Absorption of intact protein by colonic epithelial cells of the rat.

Colonic absorption of intact protein was examined in adult rats using histological and ultrastructural procedures. Horseradish peroxidase was introduced into ligated colonic loops and retained therein for 5, 10, 20, or 30 minutes prior to excision of the loops, and their processing for microscopy. Morphological evaluation revealed evidence of peroxidase absorption via pinocytosis. Tracer particles were observed adherent to the mucosal border, in apical pinocytotic vesicles, in vesicles adjacent to and fusing with lateral and basal cell membranes, in extracellular spaces throughout the mucosa, in lymphatic channels of the submucosa and occasionally in blood capillaries of mucosal and submucosal regions. The significance of these findings is discussed in light of the frequent presence of dietary and/or microbial macromolecules in the luminal milieu of the large intestine. It is suggested that pinocytotic uptake and subsequent vesicular transport and exocytosis of intact protein may occur in the colon of some species, and that such a phenomenon may be responsible for penetration of the mucosal barrier by macromolecular antigens or toxins.