#4032 RAPAMYCIN AGGRAVATES KIDNEY LESIONS IN RATS FED HIGH PHOSPHATE
Authors: Raya-Bermúdez Ana Isabel , Vidal Ángela , Castro Azahara Espartero , Martos Carmen María Pineda , Villalba Ignacio Lopez , Kolosov Stanislav , Aguilera-Tejero Escolástico 
Published: June 2023

Abstract

Background and Aims
Rapamycin is currently used to prevent rejection after kidney transplantation. mTOR blockade by rapamycin may slow the progression of diabetic kidney disease. Rapamycin promotes phosphaturia by inhibiting renal tubular transport of phosphate. Excessive tubular load of phosphate produces kidney damage by inducing tubular injury and interstitial fibrosis. The aim of this work is to evaluate the effect of rapamycin on kidney pathology in rats fed different phosphate levels.

Method
Thirty four female Wistar rats were used in this experiment. Rats, aged 2 months at the beginning of the studies, were allotted to 6 groups (n = 4-6). Rats with intact renal function were fed a standard diet containing either nomal (0.6%, NP), low (0.2%, LP) or high (1.2%, HP) phosphate. Half of the groups received placebo treatment (NP, LP, HP) and the other half were treated with rapamycin (NP-Rapa, LP-Rapa, HP-Rapa) at a dose of 1.3 mg/kg/day, for 22 days. Plasma concentrations of urea, creatinine, and phosphate as well as urine creatinine and phosphate were measured by spectrophotometry. Microscopic evaluation of the kidneys was performed in tissue sections fixed with formaldehyde solution and embedded in paraffin. Kidney sections were stained with hematoxylin-eosin, Masson Trichrome, periodic acid Schiff, and Von Kossa staining. Thirty non overlapping fields were evaluated. Chronic interstitial fibrosis and renal calcification were measured with a point counting grid and the result was expressed in percentage. Atrophic tubules were counted and the result was expressed as number of atrophic tubuli/field. Values are expressed as the mean ± standard error (SE). The difference between means was assessed by ANOVA. Fisher LSD test was used as a post-hoc procedure. A correlation study was carried out using the Pearson test. p&lt;0.05 was considered significant.

Results
Kidney lesions were more severe in rats treated with rapamycin and fed high phosphate diet. Areas of chronic interstitial fibrosis, with interstitial fibrosis and lymphoplasmacytic infiltrate, as well as atrophic tubules with thickened and wrinkled tubular basement membrane, loss of the brush border and flattening of the tubular cell cytoplasm, were observed. Calcification foci, mainly located in the corticomedullary junction, were identified with Von Kossa staining. Treatment with rapamycin did not influence renal function parameters in NP and LP rats but increased plasma creatinine and urea in HP rats. Plasma P concentration tended to decrease after rapamycin treatment. Fractional excretion of P was higher in all the groups treated with rapamycin, and the differences reached significance in HP-Rapa rats. Fractional excretion of P showed an excellent correlation with kidney lesions: chronic interstitial fibrosis (r = 0.822), calcification (r = 0.827) and tubular atrophy (r = 0.855).

Conclusion
The phosphaturic action of rapamycin aggravates renal lesions in rats fed high phosphate diets. Thus, rapamycin could be deleterious for the kidneys in the context of high phosphorus intake.