Cellular constraints to diffusion

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Abstract
These experiments were intended to evaluate the effects of antidiuretic hormone (ADH) on dissipative water transport in cortical collecting tubules isolated from rabbit kidney. In the absence of ADH, the osmotic (Pf, cm sec-1) and diffusional (PDW cm sec-1) water permeability coefficients were, respectively, 6±6 and 4.7±1.3 (SD). When ADH was added to the bathing solutions, Pf and PDW rose to, respectively, 186±38 and 14.2±1.6 (SD). In the absence of ADH, the tubular cells were flat and the lateral intercellular spaces were closed when the perfusing and bathing solutions were, respectively, hypotonic and isotonic; in the presence of ADH, the cells swelled and the intercellular spaces dilated. These data suggest that ADH increased the water permeability of the luminal membranes of the tubules. It was possible that the ADH-dependent Pf/PDW ratio was referable to the resistance of the epithelial cell layer (exclusive of luminal membranes) to water diffusion (RDW, sec cm-1). Such a possibility required that RDW be ∼ 650, i.e., approximately 25-fold greater than in an equivalent thickness of water. To test this view, it was assumed that RDi values for lipophilic solutes in lipid bilayer membranes and in luminal membranes were comparable. In lipid bilayer membranes, RDi was substantially less than 90 sec cm-1 for pyridine, n-butanol, and 5-hydroxyindole. In renal tubules, RDi for these solutes ranged from 795 to 2480 with and without ADH. It was assumed that, in the tubules, RDi was referable to cellular constraints to diffusion; for these solutes, the latter were 12-25 times greater than in water. Accordingly, it is possible that the ADH-dependent Pf/PDW ratio was also due to cellular constraints to diffusion.