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PubMed1994

Radiation protection issues in galactic cosmic ray risk assessment.

Radiation protection involves the limitation of exposure to below threshold doses for direct (or deterministic) effects and a knowledge of the risk of stochastic effects after low doses. The principal stochastic risk associated with low dose rate galactic cosmic rays is the increased risk of cancer. Estimates of this risk depend on two factors (a) estimates of cancer risk for low-LET radiation and (b) values of the appropriate radiation weighting factors, WR, for the high-LET radiations of galactic cosmic rays. Both factors are subject to considerable uncertainty. The low-LET cancer risk derived from the late effects of the atomic bombs is vulnerable to a number of uncertainties including especially that from projection in time, and from extrapolation from high to low dose rate. Nevertheless, recent low dose studies of workers and others tend to confirm these estimates. WR, relies on biological effects studied mainly in non-human systems. Additional laboratory studies could reduce the uncertainties in WR and thus produce a more confident estimate of the overall risk of galactic cosmic rays.

Adolescent↗
PubMed1987

Utilization of potatoes for life support systems in space: III. Productivity at successive harvest dates under 12-h and 24-h photoperiods.

Potatoes are among several crops under consideration for use in controlled ecological life support systems (CELSS) being proposed for space colonies. Efficient crop production for such life support systems will require near-optimal growing conditions with harvests taken when production per unit area per unit time is maximum. To determine this maximum for potato, cv. Norland plants were grown in walk-in growth rooms under 12-h and 24-h photoperiods at 16 C and harvested at 42, 63, 84, 105, 126 and 148 days from planting. At 42 days, plants were encaged in wire fence cylinders with a cross-sectional area of 0.2 m2. The dry weights (dwt) of tubers and of the entire plants increased under both photoperiods until the final harvest date (148 days), reaching 572 g tuber dwt and 704 g total dwt under 12-h, and 791 g tuber dwt and 972 g total dwt under 24-h. At a spacing of 0.2 m2 per plant, the 148-day tuber production from plants under continuous light would equate to nearly 40 t ha-1 dry matter (200 t fresh weight), approximately twice that of exceptionally high field yields. Tuber productivity (g m-2 day-1) under the 24-h photoperiod reached a maximum of 29.4 g dwt m-2 day-1 at 126 days, but continued to rise throughout the experiment under the 12-h photoperiod, reaching 19.5 g dwt m-2 day-1 at 14 days, approximately 25 m2 would continuously provide the daily dietary energy requirements for one human.

Biomass↗
PubMed1999

Combined effects of space flight factors and radiation on humans.

The probability that a dose of ionizing radiation kills a cell is about 10,000 times the probability that the cell will be transformed to malignancy. On the other hand, the number of cells killed required to significantly impact health is about 10,000 times the number that must be transformed to cause a late malignancy. If these two risks, cell killing and malignant transformation, are about equal, then the risk that occurs during a mission is more significant than the risk that occurs after a mission. The latent period for acute irradiation effects (cell killing) is about 2-4 weeks; the latent period for malignancy is 10-20 years. If these statements are approximately true, then the impact of cell killing on health in the low-gravity environment of space flight should be examined to establish an estimate of risk. The objective of this study is to synthesize data and conclusions from three areas of space biology and environmental health to arrive at rational risk assessment for radiations received by spacecraft crews: (1) the increased physiological demands of the space flight environment; (2) the effects of the space flight environment on physiological systems; and (3) the effects of radiation on physiological systems. One physiological system has been chosen: the immune response and its components, consisting of myeloid and lymphoid proliferative cell compartments. Best-case and worst-case scenarios are considered. In the worst case, a doubling of immune-function demand, accompanied by a halving of immune capacity, would reduce the endangering dose to a crew member to around 1 Gy.

Animals↗
PubMed1997

Contractile properties of rat, rhesus monkey, and human type I muscle fibers.

It is well known that skeletal muscle intrinsic maximal shortening velocity is inversely related to species body mass. However, there is uncertainty regarding the relationship between the contractile properties of muscle fibers obtained from commonly studied laboratory animals and those obtained from humans. In this study we determined the contractile properties of single chemically skinned fibers prepared from rat, rhesus monkey, and human soleus and gastrocnemius muscle samples under identical experimental conditions. All fibers used for analysis expressed type I myosin heavy chain as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Allometric coefficients for type I fibers from each muscle indicated that there was little change in peak tension (force/fiber cross-sectional area) across species. In contrast, both soleus and gastrocnemius type I fiber maximal unloaded shortening velocity (Vo), the y-intercept of the force-velocity relationship (Vmax), peak power per unit fiber length, and peak power normalized for fiber length and cross-sectional area were all inversely related to species body mass. The present allometric coefficients for soleus fiber Vo (-0.18) and Vmax (-0.11) are in good agreement with published values for soleus fibers obtained from common laboratory and domesticated mammals. Taken together, these observations suggest that the Vo of slow fibers from quadrupeds and humans scale similarly and can be described by the same quantitative relationships. These findings have implications in the design and interpretation of experiments, especially those that use small laboratory mammals as a model of human muscle function.

Animals↗
PubMed1995

Mechanism of thirst attenuation during head-out water immersion in men.

The purpose was to determine whether extracellular volume or osmolality was the major contributing factor for reduction of thirst in air and head-out water immersion in hypohydrated subjects. Eight males (19-25 yr) were subjected to thermoneutral immersion and thermoneutral air under two hydration conditions without further drinking: euhydration in water (Eu-H2O) and euhydration in air, and hypohydration in water (Hypo-H2O) and hypohydration in air (3.7% wt loss after exercise in heat). The increased thirst sensation with Hypo-H2O decreased (P < 0.05) within 10 min of immersion and continued thereafter. Mean plasma osmolality (288 +/- 1 mosmol/kgH2O) and sodium (140 +/- 1 meq/l) remained elevated, and plasma volume increased by 4.2 +/- 1.0% (P < 0.05) throughout Hypo-H2O. A sustained increase (P < 0.05) in stroke volume accompanied the prompt and sustained decrease in plasma renin activity and sustained increase (P < 0.05) in plasma atrial natriuretic peptide during Eu-H2O and Hypo-H2O. Plasma vasopressin decreased from 5.3 +/- 0.7 to 2.9 +/- 0.5 pg/ml (P < 0.05) during Hypo-H2O but was unchanged in Eu-H2O. These findings suggest a sustained stimulation of the atrial baroreceptors and reduction of a dipsogenic stimulus without major alterations of extracellular osmolality in Hypo-H2O. Thus it appears that vascular volume-induced stimuli of cardiopulmonary baroreceptors play a more important role than extracellular osmolality in reducing thirst sensations during immersion in hypohydrated subjects.

Adult↗
PubMedDate not supplied

Gas exchange in NASA's biomass production chamber: a preprototype closed human life support system.

An important aspect of environmental control in a life-support system is the monitoring and regulation of atmospheric gases (Sager et al. 1988) at concentrations required for the maintenance of all life forms. It will be necessary to know the rates of CO2 use, oxygen evolution, and water flux through evapotranspiration by a crop stand under various environmental conditions, so that appropriate designs and control systems for maintaining mass balances of those gases can be achieved for a full range of environmental regimes. Mass budgets of gases will also enable evaluation of crop health by monitoring directly the rates of gas exchange and indirectly the rate of accumulation of dry matter, based on rates of carbon dioxide use. This article focuses on the unique capabilities of the NASA biomass production chamber for monitoring and evaluating gas exchange rates, with special emphasis on results with wheat and soybean, two candidate species identified by NASA for CELSS.

Biomass↗
PubMed2005

Cardiac health for astronauts: current selection standards and their limitations.

INTRODUCTION: The screening tests for coronary artery disease (CAD) for applicants and the active astronaut corps are similar to those performed in the 1960s. Due to the limited treatment and return capabilities of most space vehicles, an in-flight cardiac event would result in mission failure. Improved CAD screening of astronauts is, therefore, paramount to long-duration mission success. METHODS: Literature review was performed to compare active and retired astronaut populations to other asymptomatic low-risk cohorts. All populations were examined to determine the incidence and prevalence of CAD. Framingham risk scores were calculated in NASA's active and retired astronaut corps and compared with age- and gender-matched controls. RESULTS: The current standards used for astronaut selection have been successful in creating a cohort that has less risk than their age- and gender-matched counterparts from the general population. However, the existing astronaut cardiovascular screening and selection tests do not adequately rule out CAD for long-duration missions, and, therefore, a "significant" risk of cardiac event remains, especially as we look toward Exploration Class missions. CONCLUSIONS: The current astronaut selection and retention standards may not adequately prevent cardiac events from occurring with the longer duration flights. Future research should be directed toward increasing the primary and secondary prevention of CAD in the astronaut cohort. In the meantime, the space program should evaluate the use of more aggressive terrestrial screening tools. It is important not to remove all older, experienced pilots from spaceflight crews unless overt or predictable pathology has been clearly identified.

Acute Disease↗
PubMed1995

Immune responses in humans after 60 days of confinement.

A confinement experiment in a normobaric diving chamber was undertaken to better understand the effect of confinement and isolation on human psychology and physiology. Pre- and postconfinement blood samples were obtained from four test subjects and control donors to analyze immune responses. No modification in the levels of CD2+, CD3+, CD4+, CD8+, CD19+, and CD56+ cells was observed after confinement. Mitogen-induced T-lymphocyte proliferation and interleukin-2 receptor expression were not altered significantly. Whole blood interferon-alpha and gamma-induction and plasma cortisol levels were also unchanged, as was natural killer cell activity. These data suggest that in humans, no specific components of the immune response are affected by a 2-month isolation and confinement of a small group.

Adult↗
PubMed1986

Radiation protection standards in space.

Radiation protection standards for the individual exposed to ionizing radiation in his/her daily work have evolved over more than 50 years since the first recommendations on limits by the NCRP and the ICRP. Initial standards were based on the absence of observable harm, notably skin erythema, but have since been modified as other concerns, such as leukemia and genetic effects, became more important. More recently, the general carcinogenic effect of radiation has become the principal concern at low doses. Genetic effects are also of concern in the younger individual. Modern radiation protection practices take both of these risks into account. Quantification of these risks improves as new information emerges. The study of the Japanese survivors of the atomic bombs continues to yield new information and the recent revisions in the dosimetry are about to be completed. The special circumstances of space travel suggest approaches to limits not unlike those for radiation workers on the ground. One approach is to derive a career limit based on the risks of accident faced by many nonradiation workers in a lifetime. The career limit can be apportioned according to the type of mission. The NCRP is considering this and other approaches to the specification of radiation standards in space.

Adult↗
PubMed1997

Exercise thermoregulation with bed rest, confinement, and immersion deconditioning.

Altered thermoregulation following exposure to prolonged (12-14 days) of bed rest and 6 hr of head-down thermoneutral water immersion in humans, and cage confinement (8 weeks) in male, mongrel dogs resulted in occasional increased core temperature (Tcore) at rest, but consistent "excessive" increase in Tcore during submaximal exercise. This excessive increase in Tcore in nonexercising and exercising subjects was independent of the mode (isometric or isotonic) of exercise training during bed rest, and was associated with the consistent hypovolemia in men but not in women taking estrogen supplementation (1.25 mg premarin/ day) which restored plasma volume during bed rest to ambulatory control levels. Post-bed rest exercise sweating (evaporative heat loss) was unchanged or higher than control levels; however, calculated tissue heat conductance was significantly lower in men, and forearm venoconstriction was greater (venous volume was reduced) in women during exercise after bed rest. Because sweating appeared proportional to the increased level of Tcore, these findings suggest that one major factor for the excessive hyperthermia is decreased core to periphery heat conduction. Exercising dogs respond like humans with excessive increase in both rectal (Tre) and exercising muscle temperatures (Tmu) after confinement and, after eight weeks of exercise training on a treadmill following confinement, they had an attenuated rate of increase of Tre even below ambulatory control levels. Intravenous infusion of glucose also attenuated not only the rise in Tre during exercise in normal dogs, but also the excessive rise in Tre and exercising Tmu after confinement. Oral glucose also appeared to reduce the rate of increase in excessive Tre in men after immersion deconditioning. There was a greater rate of rise in Tcore in two cosmonauts during supine submaximal exercise (65% VO2 max) on the fifth recovery day after the 115-day Mir 18 mission. Thus, the excessive rise in core temperature after deconditioning appears to be caused by decreased peripheral vasodilation in humans. Factors related to glucose metabolism may influence this mechanism.

Adaptation, Physiological↗
PubMed1995

Pharmacologic atrial natriuretic peptide reduces human leg capillary filtration.

Atrial natriuretic peptide (ANP) is produced and secreted by atrial cells. We measured calf capillary filtration rate with prolonged venous-occlusion plethysmography of supine healthy male subjects during pharmacologic infusion of ANP (48 pmol/kg/min for 15 min; n = 6) and during placebo infusion (n = 7). Results during infusions were compared to prior control measurements. ANP infusion increased plasma [ANP] from 30 +/- 4 to 2,568 +/- 595 pmol/l. Systemic hemoconcentration occurred during ANP infusion: mean hematocrit and plasma colloid osmotic pressure increased 4.6 and 11.3%, respectively, relative to preinfusion baseline values (p < 0.05). Mean calf filtration, however, was significantly reduced from 0.15 to 0.08 ml/100 ml/min with ANP. Heart rate increased 20% with ANP infusion, whereas blood pressure was unchanged. Calf conductance (blood flow/arterial pressure) and venous compliance were unaffected by ANP infusion. Placebo infusion had no effect relative to prior baseline control measurements. Although ANP induced systemic capillary filtration, in the calf, filtration was reduced with ANP. Therefore, pharmacologic ANP infusion enhances capillary filtration from the systemic circulation, perhaps at upper body or splanchnic sites or both, while having the opposite effect in the leg.

Adult↗
PubMed1991

Waste streams in a crewed space habitat.

A compilation of generation rates and chemical compositions of potential waste streams in a typical crewed space habitat was made in connection with the waste-management aspect of NASA's Physical/Chemical Closed-Loop Life Support Program. Waste composition definitions are needed for the design of waste-processing technologies involved in closing major life support functions in future, long-duration, human space missions. Data for the constituents and chemical formulae of the following waste streams are presented and/or discussed: human urine, feces, hygiene (laundry and shower) water, cleansing agents, trash, humidity condensate, dried sweat, and trace contaminants. Data on dust generation are also presented and discussed.

Dust↗
PubMed1996

Noninvasive markers of bone metabolism in the rhesus monkey: normal effects of age and gender.

Measurement of bone turnover in conditions such as osteoporosis has been limited by the need for invasive iliac bone biopsy to reliably determine parameters of bone metabolism. Recent advances in the area of serum and urinary markers of bone metabolism have raised the possibility for noninvasive measurements; however, little nonhuman primate data exist for these parameters. The purpose of this experiment was to define the normal range and variability of several of the newer noninvasive bone markers which are currently under investigation in humans. The primary intent was to determine age and gender variability, as well as provide some normative data for future experiments in nonhuman primates. Twenty-four rhesus macaques were divided into equal groups of male and female according to the following age groupings: 3 years, 5-10 years, 15-20 years, and > 25 years. Urine was collected three times daily for a four-day period and measured for several markers of bone turnoverm including pyridinoline (PYD), deoxypyrodinoline (DPD), hydroxyproline, and creatinine. Bone mineral density measurements of the lumbar spine were performed at the beginning and end of the study period. Serum was also obtained at the time of bone densitometry for measurement of osteocalcin levels by radioimmunoassay. There were no significant differences in bone mineral density, urine PYD, or urine DPD based on gender. Bone density was lowest in the youngest animals, peaked in the 15-20-year group, but again decreased in the oldest animals. The osteocalcin, PYD, and DPD levels followed an inversely related pattern to bone density. The most important result was the relative age insensitivity of the ratio of PYD:DPD in monkeys up to age 20 years. Since bone density changes take months or years to become measurable and iliac biopsies are invasive, the PYD/DPD marker ratio may have important implications for rapid noninvasive measurement of the effects of potential treatments for osteoporosis in the non-human primate model.

Age Factors↗
PubMed1994

Cardiovascular responses to KC-135 hyper-gravity.

The present study was designed with two intentions; Are the effects of angular velocity detectable in the cardiovascular responses during the hyper-G? Another is object to examine how the otolith signal could modify the cardiovascular responses provoked by the exposure to the hyper-G. NASA/KC-135 hyper-gravity flight was used to generate high gravito-inertial forces to exclude a possible effect of angular velocity. Six healthy subjects was indicated to make dorsal flexion of the neck to reduce the otolith input. An exposure to +l.8Gz stress resulted in a remarkable increase of systolic and diastolic blood pressure, thereby pulse pressure became a little bit narrower. R-R interval revealed a tachycardia during the hyper-G except one subject. The present experiment bore the similar cardiovascular responses as those observed in the previous studies with a short rotating radius, suggesting that almost no effect of angular velocity acts on their responses. A weaker otolith input could possibly work on them. However a systematical observation can not recognize among the subjects for the vestibular effect on the cardiovascular responses. This fact of vestibular qualification leads us to speculate that it would depend on the subjects or other factors.

Adult↗
PubMed1998

Localization of virtual objects in the near visual field.

We examined errors in the localization of nearby virtual objects presented via see-through helmet-mounted displays as a function of viewing conditions and scene content in four experiments using a total of 38 participants. Monocular, biocular, and stereoscopic presentation of the virtual objects, accommodation (required focus), participants' age, and the position of physical surfaces were examined. Nearby physical surfaces were found to introduce localization errors that differ depending on the other experimental factors. These errors apparently arise from the occlusion of the physical background by the optically superimposed virtual objects, but they are modified by participants' accommodative competence and specific viewing conditions. The apparent physical size and transparency of the virtual objects and physical surfaces, respectively, are influenced by their relative position when superimposed. The design implications of the findings are discussed in a concluding section. Head-mounted displays of virtual objects are currently being evaluated as aids for mechanical assembly and equipment maintenance. Other applications include telesurgery, surgical planning, telerobotics, and visualization aids for robotic programming.

Accommodation, Ocular↗
PubMed2001

Effect of simulated microgravity on human lymphocytes.

During space flight the function of the immune system changes significantly. Several papers reported that postflight the number and the proportion of circulating leukocytes in astronauts are modified (Leach, 1992), the in vitro mitogen induced T cell activation is depressed (Cogoli et al., 1985; Konstantinova et al. 1993) and there are detectable differences in cytokine production of leukocytes as well (Talas et al. 1983; Batkai et al. 1988; Chapes et al. 1992). One of the possible modifying forces is the microgravity condition itself. Our aim was to analyse mechanisms responsible for changing leukocyte functions in low gravity environment. For terrestrial simulation of microgravity we used a Rotary Cell Culture System (RCCS) developed by NASA. We investigated the effect of simulated microgravity on separated human peripheral blood mononuclear cells (PBMCs). We detected the populations of different cells by antibodies conjugated to fluorofors using a Flow Cytometer. Since space flight reduces the number of peripheral blood lymphocytes (Stowe et al., 1999) we supposed that apoptotic (programmed cell death) processes might be involved. This hypothesis was supported by the result of our earlier experiment demonstrating that simulated microgravity increased the level of secreted Tumor Necrosis Factor-alpha (TNFalpha, a known apoptotic signal molecule) significantly (Batkai et al. 1999).

Apoptosis↗
PubMed1994

Human cardiovascular response to sympathomimetic agents during head-down bed rest: the effect of dietary sodium.

Changes in sympathoadrenal function and cardiovascular deconditioning have long been recognized as a feature of the physiological adaptation to microgravity. The deconditioning process, coupled with altered hydration status, is thought to significantly contribute to orthostatic intolerance upon return to Earth gravity. The cardiovascular response to stimulation by sympathomimetic agents before, during, and after exposure to simulated microgravity was determined in healthy volunteers equilibrated on normal or high sodium diets in order to further the understanding of the deconditioning process.

Adult↗
PubMed1991

The state of human bone tissue during space flight.

The results of studying the bone tissue of cosmonauts after the flights (4-8 month) have been compared to the data of investigating the healthy individuals during head-down tilt (HDT, 370 days). Noninvasive methods (computer tomography, gammaphoton absorptiometry) revealed a decrease in the vertebral spongy mineral density or a increase of this parameter by a similar magnitude versus the individual preflight values in some cosmonauts. During studies of clinical cases of osteoporosis it was shown that the vertebral mineral density ratios and presence or absence of vertebral compression fractures in different age groups are nonequal.

Absorptiometry, Photon↗