Effects of Space Flight and −6° Bed Rest on the Neuroendocrine Response to Metabolic Stress in Physically Fit Subjects

Thyroid cell growth: sphingomyelin metabolism as non-invasive marker for cell damage acquired during spaceflight

Prolonged spaceflights are known to elicit changes in human cardiovascular, musculoskeletal, and nervous systems, whose functions are regulated by the thyroid gland. It is known that sphingomyelin metabolism is involved in apoptosis (programmed cell death) of thyroid cells induced by UVC radiation, but at present no data exists with regard to this phenomenon, which occurs during space missions. The aim of this study was to analyze, for the first time, the effect of spaceflight on the enzymes of sphingomyelin metabolism, sphingomyelinase, and sphingomyelin synthase, and to determine whether the ratio between the two enzymes might be used as a possible marker for thyroid activity during space missions. Both quiescent thyroid cells and thyroid cells stimulated to proliferate with thyrotropin (TSH) were cultured during the Eneide and Esperia missions on the International Space Station. The results show that during space missions the cells treated with TSH grew only 1.5 ± 0.65-fold and, thus, behave similarly to quiescent cells, while on the ground the same cells, maintained in experimental conditions that reproduced those of the flight, grew 7.71 ± 0.67-fold. Comparison of the sphingomyelinase/sphingomyelin-synthase ratio and the levels of Bax, STAT3, and RNA polymerase II in proliferating, quiescent, pro-apoptotic, or apoptotic cells demonstrated that thyroid cells during space missions were induced into a pro-apoptotic state. Given its specificity and the small amount of cells needed for analysis, we propose the use of the sphingomyelinase/sphingomyelin-synthase ratio as a marker of functional status of thyroid cells during space missions. Further studies could lead to its use in real time during prolonged spaceflights.

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.