ABSTRACT
Human cellular function depends on constant tonicity of the extracellular fluid. Water homeostasis is very accurately controlled to maintain plasma osmolality within a remarkably narrow range of 282-298 mOsm/kg. Water homeostasis is maintained through arginine vasopressin (AVP) and thirst. AVP is a nine-amino acid peptide hormone that is synthesized in the magnocellular and parvocellular neurosecretory neurons of the paraventricular nuclei and supraoptic nuclei of the hypothalamus. Aquaporin 2 is the predominant AVP-regulated water channel in the kidney and plays a key role in the control of collecting duct water permeability. Tolvaptan, the only drug of this class licensed in Europe, is an oral, selective, nonpeptide antagonist that blocks AVP binding to V2 receptors and activation of receptors at the renal collecting tubules. The result is water excretion without changing the total electrolyte excretion. Hypernatremia is a common electrolyte disorder in critically ill patients in intensive care units.
