Brain and skin do not contribute to the systemic rise in erythropoietin during acute hypoxia in humans

Erythropoietin (EPO) preserves arterial oxygen content by controlling red blood cell and plasma volumes. Synthesis of EPO was long thought to relate inversely to renal oxygenation, but in knockout mice, brain and skin have been identified as essential for the acute hypoxic EPO response. Whether these findings apply to humans remains unknown. We exposed healthy young subjects to hypoxia (equivalent to 3800 m) and measured EPO in arterial and jugular venous plasma and in cerebrospinal fluid. To examine the role of the skin for EPO production during hypoxia, subjects were exposed to 8 h of hypobaric hypoxia with or without breathing oxygen-enriched air to ensure systemic normoxemia. With 9 h of hypoxia, arterial EPO increased (from 6.0±2.2 to 22.0±6.0 mU/ml, n=11, P<0.0001) and jugular venous EPO displayed a similar response (to 22.2±6.0 mU/ml, n=11). Thus, the arterio-jugular venous EPO difference was unaffected by hypoxia and also in cerebrospinal fluid EPO remained stable following hypoxic exposure (0.33±0.15 mU/ml, n=9 in normoxia vs. 0.41±0.20 mU/ml, n=9 in hypoxia, P=0.40). No change in plasma EPO was observed when only skin was exposed to hypobaric hypoxia (n=8). Thus, neither dermal oxygen exposure nor cerebral EPO production appears to be important for the systemic EPO response to acute hypoxia in healthy humans.