Background: Apelin is a hormone that regulates cardiovascular function, and its concentration is increased by hypoxia based on cell culture and animal studies. As it remains unknown as to whether hypoxia could affect apelin levels in humans, we investigated whether breathing normobaric hypoxic gas mixture increases the circulating apelin concentration in healthy male subjects. Methods: Ten healthy young men (age 29 ± 5 years, body mass index 24.7 ± 2.8 kg/m2) breathed normobaric hypoxic gas mixture (11% O2/89% N2) for 1 hour. Venous blood samples were obtained immediately before, and 2 and 24 hours after the start of the hypoxic exposure and analyzed for circulating apelin concentrations. Results: Arterial oxygen saturation decreased steadily from a baseline value of 99% ± 1% after the initiation hypoxia challenge and reached a steady-state level of 73% ± 6% within 20-30 minutes. Baseline apelin concentration was 3.3 ± 1.3 pmol/L and remained comparable (3.3 ± 1.4 pmol/L) to the baseline concentration at a 2-hour time point. However, apelin concentration at the 24-hour time point (5.5 ± 2.8 pmol/L) was significantly (by ~67%) higher as compared with at both baseline and 2-hour time points (p < 0.05). Conclusion: In conclusion, in line with cell culture and animal studies, acute systemic hypoxia increases circulating apelin concentration also in humans
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