PROTEIN ENDOCYTOSIS BY A KIDNEY TUBULE SUSPENSION - METABOLIC REQUIREMENTS

Endocytosis in the renal tubular cell is a permanent process serving t he role of saving nitrogen from plasma peptides that are continuously cleared away by kidney glomerulus. Since small proteins appear in urin e after strenuous exercise, it was hypothesized that renal ischemia im pairs the tubular endocytic reabsorption of proteins. The aim of this paper is to describe a simple in vitro model of renal endocytosis and to use it in studies of endocytic metabolic requirements. The results show that rabbit renal proximal tubules in suspension are able to take up I-125-lysozyme, as well as RITC-lactalbumin. The fluorescent prote in was taken up only by the ends of the everted tubule fragments, and accumulated into intracellular vesicles, demonstrating the luminal pat hway of endocytosis. The amount of I-125-lysozyme taken up was equival ent to that taken up by isolated perfused tubules (Nielsen et al. (198 6) Am. J. Physiol. 251, F822-F830). Anoxia decreased 12-fold the intra cellular accumulation of I-125-lysozyme; however, the time-course of i nhibition shows that only the late steps of endocytic accumulation are energy-dependent. Substrate deprivation studies suggest a specific ro le of glucose to sustain endocytosis. Lastly, renal uptake of I-125-ly sozyme was shown to be strongly depressed by chloroquine, an alkaliniz ing agent of endosomes and lysosomes. We conclude that (1) renal tubul es in suspension are a satisfactory model for endocytic studies in kid ney; (2) suppressing oxygen and substrate supplies to kidney impairs e ndocytic tubular reabsorption of proteins.