LONG-TERM PROTEIN EXPOSURE REDUCES ALBUMIN-BINDING AND UPTAKE IN PROXIMAL TUBULE-DERIVED OPOSSUM KIDNEY-CELLS

To avoid renal loss of large amounts of proteins, filtered proteins ar e reabsorbed by endocytosis along the proximal tubule. However, althou gh protein reabsorption is a task of proximal tubular cells, it is als o a threat because it may cause cell injury. This study determines whe ther exposure to bovine serum albumin (BSA) leads to regulatory change s in endocytosis of FITC-BSA in proximal tubule-derived opossum kidney cells. Preincubation with BSA led to a decrease of FITC-BSA endocytos is with an IC50 value of 0.58 g/L. Specific binding of FITC-BSA to the apical membrane was also reduced (IC50 = 0.69 g/L). Kinetic analyses revealed that maximal uptake rate and maximal binding capacity were de creased with no change in affinity. Similar effects were observed afte r preincubation with equimolar amounts of other proteins (lactalbumin, transferrin, and conalbumin), but not after preincubation with dextra n. The effect of preincubation with BSA could be mimicked by preincuba tion with some amino acids. Preincubation with L-Ala, L-Gln, or NH4Cl, but not with L-Leu, L-Glu, or L-Asp, reduced FITC-BSA endocytosis and binding. Preincubation with BSA, but not with dextran, reduced protei n degradation and increased ammonia production, vesicular pH, as well as the rate of lactate dehydrogenase release. Apical fluid-phase endoc ytosis and apical uptake of neutral amino acids were not reduced. It i s concluded that proximal tubular cells reduce the uptake rate for pro teins, but not for other substrates, in response to increased protein load. This reduction is achieved by reducing the number of apical bind ing sites, partially in response to increased ammoniagenesis with dera nged vesicular pH and enzyme activities. Thus, increased protein filtr ation could result in reduced protein reabsorption, thereby enhancing proteinuria.