Release and decay kinetics of copeptin vs AVP in response to osmotic alterations in healthy volunteers

Context: Copeptin is the C-Terminal fragment of the arginine vasopressin (AVP) prohormone whose measurement is more robust than that of AVP. Similar release and clearance characteristics have been suggested promoting copeptin as a surrogate marker. Objective: To characterize the physiology of osmotically regulated copeptin release and its half-life in direct comparison with plasma AVP. Design: Ninety-one healthy volunteers underwent a standardized three-phase test protocol including (1) osmotic stimulation into the hypertonic range by hypertonic-saline infusion followed by osmotic suppression via (2) oral water load and (3) subsequent glucose infusion. Plasma copeptin, AVP, serum sodium, and osmolality levels were measured in regular intervals. Results: In phase 1, an increase inmedian osmotic pressure [289 (286; 291) to 311 (309; 314)mOsm/kg H2O] caused similar release kinetics of plasma copeptin [4 (3.1; 6) to 29.3 (18.6; 48.2) pmol/L] and AVP [1 (0.7; 1.6) to 10.3 (6.8; 18.8) pg/mL]. Subsequent osmotic suppression to 298 (295; 301) mOsm/kg at the end of phase 3 revealedmarkedly different decay kinetics between both peptides-An estimated initial half-life of copeptin being approximately 2 times longer than that of AVP (26 vs 12 minutes). Conclusion: Copeptin is released in equimolar amounts with AVP in response to osmotic stimulation, suggesting its high potential as anAVPsurrogate for differentiation of osmotic disorders. Furthermore,we here describe the decay kinetics of copeptin in response to osmotic depression enabling to identify a halflife for copeptin in direct comparison with AVP.