AVPR2 and autosomal dominant polycystic kidney disease: Reduction in urine osmolality (Uosm) is a good marker of V2R antagonism because of the physiologic role of V2R signaling in promoting water reabsorption by the renal collecting ducts.10 The relationship of Uosm with the mechanism of action of tolvaptan, as well as ease of urine collection and Uosm measurement, support exploration of change in Uosm as an early indicator of therapeutic response to tolvaptan in ADPKD.