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PubMed1994

Postural reactions of circulation and its regulation during simulated weightlessness.

The extention and intensification of space exploration the influence of weightlessness on human organism and the formation of a new level of adaptation. The studies of blood circulation is very important because of frequent occurrence of cardiovascular disorders in the middle age subjects. In connection with extension and intensification of space exploration the influence of weightlessness on human organism and the formation of a new level of adaptation mechanisms acquires a special significance. The data obtained in recently undertaken model experiments and also during space flights indicate that weightlessness in many ways affects various physiological systems of organism, and first of all cardiovascular system with the development of reflex, humeral and metabolic reactions. It also indicates, that the changes in functioning of cardiovascular system brings about the disruption of its regular responses, which is foremost expressed in decreased antigravitational response, which manifests itself in lowered orthostatic stability. It is worth mentioning, that the changes during previous investigations of haemodynamics were mainly carried out with the subjects under forty, therefore age related specific features of blood circulation system response are described in a few articles. The studies of the kind are especially important because of frequent occurrence of cardiovascular disorders such as heart and brain vessels atherosclerosis, hypertension in the middle age, which can to a great extent complicate and affect the "acute" period of adaptation to weightlessness and readaptation process.

Adaptation, Physiological↗
PubMed1994

Changes of contractile responses due to simulated weightlessness in rat soleus muscle.

Some contractile and electrophysiological properties of muscle fibers isolated from the slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles of rats were compared with those measured in SOL muscles from suspended rats. In suspended SOL (21 days of tail-suspension) membrane potential (Em), intracellular sodium activity (aiNa) and the slope of the relationship between Em and log [K]o were typical of fast-twitch muscles. The relation between the maximal amplitude of K-contractures vs Em was steeper for control SOL than for EDL and suspended SOL muscles. After suspension, in SOL muscles the contractile threshold and the inactivation curves for K-contractures were shifted to more positive Em. Repriming of K-contractures was unaffected by suspension. The exposure of isolated fibers to perchlorate (ClO4-)-containing (6-40 mM) solutions resulted in a similar concentration-dependent shift to more negative Em of activation curves for EDL and suspended SOL muscles. On exposure to a Na-free TEA solution, SOL from control and suspended rats, in contrast to EDL muscles, generated slow contractile responses. Suspended SOL showed a reduced sensitivity to the contracture-producing effect of caffeine compared to control muscles. These results suggested that the modifications observed due to suspension could be encounted by changes in the characteristics of muscle fibers from slow to fast-twitch type.

Animals↗
PubMed1974

Histological studies on the vestibular organ of frog embryos and larvae after simulated weightlessness.

The ontology of gravity receptors is discussed. As well as phylogenetic aspects of the question, we may ask whether the different gravity receptors in plants and animals derive from the same source; did they evolve from a general sensitivity of the living cell or do they represent parallel inventions. One may also ask whether during ontogeny of the receptor it receives information from gravity or whether the development of function of the organ is an autonomous process. In many cases ontogeny shows traces of phylogeny. Statoliths, e.g. the differentiating statolith membranes of the frog embryo, look somewhat like sedimentational structures. Furthermore the amphibian embryo in its relatively early stages takes up a special position in relation to the direction of gravity. Which mechanism is involved here? The materials studied were frog embryos and larvae which developed within a fast rotating clinostat. The results may help to answer the question as to whether the differentiation of the frog statolith membrane is an autonomous process.

Animals↗
PubMed1979

Responses of roots to simulated weightlessness on the fast-rotating clinostat.

Sedimentable cell particles are distributed randomly along the horizontal axis of the fast-rotating clinostat. They neither sediment in the direction of gravity, nor in the direction of the centrifugal force, nor in the direction of the resultant force of both. The effect of this "weightlessness" and that of very small centrifugal forces on the perception of mass acceleration was examined using young primary roots of Lepidium sativum L. (Cruciferae) during their early development on the fast-rotating clinostat. The results of the experiments show: 1) there is no response of the roots in the direction of gravity, 2) at small centrifugal forces (< 2.2 x 10(-2) g) a curvature response occurs in the direction of the stimulus, 3) the threshold value for the perception of mass acceleration lies at 4.3 x 10(-3) g, and 4) below the threshold value the existence of an autonomous root curvature has been proved for the first time, which is not caused by mass acceleration.

Gravitation↗
PubMed1998

Postnatal development under conditions of simulated weightlessness and space flight.

The adaptability of the developing nervous system to environmental influences and the mechanisms underlying this plasticity has recently become a subject of interest in space neuroscience. Ground studies on neonatal rats using the tail suspension model of weightlessness have shown that the force of gravity clearly influences the events underlying the postnatal development of motor function. These effects depend on the age of the animal, duration of the perturbation and the motor function studied. A nine-day flight study has shown that a dam and neonates can develop under conditions of space flight. The motor function of the flight animals after landing was consistent with that seen in the tail suspension studies, being marked by limb joint extension. However, there were expected differences due to: (1) the unloading of the vestibular system in flight, which did not occur in the ground-based experiments; (2) differences between flight and suspension durations; and (3) the inability to evaluate motor function during the flight. The next step is to conduct experiments in space with the flexibility and rigor that is now limited to ground studies: an opportunity offered by the International Space Station.

Animals↗
PubMed2000

Leptin responses to physical inactivity induced by simulated weightlessness.

Physical inactivity induced by head-down bed rest (HDBR) affects body composition (BC). Leptin is involved in BC regulation by acting on fuel homeostasis. We investigated whether leptin and counterregulatory hormone levels are affected by a 7-day HDBR. Fasting blood was sampled daily (0700) in males (n = 8) and on alternating days in females (n = 8) for measurements of leptin, insulin, norepinephrine (NE), epinephrine (Epi), growth hormone (GH), cortisol, nonesterified fatty acid (NEFA), and glucose. BC was measured by H(2)(18)O dilution. Energy intake (men 10.5 +/- 0.2 MJ/day, women 7.9 +/- 0.3 MJ/day) and BC were unchanged by HDBR. Increased levels of leptin (men 40%, P = 0.003; women 20%, P = 0. 050), insulin (men 34%, P = 0.018; women 25%, P = 0.022), and the insulin-to-glucose ratio (men 30%, P = 0.049; women 25%, P = 0.031) were noted. GH, NE, Epi, and cortisol levels were unaltered. NEFA dropped in both sexes, but glucose decreased only in women. In conclusion, HDBR increased leptin levels independently of stress response, changes in fat mass, energy intake, or gender. These changes were correlated to the insulin-resistance development in men. Further analyses are required, but the results have to be considered for longer HDBR periods with 1) the well-described drop in energy intake and 2) the BC changes.

Adult↗
PubMed1994

Changes in potassium contractures due to simulated weightlessness in rat soleus muscle.

Some contractile properties of soleus muscle (SOL) fibers isolated from tail-suspended (21 days) rats were compared with those determined in the slow-twitch SOL and the fast-twitch extensor digitorum longus muscle (EDL) of control rats. In SOL from suspended rats, the resting membrane potential and the intracellular Na+ activity were typical of fast-twitch muscles. The relationship between the amplitude of K+ contractures and the membrane potential was steeper for control SOL than for control EDL and suspended SOL. The inactivation curve was also shifted to more positive potentials after suspension. In the presence of perchlorate anions, the tension activation curves of control EDL and suspended SOL were similarly shifted to more negative potentials. Thus, in the present study, modifications induced by suspension in SOL, mainly related to changes in the voltage-sensing process involved in the excitation-contraction coupling mechanism, were that the SOL assumed some of the characteristics of fast-twitch muscles.

Animals↗
PubMed2005

Amino acid supplementation alters bone metabolism during simulated weightlessness.

High-protein and acidogenic diets induce hypercalciuria. Foods or supplements with excess sulfur-containing amino acids increase endogenous sulfuric acid production and therefore have the potential to increase calcium excretion and alter bone metabolism. In this study, effects of an amino acid/carbohydrate supplement on bone resorption were examined during bed rest. Thirteen subjects were divided at random into two groups: a control group (Con, n = 6) and an amino acid-supplemented group (AA, n = 7) who consumed an extra 49.5 g essential amino acids and 90 g carbohydrate per day for 28 days. Urine was collected for n-telopeptide (NTX), deoxypyridinoline (DPD), calcium, and pH determinations. Bone mineral content was determined and potential renal acid load was calculated. Bone-specific alkaline phosphatase was measured in serum samples collected on day 1 (immediately before bed rest) and on day 28. Potential renal acid load was higher in the AA group than in the Con group during bed rest (P < 0.05). For all subjects, during bed rest urinary NTX and DPD concentrations were greater than pre-bed rest levels (P < 0.05). Urinary NTX and DPD tended to be higher in the AA group (P = 0.073 and P = 0.056, respectively). During bed rest, urinary calcium was greater than baseline levels (P < 0.05) in the AA group but not the Con group. Total bone mineral content was lower after bed rest than before bed rest in the AA group but not the Con group (P < 0.05). During bed rest, urinary pH decreased (P < 0.05), and it was lower in the AA group than the Con group. These data suggest that bone resorption increased, without changes in bone formation, in the AA group.

Adult↗
PubMed1995

Cerebellar afferents to neuroendocrine cells: implications for adaptive responses to simulated weightlessness.

The hypothalamo-neurohypophyseal system as well as the autonomic nervous system is involved in homeostatic responses associated with changes in head position and orthostatic reflex. The responses induced by body tilt on earth are thought to be attributed to changes in inputs from baroreceptors, vestibular organs and proprioreceptors that are normally required for postural control. The information from these organs is sent to the hypothalamus which thereby influences both neuroendocrine and autonomic systems as well as various kinds of emotional behavior. Our findings showing the fastigial input to the hypothalamus suggested that the FN plays a significant role in these homeostatic responses through its connections with the brain stem and the hypothalamus. Figure 4 shows the input-output organization among the hypothalamus, cerebellum and brain stem, described in detail in sections III to V. This hypothesis may help to account for the autonomic and endocrine disorders often observed in weightlessness.

Adaptation, Physiological↗
PubMedDate not supplied

Role of digitalis-like substance in the hypertension of streptozotocin-induced diabetes and simulated weightlessness in rats.

We have examined the role of plasma Na+-K+ pump inhibitor (SPI) in the hypertension of streptozotocin induced insulin dependent diabetes (IDDM) in reduced renal mass rats. The increase in blood pressure (BP) was associated with an increase in extracellular fluid volume (ECFV), and SPI and a decrease in myocardial Na+,K+ATPase (NKA) activity, suggesting that increased SPI, which inhibits cardiovascular muscle (CVM) cell NKA activity, may be involved in the mechanism of IDDM-hypertension. In a second study, using prolonged suspension resulted in a decrease in cardiac NKA activity, suggesting that cardiovascular deconditioning following space flight might in part result from insufficient SPI.

Animals↗
PubMed1997

Physiological properties of rat hind limb muscles after 15 days of simulated weightless environment.

Weightlessness during space mission results in atrophic changes in those muscles which have maximum weight bearing function and consist primarily of slow twitch fibres. In the present study an animal model was designed to evaluate the effects of 15 days of hindlimb unloading (HU) in rats by tail suspension on the (i) weight of gastrocnemius (G), plantaris (P), both predominantly having fast twitch fibres and soleus (S) muscle, predominantly having fast twitch fibres and (ii) contractile properties viz peak twitch contraction (Pt) and peak tetanic contraction (Po) of GPS muscle. HU rats showed significant weight reductions of G (-17.9%), P (-13.3%) and S (-41.2%) muscles. Pt and Po were also reduced in HU group but when these were expressed per gm of GPS muscle, no significant changes in Pt and Po were observed. These findings confirm that HU in rats result in maximum atrophic change in those muscles which have predominantly slow twitch fibres and reductions in contractile properties of muscles are in proportion to reduction in muscle weight. Also, HU by tail suspension provides a good ground based model for developing the deconditioning of muscles as applicable to weightlessness of space and offers a scope for the development of various countermeasures.

Animals↗
PubMed1999

[Changes in the NO-dependent regulation of the local cerebral blood flow in rats during adaptation to the conditions of simulated weightlessness].

The nitrogen oxide NO-dependent regulation of the cerebral blood flow was studied before and after 24-hr head-down immobilization (HDI) of intact and pre-trained rats. Training consisted in 2-hr tail-suspension each day of the 2-wk period. Blood flow was determined with the laser Doppler flowmetry following local injection of a NO synthesis blocker (L-NAME), and NO (sodium nitroprusside). Neither HDI nor pre-training per se influenced NO tonic production in the cortex of large hemispheres and cerebellum. However, in pre-trained animals HDI resulted in a significant blood flow response to the local blockade of NO synthesis in the cerebellum. None of the animals changed the reaction of the blood flow to the local injection of sodium nitroprusside. The conclusion was drawn that alteration in the NO-dependent regulation of the brain blood flow in pre-trained animals could manifest of adaptation to HDI in the course of 24-hr suspension.

Adaptation, Physiological↗
PubMed2000

Bone loss during simulated weightlessness: a biomechanical and mineralization study in the rat model.

BACKGROUND: Astronauts exposed to weightlessness for extended periods experience significant decreases in bone mineral density. The clinical implications of this demineralization are not entirely clear, and the biomechanics involved are not completely understood. HYPOTHESIS: Local (rather than global) measurements of geometry and calcium concentration effectively predict femur strength in adult rats exposed to a hind-limb suspension model of weightlessness. METHODS: Female Fischer rats (6-mo-old) were divided into groups of control and hind-limb-suspended animals. Animals were sacrificed after 2 or 4 wk of hind-limb suspension, and both femurs removed from each animal. The 3-point bending strength and total bone mineralization were determined for one femur from each animal, and the mid-shaft cross-sectional geometrical properties and distribution of calcium were determined for the contralateral femur. RESULTS: Although suspension led to significant decreases in total bone mineralization, the concentration of calcium at the anterior periosteal surface was unaffected. Total bone percent mineralization was not well correlated with structural properties, but bone geometrical properties (particularly cross-sectional moment of inertia and length) correlated strongly with ultimate bending strength (r2 = 0.81). Differences in bone geometry due to suspension were consistent with a distribution of bone material closer to the axis of the femur. CONCLUSIONS: Structural properties of bone are predicted well by bone geometry and poorly by total bone percent mineralization. Decreased bone mechanical strength in this model of weightlessness is primarily due to a distribution of bone material nearer the axis of the bone.

Animals↗
PubMed1984

Role of cardiac volume receptors in the control of ADH release during acute simulated weightlessness in man.

Hemodynamic responses and antidiuretic hormone (ADH) were measured during body position changes designed to induce central blood volume shifts in ten cardiac and one heart-lung transplant recipients to assess the contribution of cardiac volume receptors in the control of ADH release during the initial acute phase of exposure to weightlessness. Each subject underwent 15 min of a sitting-control period (C) followed by 30 min of -6 degrees headdown tilt (T) and 30 min of resumed sitting (S). Venous blood samples and cardiac dimensions were taken at 0 and 15 min of C; 5, 15, and 30 min of T; and, 5, 15, and 30 min of S. Blood samples were analyzed for hematocrit, plasma osmolality, plasma renin activity (PRA), and ADH. Heart rate (HR) and blood pressure (BP) were recorded every two min. Plasma osmolality was not altered by posture changes. Mean left ventricular end-diastolic volume increased (P < .05) from 90 ml in C to 106 ml in T and returned to 87 ml in S. Plasma ADH was reduced by 20% (P < .05) with T and returned to control levels with S. These responses were similar in six normal cardiac-innervated control subjects. These data may suggest that cardiac volume receptors are not the primary mechanism for the control of ADH release during acute central volume shifts in man.

Adult↗
PubMed1997

Integrated regulation in response to simulated weightlessness.

To investigate physiological effects of tail suspension, Ca2+ concentration, immune factors, erythrocyte rheological properties, and growth hormone were determined in rats suspended for 15 and 30 d. The results showed that inhibitory changes of both local factors (proteins secreted by bone cell, Ca(2+)-ATPase in sarcoplasmic reticium) and integrated regulative factors (immune factor, growth hormone) were observed simultaneously with the decrease of bone mineral content, calcium transportation in skeletal muscles as well as erythrocyte deformability. It suggests that both local and integrated regulative processes are functioning in response to the effects of weightlessness.

Animals↗