Introduction: 

Prolonged exposure to conditions of microgravity (micro-g) as in space flights induces various physiological adaptive responses, which may cause severe health problems. Among these, orthostatic intolerance occurs in about 50% of astronauts upon re-exposure to gravity after micro-g for 14 days or longer. A variety of underlying mechanisms has been proposed including reduced blood volume, impaired baro-receptor-reflex, reduced sympathetic efferent activity to peripheral blood vessels, increased eNO-synthase activity, or increased expression of ß₂-adrenoceptors. We, therefore, investigated, whether microvascular responses to agonists of the NO-system and of adrenoceptors were altered after 60 days of 6° head down tilt (HDT) bed rest, which represents an accepted model of prolonged micro-g.

Methods: 

24 healthy male subjects were recruited to participate in the 2^nd Berlin BedRest study, and were randomized into 3 groups: control (CTR), resistive exercise (RES) and resistive exercise with whole body vibration (VRE). Measurements were performed before and after bed rest. Skin perfusion was measured on the volar aspect of the forearm with laser-Doppler flowmetry. Acetylcholine (Ach, endothelium dependent dilator) Na-nitroprusside (SNP, endothelium independent dilator) salbutamol (Salb, ß₂-adrenoceptor agonist) and Phenylephrine (PE, alpha-adrenoceptor agonist) were locally applied by iontophoresis (1% solution, 20mA). The vasoconstrictor effect of PE was determined by measuring the reactive hyperemic response to 1 min of ischemia before and after PE.

Results: 

All subjects performed the tilt table test before bed rest without problems. After bed rest 12 (50%) of subjects exhibited symptoms of presyncope such as: systolic blood pressure < 80 mmHg, heart rate >140 bpm, nausea, dizziness, pallor or profuse sweating. Exercise tended to improve orthostatic tolerance with only 33% of exercising subjects (RES: 3/8; VRE: 2/7) but 78% of CTR subjects showing signs of presyncope (p=0.089). Ach and NP induced flow increase did not change after bed rest. In contrast, Salb induced flow increase changed from 16050% before to 36572% after bed rest (p=0.011), while PE induced vasoconstriction was similar before and after bed rest. Exercise had no effect on these alterations.

Conclusions: 

Simulated micro-g induced orthostatic intolerance in 50% of subjects, which seems to be ameliorated by exercise. While endothelial function or effects of NO or alpha-adrenoceptor agonist on vascular smooth muscle tone were not affected by bed rest, ß₂-adrenoceptor dilation was markedly enhanced. This may shift the balance between vasoconstrictive and -dilatory effects of sympathetic activation towards vasodilation and thus contribute to orthostatic intolerance.