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Researchers have measured the transepithelial electrical resistance and isotopic water and urea fluxes for Madin-Darby canine kidney (MDCK) type I and II cells. Attention was also given to type I cells expressing MUC1, the mucin protein, in their apical membranes. It was found that the apical membrane permeability of type I cells resembles that of other barrier epithelia. Abrupt introduction of water-permeable pores with nystatin into the apical membranes of barrier epithelia gives rise to prompt increases in water permeability.

Estimations of the permeability properties of dissected rabbit bladder epithelium were conducted to determine the role of the apical membrane in the permeation of water, urea, ammonia and protons. Measurements of bladder membrane permeability indicated low permeations of water, urea and ammonia. However, the differences in the intrinsic properties of the apical membranes contributed to the varied measurements of water permeability.