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PubMedDate not supplied

[Quantitative study of the osteoblasts and osteoclasts in the bones of rats during the simulation of weightlessness].

Tibia and vertebrae of rats exposed to hypokinesia or head-down suspension were investigated by quantitative histomorphometry. It was found that 35- and 60-day hypokinesia as well as 35-day suspension caused osteoporosis in the tibial and vertebral spongiosa. The development of osteoporosis was accompanied by a significant reduction in the number of osteoblasts in the primary spongiosa of tibia and vertebrae whereas no noticeable changes in osteoclasts were observed either in hypokinetic or suspended rats. The only exception was lumbar vertebrae in which the amount of osteoclasts decreased as a result of 60-day hypokinesia. It is assumed that the reduction in the number and activity of osteoblasts plays the major part in the development of osteoporosis during hypokinesia and suspension.

Animals↗
PubMedDate not supplied

Usefulness of daily +2Gz load as a countermeasure against physiological problems during weightlessness.

UNLABELLED: Adaptation to head-down-tilt bed rest as a simulated microgravity leads to an abnormality of reflex control of circulation, hypovolemia and reduction of exercise capacity. We hypothesized that this cardiovascular deconditioning and reduction of exercise capacity could be prevented by a daily 1 hr centrifugation at +2Gz. To test this hypothesis, twenty healthy male subjects underwent 4 day of 6 degrees head-down-tilt bed rest. Ten of them were exposed to a +2Gz load for up to 30 min twice per day (the Gz group). The remaining 10 were not exposed to a Gz load (the no-Gz group). We estimated autonomic cardiovascular control by power spectral analysis of blood pressure and R-R interval variability, and baroreflex regulation by the transfer function analysis and the sequence method, before and after bed rest. Further, we measured hematocrit as an index of changes in plasma volume and maximal oxygen consumption as an index of exercise capacity, before and after bed rest. RESULT: In the no-Gz group, heart rate increased after bed rest. The high frequency power of R-R interval variability as an index of cardiac parasympathetic nervous activity, baroreflex gains estimated by transfer function analysis and the sequence method as index of the integrated arterial-cardiac baroreflex function decreased significantly. Associated with these changes, the ratio of low to high frequency power of R-R as an indicator of cardiac sympathovagal balance tended to increase after bed rest in the no-Gz group. However, those showed no significant changes after bed rest in the Gz group. Hematocrit increased after bed rest in the no-Gz group. It also tended to increase in the Gz group, however it did not achieve statistical significance. Maximal oxygen consumption decreased significantly to similar extent in both the groups. CONCLUSION: This result suggested that 1) a daily 1hr +2Gz load produced by a centrifuge might eliminate the changes in autonomic cardiovascular control during simulated weightlessness; 2) furthermore, it might partly reverse hypovolemia induced by bed rest; 3) however, it could not prevent the decreases in exercise capacity.

Baroreflex↗
PubMed2000

[Effects of -30 degrees head down tilt on lung function].

OBJECTIVE: To investigate the effects of short-term simulated weightlessness on lung function in healthy males. METHOD: -30 degrees head down tilt for 45 min was used to simulate short-period weightlessness. Lung function of 12 healthy males, aged 18-21, were studied with plethysmography during seating, supine and head down tilt positions. At the same time, blood flow in pulmonary artery and function of right ventricle were measured with Doppler Echo-Cardiography. Comparative analysis was done. RESULT: As body position changed from seating or supine into head down tilt, FVC, FEV1, FEV1%, MVV, VA and IVC decreased. The change of MVV was the most prominent (P < 0.000). As the position changed, pulmonary diffusion increased dramatically (DL(CO) P<0.001, K(CO) P<0.000). CONCLUSION: HDT may lead to a decrease of pulmonary ventilation and lung capacity. The increased pulmonary diffusion might be related to uniform distribution of pulmonary blood flow and increased effective pulmonary vascular bed.

Adolescent↗
PubMed1991

Tropistic responses of Avena seedlings in simulated hypogravity.

Results from investigation of the gravitropic responses of the Avena coleoptile under simulated weightlessness are presented. The tests were conducted using the flight hardware, identified by NASA as the Gravitational Plant Physiology Facility (GPPF), designed to support the Spacelab experiment, GTHRES. Weightlessness or so-called microgravity-conditions were simulated by the use of a so-called somersault clinostat. The plants were gravitropically stimulated with different g-forces and stimulation durations on a centrifuge. Timelapse video pictures of the plants were taken during the last hour prior to the stimulation and in a period of 3 hours after. The gravitropic responses of the plants were analyzed with the aid of an image analyzing system. Dose-response curves for stimulation forces 0.2, 0.4, 0.6, 0.8, and 1.0 g with different stimulation times ranging from 2 to 250 min were achieved. The results show that the reciprocity between force and stimulation time may be valid for small stimulation doses (lower than 5 g min), but not for larger doses. Furthermore, the threshold values (i.e. the smallest stimulation the plants are able to detect) found by extrapolation of the dose response curves, are less than 30 g s. The results are discussed with respect to an accepted space experiment, GTHRES, where the corresponding experiments will be carried out in the same apparatus under true microgravity conditions.

Avena↗
PubMed2000

Salt-loading and simulated microgravity on baroreflex responsiveness in rats.

Cardiovascular adaptations observed during exposure to microgravity results in impairment of baroreflex activity partially as a result of fluid and electrolyte shifts. The head-down tilt rat model mimics some of the physiological observations that have been made in astronauts. We examined the effects of salt-loading on baroreflex activity after 7 day simulated microgravity (30 degrees tail-suspension) and the subsequent 6 hr post-suspension in Sprague-Dawley (SD) rats, using low salt (0.3% NaCl) and high salt (8% NaCl) diets. In suspended animals on a low salt diet, the baroreflex response curve was shifted to the left, while the heart rate (HR) range and MAP50 values were reduced compared to their parallel tethered, non-suspended controls. For non-suspended animals, salt-loading shifted the curve to the right with a reduced HR range. In salt-loaded, suspended animals, the curve and its parameters resemble those of non-suspended animals on a low salt diet. In summary, these data have demonstrated that a short-term (seven days) simulated weightlessness may elicit cardiovascular deconditioning in rats after release from the simulation manifested as an altered responsiveness in baroreceptor-heart rate reflex and a lowered blood pressure while the rats are tethered and horizontal. Our results also suggest the counteracting effect of salt loading on cardiovascular deconditioning.

Animals↗
PubMed2002

Changes in kinetics of cerebral auto-regulation with head-down bed rest.

Thoraco-cephalic fluid shift induced by weightlessness may influence cerebral autoregulation. Our objective was to assess effects of simulated weightlessness by a 7-day head-down bed rest (HDBR) on the kinetics of cerebral blood flow (CBF) autoregulation in eight healthy women (27.9 +/- 0.9 years). This was studied by transcranial Doppler (TCD) of the middle cerebral artery (MCA) during the sudden decrease in blood pressure (BP) induced by quickly deflating thigh cuff aftera 4-min arterial occlusion before (D - 3), during (D2, D5) and after the HDBR (D + 1). BP (Finapres) and MCA maximal blood flow velocity were continuously recorded. Cerebrovascular resistance (CR) was expressed as the ratio of mean BP to mean MCA velocity. The CR slope was defined as changes in CR per second during the BP decrease. The magnitude of the relative decrease in mean BP and MCA velocity as well as the CR slope did not differ significantly before, during and after the HDBR, showing no major impairment of cerebral autoregulation during short-term HDBR. The time to maximum decrease in CR (T1 in s), corresponding to the maximum vasodilation was reduced on D2 (7-2 +/- 0.6) versus D - 3 (9.9 +/- 1-3), D5 (9-6 +/- 0.8) and R + 1 (11.7 +/- 11) probably as a result of the fluid shift. We also looked if the responses during the thigh cuff release differed in women according to their tolerance to the 10 min stand test performed after the HDBR: T1 was larger in the five women who presented orthostatic intolerance suggesting that some differences in cerebral autoregulatory responses may be related to orthostatic intolerance.

Adult↗
PubMed1997

Microneurography as a tool to investigate sympathetic nerve responses to environmental stress.

Microneurography is an electrophysiological method to record impulse traffic in human peripheral nerve in situ. Using this method, not only sensory afferent nerve activity, but also postganglionic sympathetic efferent outflow leading to muscle (muscle sympathetic nerve activity-MSNA) and skin (skin sympathetic nerve activity-SSNA) can be recorded in human subjects. In this paper, the methodology of microneurography and following findings obtained by microneurography on sympathetic nerve responses to environmental stress in humans are reviewed. 1. MSNA is enhanced by gravitational stress, while being suppressed by simulated weightlessness through baroreflex mechanism to maintain hemodynamic homeostasis. 2. MSNA is enhanced by simulated high altitude through chemoreflex mechanism. 3. SSNA, which is composed of sudomotor and vasomotor discharges, plays an essential role in thermoregulation. There exist regional differences between SSNA responses to ambient temperature in the nerves innervating hairy and glabrous skin. The function of the sympathetic nervous system in humans has been so far generally analyzed indirectly by observing the effector organ activity or by measuring the plasma noradrenaline level. Meanwhile, a more direct method to approach the sympathetic nervous function in man, called microneurography, has been developed. By applying this technique, it is possible to investigate how the human sympathetic nervous system responds to different kinds of environmental stress (Mano, 1990, 1994). In this paper, the usefulness of microneurography as a research tool in environmental physiology is shown with a review of microneurographic findings on sympathetic nerve responses to environmental stress in human subjects.

Altitude↗
PubMed2005

[Effects of rotation on osteonectin and osteopontin mRNA level of cultured osteoblasts in rats].

Conditions of disuse such as bed rest, space flight, and immobilization result in decreased mechanical loading of bone, which is associated with reduced bone mineral density and increased fracture risk. Mechanisms involved in this process are not well understood except the suppression of osteoblast function. To investigate the effect of simulated weightlessness on mRNA level of extracellular matrix proteins, osteoblasts were rotated in horizontal plane as a model of simulated microgravity. Primordial osteoblasts of rats were grown for 2 d and then rotated for 24, 48 and 72 h, respectively. After isolating total RNA in cells, reverse transcription polymerase chain reaction (PT-PCR) was made to examine the mRNA level of osteopontin (OPN) and osteonectin (ON). Meanwhile, the levels of alkaline phosphatase (ALP) and osteocalcin (BGP) in the cultured medium were measured to evaluate the calcific function of cell. The expression of OPN and ON mRNA fell significantly after rotating for 24, 48 and 72 h, respectively. The contents of BGP descended significantly, meanwhile, the activity of ALP also showed a degressive tendency. Horizontal rotation decreased the expression of ON and OPN as well as diminished the secretion of BGP and ALP, which affected the calcific function of osteoblast. The results obtained suggest that depression of extracellular matrix proteins expression plays a key role in bone loss during weightlessness.

Animals↗
PubMed2002

[Effects of head down tilt on intra-ocular pressure, near vision, and visual field and the protection effect of Chinese herbs].

Objective. To observe the influences of 21 d head down tilt (HDT) bed rest on the intra-ocular pressure, visual field and near vision in human and to study the countermeasure of Chinese herb against weightlessness. Method. Ten subjects were randomly divided into control group and Chinese herb group. -6 degrees HDT was used to simulate weightlessness. Intra-ocular pressure, near vision and vision field were measured before, during and after bed rest in both groups. Result. Intra-ocular pressure and near vision showed a wave-like decrease change during bed rest, and there exists a certain coherence between them. Visual field showed no obvious changes. Taking Chinese herb was able to antagonize the decreasing of intra-ocular pressure and near vision during various phases of bed rest. Conclusion. 1) Bed rest could lead to the decreasing of intra-ocular pressure and near vision; 2) Taking Chinese herb was able to antagonize the negative influences of bed rest on visual function.

Adult↗
PubMed1979

Comparison of physiological effects of head-down tilting and immersion on the human body.

Among the methods simulating weightlessness, effects on the human body, head-down tilting and water immersion are very useful. The purpose of the present investigation was to carry out a comparative study of water balance and water-protein composition of the blood using the above two methods to simulate the physiological effects typical of an acute stage of weightlessness adaptation. The results of the 7-d head-down tilting and immersion experiments allow the following conclusions: More pronounced changes in water balance and water-protein composition of the blood during immersion seem to indicate that immersion produces a greater effect on the human body; The pattern of changes during immersion and tilting suggests that the adaptation period to immersion takes a longer time; These findings give evidence that immersion, compared with head-down tilting, reproduces more closely effects of acute adaptation to simulated weightlessness.

Adaptation, Physiological↗
PubMed1997

[Changes in morphology and calcium content of otoconia in rats after 120 d tail-suspension].

The morphology and calcium content of otoconia in rats after long term (120 d) tail-suspension were studied using scanning electron microscopy (SEM) and X-ray microanalysis, respectively. The results showed that after 120 d simulated weightlessness otoconia were round, irregularly shaped, or with fissures, and there were rough and fine granular or small globular substances on the surface. X-ray microanalysis showed that the calcium content in the otoconia of both utricule and saccule was significantly decreased in the 120 d tail-suspended rats than that in control (P < 0.01). These results suggest that a long term(120 d) simulation of the headward distribution of blood volume and hindlimb underloading effect induced by weightlessness may cause the morphological changes and lower calcium content of the otoconia. Finally, the possible mechanism and the physiologic meaning of these findings are discussed.

Animals↗
PubMed2003

[Effects of clinorotation-induced weightlessness on growth factor and ERK signal transduction in cultured cardiac fibroblasts].

OBJECTIVE: To study the changes of expression and secretion of some growth factors and ERK1/2 activation in cardiac fibroblasts under clinorotation conditions. METHOD: Primary cultured neonatal rat cardiac fibroblasts were clinorotated to simulate weightlessness. The bFGF and TGF(beta)1 expressions in the cells and Ang II concentration in the medium were detected by western blotting and radioimmunoassay respectively. The expression and phosphorylated level of ERK1/2 were analyzed by western blotting. RESULT: Compared to the control group, bFGF expression and Ang II concentration increased in clinorotation group, but there was no evident change in TGF(beta)1 expression. At the same time, the expression of ERK1/2 increased but their phosphorylated level decreased. CONCLUSION: Different growth factors responds to clinorotation by different ways. As one of the most important reactions of growth factor signal downstream pathways, ERK1/2 activation is inhibited in clinorotation.

Angiotensin II↗