Integrative physiology of basal water permeability in the distal nephron: implications for the syndrome of inappropriate secretion of antidiuretic hormone

Patients who drink more electrolyte-free water than they can excrete may de velop hyponatremia. A subgroup of hyponatremic patients has a reduced excre tion of electrolyte-free water and a low rate of excretion of solutes even though vasopressin is not detected in their plasma. Basal water permeabilit y in the distal nephron, by permitting a limited volume of electrolyte-free water to be reabsorbed, offers a way to help explain these findings. Basal water permeability will also be considered from the perspective of integra tive physiology in evolutionary and developmental biology settings. Its pos sible clinical importance will be explored in patients with chronic hyponat remia who have a low distal volume delivery. These patients may develop osm otic demyelination if a large solute load leads to a very rapid excretion o f electrolyte-free water.