Routes of nonelectrolyte permeation across epithelial membranes.

Abstract

Radioactive tracer techniques were used to study the permeability of three epithelial membranes, the toad urinary bladder, frog choroid plexus, and rabbit gallbladder, to 16 nonelectrolytes. The general patterns of nonelectrolyte permeation were similar for all three membranes, and similar to those in other biological membranes. The permeability of lipophilic solutes was roughly proportional to their bulk phase oil/water partition coefficients, but the slope was greater in the toad bladder than in the gallbladder and plexus. Branched nonelectrolytes were less permeable than their straight-chain isomers in both the urinary bladder and gallbladder, but not in the choroid plexus. Small polar solutes permeated more rapidly than expected, and in the urinary bladder and gallbladder the permeation of urea and acetamide, but not water was inhibited by phloretin. This agent also increased 1,7-heptanediol permeability in the urinary bladder but in the gallbladder there was a marked inhibition. In all three epithelia a separate pathway exists for the permeation of large polar solutes, but quantitatively this is least important in the toad bladder. It is concluded that variations in passive nonelectrolyte permeation across epithelia are due to (i) variations in the composition and configuration of membrane lipids, (ii) the presence or absence of pathways for small solutes, and (iii) the absence of presence of pathways for larger polar solutes. We also conclude that there are at least two effects of phloretin on the permeation of nonelectrolytes across biological membranes, and that there are variations in each effect from one membrane to another.