Vasopressin and copeptin: measurement, determinants and effects

The hormone vasopressin is essential for the human body. Vasopressin’s ability to preserve water, by decreasing urine production by the kidneys, is vital to maintain homeostasis. In addition to this well-known effect, vasopressin is implicated to have widespread effects on various other regulatory processes. However, studies aiming to unravel the various actions of vasopressin have been impeded by the fact that measurement of vasopressin plasma concentrations is difficult. Therefore, as alternative, copeptin is measured. Copeptin is released into the circulation by the pituitary gland in a 1:1 ratio with vasopressin and is therefore seen as reliable surrogate marker. In this thesis, measurement and interpretation of plasma copeptin was studied. The stability of vasopressin in blood samples is limited, thus introducing a significant bias during the pre-analytical phase, when these samples are drawn and stored. In contrast, copeptin is very stable ex-vivo. However, when kidney function decreases, the clearance of copeptin from the circulation also decreases. This may result in overestimation of the importance of vasopressin in kidney patients. Notwithstanding this note on the correct interpretation of its plasma concentrations, this thesis demonstrates that copeptin can aid clinical practice. Measurement of copeptin was used to study water handling by the kidneys, but also the role of vasopressin in other physiological processes, such as the release of the stress hormone cortisol, acid-base handling by the kidney and blood pressure regulation. Copeptin gave insight into the clinical relevance of the numerous effects of vasopressin.