The role of water diffusion in the action of vasopressin

Vasopressin produces a large increase in the osmotic flow of water across the toad bladder, with little apparent change in the diffusion rate of tritiated water. This discrepancy between osmotic and diffusional net flow is the basis of the pore theory of vasopressin action. The present studies show that there is in fact a large (at least 10-fold) increase in water diffusion subsequent to addition of vasopressin, which is masked by unstirred layers and by the resistance offered to diffusion by the thick layer of connective tissue and muscle supporting the bladder epithelial cells. An even higher diffusion rate would be anticipated with the complete elimination of unstirred layers, and of barriers to diffusion remaining within the epithelial layer itself. An alternative to the pore hypothesis is considered, in which vasopressin acts solely by increasing the diffusion rate of water across the luminal membrane of the epithelial cell.