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At least 199 records · Page 11Linked to original sources
PubMed1997

Plasticity of arterial vasculature during simulated weightlessness and its possible role in the genesis of postflight orthostatic intolerance.

Even after several decades of extensive research, the basic mechanism of postflight cardiovascular dysfunction has not yet been fully elucidated. It is now well recognized that multiple mechanisms might account for the frequent occurrence of significant postflight orthostatic intolerance. It has been found that all tissues adapt their design when exposed to sustained alteration in local activity and/or stress. The most obvious example is the musculo-skeletal system, structure and function of which might be severely affected during microgravity exposure. In an attempt to elucidate whether structure and function of cardiac and vascular smooth muscle might be affected by simulated by microgravity, a serial work was started several years ago. In this paper, we present our more recent findings on plasticity of arterial vasculature and its innervation state during and after simulated microgravity and its time course.

Adaptation, Physiological↗
PubMed1999

Low-dose T(3) improves the bed rest model of simulated weightlessness in men and women.

This study tested the hypothesis that low-dose 3,5, 3'-triiodothyronine (T(3)) administration during prolonged bed rest improves the ground-based model of spaceflight. Nine men (36.4 +/- 1. 3 yr) and five women (34.2 +/- 2.1 yr) were studied. After a 5-day inpatient baseline period, subjects were placed at total bed rest with 6 degrees head-down tilt for 28 days followed by 5-day recovery. Fifty micrograms per day of T(3) (n = 8) or placebo (n = 6) were given during bed rest. Serum T(3) concentrations increased twofold, whereas thyroid-stimulating hormone was suppressed in treated subjects. T(3)-treated subjects showed significantly greater negative nitrogen balance and lost more weight (P = 0.02) and lean mass (P < 0.0001) than placebo subjects. Protein breakdown (whole body [(13)C]leucine kinetics) increased 31% in the T(3) group but only 8% in the placebo group. T(3)-treated women experienced greater changes in leucine turnover than men, despite equivalent weight loss. Insulin sensitivity fell by 50% during bed rest in all subjects (P = 0.005), but growth hormone release and insulin release were largely unaffected. In conclusion, addition of low-dose T(3) to the bed rest model of muscle unloading improves the ground-based simulation of spaceflight and unmasks several important gender differences.

Adult↗
PubMed2005

[Serum creatine kinase levels and the number of sarcolemmal dystrophin disruptions in human skeletal muscle fibers under conditions of weightlessness simulation].

9 male volunteers took part in the experiment. They were divided in two groups. 5 volunteers (control group) have been in "dry" immersion for 7 days. 4 volunteers (stimulated group) in addition to "dry" immersion were treated with artificial support stimulation. We investigated the number of muscle fibers with the disruptions of sarcolemmal dystrophin and serum creatine kinase levels. 7-day "dry" immersion did not change the mean number of muscle fibers with dystrophin disruptions, it led to significant decrease of serum creatine kinase levels and did not influence on the sensitivity of sarcolemma to injury. Artificial support stimulation did not influence on these parameters.

Adult↗
PubMed2005

[Regulation of blood volume during weightlessness simulation of long duration].

To study the effects of microgravity on the mechanisms involved in the regulation of body hydrous status, total body water (TBW), plasma volume (PV), and its main regulating hormones (plasma renin, aldosterone, atrial natriuretic peptide (ANP), anti-diuretic hormone (ADH)) were determined, by isotopic dilution, Dill and Costill's formula, and radio-immunologic dosages, in 9 male subjects submitted to a 90-d head-down bed rest (HDBR). ADH was determined in 24 h urinary collection as well as osmolality, sodium, and potassium. Body mass decreased (-2.8 +/- 0.8 kg) as well as TBW(-7.2% +/- 0.9%, i.e., -2.6 +/- 0.7 kg) and PV (-4.7% +/- 1.8%). Renin and aldosterone were enhanced (+109.0% +/- 15.4% and +87.2% +/- 38.9%, respectively). Simultaneously, we observed a decrease in ANP (-33.2% +/- 20.4%). Other variables, including ADH, were not affected by HDBR. Body mass and TBW decrease (and consequently lean body mass) are associated with muscle atrophy. Renin, aldostrerone, and ANP modifications are well explained by the decrease in PV, which was not enough to induce ADH changes. It suggests that in man, the main regulatory factor for ADH secretion is osmolality, when PV is modestly and progressively decreased without arterial pressure modification, which was the case in the present protocol.

Adult↗
PubMed1991

Human autonomic responses to actual and simulated weightlessness.

Orthostatic dysfunction occurs after exposure to microgravity, and is not completely understood. The authors developed a device for stimulating carotid baroreceptors to test the hypothesis that exposure to microgravity leads to impairment of arterial baroreflex mechanisms. Data obtained before and after two head-down bedrest studies and before and after brief Space Shuttle missions indicate that baroreceptor-cardiac reflex control is impaired by simulated or actual weightlessness. The authors speculate that arterial baroreflex derangements combine with blood volume reductions and increased venous compliance to provoke orthostatic hypotension after microgravity exposure. Altered baroreflex function after missions may result from autonomic neuronal plasticity that develops during missions secondary to changes of cardiopulmonary and arterial dimensions and consequent changes of autonomic sensory input profiles.

Autonomic Nervous System↗
PubMed1989

Effects of simulated weightlessness on rat osteocalcin and bone calcium.

Some of the musculoskeletal changes that occur in growing rats during spaceflight are simulated by a model that selectively unloads the hindlimbs while maintaining normal weight bearing on the forelimbs. Using this model we studied the response of mineral and the mineral-binding protein osteocalcin (OC) in the third lumbar vertebra (L3) and the femoral midshaft to periods of unweighting from 2 to 28 days. Serum OC decreased by 25%, consistent with a decreased rate of bone growth, during the first week of suspension and returned toward control values after 15 days. The L3 and femur weighed 20% less than control bones after 10-28 days. OC content of L3 and femur diaphysis were lower after 7 days of suspension and returned to normal levels at 28 days, whereas Ca content rose slightly at 5 days then decreased sharply. OC:Ca ratio was also affected. The data suggest that unweighting affects formation and deposition of OC and Ca differently depending on bone location and duration of unweighting. Both serum and bone OC are highly sensitive indicators of disruption of osteoblast activity by altered skeletal loading.

Animals↗
PubMed1984

Effects of simulated weightlessness on bone mineral metabolism.

Space flight and bedrest result in a negative calcium balance and osteopenia. The mechanisms underlying these events are not well understood. In particular, it is not clear whether systemic or local factors are preeminent in mediating the effects of gravity on bone mineral content. Using a rat model that unweights only the hindlimbs, we determined whether the osteopenia induced in these rats was generalized or restricted to the unweighted limbs, and whether changes in intestinal calcium transport contributed to the alterations in bone calcium content. In this model, the hindquarters were elevated using the tail at an approximately 40 degrees angle for up to 15 days. The tibia and lumbar vertebra from experimental rats contained substantially less calcium than the same bones from pair-fed controls; at 15 days, the tibia was 86.2 +/- 2.5% of the control value (mean +/- SE), and the vertebra was 75.5 +/- 3.5% of control value (mean +/- SE). However, these differences were found only for the unweighted bones; the mandible and humerus showed no differences between experimental and pair-fed control rats. When calcium uptake by bone was evaluated after 45Ca administration, we observed an initial decrease in uptake at 5 days only in the tibia [percentage of control value, 60.5 +/- 5.5, (mean +/- SE)] and vertebra (percentage of control value, 74.3 +/- 3.7) when experimental animals were compared to pair-fed controls; there was no decrease in the humerus and mandible. However, after 10 days of unweighting , calcium uptake in the tibia and vertebra of experimental animals returned to control levels and by 15 days exceeded control levels (the tibia was 125.8 +/- 5.8% of the control value and the vertebra was 136.2 +/- 9.6% of the control value) despite the progressive decrease in total bone calcium compared to that in pair-fed controls. At no time could we demonstrate a difference in duodenal calcium transport between experimental and control animals. These data suggest that in this model, which simulates certain aspects of weightlessness, changes in local factors within the unweighted bones may have a greater impact on bone mass and turnover than changes in systemic factors that regulate overall bone mineral homeostasis.

Animals↗
PubMedDate not supplied

[Psychophysiological characteristics of the sensorimotor activity of operators after short-term simulated weightlessness].

Changes in operator's sensorimotor activity and their effect on the control of a dynamic object after short-term exposure to simulated hypogravity were investigated. Hypogravity effects were simulated by a 6-day head-down tilt (-10 degrees) study and a 1-day water immersion study. Changes in the structure of actions seen after study (greater use of controllers in the longitudinal axis, alteration of bioelectric activity of muscles involved in movement arrangement, shift of their spectral power toward higher frequencies) are considered as objective indicators of disrupted automatism of controlling skills. A peculiar psychic compensation of disorders in the coordination structure of movements was a longer time used to control the position and movement of an object in the longitudinal plane. Partial disruption of automatized controlling skills after short-term exposure to simulated hypogravity facilitates a decrease in operator's reliability, when he is to perform sophisticated and precise functions.

Humans↗
PubMedDate not supplied

[Central and general hemodynamics in healthy persons during simulated weightlessness].

The pumping and contractile functions of the left ventricle were investigated during combined exposure to water immersion and head-down tilt (at -6 degrees) that simulated microgravity. This 7-day exposure caused noticeable changes in central and systemic circulation which developed as a function of time. It is assumed that the deficiency of venous blood return seen on test days 1-3 was a compensatory response to initial blood displacement which was induced by a reduction of the circulating blood volume. The reduction was in turn a result of responses from volumoreceptors of the heart and intrathoracic vessels to "excessive" intrathoracic blood volume.

Adult↗
PubMedDate not supplied

[Effect of simulated weightlessness on calcium metabolism and the condition of bone tissue in experimental animals].

Male rats of the Wistar strain were for 40 days exposed to hypokinesia and hypodynamia. After exposure bone biochemical, physical, chemical, morphological and strength parameters were investigated. Both simulation studies led to changes in calcium and phosphorus metabolism but the nature of bone changes was different. It is concluded that the efficacy of drugs used as countermeasures in animal simulation experiments should be assessed by both exposures, viz. small size cages and suspension, because they reproduce different mechanisms of potential bone lesions in microgravity. It is also inferred that various countermeasures should be used in combination to yield the best results.

Animals↗
PubMed1995

Characteristics of the venous hemodynamics of the leg under simulated weightlessness: effects of physical exercise as countermeasure.

In order to test the hypothesis that increases in calf venous distensibility in microgravity are partly due to the changes affecting the surrounding skeletal muscles (muscular atrophy), 12 healthy volunteers were exposed for 28 d to microgravity simulated by -6 degrees head-down bed rest. Half these subjects were exposed to countermeasures during bed rest: a) repeated LBNP (Lower Body Negative Pressure) sequences starting on the 15th d with one 15 min sequence at -35 mb, every other day from the 15th until the 21st d, and then every day until the end of bed rest; b) physical training including isotonic type exercise and isometric or isokinetic work by all muscle mass of upper and lower limbs (from the 8th until the 28th d). The other six subjects forming the control group were not subjected to any countermeasure. Calf venous hemodynamics were determined by mercury strain gauge plethysmography with venous occlusion. Distensibility (delta Vmax) and venous emptying (venous outflow at the 6th s of emptying: VO6, half-emptying time: T1/2, maximum venous outflow (MVO) could also be measured. Nuclear magnetic resonance (NMR) was used to study changes in volume of calf muscles. Plethysmographic measurements made for each subject prior to, during (once a week), and after bed rest show a parallel increase in calf venous distensibility in both groups of subjects until the 20th d of bed rest. Filling and emptying times then tended to stabilize in the group treated with countermeasures (group CM) whereas high venous distensibility was observed until the end of bed rest and 5 d thereafter in the control group (group C).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1976

Antiorthostatic hypokinesia as a method of weightlessness simulation.

Physiological effects seen in eight test subjects during a 5-d bedrest experiment in the head-down position (0,-4,-8,-12degrees) were studied. It was shown that the antiorthostatic hypokinesia at -12degrees could reproduce physiological responses shown by space crewmembers more closely than recumbent bedrest. Our observations help to stimulate an acute stage of human adaptation to the weightless state and to assess the part played by gravity-induced blood redistribution in the development of physiological changes.

Adult↗
PubMedDate not supplied

[Morphological study of early changes in rat bones during exposure to simulated weightlessness].

By histomorphometric methods tibial bones and lumbar vertebrae of rats exposed for 7 days to hypokinesia or head-down suspension were investigated. Both hypokinesia and suspension led to osteoporosis of the tibial metaphyseal spongiosa which was primarily induced by bone growth inhibition and probably by increased bone resorption. No signs of osteoporosis were seen in tibial diaphyses. In contrast to tibial bones, osteoporosis of the spongiosa of lumbar vertebrae was found only in hypokinetic rats. It is concluded that during exposure to simulated microgravity early signs of osteoporosis occur in the tibial spongiosa and that changes in the spongy matter of tubular bones and vertebrae are similar and systemic. It is suggested that an acute stress-reaction in response to simulated microgravity plays a certain role in the development of osteoporosis.

Animals↗
PubMed1986

Skeletal response to dietary calcium in a rat model simulating weightlessness.

Unweighting the hindlimbs of a rat by tail suspension leads to a decrease in bone in the unweighted hindlimbs, but not in the normally weighted forelimbs. We evaluated whether increments in dietary calcium could prevent this. Growing rats were fed diets ranging in calcium content from 0.1% to 2.4%. After the rats were suspended for two weeks, we found no differences between suspended and control animals fed the same diet with respect to calcium transport or serum levels of calcium, phosphorus, 1,25-dihydroxyvitamin D, and parathyroid hormone. In both groups, increasing dietary calcium reduced active intestinal calcium transport and serum 1,25-dihydroxyvitamin D levels. The calcium content of the tibia and lumbar vertebra (but not the humerus) was reduced in suspended rats compared to control rats fed the same diet. However, increasing dietary calcium increased the calcium content of all bones in both suspended and control animals. The bone formation rate at the tibiofibular junction (measured by double-label tetracycline) was reduced in the suspended animals compared to controls and was not altered by dietary calcium. However, the marrow area of the tibia, an indication of bone resorption, did not differ between suspended and control animals and was equally reduced in both groups when dietary calcium was increased. Our data suggest that the deleterious effects of skeletal unweighting on bone formation cannot be explained by changes in the calciotropic hormones and are not reversed by increments in dietary calcium. However, increasing dietary calcium can increase bone calcium, even in unweighted limbs, by decreasing bone resorption.

Animals↗