Intravenous phosphate loading increases fibroblast growth factor 23 in uremic rats.

Oral phosphate loading and calcitriol stimulate Fibroblast growth factor 23 (FGF23) secretion, but the mechanisms underlying the stimulation of FGF23 remain to be studied. We compared the effect of intravenous phosphate loading with that of oral loading on FGF23 levels in normal and 5/6 nephrectomized uremic rats. Uremic rats (Nx) and sham-operated rats were fed a normal phosphate diet for 2 weeks and then divided into 3 groups: 1) with the same phosphate diet (NP), 2) with a high phosphate diet (HP), and 3) NP rats with intravenous phosphate infusion using a microinfusion pump (IV). Blood and urine were obtained 1 day (early phase) and 7 days (late phase) after the interventions. In the early and late phases, serum phosphate levels and fractional excretion of phosphate (FEP) were comparable in the HP and IV groups in both Sham and Nx rats. Serum phosphate levels in the HP and IV groups were equally and significantly higher than those in the NP group only in the late phase in Nx rats. In the early phase, FGF23 levels were comparable in the NP, HP, and IV groups, but were significantly higher in the HP and IV groups compared to the NP group in the late phase in Nx rats. 1α-hydroxylase and sodium dependent phosphate co-transporter 2a expression levels in the kidney in Nx rats were equally and significantly decreased in the HP and IV groups compared with the NP group, while 24-hydroxylase expression was equally and significantly increased. These results show that chronic intravenous phosphate loading increases bioactive FGF23, indicating that an alternative pathway for FGF23 regulation, in addition to the dietary route, may be present. This pathway is clearer under conditions produced by a kidney injury in which phosphate is easily overloaded

Urinary data of uremic rats (Day 1 and Day 7).

<p>Data are means ±s.e.; UV, urine volume; Ccr, creatinine clearance; FEP, fractional excretion of phosphate; NP, normal phosphate diet; HP, high phosphate diet; IV, intravenous phosphate load.</p>†<p>P<0.05 versus Nx+NP (Day 1),</p>$<p>P<0.05 versus Nx+NP (Day 7).</p

Serum chemistries of uremic rats (Day 1 and Day 7).

<p>Data are means ±s.e.; Cre, serum creatinine; P, serum phosphate; PTH, parathyroid hormone; FGF, fibroblast growth factor; Nx, 5/6 nephrectomized; NP, normal phosphate diet; HP, high phosphate diet; IV, intravenous phosphate load.</p>†<p>P<0.05 versus Nx+NP (Day 1),</p>$<p>P<0.05 versus Nx+NP (Day 7).</p

Immunoblotting for 1α-hydroxylase, 24-hydroxylase, NPT-2a, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (a).

<p>Densitometric quantification of the corresponding bands was performed using an image analyzer, 1α-hydroxylase (b) 24-hydroxylase (c), and NPT-2a (d). Data are presented after normalization to GAPDH expression and are depicted as the percentage change from the respective controls (Sham+NP). Data are shown as mean ± s.e. (<i>n</i> = 4 each). *P<0.05 vs. Sham+NP and ^#P<0.05 versus Nx+NP.</p

Serum chemistries of sham rats (Day 1 and Day 7).

<p>Data are means±s.e.; Cre, serum creatinine; P, serum phosphate; PTH, parathyroid hormone; FGF, fibroblast growth factor; NP, normal phosphate diet; HP, high phosphate diet; IV, intravenous phosphate load.</p>#<p>P<0.05 versus Sham+NP (Day 1).</p

Effect of phosphate loading on 24-hydroxylase expression in the kidney of uremic rats on day 7.

<p>(a) Nx+NP, (b) Nx+HP, (c) Nx+IV rats, and (d) Sham+NP rats. Magnification×100.</p

Effect of phosphate loading on 1α-hydroxylase expression in the kidney of uremic rats on day 7.

<p>(a) Nx+NP, (b) Nx+HP, (c) Nx+IV rats, and (d) Sham+NP rats. Magnification×100.</p

Effect of phosphate loading on NPT-2a expression in the kidney of uremic rats on day 7.

<p>(a) Nx+NP, (b) Nx+HP, (c) Nx+IV rats, and (d) Sham+NP rats. Magnification×100.</p