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PubMed2003

Neurophysiology of developing fish at altered gravity: background--facts--perspectives.

During the entire evolution of life on Earth, the phylogenetic as well as the individual development of all organisms took place under constant gravity conditions, against which they achieved specific countermeasures for compensation and adaptation. On the one side, gravity represents a factor of physical restriction, which compelled the ancestors of all extant living beings to develop basic achievements to counter the gravitational force (e.g., elements of statics like any kind of skeleton--from actin to bone--to overcome gravity enforced size limits or to keep form). On the other side, already early forms of life possibly used gravity as an appropriate cue for orientation and postural control, since it is continuously present and has a fixed direction. Due to such a thorough adaptation to the Earthly gravity vector, both orientation behaviour as well as the ontogenetic development of animals is impaired, when they have to experience altered gravity (delta g; i.e., hyper- or microgravity). On this background, it is still an open question to which extent delta g affects the normal individual development, either on the systemic level of the whole organism or on the level of individual organs or even single cells. The present review provides information on these questions, focusing on developing fish as model systems. Special emphasis is being laid on the effect of delta g on the developing brain and vestibular system, comprising investigations on behaviour and plastic reactivities of the brain and inner ear. Moreover, clues and insights into the possible basic causes of space motion sickness-phenomena (SMS; a kinetosis) are provided. Overall, the results speak in favour of the following concept: short-term altered gravity (< or = 1 day) can induce transitional aberrant behaviour due to malfunctions of the inner ear, originating from asymmetric otoliths or, generally, from a mismatch between canal and otolith afferents. The vanishing aberrant behaviour is due to a reweighing of sensory inputs and neurovestibular compensation, probably on bioelectrical basis. During long-term altered gravity (several days and more), step by step neuroplastic reactivities on molecular basis (i.e., molecular facilitation) in the brain and inner ears obviously activate feedback mechanisms between the CNS and the vestibular organs for the regain of normal behaviour. Mainly, the following areas of research with animals at altered gravity need to be addressed in the future: (1) Maintenance of animals through two complete life cycles in the space environment (developmental deficiencies?). (2) Investigation of the peripheral and central vestibular system by ground-based studies (mutants, hypergravity experiments...), focusing on plasticity in developing animals as well as in adults. (3) Investigation of the effect of microgravity during critical developmental periods (imprinting phase for graviperception?). Answers to these questions may be of crucial interest for basic gravitational research.

Animals↗

PubMed2001

Applied horizontal force increases impact loading in reduced-gravity running.

The chronic exposure of astronauts to microgravity results in structural degradation of their lower limb bones. Currently, no effective exercise countermeasure exists. On Earth, the impact loading that occurs with regular locomotion is associated with the maintenance of bone's structural integrity, but impact loads are rarely experienced in space. Accurately mimicking Earth-like impact loads in a reduced-gravity environment should help to reduce the degradation of bone caused by weightlessness. We previously showed that running with externally applied horizontal forces (AHF) in the anterior direction qualitatively simulates the high-impact loading associated with downhill running on Earth. We hypothesized that running with AHF at simulated reduced gravity would produce impact loads equal to or greater than values experienced during normal running at Earth gravity. With an AHF of 20% of gravity-specific body weight at all gravity levels, impact force peaks increased 74%, average impact loading rates increased 46%, and maximum impact loading rates increased 89% compared to running without any AHF. In contrast, AHF did not substantially affect active force peaks. Duty factor and stride frequency decreased modestly with AHF at all gravity levels. We found that running with an AHF in simulated reduced gravity produced impact loads equal to or greater than those experienced at Earth gravity. An appropriate AHF could easily augment existing partial gravity treadmill running exercise countermeasures used during spaceflight and help prevent musculoskeletal degradation.

Adult↗

PubMed1980

Gravity Functions of Circumnutation by Hypocotyls of Helianthus annuus in Simulated Hypogravity.

For more than a decade research on the botanical mechanism responsible for circumnutation has centered on whether or not these nearly ubiquitous oscillations can be attributed to a hunting process whereby the plant organ continuously responds to the gravity force and, by overshooting each stimulus, initiates a sustained oscillation or, driven by a not yet defined autogenic mechanism, performs oscillatory activities that require no external reinforcement to maintain the observed rhythms of differential growth.We explore here the effects of altered gravity force on parameters of circumnutation. Following our earlier publication on circumnutation in hypergravity we report here an exploration of circumnutation in hypogravity.Parameters of circumnutation are recorded as functions of the axially imposed gravity force. The same method was used (two-axes clinostat rotation) to produce sustained gravity forces referred to as hypergravity (1 < g), hypogravity (0 [unk] g < 1), and negative gravity (-1 < g < 0). In these three regions of the g-parameter nutational frequency and nutational amplitude were influenced in different ways.The results of our tests describe the gravity dependence of circumnutation over the full range of real or simulated gravity levels that are available in an earth laboratory. Our results demonstrated that nutational parameters are indeed gravity-dependent but are not inconsistent with the postulate that circumnutation can proceed in the absence of a significant gravity force.

Journal Article↗

PubMed2002

Pool film boiling experiments on a wire in low gravity: preliminary results.

This paper reports preliminary results for pool film boiling on a wire immersed in almost saturated FC72 recently obtained during an experimental campaign performed in low gravity on the European Space Agency Zero-G airplane, (reduced gravity level 10(-2)). This is part of a long-term research program on the effect of gravitational and electric forces on boiling. The reported data set refers to experiments performed under the following conditions: (1) Earth gravity without electric field, (2) Earth gravity with electric field, (3) low gravity without electric field, and (4) low gravity with electric field. Although a decrease of gravity causes a heat transfer degradation, the electric field markedly improves heat exchange. This improvement is so effective that, beyond a certain field value, the heat flux is no longer sensitive to gravity. Two main film boiling regimes have been identified, both in normal and in low gravity: one is affected by the electric field and the other is practically insensitive to the field influence.

Journal Article↗

PubMed2005

The role of gravity in adaptation of the vertical angular vestibulo-ocular reflex.

The gain of the vertical angular vestibulo-ocular reflex (aVOR) was adapted in side-down and prone positions in two monkeys and tested in four planes: left-/right-side down; forward/backward; and two intermediate planes that lie approximately in the planes of the vertical semicircular canal pairs, left anterior/right posterior (LA/RP) and right anterior/left posterior (RA/LP). Gain changes, expressed as a percent of preadapted values, were plotted as a function of head orientation in the planes of tilt, and fitted with sinusoids to obtain the gravity-dependent (amplitude) and gravity-independent (bias) components of adaptation. Gravity-dependent gain changes were always maximal when tested in a plane that included the head orientation in which the aVOR gain had been adapted. Changes were minimal when the head was tilted in a plane orthogonal to the plane of adaptation, and were smaller but still significant when tested in the two intermediate planes. Gravity-independent VOR gain changes were uniform over all planes of head tilt. Thus, the gravity-dependent and gravity-independent components could be separated experimentally. The aVOR gain changes from the head tilts in different directions were utilized to reconstruct the gain changes in three dimensions. They formed a continuous surface, which peaked in and around the position of adaptation. These studies support the postulate that gain adaptation has both gravity-independent and gravity-dependent components, and further show that these gain changes have a three-dimensional structure. These results are similar to those in humans, indicating that the gravity-dependent adaptation of the aVOR is likely to be a common phenomenon across species.

Acclimatization↗

PubMed1997

The effect of reduced gravity on the kinematics of human walking: a test of the dynamic similarity hypothesis for locomotion.

To gain insight into the basic principles that govern the biomechanics of locomotion, we investigated the effect of reduced gravity on walking kinematics. We hypothesized that humans walk in a dynamically similar fashion at combinations of speed and simulated gravity that provide equal values of the Froude number, v2/gLleg, where v is forward speed, g is gravitational acceleration and Lleg is leg length. The Froude number has been used to predict the kinematics and kinetics of legged locomotion over a wide range of animal sizes and speeds, and thus provides a potentially unifying theory for the combined effects of speed, size and gravity on locomotion biomechanics. The occurrence of dynamic similarity at equal Froude numbers has been attributed previously to the importance of gravitational forces in determining locomotion mechanics. We simulated reduced gravity using a device that applies a nearly constant upward force to the torso while subjects walked on a treadmill. We found that at equal Froude numbers, under different levels of gravity (0.25g-1.0g), the subjects walked with nearly the same duty factor (ratio of contact time to stride time), but with relative stride lengths (Ls/Lleg, where Ls is stride length) that differed by as much as 67 %, resulting in the rejection of our hypothesis. To understand the separate effects of speed and gravity further, we compared the mechanics of walking at the same absolute speed at different levels of gravity (0.25g-1.0g). In lower gravity, subjects walked with lower duty factors (10 %) and shorter relative stride lengths (16 %). These modest changes in response to the fourfold change in gravity indicate that factors other than gravitational forces are the primary determinants of walking biomechanics.

Biomechanical Phenomena↗

PubMed2000

[The cell as a gravity-dependent biomechanic system].

In the period of 1995-1997 experimental and theoretical studies with various biomechanic objects, i.e. individual cells and cell associations, were performed under changed gravity (0.00001-5 g). Experimental investigations were conducted using clinostats and centrifuges to model effects of hypo- and hypergravity, and aboard space vehicles in real microgravity. Cell cultures in vitro including fibroblasts and osteoblasts on a solid glass or plastic substrate served as objects of the studies. Changes in value and direction of the gravity vector were found to modify the morphophysiological characteristics of cells: structural organization (spatial rearrangement of the intracell component, changes in forms, sizes and quantity of cells) and functional activity (alterations in energy expenditure and intensity of intracellular metabolism). The data suggest that there should be mechanisms of gravitational sensitivity in living systems on the cellular level. As was stated, sensitivity of unicellular free-living organisms to gravity is mostly defined by the motor activity determined by the level of general metabolism. Morphological characteristics (form, size and mass) are of secondary importance. Theoretical analysis resulted in correction of one of the principle postulates of gravitational biology stating a direct link between size (mass) and gravitational sensitivity of organism. Described were consistent patterns of growth, development, and behavior of unicellular cultures in gravitational fields. Strengthening of the force of gravity (hypergravity) leads to eventual deceleration of cell growth and diminution of biomass gain. On the other hand, the spaceflight environment (microgravity) stimulates growth mechanisms. In our opinion, behind these gravitational effects are altered levels of energy spent by cells to overcome the force of gravity. Opposite trends were observed in experiments with cell cultures in vitro. During space microgravity, fibroblast cultures on the solid substrate decreased the growth rate, and inhibited cell division and migration within the substrate. Compared to the Earth's gravity, under elevated gravity these parameters were noticeably higher. It was demonstrated that the main cause of the unfavorable effects of space microgravity on the cellular level is decay in the adherence of cells to the substrate. Explored were also the most probable mechanisms of the effects of changed gravity on the cell as a biomechanic structure. Specialized and non-specialized graviceptors of various types of cells were crypt-analyzed and classified. In future, investigations should be angled for elucidation of the role of intracellular components in perception and implementation of the gravitational stimulus, and description of quantitative characteristics of energy exchange and metabolism in cells as a function of gravity force and direction.

Biomechanical Phenomena↗

PubMed2005

Magnetic levitation-based Martian and Lunar gravity simulator.

Missions to Mars will subject living specimens to a range of low gravity environments. Deleterious biological effects of prolonged exposure to Martian gravity (0.38 g), Lunar gravity (0.17 g), and microgravity are expected, but the mechanisms involved and potential for remedies are unknown. We are proposing the development of a facility that provides a simulated Martian and Lunar gravity environment for experiments on biological systems in a well controlled laboratory setting. The magnetic adjustable gravity simulator will employ intense, inhomogeneous magnetic fields to exert magnetic body forces on a specimen that oppose the body force of gravity. By adjusting the magnetic field, it is possible to continuously adjust the total body force acting on a specimen. The simulator system considered consists of a superconducting solenoid with a room temperature bore sufficiently large to accommodate small whole organisms, cell cultures, and gravity sensitive bio-molecular solutions. It will have good optical access so that the organisms can be viewed in situ. This facility will be valuable for experimental observations and public demonstrations of systems in simulated reduced gravity.

Animals↗

PubMed2004

Escherichia coli growth under modeled reduced gravity.

Bacteria exhibit varying responses to modeled reduced gravity that can be simulated by clino-rotation. When Escherichia coli was subjected to different rotation speeds during clino-rotation, significant differences between modeled reduced gravity and normal gravity controls were observed only at higher speeds (30-50 rpm). There was no apparent affect of removing samples on the results obtained. When E. coli was grown in minimal medium (at 40 rpm), cell size was not affected by modeled reduced gravity and there were few differences in cell numbers. However, in higher nutrient conditions (i.e., dilute nutrient broth), total cell numbers were higher and cells were smaller under reduced gravity compared to normal gravity controls. Overall, the responses to modeled reduced gravity varied with nutrient conditions; larger surface to volume ratios may help compensate for the zone of nutrient depletion around the cells under modeled reduced gravity.

Colony Count, Microbial↗

PubMed1997

Bioelectricity, gravity and plants.

This brief review summarizes gravity-induced changes in bioelectric parameters and evaluates their contribution to our understanding of the sensing of gravity, and the transduction and transmission of the gravity stimulus in plants. During the last few decades, information has accumulated demonstrating gravity-induced changes in surface potentials, membrane voltages, endogenous electric currents and ion fluxes. These changes point to the plasma membrane as the site of perception and transduction of the gravity signal. To date, it is reasonable to assume that gravity affects the state of ion channels (in particular, Ca2+ channels) and the activity of ion pumps (in particular, the electrogenic H(+)-ATPase) in the plasma membrane leading to intracellular and apoplasmic changes in ion activities and in membrane voltages. The flow of H+ and Ca2+ currents is probably the means by which information about gravity is amplified and transmitted from sensing to responding cells. No data are available so far about the effect of microgravity on bioelectric parameters. However, it would be interesting to learn if plants become hypersensitive to gravity during a prolonged stay in microgravity. If so, such plants might fire action potentials after return to earth, because more Ca2+ channels than usual may be activated by 1 g in microgravity-adapted plants.

Animals↗

PubMed1991

The urine specific gravity dipstick: a useful tool to increase fluid intake in stone forming patients.

High fluid intake is the only preventive dietary measure that can be recommended to all patients with stones. However, the efficacy of dietary advice given to patients is unknown. We compared the impact of dietary advice to increase hydration (group 1, 57 patients) and of no dietary advice (group 2, 83 patients) on 24-hour urine volume. No significant difference was noted between groups 1 (1,624 ml.) and 2 (1,732 ml.). We then determined if urine specific gravity dipsticks could help patients increase the 24-hour urine volume. A correlation between 24-hour urine volume and mean urine specific gravity was performed on 263 randomly chosen patients. There was an inverse relationship between urine specific gravity and 24-hour urine volume with a correlation coefficient of 0.522 (y = 1.0207 - 0.00374x). Most patients (81.6%) with 24-hour urine volumes of less than 2.1 had a urine specific gravity of more than 1.010. The use of specific gravity dipsticks was evaluated as a tool to help 24 patients increase the 24-hour urine volume. The 24-hour urine volume increased significantly (p less than 0.05, paired Student's t test) in patients after feedback from specific gravity dipsticks when they were instructed to keep the urine specific gravity at or less than 1.010 (average 24-hour urine volume increased 192%). We conclude that dietary advice may be insufficient to modify fluid intake habits in stone patients. However, modifications of fluid intake habits may be improved by feedback from specific gravity dipsticks.

Adolescent↗

PubMed1988

Specific gravity test strips used in monitoring urine concentrations of urolithiasis patients.

Current therapy for urolithiasis patients includes instructions to increase water intake and 24-hour urine output. Previous studies have measured changes in the 24-hour urine volume to evaluate the efficacy of fluid therapy in each patient. We used paper test strips to monitor urine pH and specific gravity in 22 of our stone clinic patients: 10 were instructed to increase water intake just before the study (group 1) and 12 were not so instructed (group 2). Mean specific gravities of 1.0222 (1.0238 corrected for pH) for group 1 and 1.0197 (1.0220 corrected for pH) for group 2 did not differ significantly. Urine specific gravities also were compared for 3 intervals: 1 to 9 a.m., 9 a.m. to 5 p.m. and 5 p.m. to 1 a.m. Of the 22 patients 10 (3 from group 1 and 7 from group 2) had significant diurnal variations in the urine specific gravities, corrected and uncorrected, among these 3 periods. In addition, both groups had a significantly higher mean specific gravity from 1 to 9 a.m. (1.0234 uncorrected and 1.0248 corrected) than from 9 a.m. to 5 p.m. (1.0194 uncorrected and 1.0218 corrected). The 5 p.m. to 1 a.m. (mean of 1.0220 uncorrected and 1.0239 corrected) specific gravity did not differ significantly in either group. If 1.015 is the highest acceptable specific gravity of urine in stone patients, the findings suggest inadequate dilution of urine in these patients, whether or not they were instructed to increase water intake. Also, the significant diurnal variation in urine specific gravity would allow a nighttime triggering event at these hours of higher urine concentration.

Adult↗

PubMed2003

New aspects of gravity responses in plant cells.

Plants show two distinct responses to gravity: gravity-dependent morphogenesis (gravimorphogenesis) and gravity resistance. In gravitropism, a typical mechanism of gravimorphogenesis, gravity is utilized as a signal to establish an appropriate form. The response has been studied in a gravity-free environment, where plant seedlings were found to perform spontaneous morphogenesis, termed automorphogenesis. Automorphogenesis consists of a change in growth direction and spontaneous curvature in dorsiventral directions. The spontaneous curvature is caused by a difference in the capacity of the cell wall to expand between the dorsal and the ventral sides of organs, which originates from the inherent structural anisotropy. Gravity resistance is a response that enables the plant to develop against the gravitational force. To resist the force, the plant constructs a tough body by increasing the cell wall rigidity that suppresses growth. The mechanical properties of the cell wall are changed by modification of the cell wall metabolism and cell wall environment, especially pH. In gravitropism, gravity is perceived by amyloplasts in statocytes, whereas gravity resistance may be mediated by mechanoreceptors on the plasma membrane.

Cell Wall↗

PubMed1991

Effect of source and amount of fiber on kinetics of digestion and specific gravity of forage particles in the rumen.

This experiment investigated the relationship between kinetics of digestion and change in specific gravity during in situ incubation. Nine cows were fed three sources of fiber (corn silage, alfalfa silage, or alfalfa hay) in diets formulated to contain 25, 30, or 35% NDF in three simultaneous 3 x 3 Latin squares. Method of alfalfa preservation did not influence rate of digestion or rate of increase in specific gravity of forage particles measured by a flotation technique. Prior to incubation, specific gravity of forage particles was in increasing order: alfalfa hay, alfalfa silage, and then corn silage. Essentially, all particles with a specific gravity less than 1.0 shifted to a higher specific gravity fraction by hydration within the first 4 h of incubation. From 4 to 56 h of incubation, percentage of residual DM that settled in solution having specific gravity of 1.3 increased linearly from 21 to 27% for corn silage but exponentially from 3 to 20% for alfalfa forages. Fractional rates of DM and NDF digestion and increase in percentage of residual DM having a specific gravity greater than 1.3 increased with the amount of fiber in the alfalfa diets and were correlated positively, suggesting that rate of increase in specific gravity, which affects rate of passage from the rumen, is influenced by rate of digestion of forage particles.

Animal Feed↗

PubMed1980

Effects of radiographic contrast media on results of urinalysis, with emphasis on alteration in specific gravity.

The effect of radiographic contrast medium on the urine specific gravity of ten clinically normal dogs was evaluated. Urinary excretion of different dosages of intravenously administered triiodinated contrast medium caused significant alterations of urine specific gravity. The magnitude of change was related primarily to preinjection urine specific gravity values rather than dosage of contrast medium. In general, preinjection urine specific gravities < 1.040 were increased 15 minutes after injection of contrast material, whereas preinjection urine specific gravities > 1.040 were decreased 15 minutes after injection of contrast medium. A linear relationship was identified between the concentration of radiographic contrast medium in either distilled water or urine and the specific gravity of these mixtures. This simulated the effects of radiologic contrast medium on urine specific gravity when used for retrograde urethrocystography. Results of the investigation indicated that urinary excretion of radiographic contrast media significantly alters urine specific gravity values in normal dogs and renders postinjection urine samples unsuitable for diagnostic evaluation of renal tubular concentrating capacity. In addition, previously reported errors in urine protein, glucose, and sediment evaluations attributed to radiographic contrast media were reviewed.

Animals↗

PubMed2003

The effect of gravity on surface temperatures of plant leaves.

A fundamental study was conducted to develop a facility having an adequate air circulation system for growing healthy plants over a long-term under microgravity conditions in space. To clarify the effects of gravity on heat exchange between plant leaves and the ambient air, surface temperatures of sweet potato and barley leaves and replica leaves made of wet paper and copper were evaluated at gravity levels of 0.01, 1.0, 1.5 and 2.0 g for 20 s each during parabolic aeroplane flights. Thermal images were captured using infrared thermography at an air temperature of 26 degrees C, a relative humidity of 18% and an irradiance of 260 W m-2. Mean leaf temperatures increased by 0.9-1.0 degrees C with decreasing gravity levels from 1.0 to 0.01 g and decreased by 0.5 degrees C with increasing gravity levels from 1.0 to 2.0 g. The increase in leaf temperatures was at most 1.9 degrees C for sweet potato leaves over 20 s as gravity decreased from 1.0 to 0.01 g. The boundary layer conductance to sensible heat exchange decreased by 5% when the gravity decreased from 1.0 to 0.01 g at the air velocity of 0.2 m s-1. The decrease in the boundary layer conductance with decrease in the gravity levels was more significant in a lower air velocity. Heat exchange between leaves and the ambient air was more retarded at lower gravity levels because of less sensible and latent heat transfers with less heat convection.

Air Conditioning↗

PubMed1986

Importance of correcting isokinetic peak torque for the effect of gravity when calculating knee flexor to extensor muscle ratios.

The purpose of our investigation was to compare, for the hamstring and quadriceps femoris muscles, peak torque values uncorrected for gravity with the peak torque values corrected for gravity and to determine the effect of making this correction on the hamstring to quadriceps femoris muscle peak torque ratio at slow and fast isokinetic speeds. We measured peak torques isokinetically at 60 degrees/sec (slow) and 240 degrees/sec (fast) in 25 female university soccer players. The gravity effect torque (GET) is the torque resulting from the effect of gravity on the combined weight of the leg and dynamometer arm at the precise angle of extension and flexion peak torque. The GET was added to the measured quadriceps femoris muscle peak torque and subtracted from the hamstring muscle peak torque to yield gravity corrected values. Failure to consider GET greatly underestimated quadriceps femoris muscle torque and overestimated hamstring muscle torque and the ratio between these torques at both speeds. Whereas the uncorrected hamstring to quadriceps femoris muscle peak torque ratio increased as speeds went from 60 degrees/sec to 240 degrees/sec, the gravity corrected ratio significantly decreased. Clinicians must remember the importance of making the gravity correction in patients with reduced torque output where the gravitational torque is a greater percentage of the measured torque to ascertain correctly the relative strength of antagonists inversely affected by gravity.

Adolescent↗

PubMed2003

Methods to locate center of gravity in scoliosis.

STUDY DESIGN: Prospective evaluation of the location of the center of gravity during supine, standing, and gait. OBJECTIVE: Develop methods to quantify center of gravity locations in patients with scoliosis and controls and to evaluate the merit of the quantitative assumptions relative to spinal fusion surgery. SUMMARY AND BACKGROUND DATA: The center of gravity, or balance point of the body, is generally considered to be the single best estimate of the body's location. To date, investigators have not examined the body's center of gravity location to assist surgical planning to maintain and/or restore coronal and sagittal plane balance, nor have they used center of gravity location to help assess surgical outcomes. MATERIALS AND METHODS: The whole-body center of gravity (MR-COG) was determined for three subjects from magnetic resonance imaging data obtained supine. The whole-body center of gravity was also determined using subject specific (SS-COG) and literature-based (STD-COG) segment center of gravity locations in conjunction with a video motion capture system obtained supine, standing and during gait. RESULTS: Differences existed among the three methods of determining COG locations in supine, with the SS-COG and MR-COG being most closely aligned. Results from gait data indicated typical anterior/superior and right/left COG shifts during the gait cycle. The SS-COG method consistently determined a COG location inferior to the STD-COG method; however, variation within the gait cycle was similar. Shifts in COG locations relative to a coordinate system fixed in the pelvis were more than 5 cm in the superior/inferior direction, approximately 4 cm in the anterior/posterior direction, and minimal in the left/right direction. CONCLUSIONS: Methods have been developed to determine locations of the whole body COG in both preoperative and postoperative subjects undergoing spinal fusion surgery and controls. The methods are robust to include men and women, subjects with and without instrumentation, and subjects in various positions including gait.

Adult↗