Assessing the characteristic between length of hypoxic exposure and serum erythropoietin levels.

We examined the relationship between the duration of hypoxic exposure and serum erythropoietin (EPO) production. Adult male Long-Evans rats were exposed to hypobaric hypoxia (HH = 0.5 atm) for a period of 15 min to 20 days. Serum for EPO by radioimmunoassay was collected immediately, 1 or 2 h after HH exposure. A significant rise in EPO levels occurred 1 h after a 30-min HH exposure that was not sustained 2 h after termination. One hour of HH exposure resulted in increased EPO levels 1 h after termination of exposure and further increased levels 2 h after termination of exposure. With prolonged exposure, serum levels remained elevated at 6 and 20 days, despite the development of polycythemia. We concluded that the hypoxic stimulus for elevation of serum EPO could be as short as 30 min and that EPO levels remained elevated after chronic HH. The experimental data were consistent with a mathematical model in which stimulated EPO production was proportional to the time of HH stimulus.