Effect of weightlessness on sympathetic-adrenomedullary activity of rats

Three cosmic experiments were performed in which rats spent 18-20 days in space on board the biosatellites "COSMOS 782", "COSMOS 936" and "COSMOS 1129". The following indicators of the sympathetic-adrenomedullary system (SAS) activity were measured: tissue and plasma catecholamines (CA), CA-synthesizing enzymes--tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), phenylethanolamine-N-methyltransferase (PNMT)--as well as CA-degrading enzymes-monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT). Adrenal epinephrine (EPI) and norepinephrine (NE) as well as CA-synthesizing and degrading enzymes were not significantly changed in the animals after flight on COSMOS 782. On the other hand, a significant increase was found in heart CA, the indicator which is usually decreased after stress. 26 days after landing all values were at control levels. The results obtained, compared to our previous stress experiments on Earth, suggest that prolonged weightlessness does not appear to be a pronounced stressful stimulus for the SAS. Heart and plasma CA, mainly NE, were increased both in the group living in the state of weightlessness and the group living in a centrifuge and exposed to artificial gravitation 1 g (COSMOS 936), suggesting again that prolonged weightlessness is not an intensive stressful stimulus for the SAS. The animals exposed after space flight on COSMOS 1129 to repeated immobilization stress on Earth showed a significant decrease of adrenal EPI and an expressive increase of adrenal TH activity compared to stressed animals which were not in space. Thus, the results corroborate that prolonged state of weightlessness during space flight though not representing by itself an intensive stressful stimulus for the sympathetic-adrenomedullary system, was found to potentiate the response of "cosmic rats" to stress exposure after return to Earth.