Nature of the water permeability increase induced by antidiuretic hormone (ADH) in toad urinary bladder and related tissues.

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Abstract
In artificial lipid bilayer membranes, the ratio of the water permeability coefficient (Pd(water)) to the permeability coefficient of an arbitrary nonelectrolyte such as n-butyramide (Pd(n-butyramide)) remains relatively constant with changes in lipid composition and temperature, even though the individual Pd''s increase more than 100-fold. This may be a general rule that also holds for the lipid bilayers of cells and tissues, and therefore if Pd(water)/Pd(solute) greatly exceeds the value found for artificial lipid bilayers (where solute is a molecule, such as 1,6 hexanediol or n-butyramide, that crosses the cell membrane by a solubility-diffusion mechanism without the aid of a special transporting system), then water crosses the cell membrane via aqueous pores. Applying this criterion to the toad urinary bladder, even in the unstimulated bladder, water probably crosses the luminal membrane primarily through small aqueous pores, and this is almost certainly the case after antidiuretic hormone (ADH) stimulation. ADH stimulation ultimately may lead either to formation (or enlargement) of pores, by the rearrangement of preexisting subunits, or to an unplugging of these pores.

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