In ulcerative colitis (UC), the epithelial barrier is impaired by erosion/ulcer-type lesions and epithelial apoptosis causing local leaks, and generalized tight junction alterations increasing the basal permeability. We quantified the contribution of these mechanisms to the increased colonic ion permeability.

Sigmoid colon was stripped, and the spatial distribution of current clamped across the viable epithelium was recorded by a microelectrode probe, using the conductance scanning method. Local leaks (circumscribed conductive peaks) were marked, and structural changes were studied in H&E-stained series sections.

Overall conductivity increased from 8.4 ± 0.7 mS/cm² (mean ± SEM) in controls to 11.7 ± 0.6 in specimens with mild inflammation (i.e., with intact epithelium) and 34.4 ± 6.2 mS/cm² in moderate-to-severe inflammation (i.e., with visible epithelial lesions). Only in part this was caused by a generalized increase in basal conductivity (12.2 ± 1.5 mS/m² in moderate-to-severe UC vs. 8.3 ± 0.7 in controls). More importantly, the spatial distribution of conductivity, which was even in controls, showed dramatic leaks in UC. Leaks found in mild inflammation without epithelial lesion turned out to be foci of epithelial apoptosis. In moderate-to-severe inflammation, leaks correlated with epithelial erosion/ulcer-type lesions or crypt abscesses.

In early UC, but not in controls, seemingly intact epithelium comprises leaks at apoptotic foci. With more intensive inflammation, erosion/ulcer-type lesions are highly conductive, even if covered with fibrin. Local leaks contribute 19% to the overall epithelial conductivity in mild and 65% in moderate-to-severe inflammation.