EFFECT OF ANISOTONIC CONDITIONS ON THE TRANSPORT OF HYDROPHILIC MODELCOMPOUNDS ACROSS MONOLAYERS OF HUMAN COLONIC CELL-LINES

The effect of anisotonic solutions on the enhancement of the transport of hydrophilic model compounds across monolayers of Caco-2 and HT-29. cl19A intestinal epithelial cells was studied. In filter-grown monolay ers of the highly differentiated villus-like Caco-2 cell line, a profo und and dose-dependent drop in the transepithelial electrical resistan ce was found after apical treatment with a 30 or a 50% hypotonic solut ion (200 and 150 mOsmol, respectively). This drop was not observed aft er basolateral and two-sided application of a 50% hypotonic solution. During apical hypotonic treatment a 12- and 8-fold increase also was o bserved in transepithelial transport of two hydrophilic model compound s, i.e., fluorescein-Na and fluorescein-isothiocyanate-labeled dextran , MW 4000, respectively. Through confocal laser scanning microscopy, i t was revealed that this enhanced transport was predominantly via the paracellular route. Moreover, morphological changes in the cell layers indicating cell swelling were observed after apical hypotonic, but no t after basolateral or bilateral treatment, probably resulting from an incomplete regulatory volume decrease response. This swelling, and sl ight lateral retraction of the cells, allowed the hydrophilic compound s to pass between the cells. The effects of hypotonic challenge also w ere studied in monolayers of the more crypt cell-like HT-29.cl19A cell line. After apical hypotonic shock, these cells showed no effect on t ransepithelial electrical resistance, whereas an increase was observed after basolateral and bilateral treatment. Hypotonic shock failed to increase the transport of the hydrophilic model compounds in this cell line. It is thus suggested that apical hypotonicity may induce an enh anced paracellular transport of hydrophilic compounds in mature villus -like but not in crypt-like enterocytes.