Effects of charge on membrane processing in the proximal nephron

To examine the effects of molecular charge on membrane processing in renal tubular cells, the distribution of cationic and anionic ferritin was characterized in microperfused proximal nephron segments. During the first 7 min of proximal tubule perfusion, cationic ferritin was observed 1) bound to the brush-border membrane, 2) in apically positioned vesicles and vacuoles, 3) in lysosomes, 4) in vesicles adjacent to the basolateral plasmalemma, and 5) bound to the basolateral plasmalemma. Compared with anionic ferritin, the distribution of cationic ferritin was characterized by 1) a smaller relative grain density for lysosomes, 2) an accumulation of granules in an enlarged pool of apical cytoplasmic vescicles and vacuoles, and 3) a greater number of granules reaching the basolateral plasmalemma. During incubation directly in the presence of isolated renal cortical microvilli, binding of cationic ferritin increased significantly as pH was lowered from 8.0 to 4.5 and was greater than that of anionic ferritin, which varied little with pH. The data indicate that the molecular charge of endocytosed substances affects routing and membrane processing in proximal tubular cells, suggesting that their membrane-binding characteristics may influence transport patterns.