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PubMed2001

Theoretical and experimental dissection of gravity-dependent mechanical orientation in gravitactic microorganisms.

Mechanisms of gravitactic behaviors of aquatic microorganisms were investigated in terms of their mechanical basis of gravity-dependent orientation. Two mechanical mechanisms have been considered as possible sources of the orientation torque generated on the inert body. One results from the differential density within an organism (the gravity-buoyancy model) and the other from the geometrical asymmetry of an organism (the drag-gravity model). We first introduced a simple theory that distinguishes between these models by measuring sedimentation of immobilized organisms in a medium of higher density than that of the organisms. Ni2+-immobilized cells of Paramecium caudatum oriented downwards while floating upwards in the Percoll-containing hyper-density medium but oriented upwards while sinking in the hypo-density control medium. This means that the orientation of Paramecium is mechanically biased by the torque generated mainly due to the anterior location of the reaction center of hydrodynamic stress relative to those of buoyancy and gravity; thus the torque results from the geometrical fore-aft asymmetry and is described by the drag-gravity model. The same mechanical property was demonstrated in gastrula larvae of the sea urchin by observing the orientation during sedimentation of the KCN-immobilized larvae in media of different density: like the paramecia, the gastrulae oriented upwards in hypo-density medium and downwards in hyper-density medium. Immobilized pluteus larvae, however, oriented upwards regardless of the density of the medium. This indicates that the orientation of the pluteus is biased by the torque generated mainly due to the posterior location of the reaction center of gravity relative to those of buoyancy and hydrodynamic stress; thus the torque results from the fore-aft asymmetry of the density distribution and is described by the gravity-buoyancy model. These observations indicate that, during development, sea urchin larvae change the mechanical mechanism for the gravitactic orientation. Evidence presented in the present paper demonstrates a definite relationship between the morphology and the gravitactic behavior of microorganisms.

Animals↗

PubMed1975

Selection of artificial gravity by animals during suborbital rocket flights.

White rats selected preferred artificial gravity levels by locomotion in centrifuges consisting of two runways mounted in the nose of sounding rockets. Roll rate of the Aerobee 150A rocket was designed to produce an angular velocity of 45 r.p.m. during 5 min of free-fall, providing a gravity range range from 0.3 to 1.5 G depending on a subject's runway position. One animal was released at the high and one at the low gravity position in each flight. Animal positions were continuously recorded. Flight subjects were selected from about 100 trained animals adapted to the simulated launch environment for several months. In two flights excessive rollrates produced gravity ranges above the designed limits. In two other flights the desired range was produced. Locomotion patterns during these flights were similar. All four animals explored the entire available G-range. One rat settled at 0.4 G after 2 min; the others crossed the 1-G location in progressively narrower excursions and were near earth gravity at the end of the test period. Data were more varible than in laboratory tests above 1 G and the observation periods were necessarily few and short. Tentatively, however, the data suggest that normal earth-reared rats select earth gravity when available magnitudes include values above and below 1 B. Modification of gravity preference by prolonged exposure to higher or lower levels remains a possibility.

Animals↗

PubMed1967

The influence of simulated low-gravity environments on growth, development and metabolism of plants.

Low-gravity environments may be simulated through appropriate horizontal clinostat rotation. This simulation is accomplished through a biological nullification of the directional component of gravitational force. Measuring biologically effective gravity force by organ response, it is readily demonstrated that biologically active gravitational force may be treated as a two-dimensional vector. Though the magnitude dimension of this vector remains virtually constant anywhere on earth, the biologically effective direction dimension may be quantitatively altered by clinostat rotation, provided appropriate angular velocities and angles of inclination of clinostat axes are employed. Using oat seedlings, a rotation rate of 2 rpm, and a horizontal axis clinostat, a 'zero g' environment may be simulated. This simulated 'zero g' condition is attested by the inability of plants to perceive unidirectional gravitational force of sufficient magnitude to elicit directional growth. Under such conditions, plants will grow in the direction imparted by the initial orientation of the plants in the system. Geotropic curvature responses to subsequent geostimulation are, however, greater in seedlings grown under these conditions, nullifying the direction dimension of gravitational force, than in seedlings grown with rotation but with normal unidirectional gravity loads. Root growth under simulated 'zero' gravity conditions is likewise enhanced as compared to plants grown with rotation but normal unidirectional gravity. These differences in magnitude of growth and response to subsequent geostimulation are inexplicable on bases of modified auxin economy or production. Respiration rates are, however, materially enhanced by the simulated 'zero g' environments. This enhancement of respiration, as well as growth, quantitatively diminishes as the unidirectional gravity load is increased. These results imply that the primary effect of low-gravity environments is likely that of modifying the magnitude of respiratory metabolism. Such a modification is partly or wholly explicable on a basis of a modified distribution pattern of metabolically active particulate cell inclusions.

Avena↗

PubMed2003

Perceptual stability during active head movements orthogonal and parallel to gravity.

We measured how much the visual world could be moved during various head rotations and translations and still be perceived as visually stable. Using this as a monitor of how well subjects know about their own movement, we compared performance in different directions relative to gravity. For head rotations, we compared the range of visual motion judged compatible with a stable environment while rotating around an axis orthogonal to gravity (where rotation created a rotating gravity vector across the otolith macula), with judgements made when rotation was around an earth-vertical axis. For translations, we compared the corresponding range of visual motion when translation was parallel to gravity (when imposed accelerations added to or subtracted from gravity), with translations orthogonal to gravity. Ten subjects wore a head-mounted display and made active head movements at 0.5 Hz that were monitored by a low-latency mechanical tracker. Subjects adjusted the ratio between head and image motion until the display appeared perceptually stable. For neither rotation nor translation were there any differences in judgements of perceptual stability that depended on the direction of the movement with respect to the direction of gravity.

Adult↗

PubMed1988

Prediction of physical workload in reduced gravity.

As we plan for long-term living and working in low-gravity environments, a system to predict mission support requirements, such as food and water, becomes critical. Such a system must consider the workload imposed by physical tasks for efficient estimation of these supplies. An accurate estimate of human energy expenditure on a space station or lunar base is also necessary to allocate personnel to tasks, and to assign work-rest schedules. An elemental analysis approach for predicting one's energy expenditure in industrial jobs was applied to low-gravity conditions in this paper. This was achieved by a reduction of input body and load weights in a well-accepted model, in proportion to lowered gravity, such as on the moon. Validation was achieved by applying the model to Apollo-era energy expenditure data. These data were from simulated lunar gravity walking studies, observed Apollo 14 walking, simulated lunar gravity upper body torquing, and simulated lunar gravity cart pulling. The energy expenditure model generally underpredicted high energy expenditures, and overpredicted low to medium energy expenditures. The predictions for low to medium workloads were, however, within 15-30% of actual values. Future developmental work will be necessary to include the effects of traction changes, as well as other nonlinear expenditure changes in reduced gravity environments.

Energy Metabolism↗

PubMed1996

The influence of center of gravity and lens mass on rigid lens dynamics.

PURPOSE: Center of gravity and lens mass have both been shown to influence rigid lens centration and stability. The purpose of this study was to investigate the relative contributions of these two factors to rigid lens dynamics. METHODS: Eight subjects (four with high-riding lenses and four with low-riding lenses) participated in the study. Each subject was fit with 12 lenses-six designs in each of two materials. Center of gravity and lens mass were recorded in every case. For each lens, four lens dynamics variables were assessed over a five-blink cycle for both the vertical and the horizontal meridians: 1) initial lens position; 2) settled lens position; 3) amount of lens movement; and 4) rate of lens movement. RESULTS: Multiple regression and correlation analysis showed that center of gravity influences initial and settled lens position, in both the horizontal and the vertical meridians. Mass alone was found not to be a significant predictor of rigid lens dynamics. The effect of center of gravity on settled vertical lens position was only apparent when high rider and low rider subgroups were analyzed separately, with a posterior movement of the center of gravity being associated with a lowering of the settled lens position for high rider subjects, and a raising of the settled lens position for low rider subjects. The results suggest that a rigid lens will become more stable as the center of gravity is shifted further behind the lens vertex, but this effect is reduced as lens mass is increased. CONCLUSIONS: The location of the center of gravity of a rigid lens influences its on-eye centration and stability more so than does lens mass.

Contact Lenses↗

PubMed2006

Effects of altered gravity on the actin and microtubule cytoskeleton of human SH-SY5Y neuroblastoma cells.

Human SH-SY5Y neuroblastoma cells were used to study the effects of altered gravity on the actin and microtubule cytoskeleton dynamics. A cholinergic stimulation of the cells during a 6 min period of changing gravity (3 parabolas) resulted in an enhanced actin-driven protrusion of evoked lamellipodia. Likewise, the spontaneous protrusive activity of nonactivated cells was promoted during exposure to changing gravity (6 up to 31 parabolas). Ground-based experiments revealed a similar enhancement of the spontaneous and evoked lamellar protrusive activity when the cells were kept at 2 g hypergravity for at least 6 min. This gravity response was independent of the direction of the acceleration vector in respect to the cells. Exposure of the cells to "simulated weightlessness" (clinorotation) had no obvious influence on this type of lamellar actin cytoskeleton dynamics. A 20 min exposure of the cells to simulated weightlessness or to changing gravity (6 to 31 parabolas) - but not to 2 g (hypergravity, centrifugation) - resulted in an altered arrangement of microtubules indicated by bending, turning, and loop formation. A similar altered arrangement was shown by microtubules which had polymerized into lamellipodia after release from a taxol block at simulated weightlessness (clinorotation) or during changing gravity (5 parabolas). Our data suggest that in human SH-SY5Y neuroblastoma cells, microgravity affects the dynamics and spatial arrangement of microtubules but has no influence on the Rac-controlled lamellar actin cytoskeleton dynamics and cell spreading. The latter, however, seems to be promoted at hypergravity.

Acetylcholine↗

PubMed2006

Rhizoids and protonemata of characean algae: model cells for research on polarized growth and plant gravity sensing.

Gravitropically tip-growing rhizoids and protonemata of characean algae are well-established unicellular plant model systems for research on gravitropism. In recent years, considerable progress has been made in the understanding of the cellular and molecular mechanisms underlying gravity sensing and gravity-oriented growth. While in higher-plant statocytes the role of cytoskeletal elements, especially the actin cytoskeleton, in the mechanisms of gravity sensing is still enigmatic, there is clear evidence that in the characean cells actin is intimately involved in polarized growth, gravity sensing, and the gravitropic response mechanisms. The multiple functions of actin are orchestrated by a variety of actin-binding proteins which control actin polymerisation, regulate the dynamic remodelling of the actin filament architecture, and mediate the transport of vesicles and organelles. Actin and a steep gradient of cytoplasmic free calcium are crucial components of a feedback mechanism that controls polarized growth. Experiments performed in microgravity provided evidence that actomyosin is a key player for gravity sensing: it coordinates the position of statoliths and, upon a change in the cell's orientation, directs sedimenting statoliths to specific areas of the plasma membrane, where contact with membrane-bound gravisensor molecules elicits short gravitropic pathways. In rhizoids, gravitropic signalling leads to a local reduction of cytoplasmic free calcium and results in differential growth of the opposite subapical cell flanks. The negative gravitropic response of protonemata involves actin-dependent relocation of the calcium gradient and displacement of the centre of maximal growth towards the upper flank. On the basis of the results obtained from the gravitropic model cells, a similar fine-tuning function of the actomyosin system is discussed for the early steps of gravity sensing in higher-plant statocytes.

Actomyosin↗

PubMed1983

Gravity and positional homeostasis of the cell.

Normally bilateralization takes place in the presence of the Earth's gravity which produces torque, shear, tension and compression acting upon the naked aggregates of cytoplasm in the zygote which is only stabilized by a weak cytoskeleton. In an initial examination of the effects of these quantities on development, an expression is derived to describe the tendency of torque to rotate the egg and reorganize its constituents. This expression yields the net torque resulting from buoyancy and gravity acting upon a dumbbell shaped cell with heavy and light masses at either end and "floating" in a medium. Using crude values for the variables, torques of 2.5 x l0(-13) to 8.5 x 10(-1) dyne-cm are found to act upon cells ranging from 6.4 micrometers to 31 mm (chicken egg). By way of camparison six microtubules can exert a torque of 5 x 10(-9) dyne-cm. (1) Gravity imparts torque to cells; (2) torque is reduced to zero as gravity approaches zero; and (3) torque is sensitive to cell size and particulate distribution. Cells must expend energy to maintain positional homeostasis against gravity. Although not previously recognized, Skylab 3 results support this hypothesis: tissue cultures used 58% more glucose on Earth than in space. The implications for developmental biology, physiology, genetics, and evolution are considered. At the cellular and tissue level the concept of "gravity receptors" may be unnecessary.

Animals↗

PubMed1986

Classification of gravity effects on "free" cells.

When cell physiologists detect gravity related reactions of their objects it is often difficult to decide where the receptors for the observed effects are located. Answering this question is necessary for any further analysis of a detected gravity effect on cells. In previous papers we have discussed direct and indirect gravity effects in relation to the smallest functional units where the primary receptor, which interacts with gravity, is positioned inside and outside of such a unit, respectively. So, in a first approximation we can conclude that in a multicellular aquatic organism, which changes its metabolism in weightlessness, the primary receptors of gravity are located inside the cells of that organism. A special approach is necessary when free living cells, the density of which may be higher than the one of the (liquid) medium, or even cells living on a free surface are observed. In these two cases also indirect effects have to be taken into account, which will be demonstrated with the aid of the slime mold Physarum polycephalum. Additionally the environment of the organisms can be changed directly and indirectly by gravity.

Animals↗

PubMed1992

Possible actions of gravity on the cellular machinery.

Since the first flight of the ESA Biorack on the German Spacelab Mission D1 in 1985 evidence has been obtained that biological cells and small unicellular organisms function differently under conditions of microgravity. However, there is still lack of scientific proof that these effects are caused by a direct influence on the cells in the weightlessness condition. The question how normal gravity may play a role in cellular activity is being addressed and the results show that gravity may provide important signals during certain state transitions in the cell. These would be gravity-sensitive windows in the biological process. Also, by amplification mechanisms inside the cell, the cell may assume a state that is typical for normal gravity conditions and would change in microgravity. Experimental tools are discussed that would provide the conditions to obtain evidence for direct action of gravity and for the possible existence of gravity-sensitive windows.

Animals↗

PubMedDate not supplied

Creatinine versus specific gravity-adjusted urinary cadmium concentrations.

The aim was to assess how urinary creatinine is affected by age, gender, body size and meat intake, and to determine to what extent such factors might affect the creatinine adjustment of urinary cadmium. The study was based on three Swedish studies: (1) 67 non-smoking women aged 20-50 years (24-h urine samples); (2) 289 men and 434 women aged 16-81 years (spot urine samples); and (3) 98 men and 105 women aged 19-72 years (spot urine samples). The effects of age, body surface area (as an indicator of muscle mass), and meat intake on urinary creatinine and cadmium were analysed using multiple regression analyses. Gender- and age-related variations in urinary creatinine and cadmium adjusted for creatinine or specific gravity were compared by ANOVA or ANCOVA. In the multiple regression analyses, body surface area, gender, age and meat intake were the major determinants of urinary creatinine. Urinary cadmium adjusted for creatinine and specific gravity were also dependent on body size, gender and age. Urinary cadmium adjusted for creatinine was 15-92% higher in women or older individuals than in men or younger individuals. Women or older individuals had -3 to 79% higher urinary cadmium adjusted for specific gravity than men or younger individuals had, and such a difference between gender or age group was less obvious in specific gravity adjustment than in creatinine adjustment. Thus, urinary cadmium adjusted for creatinine is more affected by age, gender, body size and meat intake than is specific gravity adjustment. When comparing individuals or populations with large differences in muscle mass or meat intake, such effects can be especially important. In such studies, specific gravity adjustment seems to be more appropriate.

Adult↗

PubMed2005

Effect of lower body negative pressure and gravity on regional lung ventilation determined by EIT.

The aim of our study was to check the effect of varying blood volume in the chest and gravity on the distribution of ventilation and aeration in the lungs. The change in intrathoracic blood volume was elicited by application of lower body negative pressure (LBNP) of -50 cmH2O. The variation of gravity in terms of hypogravity (approximately 0g) and hypergravity (approximately 2g) was induced by changes in vertical acceleration achieved during parabolic flights. Local ventilation magnitude and end-expiratory lung volume were determined in eight human subjects in the ventral and dorsal lung regions within a transverse cross-section of the lower chest by electrical impedance tomography. The subjects were studied in a 20 degrees head-down tilted supine body position during tidal breathing and full forced expirations. During tidal breathing, a significant effect of gravity on local magnitude of ventilation and end-expiratory lung volume was detected in the dorsal lung regions both with and without LBNP. In the ventral regions, this gravity dependency was only observed during LBNP. During forced expiration, LBNP had almost no effect on local ventilation and end-expiratory lung volume in either lung region. Gravity significantly influenced the end-expiratory lung volumes in dorsal lung regions. The results indicate that exposure to LBNP exerts a less appreciable effect on regional lung ventilation than the acute changes in gravity.

Adult↗

PubMed2003

Does canopy position affect wood specific gravity in temperate forest trees?

The radial increases in wood specific gravity known in many tree species have been interpreted as providing mechanical support in response to the stresses associated with wind loading. This interpretation leads to the hypothesis that individuals reaching the canopy should (1) be more likely to have radial increases in specific gravity and (2) exhibit greater increases than individuals in the subcanopy. Wood specific gravity was determined for three species of forest trees (Acer rubrum, Fagus grandifolia and Tsuga canadensis) growing in central Massachusetts, USA. Acer rubrum shows radial increases in specific gravity, but these increases are not more pronounced in canopy trees; the other two species show a pattern of radial decreases. The degree of radial increase or decrease is influenced by tree height and diameter. Of the dominant tree species for which we have data, A. rubrum, Betula papyrifera and Pinus strobus show radial increases in specific gravity, whereas F. grandifolia, T. canadensis and Quercus rubra show decreases. The occurrence of radial increases in B. papyrifera and P. strobus, which are often canopy emergents, suggests that it is overall adaptive strategy that is important rather than position (canopy vs. subcanopy) of any individual tree. It is suggested that radial increases in specific gravity are associated with early-successional status or characteristics and decreases with late-successional status or persistence in mature forest.

Acer↗

PubMed1992

Specific gravity of bovine colostrum immunoglobulins as affected by temperature and colostrum components.

The effects of temperature and colostrum components on specific gravity in bovine colostrum were investigated. Thirty-nine first milking colostrum samples were collected from Holstein cows. The samples were assayed for alpha-tocopherol, fat, protein, total solids, and IgG. The concentrations of total solids, total protein, total IgG, and fat in colostrum were 26.6, 12.5, 3.7, and 9.4 g/100 g, respectively. A range of 1.8 to 24.7 micrograms/ml for alpha-tocopherol was measured in the colostrum samples. Specific gravity of the colostrum was measured using a hydrometer in increments of 5 degrees C from 0 to 40 degrees C. Specific gravity explained 76% of the variation in colostral total IgG at a colostrum temperature of 20 degrees C. The regression model was improved only slightly with the addition of protein, fat, and total solids. The model for samples at 20 degrees C was IgG (milligrams per milliliter) = 958 x (specific gravity) - 969. Measurement of specific gravity at variable temperatures necessitated inclusion of temperature in the model for estimation of IgG. Inclusion of the other components of colostrum into the model slightly improved the fit. The regression model for samples at variable temperatures was as follows: IgG (milligrams per milliliter) = 853 x (specific gravity) + .4 x temperature (Celsius degrees) - 866.

Animals↗

PubMed1994

Immunoglobulin concentration, specific gravity, and nitrogen fractions of colostrum from Jersey cattle.

Colostrum samples from 88 Jersey cows were analyzed for concentrations of IgG, IgM, IgA, total solids, specific gravity, and N fractions. Colostrum (50 ml) was sampled from each cow as soon as possible after parturition, and specific gravity was determined immediately using a hydrometer. Samples then were frozen prior to analysis of Ig, fat, and N fractions. Mean concentrations of IgG, IgM, and IgA were 65.8, 2.4, and 1.7 g/L, respectively. Concentration of IgG was lower, and IgA was higher, in colostrum from second lactation cows than from first lactation cows or from cows in third or later lactations; IgM increased linearly as lactation number increased. Total N, protein N, noncasein N, and fat contents also were lower in second lactation cows. Regression of total Ig (grams per liter) on specific gravity was -1172 + 1180 x specific gravity (r2 = .38). Relationship of total Ig to specific gravity differed from colostrum of Holstein cattle and may have been related to differences in fat and noncasein N concentrations. Use of specific gravity hydrometer to estimate Ig concentration using equations derived from Holstein cattle appears to underestimate Ig concentration in colostrum from Jersey cattle.

Animals↗

PubMed1985

Brain specific gravity and CT scan density measurements after human head injury.

White matter specific gravity was measured using the microgravimetric method in 20 comatose patients with diffuse head injury who were undergoing intracranial pressure (ICP) monitoring, and in 19 patients with focal injuries who were undergoing evacuation of contusions or intracerebral hematomas. Computerized tomography (CT) density readings were obtained for each site of white matter sampling by locating the sampling site on the preoperative CT scan. A significant correlation was found between the specific gravity values and the CT density numbers (r = 0.775; p less than 0.001). Patients with focal injuries demonstrated reduced perifocal specific gravity, suggesting brain edema. The mean specific gravity in patients with diffuse injury was within the normal range. In 10 of 12 patients in whom the specific gravity was above the normal range, the CT density was also above the normal range. These data suggest that cerebral vascular engorgement is the cause of the high specific gravity. Six (60%) of this small subgroup of 10 patients also demonstrated a high ICP.

Adult↗

PubMed1997

The effect of the external medium on the gravity-induced polarity of cytoplasmic streaming in Chara corallina (Characeae).

Gravity induces a polarity of cytoplasmic streaming in vertical internodal cells of Chara such that the downwardly directed stream moves faster than the upwardly directed stream. In order to determine whether the statolith theory (in which intracellular sedimenting particles are responsible for gravity sensing) or the gravitational pressure theory (in which the entire protoplast acts as the gravity sensor) best explain the gravity response in Chara internodal cells, we controlled the physical properties of the external medium, including density and osmolarity, with impermeant solutes and examined the effect on the polarity of cytoplasmic streaming. As the density of the external medium is increased, the polarity of cytoplasmic streaming decreases and finally disappears when the density of the external medium is equal to that of the cell (1015 kg/m3). A further increase in the density of the external medium causes a reversal of the gravity response. These results are consistent with the gravitational pressure theory of gravity sensing since the buoyancy of the protoplast is dependent on the difference between the density of the protoplast and the external medium, and are inconsistent with the statolith theory since the buoyancy of intracellular particles are unaffected by changes in the external medium.

Biophysical Phenomena↗