Supplementary Material for: Phosphorus restriction prevents rapamycin-induced kidney damage in rats

Introduction: There are conflicting reports about the effect or rapamycin on the kidneys. Rapamycin is known to promote phosphaturia that may be associated to renal injury. Methods: Detailed histopathological studies were performed on the kidneys of rats with normal (Control) and reduced (Nx) renal mass that were treated with rapamycin (1.3 mg/kg for 22 days) or placebo. The effect of rapamycin was also evaluated in Control and Nx rats fed different amounts of phosphorus: 0.6% P (NP), 1.2% P (HP) and 0.2% P (LP). Quantitative scores of kidney lesions were obtained for: interstitial nephritis (IN), tubular damage (TD), and nephrocalcinosis (NC). Results: When compared with placebo, rapamycin administration to Nx rats resulted in significant increases in IN (4.17±0.74 vs 1.51±0.53 %) and TD (14.45±1.51 vs 8.61±1.83 %). Rapamycin also increased NC both in Control (0.86±0.23 vs 0.14±0.06 %) and Nx (0.86±0.32 vs 0.15±0.14 %) rats. In Control rats receiving rapamycin, feeding HP aggravated IN (3.25±0.48 %), TD (22.47±4.56 %) and NC (3.66±0.75 %) while feeding LP prevented development of any renal lesions. In Nx rats treated with rapamycin, HP intake also increased IN (8.95±1.94 %), TD (26.86±3.95 %) and NC (2.77±0.60 %) whereas feeding LP reduced all lesions to lower levels than in rats fed NP. Rapamycin treatment increased fractional excretion of P (FEP) and an excellent correlation between scores for renal lesions and FEP was found. Conclusion: Rapamycin has deleterious effects on kidney pathology causing lesions that are located mainly at tubular and tubulo-interstitial level. Rapamycin-induced kidney damage is more evident in rats that already have decreased renal function and seems to be related to the phosphaturic effect of the drug. Dietary P restriction prevents kidney damage in rats treated with rapamycin.