AVPR2 and Hyponatremia: Tolvaptan is a selective vasopressin V2 receptor (V2R) antagonist that was approved by the Food and Drug Administration (FDA) in 2009 for the treatment of euvolemic and hypervolemic hyponatraemia (serum sodium < 125 mEq/l), associated with heart failure, cirrhosis and the SIAD 1, 2, 3.