Trans/paracellular, surface/crypt, and epithelial/subepithelial resistances of mammalian colonic epithelia.

The epithelial barrier function of the large intestine resides in the trans- and paracellular pathways of the surface epithelium and crypts. Conventional transmural resistance and permeability measurements, however, yield only the resistance of the whole tissue and not that of its individual components. Combining conductance scanning techniques and impedance analysis, we determined the resistance of epithelial and subepithelial tissues, crypts and surface epithelium, and trans- and paracellular pathways of the mouse distal colon. The subepithelial tissue contributed 15% to the transmural resistance of 118+/-9 omega x cm2. In the epithelium proper the resistance of crypts (429+/-86 omega x cm2) exceeded that of the surface epithelium (132+/-15 omega x cm2). The paracellular resistance (3.2+/-0.4 k omega x cm2) of the surface epithelium was 23-fold higher than the transcellular resistance (137+/-16 omega x cm2), and thus the epithelium was classified as "medium tight". In order to investigate the trans- and paracellular resistances of the crypt epithelium as well, flat monolayers of HT-29/B6 cultured colon crypt cells were studied, which had a transepithelial resistance of 349+/-32 omega x cm2. With transcellular resistance (377+/-41 omega x cm2) tenfold lower than the paracellular resistance (3.9+/-1.3 k omega x cm2), this cryptal monolayer was also classified as "medium tight". Hence, considering the 1.2 times larger area of the crypt epithelium, the surface epithelium has a 4 times larger ion permeability than the crypt epithelium. However, the paracellular resistances are not different. Thus the lower transcellular resistance of the surface compared to the crypt epithelium suggests a higher density of ion channels in the apical membrane of surface cells.