Background: The underlying mechanisms of renal transplant dysfunction are poorly understood. There is little information on tubular function in kidney grafts. The cDNAs encoding kidney-specific cell surface proteins required for renal reabsorption of sodium (sodium cotransporter in thick ascending limb of Henle, rBSC1) and water (apical water channel in collecting duct, AQP2) have been recently identified. Since transcripts of these proteins are up-regulated in dehydration in association with maximal concentration of urine, we examined urinary concentrating ability and expression levels of mRNA of these proteins in kidney isografts.
Methods: Male Sprague-Dawley rats underwent syngeneic renal transplantation or unilateral nephrectomy (UNX) and were deprived of water for 24 hours at six weeks after the operation when histological and functional compensation of the intact kidney was complete. Blood and urinary samples were collected before and after dehydration. The amount of rBSC1 or AQP2 mRNA was measured using competitive polymerase chain reaction (PCR) by inducing a point mutation at the middle of PCR product for rBSC1 or by deleting 180 bp from 780 bp PCR product for AQP2, respectively. The protein expression was examined by Western blot analysis.
Results: Both groups of rats demonstrated the same levels of compensatory renal hypertrophy (approximately 60% weight increase) and plasma creatinine values. Histological examination revealed enlarged glomeruli and tubules, but no findings of ischemic damage, such as tubular atrophy or interstitial changes. Urinary concentration was noted in the UNX rats but not in rats with kidney grafts. Competitive PCR demonstrated that dehydration did not increase rBSC1 and AQP2 transcripts in rats with kidney transplantation. Immunoblot analysis confirmed that the marked increase of both rBSC1 and AQP2 proteins was noted only in the remnant kidney of dehydrated rats.
Conclusions: Rats with kidney isografts have a limited capacity to concentrate urine and, at the same time, fail to increase rBSC1 and AQP2 transcripts. This suggests that there is a prolonged damage of renal tubules by ischemia or denervation of the donor kidney, both of which are inevitable in the transplantation procedure.