Macromolecular absorption. Mechanism of horseradish peroxidase uptake and transport in adult and neonatal rat intestine.

The immature small intestine of neonatal mammals is permeable to gamma globulins as a source of passive immunity. Allegedly, macromolecular absorption ceases when the epithelial cell membrane matures. However, some evidence exists that adult animals retain a limited capacity to transport antigenic and biologically active quantities of large molecules. In this study, the mechanism of absorption of the tracer protein, horseradish peroxidase (HRP), was tested in neonatal and adult rat gut sacs. Transport into serosal fluid was quantitated by enzymatic assay and monitored morphologically by histochemical techniques. A greater transport of HRP was noted in the adult jejunum compared to adult ileum and neonatal intestine. Morphologically, the uptake mechanism in adult intestine was similar to the endocytosis previously reported in neonatal animals Like other endocytotic processes, HRP uptake in adult rats is an energy-dependent process as determined by metabolic inhibitors and temperature-controlled studies. An understanding of the mechanism whereby macromolecules are bound to intestinal membranes and engulfed by them is necessary before the action of physiologic macromolecules such as enterotoxins can be appreciated.