Quantitative studies of protein uptake in microperfused renal proximal tubules.

Proximal convoluted tubules were microperfused with an iodine 125 egg-white-lysozyme containing perfusion solution. The intraluminal lysozyme concentration decreased about 10% per mm tubular length over a wide range of intraluminal lysozyme concentrations tested indicating a constant fractional reabsorption over a wide concentration range. Lysozyme reabsorption at a high intraluminal lysozyme concentration of 10 mg/ml indicated that the tubular protein uptake by endocytosis is a saturable process. High intraluminal concentrations of basic amino acids inhibited the cellular uptake of lysozyme, while neutral amino acids had no effect.