Responses of the photosynthetic flagellate,Euglena gracilis,to hypergravity

Motility and orientation has been studied in the unicellular photosynthetic flagellate, Euglena gracilis, using real time image analysis capable of tracking up to 200 cells simultaneously in the slow rotating centrifuge microscope (NIZEMI) which allows one to observe the cells' swimming behavior during centrifugation accelerations between 1 g and 5 g. At 1 g the cells show a weak negative gravitaxis, which increases significantly at higher accelerations up to about 3 g. Though most cells were capable of swimming even against an acceleration of 4.5 g, the degree of gravitaxis decreased and some of the cells were passively moved downward by the acceleration force; this is true for most cells at 5 g. The velocity of cells swimming against 1 g is about 10% lower than that of cells swimming in other directions. The velocity decreases even more drastically in cells swimming against higher acceleration forces than those at 1 g. The degree of gravitactic orientation drastically decreases after short exposure to artificial UV radiation which indicates that gravitaxis may be due to an active physiological perception rather than a physical effect such as an asymmetry of the center of gravity within the cell. Offprint requests to: D.-P. Häder     

Reduced orthostatic tolerance following 4 h head-down tilt

The cardiovascular responses to a 10-min 1.22 rad (70 degrees) head-up tilt orthostatic tolerance test (OST) was observed in eight healthy men following each of a 5-min supine baseline (control), 4 h of 0.1 rad (6 degrees) head-down tilt (HDT), or 4 h 0.52 rad (30 degrees) head-up tilt (HUT). An important clinical observation was presyncopal symptoms in six of eight subjects following 4 h HDT, but in no subjects following 4 h HUT. Immediately prior to the OST, there were no differences in heart rate, stroke volume, cardiac output, mean arterial pressure and total peripheral resistance for HDT and HUT. However, stroke volume and cardiac output were greater for the control group. Mean arterial pressure for the control group was less than HDT but not HUT. Over the full 10-min period of OST, the mean arterial pressure was not different between groups. Heart rate increased to the same level for all three treatments. Stroke volume decreased across the full time period for control and HDT, but only at 3 and 9 min for HUT. There was a higher total peripheral resistance in the HDT group than control or HUT. The pre-ejection period to left ventricular ejection time ratio was less in HDT than for control or HUT groups. These data indicate a rapid adaptation of the cardiovascular system to 4 h HDT that appears to be inappropriate on reapplication of a head to foot gravity vector. We speculate that the cause of the impaired orthostatic tolerance is decreased tone in venous capacitance vessels so that venous return is inadequate.     

失重/模拟失重对内皮细胞的影响实验研究进展

血管内皮作为血管壁的衬里,参与调节组织器官的局部血流和机体其它生理进程,在维持血管完整性和内环境稳定中发挥关键作用。内皮细胞对包括重力在内的机械应力刺激极为敏感,重力变化可对其形态和功能构成不同程度的影响。研究发现,失重/模拟失重通过诱导内皮细胞细胞骨架重塑、质膜caveolae重布,使其合成分泌血管活性物质、炎性介质的能力以及细胞表面粘附分子表达发生改变,这些分子变化又对内皮细胞的生长、增殖、凋亡、迁移和血管生成等具有精细调控作用。本文综合评述了失重/模拟失重对内皮细胞功能的影响,同时围绕文献报道中一些尚存争议的观点进行了适当讨论。     

Accumulation of a tumor suppressor p53 protein in rat muscle during a space flight

We previously reported that the space environment consisting of microgravity and space radiation induced an increased level of p53 protein, a tumor suppressor gene product, in rat skin. Here, we report the increase of p53 protein in the muscles of rats that traveled into space. Rats were divided into three groups. The first group remained on earth (VC), and did not show any change in p53 protein level. The second group made a 14-day flight into space on the Second Spacelab Life Science (SLS-2) Mission (F). The third group was experimentally subjected to the same kinds of stress as those in the second group without making a space flight (SC). F and SC rats were sacrificed on day zero (F-0, SC-0) and day nine (F-9, SC-9) after return from space. F-0 rats showed a 1.5-fold increase in p53 protein level compared with that of SC-0 rats, whereas, F-9 rats showed a 1.35-fold increase in p53 protein compared with that of SC-9 rats. These results suggest that the accumulation of cellular p53 protein induced by space environments occurs not only in rat skin cells, but also in rat muscle cells.     

Gravity and light effects on the circadian clock of a desert beetle,Trigonoscelis gigas

Circadian function is affected by exposure to altered ambient force environments. Under non-earth gravitational fields, both basic features of circadian rhythms and the expression of the clock responsible for these rhythms are altered. We examined the activity rhythm of the tenebrionid beetle, Trigonoscelis gigas, in conditions of microgravity (microG; spaceflight), earth's gravity (1 G) and 2 G (centrifugation). Data were recorded under a light-dark cycle (LD), constant light (LL), and constant darkness (DD). Free-running period (tau) was significantly affected by both the gravitational field and ambient light intensity. In DD, tau was longer under 2 G than under either 1 G or microG. In addition, tauLL was significantly different from tauDD under microG and 1 G, but not under 2 G.     

Lentil root statoliths reach a stable state in microgravity

 The kinetics of the movement of statoliths in gravity-perceiving root cap cells of Lens culinaris L. and the force responsible for it have been analysed under 1 g and under microgravity conditions (S/MM-03 mission of Spacehab 1996). At the beginning of the experiment in space, the amyloplasts were grouped at the distal pole of the statocytes by a root-tip-directed 1-g centrifugal acceleration. The seedlings were then placed in microgravity for increasing periods of time (13, 29, 46 or 122 min) and chemically fixed. During the first 29 min of microgravity there were local displacements (mean velocity: 0.154 μm min⁻¹) of some amyloplasts (first at the front of the group and then at the rear). Nevertheless, the group of amyloplasts tended to reconstitute. After 122 min in microgravity the bulk of amyloplasts had almost reached the proximal pole where further movement was blocked by the nucleus. After a longer period in microgravity (4 h; experiment carried out 1994 during the IML 2 mission) the statoliths reached a stable position due to the fact that they were stopped by the nucleus. The position was similar to that observed in roots grown continuously in microgravity. Treatment with cytochalasin D (CD) did not stop the movement of the amyloplasts but slowed down the velocity of their displacement (0.019 μm min⁻¹). Initial movement patterns were the same as in control roots in water. Comparisons of mean velocities of amyloplast movements in roots in space and in inverted roots on earth showed that the force responsible for the movement in microgravity (F_c) was about 86% less (F_c = 0.016 pN) than the gravity force (F_g = 0.11 pN). Treatment with CD reduced F_c by two-thirds. The apparent viscosity of the statocyte cytoplasm was found to be 1 Pa s or 3.3 Pa s for control roots or CD treated roots, respectively. Brownian motion or elastic forces due to endoplasmic reticulum membranes do not cause the movement of the amyloplasts in microgravity. It is concluded that the force transporting the statoliths is caused by the actomyosin system. Received: 22 March 1999 / Accepted: 18 December 1999     

Gravity independence of seed-to-seed cycling in Brassica rapa

 Growth of higher plants in the microgravity environment of orbital platforms has been problematic. Plants typically developed more slowly in space and often failed at the reproductive phase. Short-duration experiments on the Space Shuttle showed that early stages in the reproductive process could occur normally in microgravity, so we sought a long-duration opportunity to test gravity's role throughout the complete life cycle. During a 122-d opportunity on the Mir space station, full life cycles were completed in microgravity with Brassica rapa L. in a series of three experiments in the Svet greenhouse. Plant material was preserved in space by chemical fixation, freezing, and drying, and then compared to material preserved in the same way during a high-fidelity ground control. At sampling times 13 d after planting, plants on Mir were the same size and had the same number of flower buds as ground control plants. Following hand-pollination of the flowers by the astronaut, siliques formed. In microgravity, siliques ripened basipetally and contained smaller seeds with less than 20% of the cotyledon cells found in the seeds harvested from the ground control. Cytochemical localization of storage reserves in the mature embryos showed that starch was retained in the spaceflight material, whereas protein and lipid were the primary storage reserves in the ground control seeds. While these successful seed-to-seed cycles show that gravity is not absolutely required for any step in the plant life cycle, seed quality in Brassica is compromised by development in microgravity. Received: 3 August 1999 / Accepted: 27 August 1999     

微重力及模拟微重力对植物生长的影响

重力对地球上生物的生长、发育、代谢及繁殖等具有重要影响.植物细胞的重力敏感性已被众多研究所证明,在空间微重力环境或地面模拟微重力环境下,植物表现特殊的微重力反应.微重力或模拟微重力会对植物体生长产生一系列的影响.综述微重力及模拟微重力对植物生长的影响,并对近期这一领域的研究进行了概括.     

Tissue culture in synthetic atmospheres: diffusion rate effects on cytokinin-induced callus growth and isoflavonoid production in soybean [Glycine max (L.) Merr. cv. Acme]

Concentration is one factor that is known to determine how metabolic gases influence the growth and secondary metabolism of plant tissues in culture. How actual gas bioavailability influences these processes has not been studied despite its potential importance in specialized applications. A simple model system, soybean [Glycine max (L.) Merr. cv. Acme] callus culture, was selected for experiments because exogenous cytokinin (6-benzylaminopurine; BAP) elicits two types of responses: (1) enhanced callus proliferation, and (2) rapid induction of the isoflavonoid daidzein (7,4′-dihydroxyisoflavone). Synthetic atmospheres supplying metabolic gases with higher or lower bioavailability than in air were created by replacing the nitrogen moiety in standard air with either helium or argon, respectively. Callus was cultured on agar or in liquid shake cultures according to standard procedures. At an optimal cytokinin concentration for stimulation of callus proliferation, 4.4 × 10⁻⁷ M BAP, increased diffusion rates for the metabolic gases resulted in greater weight gain in agar cultures. Weight gain was 11% higher for He-treated and 39% lower for Ar-treated cultures than for the nitrogen control. In contrast, there was no significant effect of metabolic gas diffusion rate on daidzein production in either agar or liquid cultures. Apart from the potential application of these synthetic atmospheres for enhancing plant tissue culture growth, they may have unique value for the space program as an effective way of replicating the gas exchange limitations posed for plants by microgravity (Ar atmosphere), and as a countermeasure for this limitation (He atmosphere).     

Simulation of the effects of microtubules in the cortical rotation of amphibian embryos in normal and zero gravity

This paper reports the results of computer modeling of microtubules that end up in the cortical region of a one-cell amphibian embryo, prior to the first cell division. Microtubules are modeled as initially randomly oriented semi-flexible rods, represented by several lines of point-masses interacting with one another like masses on springs with longitudinal and transverse stiffness. They are also considered to be space-filling rods floating in a viscous fluid (cytoplasm) experiencing drag forces and buoyancy from the fluid under a variable gravity field to test gravitational effects. Their randomly distributed interactions with the surrounding spherical container (the cell membrane) have a statistical nonzero average that creates a torque causing a rotational displacement between the cytoplasm and the rigid cortex. The simulation has been done for zero and normal gravity and it validates the observation that cortical rotation occurs in microgravity as well as on Earth. The speed of rotation depends on gravity, but is still substantial in microgravity.