Basolateral endocytosis of protein in isolated perfused proximal tubules

Luminal uptake and degradation of protein in proximal tubules is well documented. However, abluminal uptake has only been demonstrated in a few species and probably only amounts to a few percent of luminal absorption. To investigate this absorptive pathway, isolated perfused proximal tubules from rabbit kidney were exposed to either cationized ferritin or horseradish peroxidase in the bath for 30 min. The tubules were then fixed and processed for electron microscopy. Peroxidase and small amounts of ferritin were found in the intercellular spaces, in endocytic vesicles located in the abluminal part of the cells and in multivesicular bodies. No tracer was found in the lumina or in the apical part of the cells. The tubules were ultrastructurally intact thus excluding the possibility that the proteins were absorbed via the luminal endocytic pathway or as a result of damaged cell membranes. In conclusion, this study presents evidence that ferritin and peroxidase can be absorbed via the basolateral membranes in rabbit proximal tubules.