Effects of real and simulated microgravity on response of sympathoadrenal system to various stress stimuli.

Changes in plasma levels of epinephrine (EPI) and norepinephrine (NE) were investigated in humans exposed to physical exercise (WL), to psychic stressor (mental arithmetic test, MAT), and to oral glucose administration (oGTT) before and during a stay in microgravity (real space flight, SF) or in simulated microgravity (head-down bed rest, HDBR). A permanent cannula inserted into the cubital vein and a special appliance, Plasma-03, were used for blood collection, plasma separation, and freezing of samples during SF. Plasma EPI, NE, dihydroxyphenylglycol (DHPG), and dihydroxyphenylalanine (DOPA) levels were measured by the high-pressure liquid chromatography (HPLC) method. Basal plasma EPI, NE, DHPG, and DOPA levels were found within the range of control values during SF. Preflight WL produced high increase in plasma NE and moderate elevation of plasma EPI, DHPG, and DOPA levels. Exaggerated exercise induced increases in plasma NE, DHPG, EPI, and DOPA levels were demonstrated in real microgravity. A return to preflight responses of sympathoadrenal system was seen after the landing. Plasma EPI, NE, and DHPG responses to MAT were relatively small, but increased during SF. During the oGTT the plasma EPI levels were slightly reduced in microgravity. Similarly as in SF, WL in HDBR was followed by significantly exaggerated responses of plasma catecholamines. These results show that both somatic and psychological stressors are able to induce an increased activation of sympathoadrenal system during SF or simulated microgravity in HDBR.