Some Considerations of the Role of Antidiuretic Hormone in Water Homeostasis

Abstract

Publisher Summary This chapter discusses the effects of antidiuretic hormone (ADH) on water and solute permeation rates in mammalian collecting tubules and effects of ADH on water and hydrophilic solute transport coefficients. The chapter also discusses pathway for water permeation, origin of cellular constraints to diffusion, transcellular versus paracellular water flow, permeation of moderately lipophilic solutes, activation energies for water and solute permeation, and comparison of ADH effects in collecting tubules and amphibian epithelia. It has been found that in cortical collecting tubules and possibly other ADH-sensitive tissues, water and moderately lipophilic solutes traverse luminal plasma membranes by parallel diffusion pathways narrow aqueous channels for water and hydrophobic regions for moderately lipophilic solutes. Both regions, but particularly the hydrophobic pathway for moderately lipophilic solute transport, may be in a higher entropy state than the remaining, impermeant regions of luminal membranes. Moreover, ADH increases the number of these specialized pathways without necessarily affecting the ordering of the regions of luminal membranes that are impermeant to water and solutes. Thus, the total ADH-dependent change in the entropy of luminal membranes will depend on the magnitude of the hormone-mediated increase in the fractional area occupied by the specialized water and solute permeation pathways.