ERYTHROPOIETIN UNDER REAL AND SIMULATED MICROGRAVITY CONDITIONS IN HUMANS

The aim of this study was to analyze the time course of erythropoietin (EPO) during Earth-bound microgravity simulations such as bed rest, i solation and confinement (IC), head-down tilt (HDT; -6 degrees), and i mmersion to evaluate which factors could contribute to alterations in EPO under real microgravity conditions during and after short- (<10 da ys) and long-term (>6 mo) spaceflights. During bed rest (24 h), no sig nificant changes in EPO could be observed. Subjects confined in a divi ng chamber facility for 60 days showed a decrease in EPO. In the recov ery period a slight increase was observed, but EPO concentrations did not reach the pre-IC control level. In the control period before HDT, subjects showed normal resting values for EPO, but on day 2 of HDT the EPO concentrations were decreased (P < 0.01). Later the EPO levels re mained below the control value and were increased after KDT (P < 0.05) . After immersion (24 h) increased EPO concentrations could be determi ned (P < 0.05). During a short-term spaceflight the astronauts showed in-flight (day 4) decreased and unchanged EPO concentrations. During a long-term spaceflight, 24 h after recovery, the cosmonaut showed slig htly elevated EPO concentration, which increased markedly during the f ollowing days. It is concluded that 1) HDT (-6 degrees) causes a rapid decrease in EPO in humans, 2) IC per se leads to diminished EPO conce ntrations, 3) EPO regulation in humans during short- and long-term spa ceflights might be different, 4) changes in central blood volume, i.e. , central venous pressure, seem to be involved in the modulation of EP O production and release under simulated and real microgravity conditi ons, and 5) the HDT(-6 degrees) Earth-bound simulation reflects mostly the changes in EPO production and release observed under real microgr avity conditions in humans.