Effects of exposure to space flight on endocrine regulations in experimental animals.

This minireview summarizes the results of the observations on changes in endocrine functions of rats exposed to space flights for various periods. The results found after space flights are compared with those obtained from rats in acute or repeated restrain stress. A slight increase of plasma catecholamine levels was observed in rats after space flight of longer duration (>14 days), but no changes in catecholamine content in the activity of catecholamine synthesizing enzymes were noted in adrenal medulla and in hypothalamus. The norepinephrine content was, however, decreased in several nuclei selected from hypothalamus of flight rats. Plasma corticosterone levels were increased after space flight and morphological examination of pituitary showed elevated activity of corticotrophs. However, the plasma levels of ACTH were not increased in rats 6 hours after space flight. These changes in plasma hormone levels affected the activity of enzymes involved in metabolism of amino acids in liver and lipolysis in adipose tissue. The plasma levels of testosterone and triiodothyronine were diminished after space flight suggesting the suppression of the thyroid and gonadal activity. Increase of plasma insulin and glucose levels were found in rats after space flight, but the glucagon values were not changed. Comparing these results from flight rats with the animals exposed to acute or repeated stress indicate that long stay in microgravity do not represent very intensive stressogenic stimulus for adrenocortical and sympatho-adrenomedullar systems, and hormone alterations observed after space flight may be due to acute gravitational stress resulting from a return to Earth gravity. Therefore further studies including the inflight animal experiments on a board of International Space Station are necessary for elucidation of the effects of microgravity on endocrine functions.