Surface binding and uptake of polycationic ferritin by neonatal piglet intestinal epithelium

Pieces of small intestine from newborn, I- and 6-day-old piglets were incubated in vitro and ligated segments were incubated in vivo with polycationic ferritin (PCF) to determine the distribution and possible role in protein uptake of anionic sites on membranes at the luminal surfaces of the epithelial cells. Most PCF binding to the absorptive cells occurred on the upper third or half of the villi and to some non-absorptive cells (tuft cells) throughout the length of the villi. Pieces of intestine which were fixed before incubation had PCF on microvilli and apical invaginations of absorptive cells, but none in the sub-apical tubules. When samples were incubated with PCF in vitro before fixation PCF was bound to the surfaces of microvilli of absorptive cells and to the membranes lining the apical invaginations, some sub-apical vesicles and some sub-apical tubules in all age-groups. In vivo experiments with longer incubation times resulted in a similar distribution, with increased amounts of PCF in the sub-apical tubules in samples from newborn and 1-day-old piglets only. In the newborn there were small vesicles containing PCF which had apparently moved further into the cells. At the same concentration (2 mg/ml) ferritin did not enter the sub-apical tubules; but it did enter and was taken up by the cells in the presence of 4% (w/v) serum albumin. It is concluded that the mechanism of protein uptake does not involve the microvillar membrane and that non-specific interaction with the invaginated plasma membrane, as a function of charge-density, leads to opening of the sub-apical tubular system to protein. Movement further into the cell depends on other factors.