodium-Phosphate Cotransporter Proteins; D050610:Sodium-Phosphate Cotransporter Proteins, Type II; D027981:Symporters; D013935:Thymic Factor, Circulating