Autoradiographic study of the permeability characteristics of the small intestine.

Abstract

SUMMARY This autoradiographic study demonstrates the distribution of a range of small solutes and macromolecules in the mucosa of the guinea-pig small intestine after intracardiac injection. The substances investigated were: 14C-urea, 3H-mannose, and 125I polyvinylpyrrolidone (PVP). Small bowel biopsies were taken at intervals from one to 60 minutes after injection and the tissues processed for autoradiography. Light microscopic examination of the autoradiographs showed that the compartmental distribution depended on the molecular size of the substances being studied. Urea and mannose, as small solutes, were uniformly distributed throughout the intravascular, extravascular, and epithelial compartments. Inulin was evenly distributed in the vessel lumen and extravascular space but there was a considerable drop in concentration in the epithelium. PVP exhibited the most marked gradients, the concentration being greatest in the vascular lumina, lower in the extravascular space, least in the epithelium. Thus there appear to be two barriers to macromolecular passage which are freely permeable to small solutes: the capillary wall and the epithelium. At a light microscopical level it is not possible to observe whether the limiting membrane of each of these barriers is the cell plasmalemmal membrane or the basement membrane. The selectivity of the epithelial barrier is greater than that of the capillary barrier.It is widely appreciated that the small intestine represents a semi-permeable barrier between the vascular lumen and the external environment of the intestinal lumen. This is a complex multiple barrier whose anatomical components comprise the vascular endothelium, the vascular basement membrane, the epithelial basement membrane, and the epitlelium of the intestinal mucosa. The small intestine is a relatively impermeable barrier to the solutes whose molecular weight is greater than 180 (Fordtran et al., 1965), but it is well recognised that substances greatly in excess of this size may permeate the mucosa in minor degrees. Plasma proteins up to the size of IgM immunoglobulins certainly pass from the circulation into the intestinal lumen (Barth et al., 1964;Jarnum and Jensen, 1972) and substances as large as ferritin (Bockman and Winborn, 1966) and IgG (Kraehenbuhl and Campiche, 1969) are absorbed from the small intestine by passive sorption. Loehry