Routes of nonelectrolyte permeation across epithelial membranes

Abstract

Summary. 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 oilJwater partition coefficients, but the slope was greater in the toad Nadder than in the gallbladder and plexus. Branched nonelectrolytes were Iess 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 or 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.Simple epithelial membranes, such as those of the intestine, gallbladder, choroid plexus, renal tubule, and toad urinary bladder, are composed of a sheet of cells held together at the apical surface by tight junctions (zonula occludens). These junctions are formed by the fusion of the plasma membranes of adjacent cells completely around the perimeter of each cell (see

Urea secretion by the straight segment of the proximal tubule.

“…However, phloretin has no effect on transpithelial water permeation across the toad bladder (22). In addition, urea permeability is inhibited by phloretin across the gall bladder (23).…”

“…These findings would suggest that under certain circumstances the collecting duct is capable of active urea reabsorption. No mammalian renal studies have suggested active urea secretion; however, active urea secretion has been demonstrated by frog kidney (21), and carrier-mediated urea transport inhibited by phloretin has been shown in the toad bladder (22,23). However, phloretin has no effect on transpithelial water permeation across the toad bladder (22).…”

The permeability of the secretin stimulated exocrine pancreas to non‐electrolytes.

“…Compounds with five and six carbon atoms have o-A lying between 0-91 and 0-99. For all compounds tested -A is also plotted against molecular volume (Wright & Pietras 1974). For compounds with a molecular volume greater …”

Uptake of [14C]urea by the in vivo choroid plexus—cerebrospinal fluid—brain system: identification of sites of molecular sieving

“…via a transcellular pathway in the choroid plexus. Similarly, Wright & Pietras (1974) concluded that there is transcellular penetration of urea through the frog choroid plexus. Although it is not possible to distinguish between transcellular and paracellular routes (Smulders & Wright, 1971) on the basis of the half-time data, the relatively short half-time of 0 3 hr for the rate of appearance of tracer urea into the c.s.f.…”