Absence of significant cellular dilution during ADH-stimulated water reabsorption.

Water reabsorption across many "tight" urinary epithelia is driven by large transepithelial osmotic gradients and is controlled by antidiuretic hormone (ADH). Numerous investigators have concluded that ADH-induced water reabsorption causes large apparent increases in cell volume with concomitant cytoplasmic dilution. A central question in renal physiology has been how cellular homeostasis is maintained in tight urinary epithelia during antidiuresis. Previous direct measurements of cell membrane permeability to water and the present direct measurements of cell volume in collecting tubules of rabbit kidney cortex by quantitative light microscopy show that cell volume does not change significantly during transcellular water flow. Fluid transported across the epithelium accumulated in lateral and basal intercellular spaces; the effect was an increase in cell height and tubule wall thickness accompanied by maintenance of nearly constant cell volume. The stability of cell volume is a consequence of the relatively high water permeability of the blood-facing cell membrane.