Days Of Spaceflight Research Articles

Variations in digestive physiology of rats after short duration flights aboard the US space shuttle.

The purpose of this work was to assess the influence of microgravity on several endogenous and microbial parameters of digestive physiology. On the occasion of two Spacelab Life Sciences missions, SLS-1 (a 9-day space flight) and SLS-2 (a 14-day space flight), Sprague-Dawley rats flown aboard the US space shuttle were compared to age-matched ground-based controls. In both flights, exposure to microgravity modified cecal fermentation: concentration and profile of short-chain fatty acids were altered, whereas urea and ammonia remained unchanged. Only in SLS-1 was there an induction of intestinal glutathione-S-transferase. Additional analyses in SLS-2 showed a decrease of hepatic CYP450 and of colonic goblet cells containing neutral mucin. After a postflight recovery period equal to the mission length, only modifications of the hepatic and intestinal xenobiotic metabolizing enzymes still persisted. These findings should help to predict the alterations of digestive physiology and detoxification potential likely to occur in astronauts. Their possible influence on health is discussed.

Changes in Ribbon Synapses and Rough Endoplasmic Reticulum of Rat Utricular Macular Hair Cells in Weightlessness

This study combined ultrastructural and statistical methods to learn the effects of weightlessness on rat utricular maculae. A principle aim was to determine whether weightlessness chiefly affects ribbon synapses of type II cells, since the cells communicate predominantly with branches of primary vestibular afferent endings. Maculae were microdissected from flight and ground control rat inner ears collected on day 13 of a 14-day spaceflight (F13), landing day (R0) and day 14 postflight (R14) and were prepared for ultrastructural study. Ribbon synapses were counted in hair cells examined in a Zeiss 902 transmission electron microscope. Significance of synaptic mean differences was determined for all hair cells contained within 100 section series, and for a subset of complete hair cells, using SuperANOVA? software. The synaptic mean for all type II hair cells of F13 flight rats increased by 100% and that for complete cells by 200%. Type I cells were less affected, with synaptic mean differences statistically insignificant in complete cells. Synapse deletion began within 8 h upon return to Earth. Additionally, hair cell laminated rough endoplasmic reticulum of flight rats was reversibly disorganized on R0. Results support the thesis that synapses in type II hair cells are uniquely affected by altered gravity. Type II hair cells may be chiefly sensors of gravitational and type I cells of translational linear accelerations.

Yaw and pitch visual-vestibular interaction in weightlessness

Both yaw and pitch visual-vestibular interactions at two separate frequencies of chair rotation (0.2 and 0.8 Hz) in combination with a single velocity of optokinetic stimulus (36 degrees/s) were used to investigate the effects of sustained weightlessness on neural strategies adopted by astronaut subjects to cope with the stimulus rearrangement of spaceflight. Pitch and yaw oscillation in darkness at 0.2 and 0.8 Hz without optokinetic stimulation, and constant velocity linear optokinetic stimulation at 18, 36, and 54 degrees/s presented relative to the head with the subject stationary, were used as controls for the visual-vestibular interactions. The results following 8 days of space flight showed no significant changes in: (1) either the horizontal and vertical vestibulo-ocular reflex (VOR) gain, phase, or bias; (2) the yaw visual-vestibular response (VVR); or (3) the horizontal or vertical optokinetic (OKN) slow phase velocity (SPV). However, significant changes were observed: (1) when during pitch VVR at 0.2 Hz late inflight, the contribution of the optokinetic input to the combined oculomotor response was smaller than during the stationary OKN SPV measurements, followed by an increased contribution during the immediate postflight testing; and (2) when during pitch VVR at 0.8 Hz, the component of the combined oculomotor response due to the underlying vertical VOR was more efficiently suppressed early inflight and less suppressed immediately postflight compared with preflight observations. The larger OKN response during pitch VVR at 0.2 Hz and the better suppression of VOR during pitch VVR at 0.8 Hz postflight are presumably due to the increased role of vision early inflight and immediately after spaceflight, as previously observed in various studies. These results suggest that the subjects adopted a neural strategy to structure their spatial orientation in weightlessness by reweighting visual, otolith, and perhaps tactile/somatic signals.

THE EFFECT OF SLS-2 SPACEFLIGHT ON GLUCOSE TRANSPORTERS 1 AND 4 IN RAT GASTROCNEMIUS MUSCLE.

1635 It has been previously shown that insulin-stimulated glucose transport is increased in rat soleus after 5.4 days of spaceflight and earth-based unweighting (Tischler et al., 1993, J. Appl. Physiol., 74: 2161-2165 ). We examined the effect of 14 days of spaceflight and 14 days of recovery on glucose transporters (Glut) 1 and 4 in rat gastrocnemius muscle that were flown on Spacelab Life Sciences − 2. As well, a group of rats that were sacrificed in space on day 13 were also included in the analyses. The number of rats in each group was 5. Glut 1 and 4 content was determined using Western blotting and image densitometry. There were no significant differences between any of the spaceflight groups and their respective simulated cage, ground based controls: 13 day in-flight Glut 1 = 1.14 vs. control Glut 1 = 1.49 OD/mm2; 13 day in-flight Glut 4 = 1.10 vs. control Glut 4 = 1.44 OD/mm2; 14 day post-flight Glut 1 = 1.48 vs. control Glut 1 = 1.50 OD/mm2; 14 day post-flight Glut 4 = 0.99 vs. control Glut 4 = 1.03 OD/mm2; 14 day recovery post-flight Glut 1 = 1.76 vs. control Glut 1 = 2.00 OD/mm2; and, 14 day recovery post-flight Glut 4 = 0.87 vs. control Glut 4 = 0.77 OD/mm2. These findings indicate that this duration of spaceflight does not significantly alter the content of glucose transporters in rat gastrocnemius muscle. Supported by a tissue-sharing grant with the C.S.A. and N.A.S.A.

Counteraction of spaceflight-induced changes in the rat central serotonergic system by adrenalectomy and corticosteroid replacement

The effects of a 17-day spaceflight duration on serotonergic measures in various parts of rat brain have been studied (flight-SHAM group). The contribution of the activation of the hypothalamo-pituitary-adrenal axis (HPA) related to the response of the central serotonin system was evaluated in adrenalectomized with chronic corticosterone replacement rats (flight-ADX+CORT group). These two groups of rats were compared to their respective ground-based controls. Physiological parameters (body, adrenal and thymus weights) and corticosterone levels were measured. In flight-SHAM group as compared to controls, adrenal hypertrophy and elevation in plasma corticosterone levels (174%) were observed, without change in thymus mass. In most brain areas studied, significant decreases in TRP, 5-HTP and 5-HIAA were found associated with lower levels of 5-HT in cortex, thalamus and striatum. Conversely, there were elevations in TRP, 5-HTP levels in striatum and increases in 5-HIAA\\5-HT ratios, an index of 5-HT turnover, in cortex, striatum and olfactory bulb while the hypothalamus was the sole region where a fall was observed. In ADX rats with chronic corticosterone replacement these effects were not observed in the majority of brain areas. It is concluded that a 17-day spaceflight exerted an inhibitory effect on serotonin metabolism, probably by activation of the HPA axis. The results could not distinguish between the effects of microgravity and the stress associated with landing.

Alteration of renal function of rats following spaceflight.

Following spaceflight, changes in renal function of humans have been suggested. To assess the effects of readaptation on renal function, urine was collected from male rats ( approximately 245 g) over a 2-wk period following a 14-day spaceflight. Rats were assigned to three groups: flight animals (n = 6), flight controls (n = 6) housed in the flight cages on the ground, and vivarium controls (n = 5) housed in standard shoe box cages. Animals were placed into individual metabolic cages for urine collection. Urine output was significantly increased for 3 days following flight. Excretion rates of Na+ and K+ were increased, resulting in an increased osmotic excretion rate. Creatinine excretion rate increased over the first two postflight days. Glomerular filtration rate increased immediately following spaceflight without changes in plasma creatinine, Na+, K+, or osmolality. Increased excretion of solute was thus the result of increased delivery and a decreased percent reabsorption of the filtered load. Osmolal clearance was increased immediately postflight while free water clearance was decreased. In growing rats, the diuresis after short-duration spaceflight is the result of an increase in solute excretion with an accompanying reduction in free water clearance.

Effects of microgravity and bone morphogenetic protein II on GFAP in rat brain.

This study evaluated effects of bone morphogenetic protein II (BMP) on glial fibrillary acidic protein (GFAP) in the brain of female Fischer 344 rats during 14 days of spaceflight. GFAP mRNA decreased in vehicle-implanted rats flown on the space shuttle by 53 and 48% in the stratum moleculare and stratum lacunosum moleculare hippocampal subregions, respectively. GFAP mRNA was not significantly affected by BMP implantation during spaceflight. Rats returning from space exhibited a 56% increase in serum corticosterone. BMP treatment did not additively increase corticosterone elevations in microgravity but appeared to increase serum corticosterone and reduce GFAP mRNA in the stratum moleculare in control rats. These data suggest that exposure to microgravity reduces GFAP expression in hippocampal astrocytes.

Spaceflight has compartment- and gene-specific effects on mRNA levels for bone matrix proteins in rat femur.

In the present study, we evaluated the possibility that the abnormal bone matrix produced during spaceflight may be associated with reduced expression of bone matrix protein genes. To test this possibility, we investigated the effects of a 14-day spaceflight (SLS-2 experiment) on steady-state mRNA levels for glyceraldehyde-3-phosphate dehydrogenase (GAPDH), osteocalcin, osteonectin, and prepro-alpha(1) subunit of type I collagen in the major bone compartments of rat femur. There were pronounced site-specific differences in the steady-state levels of expression of the mRNAs for the three bone matrix proteins and GAPDH in normal weight-bearing rats, and these relationships were altered after spaceflight. Specifically, spaceflight resulted in decreases in mRNA levels for GAPDH (decreased in proximal metaphysis), osteocalcin (decreased in proximal metaphysis), osteonectin (decreased in proximal and distal metaphysis), and collagen (decreased in proximal and distal metaphysis) compared with ground controls. There were no changes in mRNA levels for matrix proteins or GAPDH in the shaft and distal epiphysis. These results demonstrate that spaceflight leads to site- and gene-specific decreases in mRNA levels for bone matrix proteins. These findings are consistent with the hypothesis that spaceflight-induced decreases in bone formation are caused by concomitant decreases in expression of genes for bone matrix proteins.

ESTIMATION OF ELBOW JOINT POSITION BEFORE, DURING, AND FOLLOWING A 17-DAY SPACEFLIGHT.

1920 The ability to estimate elbow joint position was tested in four male astronauts before (-30 and -12 days), during (1, 4, 9, and 15 days), and after(+1, +5, +9, and +16 days) a 17-day spaceflight (STS-78). Subjects were asked to estimate 90 ° or 135 ° joint angles during passive (relaxed) and active (10% maximal agonist effort) isokinetic extension (EXT) and flexion(FLX). Subjects also estimated these joint positions during low resistance isotonic EXT. For each angle, 6 (isokinetic) and 3 (isotonic) trials were requested in random order. Across all test conditions, the mean angle estimate ranges were 93 °-118 ° and 119 °-152 ° for requested angles of 90 ° and 135 °, respectively. Subjects overestimated the 90 ° position across all conditions, whereas the accuracy of the 135 ° estimate varied. Angle estimates were most accurate during isokinetic FLX for 90 °(mean error +9.8 °) and isokinetic EXT for 135 ° (mean error +0.7°). No differences occurred between passive vs. active isokinetic EXT or FLX. Estimates of each joint angle during isokinetic EXT were less than isotonic EXT, but greater than isokinetic FLX (P<0.05). During spaceflight, joint angle estimates were lower during isokinetic FLX and EXT for 90 °(P<0.05) and 135 ° (P≤0.10) compared to preflight. Spaceflight also decreased angle estimates during isotonic FLX and EXT (P<0.05). Elbow angle estimates had returned to preflight values one day after return to 1G. These data demonstrate that elbow joint angle estimation was: 1) decreased during spaceflight, particularly for isotonic conditions 2) rapidly restored to preflight values upon return to 1G; and 3) not changed by low level agonist muscle exertion during isokinetic movements. These data indicate that the proprioception of elbow joint position is altered by microgravity, but rapidly readapts to preflight control values after return to 1G.

Effects of space flight on GLUT-4 content in rat plantaris muscle.

The effects of 14 days of space flight on the glucose transporter protein (GLUT-4) were studied in the plantaris muscle of growing 9-week-old, male Sprague Dawley rats. The rats were randomly separated into five groups: pre-flight vivarium ground controls (PF-VC) sacrificed approximately 2 h after launch; flight groups sacrificed either approximately 5 h (F-R0) or 9 days (F-R9) after the return from space; and synchronous ground controls (SC-R0 and SC-R9) sacrificed at the same time as the respective flight groups. The flight groups F-R0 and F-R9 were exposed to micro-gravity for 14 days in the Spacelab module located in the cargo bay of the shuttle transport system--58 of the manned Space Shuttle for the NASA mission named "Spacelab Life Sciences 2". Body weight and plantaris weight of SC-R0 and F-R0 were significantly higher than those of PF-VC. Neither body weight nor plantaris muscle weight in either group had changed 9 days after the return from space. As a result, body weight and plantaris muscle weight did not differ between the flight and synchronous control groups at any of the time points investigated. The GLUT-4-content (cpm/microgram membrane protein) in the plantaris muscle did not show any significant change in response to 14 days of space flight or 9 days after return. Similarly, citrate synthase activity did not change during the course of the space flight or the recovery period. These results suggest that 14 days of space flight does not affect muscle mass or GLUT-4 content of the fast-twitch plantaris muscle in the rat.

Space-related bone mineral redistribution and lack of bone mass recovery after reambulation in young rats.

This study reports the effects of a 14-day spaceflight followed by a 14-day reambulation period on bones of 56-day-old male rats compared with synchronous (S) and vivarium (V) control animals. Femur, tibia, and humerus bone mineral densities (BMD); bone calcium and phosphorus concentrations ([Ca2+] and [P]), measured by X-ray microanalysis (XRM), on tibia, vertebra, and calvaria; and histomorphometric data on proximal primary and secondary spongiosae (I and II SP, respectively) of the tibia and humerus were measured. After the flight in flown rats (compared with S), BMD was lower in the distal femur and remained similar to S in humerus and tibia, [Ca2+] and [P] were lower in tibia II SP and higher in calvaria, tibia I SP width and II SP bone volume were lower, resorption was markedly higher in tibia II SP, and no difference in formation parameters was observed. After reambulation, BMD was lower in long bones of both flight and S groups compared with V. Bone loss appeared in humeral II SP and worsened in tibial II SP in flown rats. Tibial formation parameters were higher in flown rats compared with V and S, indicating the onset of an active recovery. Tibial XRM [Ca2+] and [P] in flown rats remained below control levels.

Regulation of autonomic nervous system in space and magnetic storms

Variations in the earth's magnetic field and magnetic storms are known to be a risk factor for the development of cardiovascular disorders. The main “targets” for geomagnetic perturbations are the central nervous system and the neural regulation of vascular tone and heart rate variability. This paper presents the data about effect of geomagnetic fluctuations on human body in space. As a method for research the analysis of heart rate variability was used, which allows evaluating the state of the sympathetic and parasympathetic parts of the autonomic nervous system, vasomotor center and subcortical neural centers activity. Heart rate variability data were analyzed for 30 cosmonauts at the 2-nd day of space flight on transport spaceship Soyuz (32nd orbit). There were formed three groups of cosmonauts: without magnetic storm (n=9), on a day with magnetic storm (n=12) and 1–2 days after magnetic storm (n=9). The present study was the first to demonstrate a specific impact of geomagnetic perturbations on the system of autonomic circulatory control in cosmonauts during space flight. The increasing of highest nervous centers activity was shown for group with magnetic storms, which was more significant on 1–2 days after magnetic storm. The use of discriminate analysis allowed to classify indicated three groups with 88 % precision. Canonical variables are suggested to be used as criterions for evaluation of specific and non-specific components of cardiovascular reactions to geomagnetic perturbations. The applied aspect of the findings from the present study should be emphasized. They show, in particular, the need to supplement the medical monitoring of cosmonauts with predictions of probable geomagnetic perturbations in view of the prevention of unfavorable states appearances if the adverse reactions to geomagnetic perturbations are added to the tension experienced by regulatory systems during various stresses situations (such as work in the open space).

Effects of microgravity on muscle and cerebral cortex: A suggested interaction

The “slow” antigravity muscle adductor longus was studied in rats after 14 days of spaceflight (SF). The techniques employed included standard methods for light microscopy, neural cell adhesion molecule (N-CAM) immunocytochemistry and electron microscopy. Light and electron microscopy revealed myofiber atrophy, segmental necrosis and regenerative myofibers. Regenerative myofibers were N-CAM immunoreactive (N-CAM-IR). The neuromuscular junctions showed axon terminals with a decrease or absence of synaptic vesicles, degenerative changes, vacant axonal spaces and changes suggestive of axonal sprouting. No alterations of muscle spindles was seen either by light or electron microscopy. These observations suggest that muscle regeneration and denervation and synaptic remodeling at the level of the neuromuscular junction may take place during spaceflight. In a separate study, GABA immunoreactivity (GABA-IR) was evaluated at the level of the hindlimb representation of the rat somatosensory cortex after 14 days of hindlimb unloading by tail suspension (“simulated” microgravity). A reduction in number of GABA-immunoreactive cells with respect to the control animals was observed in layer Va and Vb. GABA-IR terminals were also reduced in the same layers, particularly those terminals surrounding the soma and apical dendrites of pyramidal cells in layer Vb. On the basis of previous morphological and behavioral studies of the neuromuscular system after spaceflight and hindlimb suspension it is suggested that after limb unloading there are alterations of afferent signaling and feedback information from intramuscular receptors to the cerebral cortex due to modifications in the reflex organization of hindlimb muscle groups. We propose that the changes observed in GABA immunoreactivity of cells and terminals is an expression of changes in their modulatory activity to compensate for the alterations in the afferent information.

Effect of weightlessness on the ultrastructure of rat striated muscles

The ultrastructure of rat striated muscles fixed on the 13th day of space flight was studied. It was found that adaptive atrophic and degenerative changes occur in parallel with atypical cellular and intracellular regeneration. The muscles are not completely restored after a 14-day readaptation on the Earth.

Effects of 14 days of spaceflight and nine days of recovery on cell body size and succinate dehydrogenase activity of rat dorsal root ganglion neurons

The cross-sectional areas and succinate dehydrogenase activities of L 5 dorsal root ganglion neurons in rats were determined after 14 days of spaceflight and after nine days of recovery. The mean and distribution of the cross-sectional areas were similar to age-matched, ground-based controls for both the spaceflight and for the spaceflight plus recovery groups. The mean succinate dehydrogenase activity was significantly lower in spaceflight compared to aged-matched control rats, whereas the mean succinate dehydrogenase activity was similar in age-matched control and spaceflight plus recovery rats. The mean succinate dehydrogenase activity of neurons with cross-sectional areas between 1,000 and 2,000 μm 2 was lower (between 7 and 10%) in both the spaceflight and the spaceflight plus recovery groups compared to the appropriate control groups. The reduction in the oxidative capacity of a subpopulation of sensory neurons having relatively large cross-sectional areas immediately following spaceflight and the sustained depression for nine days after returning to 1 g suggest that the 0 g environment induced significant alterations in proprioceptive function.

FORCE-VELOCITY AND FORCE-POWER PROPERTIES OF HUMAN MUSCLE FIBERS AFTER SPACEFLIGHT 1081

The purpose of this study was to investigate the effects of 17 days of spaceflight (SF) on the force-velocity-power properties of human muscle fibers. Pre- and post-SF soleus (SOL) and gastrocnemius (GAST) muscle biopsies were obtained from 4 male crew members of the 17 day NASA Life Microgravity Sciences Spacelab mission (STS-78). 384 single fiber segments were subjected to a series of isotonic force clamps at maximal Ca2+ -activation and subsequently run on 5% SDS polyacrylamide gels to determine myosin heavy chain(MHC) composition. After SF, shortening velocity at peak power output increased for the SOL type I (+29%) and GAST type 1 (+30%) fibers and was unchanged for the GAST-IIa fibers. In contrast, the force produced at peak power declined for SOL-I (-28%), GAST-I (-11%), and GAST-IIa fibers (-17%) due to decreases in peak force (Po) and a/Po for the SOL-I fibers, a/Po only for the GAST-I fibers, and Po only for the GAST-IIa fibers. Absolute peak power was affected by these changes as indicated intable (FL, fiber length; * P < 0.05, † P = 0.06). The increased peak power of the post-SF GAST-I fibers may be due to their preferential recruitment during the exercise countermeasures used on this flight.

The effects of orbital spaceflight on bone histomorphometry and messenger ribonucleic acid levels for bone matrix proteins and skeletal signaling peptides in ovariectomized growing rats.

A 14-day orbital spaceflight was performed using ovariectomized Fisher 344 rats to determine the combined effects of estrogen deficiency and near weightlessness on tibia radial bone growth and cancellous bone turnover. Twelve ovariectomized rats with established cancellous osteopenia were flown aboard the space shuttle Columbia (STS-62). Thirty ovariectomized rats were housed on earth as ground controls: 12 in animal enclosure modules, 12 in vivarium cages, and 6 killed the day of launch for baseline measurements. An additional 18 ovary-intact rats were housed in vivarium cages as ground controls: 8 rats were killed as baseline controls and the remaining 10 rats were killed 14 days later. Ovariectomy increased periosteal bone formation at the tibia-fibula synostosis; cancellous bone resorption and formation in the secondary spongiosa of the proximal tibial metaphysis; and messenger RNA (mRNA) levels for the prepro-alpha2(1) subunit of type 1 collagen, osteocalcin, transforming growth factor-beta, and insulin-like growth factor I in the contralateral proximal tibial metaphysis and for the collagen subunit in periosteum pooled from tibiae and femora and decreased cancellous bone area. Compared to ovariectomized weight-bearing rats, the flight group experienced decreases in periosteal bone formation, collagen subunit mRNA levels, and cancellous bone area. The flight rats had a small decrease in the cancellous mineral apposition rate, but no change in the calculated bone formation rate. Also, spaceflight had no effect on cancellous osteoblast and osteoclast perimeters or on mRNA levels for bone matrix proteins and signaling peptides. On the other hand, spaceflight resulted in an increase in bone resorption, as ascertained from the diminished retention of a preflight fluorochrome label. This latter finding suggests that osteoclast activity was increased. In a follow-up ground-based experiment, unilateral sciatic neurotomy of ovariectomized rats resulted in cancellous bone loss in the unloaded limb in excess of that induced by gonadal hormone deficiency. This additional bone loss was arrested by estrogen replacement. We conclude from these studies that estrogen alters the expression of signaling peptides believed to mediate skeletal adaptation to changes in mechanical usage and likewise modifies the skeletal response to mechanical unloading.

Myonuclear loss in atrophied soleus muscle fibers

A skeletal muscle fiber consists of many successive "territories," each controlled by the nucleus residing in that territory. Because nuclei appear to control a specific amount of territory (nuclear domain), nuclei must be added to accommodate an increase in fiber size. Because growth and hypertrophy require the addition of nuclei to fibers, it is of interest to determine whether atrophy causes a decrease in myonuclear number. This study compared the myonuclear population in the soleus muscles of rats that had undergone atrophy due to 10 days of spaceflight in the space shuttle, Endeavour, with muscles of ground-based control animals (10 rats each). Myofibrillar ATPase activity was used to determine the major skeletal muscle fiber types in control rats and those having spent 10 days in space, and dystrophin antibodies were used to label the sarcolemma to identify underlying myonuclei. Type I and II fibers were atrophied after the flight, but type I fibers were atrophied twice as much as type II. Myonuclei were counted in identified and measured fibers, and the distribution normalized to number per millimeter of fiber circumference; this was significantly greater in type II than in type I fibers in both groups of rats. However, although the muscle fibers from flight animals were significantly atrophied, the normalized number of nuclei were identical between control and flight animals, indicating that nuclei decreased in numbers as the fibers atrophied. The nuclear domain is under strict control, and a decrease in the domain, as induced by atrophy, will cause nuclear degeneration and loss, which maintains a relatively constant size of the nuclear domain.

Trace element composition and histological analysis of rat bones from the space shuttle.

The physiological and pharmacobiological changes associated with space flight are of greater concern. Exposure to a weightless environment has been shown to have numerous effects on body composition and organ functions. Alterations include decreases in muscle and liver mass, changes in bone structure and integrity, and changes in cardiovascular functions. Zero-gravity in particular has been reported to inhibit several physiological processes of bone formation, retard bone growth and impair the mechanical properties of bones. This report examines the effect of 14 days of spaceflight on the bone trace element compositions of rapidly growing rats. Marked changes of bone trace element contents were found in either weight-bearing bones or non-weight-bearing bones, depending on the metal species. Histological examination revealed an irregular thickening of the endosteal surface of the cortical bone (thoracic vertebrae) of the in-flight rat, whereas it was uniform in the ground control. We suggest that the microgravity environment causes several bone alternations, such as abnormal trace element compositions and defects in vertebral maturation.

Distribution of myosin heavy chain isoforms in non-weight-bearing rat soleus muscle fibers.

The effects of 14 days of spaceflight (SF) or hindlimb suspension (HS) (Cosmos 2044) on myosin heavy chain (MHC) isoform content of the rat soleus muscle and single muscle fibers were determined. On the basis of electrophoretic analyses, there was a de novo synthesis of type IIx MHC but no change in either type I or IIa MHC isoform proportions after either SF or HS compared with controls. The percentage of fibers containing only type I MHC decreased by 26 and 23%, and the percentage of fibers with multiple MHCs increased from 6% in controls to 32% in HS and 34% in SF rats. Type IIx MHC was always found in combination with another MHC or combination of MHCs; i.e., no fibers contained type IIx MHC exclusively. These data suggest that the expression of the normal complement of MHC isoforms in the adult rat soleus muscle is dependent, in part, on normal weight bearing and that the absence of weight bearing induces a shift toward type IIx MHC protein expression in the preexisting type I and IIa fibers of the soleus.