Microscopical determination of the filtration permeability of the mucosal surface of the goldfish intestinal epithelium

Summary The rate of shrinkage of the mucosal folds of goldfish intestine in response to mucosal hypertonicity was measured by microscopic means. Because of the geometry of the intestinal folds the rate of shrinkage could be directly related to the loss of volume from the fold through the brush border membranes and tight junctions. Experimentally a wide range of velocities was observed, reflecting the difficulty of rapidly estabilishing a uniform osmotic gradient at the preparation's mucosal surface. The initial velocity of volume loss provided a measure of the filtration permeability (P _f ) of the mucosal surface. From the highest velocities observed the filtration permeability was estimated to be approximately 14×10^–3 cm/sec related to the folded mucosal surface and 65×10^–3 cm/sec related to the straight serosal surface. Consideration of the experimental errors and unstirred layer effects make it probable that the latter value is still an underestimate of the trueP _f . The series barriers of the epithelium cause the total tissueP _f to be less than theP _f of the mucosal surface alone. In addition theP _f measured in the presence of an osmotic gradient may differ substantially from the tissue filtration permeability which exists in the absence of a change in osmolarity.