Context: Apelin and arginine vasopressin are antagonists in the regulation of body fluid and osmotic homeostasis. There are no data about apelin levels in patients with polyuria-polydipsia syndrome (PPS).
Objective: To investigate plasma apelin levels and plasma apelin to copeptin ratios in patients with PPS and healthy volunteers using copeptin as a surrogate marker for arginine vasopressin.
Design, participants, and setting: We included 41 patients with PPS in this post hoc analysis of a prospective study performed in tertiary care hospitals in Switzerland and Germany and 113 healthy volunteers as a control group.
Outcome measures: Plasma apelin and copeptin levels were measured in 15 patients with complete central diabetes insipidus (DI), seven patients with complete nephrogenic DI, 19 patients with primary polydipsia (PP), and 113 healthy volunteers.
Results: Plasma apelin levels were highest in patients with complete nephrogenic DI (413 pmol/L; interquartile range, 332-504 pmol/L; P = .01) and lower in patients with PP (190 [172-215] pmol/L; P < .001) or complete central DI (209 [174-241] pmol/L; P = .02) as compared to healthy volunteers (254 [225-311] pmol/L). Plasma apelin to copeptin ratio in patients with PP (53 [38-92] pmol/pmol; P > .9) was similar to healthy volunteers (57 [37-102] pmol/pmol). In contrast, the apelin to copeptin ratio was higher in patients with complete central DI (89 [73-135] pmol/pmol; P = .02) and lower in patients with complete nephrogenic DI (7 [6-10] pmol/pmol; P < .001) compared to healthy volunteers.
Conclusion: In PP, normal plasma apelin to copeptin ratio attests a normal water homeostasis. In contrast, in patients with central or nephrogenic DI, the increased or decreased apelin to copeptin ratio, respectively, reflects a disturbed osmotic and body fluid homeostasis.
Trial registration: ClinicalTrials.gov NCT00757276.