The effect of albumin on proximal tubule epithelial cells (PTEC) is believed to play a significant part in the development of interstitial fibrosis by various mechanisms including secretion of profibrotic and proinflammatory cytokines and epithelial mesenchymal transdifferentiation. Albumin has been shown to activate various signalling pathways and also result in secretion of TGF-β, endothelin-1, MCP-1 and RANTES by PTEC. These effects are thought to be mediated by the cell surface receptor megalin. Whether these effects of albumin requires its internalisation by megalin or can occur upon its binding followed by subsequent signalling is unclear. Opossum kidney (OK) cells have been extensively used to study albumin endocytosis. However, not much data exists in human PTEC. We investigated albumin endocytosis determined by fluoremetric analysis of Rhodamine Red-X labelled albumin up-take in human primary and transformed (HKC-8) PTEC cells and compared them with OK cells. Experiments were performed in cell culture medium, cells were lysed in 20mM TrisHCl containing 1% Triton and 0.5 % SDS, 2mM EDTA. Statistical analysis was by ANOVA followed by a Dunnett test; n=3-5. A dose-dependent increase in albumin endocytosis was observed in all the 3 cell types at 30 min at 37°C (0.025 mg/ml to 0.2 mg/ml human serum albumin). The level of endocytosis in OK cells tended to plateau at 0.05 mg/ml. However, in human PTEC no plateau was observed and endocytosis continued to increase up to 0.2 mg/ml. The OK cells endocytosed significantly more albumin than both Primary PTEC and HKC cells (4690 fluorescent units/mg protein (FU/mg) Vs 831 FU/mg Vs 220 FU/mg, respectively, at 0.2 mg/ml, P<0.01). In keeping with current literature 225 μM 5-(N-ethyl-N-isopropyl) amiloride (EIPA) reduced endocytosis by 70% in OK cells. No reduction was seen in HKC-8 by EIPA. Endocytosis in OK vs HKC-8 cells was significantly different as analysed by 2-way ANOVA where the 2 factors were cell type and concentration of EIPA (P<0.01). Primary PTEC responded to EIPA in a similar fashion to HKC-8 cells. Statins are believed to inhibit albumin endocytosis via inhibition of Rho GTPases. Simvastatin dose dependently reduced albumin endocytosis in OK cells (0.1μM-50μM; 42-88%). However, endocytosis was not inhibitable with simvastatin in human cells. Receptor Associated Protein (RAP) has a greater affinity for megalin than albumin, and consequently blocks albumin binding. A dose-dependent inhibition of albumin endocytosis by RAP was achieved in OK cells (59 ± 2.9% inhibition at 0.25μM, mean ± SEM). A similar trend was observed in HKC-8 cells. However, this did not reach statistical significance (31 ± 14.7% inhibition at 0.25μM). These experiments show a differences in albumin endocytosis in OK cells compared to human PTECs.