The transport barrier of epithelia: a comparative study on membrane permeability and charge selectivity in the rabbit

The transport barrier of the epithelial presents one of the major problems limiting the effective use of these tissues as alternate delivery routes for macromolecules such as peptides and proteins. In the present study, two membrane transport properties, namely, the permeability and permselectivity of the shunt pathway, were investigated and compared in various tissues including the nasal, tracheal, bronchial, buccal, rectal, vaginal, corneal, epidermal, duodenal, jejunal, ileal, and colonic epithelia. Membrane permeability was evaluated using a combined method based on electrical conductance and flux measurements of a hydrophilic fluorescent probe, 6-carboxy fluorescein (CF). Membrane permselectivity or the charge discriminating ability of the membrane was evaluated by KCl diffusion potential measurements. The results indicate that all epithelia under investigation possess a relatively high degree of permeation barrier and are highly selective for the absorption of positively charged solutes. Shunt path permeability was found to vary greatly among tissues from different epithelia, whereas membrane charge selectivity was relatively constant in these tissues. A good correlation was observed between membrane electrical conductance and steady-state flux of CF, indicating a paracellular transport of the compound. The rank order of the intrinsic membrane permeability was as follows: intestinal approximately nasal greater than or equal to bronchial greater than or equal to tracheal greater than vaginal greater than or equal to rectal greater than corneal greater than buccal greater than skin. Membrane permselectivity, expressed as the ratio of transport number (positive over negative), ranges from 1.78 for the buccal to 1.33 for the rectal epithelium.(ABSTRACT TRUNCATED AT 250 WORDS)