AVP and essential hypertension: Since over 90% of circulating AVP is bound to platelets and its plasma half-life is only 5 to 20 min (24), the urinary AQP2/creatinine ratio, which reflects the shedding phenomenon of AQP2 from the apical luminal membrane of collecting duct principal cells, has been considered as valid surrogate for assessing renal sensitivity to AVP action, based on results obtained in a rat model of essential hypertension (25).