[The pharmacological correction of hemodynamics during the simulation of the early period of adaptation to weightlessness]

Investigations of volunteered subjects in whom antiorthostatic hypokinesia was modeled by tilting the cranial end of the body by -15 degrees were to answer whether pharmacological countermeasures of the vestibular/autonomous syndrome (motion sickness, sea sickness) alter the orthostatic tolerance and cardiovascular parameters. As was stated, the drugs allow controlled correction of hemodynamic resistance to the head-down tilt. Ephedrine, phencarol, ephedrine combined with scopolamine and pipolphen, and stugeron normalized cerebral circulation. Medicaments were unable to moderate the sympathoadrenal reaction to the head-down tilt; however, blood concentrations of catecholamines were significantly lower than in the control. The greatest effect was achieved using ephedrine and ephedrine plus pipolphen.

Arterial pressure in humans during weightlessness induced by parabolic flights

Results from our laboratory have indicated that, compared with those of the 1-G supine (Sup) position, left atrial diameter (LAD) and transmural central venous pressure increase in humans during weightlessness (0 G) induced by parabolic flights (R. Videbaek and P. Norsk. J. Appl. Physiol. 83: 1862-1866, 1997). Therefore, because cardiopulmonary low-pressure receptors are stimulated during 0 G, the hypothesis was tested that mean arterial pressure (MAP) in humans decreases during 0 G to values below those of the 1-G Sup condition. When the subjects were Sup, 0 G induced a decrease in MAP from 93 +/- 4 to 88 +/- 4 mmHg (P < 0.001), and LAD increased from 30 +/- 1 to 33 +/- 1 mm (P < 0.001). In the seated position, MAP also decreased from 93 +/- 6 to 87 +/- 5 mmHg (P < 0.01) and LAD increased from 28 +/- 1 to 32 +/- 1 mm (P < 0.001). During 1-G conditions with subjects in the horizontal left lateral position, LAD increased compared with that of Sup (P < 0.001) with no further effects of 0 G. In conclusion, MAP decreases during short-term weightlessness to below that of 1-G Sup simultaneously with an increase in LAD. Therefore, distension of the heart and associated central vessels during 0 G might induce the hypotensive effects through peripheral vasodilatation. Furthermore, the left lateral position in humans could constitute a simulation model of weightlessness.

[Changes of blood circulation, muscle and skeletal systems in 30 d tail-suspended [correction of tail-suspented] rats]

OBJECTIVE

To confirm the tail-suspended rat model for the study of countermeasures against weightlessness.

METHOD

The changes of blood circulation, muscle and skeletal systems in control group rats (n = 15) and 30 d tail-suspended rats (n = 15) were compared.

RESULT

Compared with the control group, the hemorrheology and erythrocyte deformability decreased significantly, muscle-fiber atrophy, muscle contraction function decreased, the type I muscle fibers transformed to the type II muscle fibers, bone-mineral content of L3 and load capacity of femur declined, bone metabolism changed in the suspended rats.

CONCLUSION

The tail-suspended rat is a suitable animal model for the study of countermeasures against weightlessness.

Can centrifugation at increased G be used to predict results of 0G?

Can physiologic research on the effects of weightlessness be conducted using a centrifuge providing an increased G environment? The answer to this question, if affirmative can have extraordinary implications regarding physiologic research in space that now begs the answer to the more general and basic question, what is the 'best environment' to differentiate the biological effects of mass from weight? In considering the best environment, there are several important factors in support of conducting human and animal physiologic 'gravitational' research on centrifuges here on Earth. Increased G research using a centrifuge can be conducted for a small fraction of the cost of research in space. It can be made available to more scientists as the number of centrifuges for this type of research increases. There are far less-detailed and therefore much easier preparation requirements for animal and human studies on a centrifuge compared with space research.

The venous tone is not altered after three-week tail suspension in rats

Cardiovascular deconditioning observed in humans during spaceflight has been suggested to be related in part to changes in venous compliance, mechanisms including skeletal muscle deconditioning. However, increased venous compliance was observed during very short term simulations (24 to 48 hours), and during an over 28-day simulation the hyperdistensibility tended to decrease whereas the muscular changes were still present (2). In the first case, muscular changes can not explain the venous alterations because of the short delay. In the second case, the relationship between muscular and venous alterations disappeared. Finally, it is suggested that factors other than muscular ones could explain the changes in venous compliance observed during spaceflights. The fact that orthostatic hypotension has never been observed after hindlimb suspension in the rat raises issue with the use of tail-suspended rats as a valid model for the study of the mechanisms involved in cardiovascular deconditioning induced by spaceflight in humans. However, in vitro altered responsiveness of the vena cava to norepinephrine were observed in rat after spaceflight and tail suspension. The purpose of the experiments was to verify if any change occurs in venous tone measured in vivo in rats after three-week tail suspension.

Insight into mechanisms of reduced orthostatic performance after exposure to microgravity: comparison of ground-based and space flight data

Since the beginning of human spaceflight, the value of understanding mechanisms of physiological adaptation to microgravity became apparent to life scientists who were interested in maintining crew health and developing countermeasures agains adverse effects of the mission. However, several characteristics associated the the logistics of spaceflight presented significant limitations to the scientific study of human adaptation to microgravity. Because space missions are so infrequent and involve minimal numbers of crewmembers, meaninful statistical analysis of data are limited. Reproducibility of results from spaceflight experiments is difficult to assess since there are few repeated space missions involving the same crewmembers. Since the emphasis of space missions is placed on operations, experiments are compromised without adequate control over various factors (e.g., time, diet, physical activities, etc.) that can impact measured responses. With the mimimal opportunity to collect spaceflight data, there is a high risk of experiments that simultaneously interfere with other experiments by the increasing demand on the crewmembers to participate in mumerous experiments proposed by multiple investigators. The technology and ability to measure physiological functions necessary to test specific hypotheses can be severely limited by physical space and power constraints of the space enviroment. Finally, technical and logistical aspects of space missions such as launch delays, extended missions, and inflight operational emergencies can significantly compromise the timing and control of experiments. These limitations have stimulated scientists to develop ground-based analogs of microgravity in an effort to investigate the effects of spaceflight on physiological function in a controlled experimental setting. The purpose of this paper is to provide a selected comparison of data collected from ground-based experiments with those obtained from spaceflight in an effort to assess the adequacy of ground analogs of actual flight for the study of human physiological adaptation to microgravity. Specifically, results from ground and spaceflight will be used to provide insight into mechanisms underlying adaptations of blood pressure regulation and reduced orthostatic performance to the microgravity environment.

Sympathetic nerve responses in humans to short and long term simulation of microgravity

The present paper aimed to review findings obtained by our researches to elucidate sympathetic nerve mechanisms of cardiovascular deconditioning in humans exposed to short and long term simulation of microgravity. Sympathetic nerve activity in humans has been so far investigated using indirect methods by analyzing the activities of effector organs, such as heart rate, blood flow, blood pressure, sweating etc. or by measuring the plasma nor-adrenaline level. Now we have a technique called microneurography which has enabled us to measure directly the sympathetic nerve activity form human peripheral nerves. The microneurography technique was used for the first time before, during and after the Space Shuttle "Neurolab" mission launched in April 1998 to elucidate how sympathetic nerve activity in astronauts is modified by exposure to microgravity in space. In this paper, we would like to present our recent findings concerning sympathetic nerve responses to short and long term microgravity simulated by different methods.

Microgravity simulations with human lymphocytes in the free fall machine and in the random positioning machine

The purpose of this paper is to present the results obtained in our laboratory with both instruments, the FFM [free fall machine] and the RPM [random positioning machine], to compare them with the data from earlier experiments with human lymphocytes conducted in the FRC [fast rotating clinostat] and in space. Furthermore, the suitability of the FFM and RPM for research in gravitational cell biology is discussed.

Height increase, neuromuscular function, and back pain during 6 degrees head-down tilt with traction

BACKGROUND

Spinal lengthening and back pain are commonly experienced by astronauts exposed to microgravity.

METHODS

To develop a ground-based simulation for spinal adaptation to microgravity, we investigated height increase, neuromuscular function and back pain in 6 subjects all of whom underwent two forms of bed rest for 3 d. One form consisted of 6 degrees of head-down tilt (HDT) with balanced traction, while the other was horizontal bed rest (HBR). Subjects had a 2-week recovery period in between the studies.

RESULTS

Total body and spinal length increased significantly more and the subjects had significantly more back pain during HDT with balanced traction compared to HBR. The distance between the lower endplate of L4 and upper endplate of S1, as measured by ultrasonography, increased significantly in both treatments to the same degree. Intramuscular pressures in the erector spinae muscles and ankle torque measurements during plantarflexion and dorsiflexion did not change significantly during either treatment.

CONCLUSION

Compared to HBR, HDT with balanced traction may be a better method to simulate changes of total body and spinal lengths, as well as back pain seen in microgravity.

Physiological properties of rat hind limb muscles after 15 days of simulated weightless environment

Weightlessness during space mission results in atrophic changes in those muscles which have maximum weight bearing function and consist primarily of slow twitch fibres. In the present study an animal model was designed to evaluate the effects of 15 days of hindlimb unloading (HU) in rats by tail suspension on the (i) weight of gastrocnemius (G), plantaris (P), both predominantly having fast twitch fibres and soleus (S) muscle, predominantly having fast twitch fibres and (ii) contractile properties viz peak twitch contraction (Pt) and peak tetanic contraction (Po) of GPS muscle. HU rats showed significant weight reductions of G (-17.9%), P (-13.3%) and S (-41.2%) muscles. Pt and Po were also reduced in HU group but when these were expressed per gm of GPS muscle, no significant changes in Pt and Po were observed. These findings confirm that HU in rats result in maximum atrophic change in those muscles which have predominantly slow twitch fibres and reductions in contractile properties of muscles are in proportion to reduction in muscle weight. Also, HU by tail suspension provides a good ground based model for developing the deconditioning of muscles as applicable to weightlessness of space and offers a scope for the development of various countermeasures.

Evaluation of the three-dimensional clinostat as a simulator of weightlessness

Concerns regarding the reliability of slow-and fast-rotating uni-axial clinostats in simulating weightlessness have induced the construction of devices considered to simulate weightlessness more adequately. A new three-dimensional (3-D) clinostat equipped with two rotation axes placed at right angles has been constructed. In the clinostat, the rotation achieved with two motors is computer-controlled and monitored with encoders attached to the motors. By rotating plants three-dimensionally at random rates on the clinostat, their dynamic stimulation by gravity in every direction can be eliminated. Some of the vegetative growth phases of plants dependent on the gravity vector, such as morphogenesis, are shown to be influenced by rotation on the 3-D clinostat. The validity of 3-D clinostatting has been evaluated by comparing structural parameters of cress roots and Chara rhizoids obtained under real microgravity with those obtained after 3-D clinostatting. The parameters analyzed up to now (organization of the root cap, integrity and polarity of statocytes, dislocation of statoliths, amount of starch and ER) demonstrate that the 3-D clinostat is a valuable device for simulating weightlessness.

Influence of longitudinal whole animal clinorotation on lens, tail, and limb regeneration in urodeles

Two species of newts (Urodela) and two types of clinostats for fast clinorotation (60 rpm) were used to investigate the influence of simulated weightlessness on regeneration and to compare results obtained with data from spaceflight experiments. Seven or fourteen days of weightlessness in Russian biosatellites caused acceleration of lens and limb regeneration by an increase in cell proliferation, differentiation, and rate of morphogenesis in comparison with ground controls. After a comparable time of clinorotation the results obtained with Triturus vulgaris using a horizontal clinostat were similar to those found in spaceflight. In contrast, in Pleurodeles waltl using both horizontal and radial clinostats the results were contradictory compared to Triturus. We speculate that different levels of gravity or/and species specific thresholds for gravitational sensitivity could be responsible for these contradictory results.

Clinorotation inhibits chondrogenesis in micromass cultures of embryonic mouse limb cells

Studies of the response of mammalian chondrocytes to gravitational changes in vivo, in organ culture, and in cell culture show that chondrogenesis is reduced in microgravity or by unloading, and increased by low levels of excess g. To investigate the cellular responses to microgravity using a ground based model, micromass cultures were exposed to simulated weightlessness on two clinostats. For rotation on the large clinostat, cultures were set up in Rose chambers, and cells were videotaped and photographed at several time periods after rotation began. For the smaller clinostat, cultures were set up in T-flasks, and two axes of rotation for clinostated cultures were used. Stationary controls [+1 g, -1 g (upside-down), and sideways] as well as rotation controls were employed. Rotation rate was 30 rpm for both clinostatted cultures and rotation controls. Chondrocyte differentiation was assessed by cartilage specific alcian blue staining. Significantly fewer alcian blue stained nodules were present in clinostatted cultures than in stationary controls or rotation controls. Nodules that did not stain with alcian blue, probably due to unsulfated matrix were present in all cultures. The number of nodules in sideways controls was greater than in any other culture (108% of +1 g controls), probably due to ongoing stimulus of the cell via cytoskeletal components. The results show that chondrocytes in culture respond to changes in the gravity vector in a predictable manner, and that carefully controlled clinostat studies can be useful adjuncts to and predictors for spaceflight experiments.

Does bed rest produce changes in orthostatic function comparable to those induced by space flight?

Use of bed rest to simulate microgravity exposure is not well validated. We compared heart rate (HR) and blood pressure (BP) responses to standing in bed-rest (BR) subjects (n=11) to those of two astronaut groups. One astronaut group (n=28) fluid loaded (FL) before landing by consuming a water and salt tablet mixture, the second astronaut group (n=8) did not (NL). Bed-rest or microgravity exposure lasted approximately 7.0 days. Preexposure, the responses to standing did not differ between groups. Postexposure, all groups demonstrated an increased HR response (p<0.01), a decreased SBP response (p<0.05), no change in DBP response, and a reduced PP response (p<0.05) compared to preexposure. Change in HR response was lowest for the FL group, presumably due to increased plasma volume induced by fluid consumption. These findings generally support bed rest as a valid simulator of microgravity.

Altered baroreflex control of forearm vascular resistance during simulated microgravity

Reflex peripheral vasoconstriction induced by activation of cardiopulmonary baroreceptors in response to reduced central venous pressure (CVP) is a basic mechanism for elevating systemic vascular resistance and defending arterial blood pressure during orthostatically-induced reductions in cardiac filling and output. The sensitivity of the cardiopulmonary baroreflex response [defined as the slope of the relationship between changes in forearm vascular resistance (FVR) and CVP] and the resultant vasoconstriction are closely and inversely associated with the amount of circulating blood volume. Thus, a high-gain FVR response will be elicited by a hypovolemic state. Exposure to microgravity during spaceflight results in reduced plasma volume. It is therefore reasonable to expect that the FVR response to cardiopulmonary baroreceptor unloading would be accentuated following adaptation to microgravity. Such data could provide better insight about the physiological mechanisms underlying alterations in blood pressure control following spaceflight. We therefore exposed eleven men to 6 degrees head-down bedrest for 7 days and measured specific hemodynamic responses to low levels of the lower body negative pressure to determine if there are alterations in cardiopulmonary baroreceptor stimulus-FVR reflex response relationship during prolonged exposure to an analog of microgravity.

The simulation of microgravity conditions on the ground

This chapter defines weightlessness as the condition where the acceleration of an object is independent of its mass. Applying this definition to the clinostat, it argues that the clinostat is very limited as a simulator of microgravity because it (a) generates centrifugal forces, (b) generates particle oscillations with mass-dependent amplitudes of speed and phase shifts relative to the clinorotation, (c) is unable to remove globally the scalar effects of gravity such as hydrostatic pressure, which are independent of the direction of gravity in the first place, and, (d) generates more convective mixing of the gaseous or liquid environment of the test object, rather than eliminating it, as would true weightlessness. It is proposed that attempts to simulate microgravity must accept the simulation of one aspect of microgravity at a time, and urges that the suppression of convective currents be a major feature of experimental methods that simulate microgravity.

Electrical vestibular stimulation and space motion sickness

Electrical vestibular stimulation (EVS) in dynamic balance condition was studied in order to search for a new provocative test of space motion sickness (SMS). SMS is usually attributed to a sensory conflict caused by exposure to microgravity. Vestibular information is conflicting but also unusual and insignificant. EVS is in accordance with this feature because it is not the adequate stimulus of the vestibular receptors. EVS was achieved by means of binaural electrical stimulation. Effects of EVS were potentiated by compelling the subject to maintain dynamic balance on a seesaw. The quantification of this function was performed before during and after EVS in order to investigate a possible relationship between objective consequences of EVS i.e. dynamic balance disturbances, and the discomfort experienced by the subjects. Dynamic balancing skill was statistically worsened during EVS. Moreover EVS evoked subjective symptoms of SMS in 17 out of the 30 subjects examined. During EVS in eyes open conditions, the subjects who encountered the strongest discomfort, presented the most disturbed dynamic balance, evidencing a relationship between the level of discomfort and the imbalance arising from EVS This method could thus constitute an interesting basis of SMS ground-based test.

Theoretical and experimental investigations on the fast rotating clinostat

We have investigated both theoretically and experimentally the validity of the fast rotating clinostat to simulate microgravity for a free swimming single-cell organism such as the paramecium. Computer simulations show that cells on suspension move as cells cultivated in space. However, rotated paramecia are still affected by gravity, as shown by the variations in the rate of paramecium rotation on their axis. Using a fast clinostat, which allows to investigate simultaneously twenty cultures, we have observed a stimulating effect on cell growth rate similar to that previously reported in space. All these results point towards the fact that the fast clinostat can reproduce some of the effects of microgravity on paramecia.

Volume loading of the heart by "leg up" position and head down tilting (-6 degrees) (HDT)

Head down tilting is widely used to increase preload and to induce intrathoracic blood pooling similar to microgravity. During daily routine, this venous pooling is performed by raising the legs up. In this study, both these approaches have been compared by invasive measurement using a right heart catheter. In patients with moderate coronary artery disease, diagnostic right heart catheterization was performed by the Swan-Genz-techniques. All measurements were performed with head down tilting (-6 degrees) and with "leg up" position. Patients then received Nitroglycerin to countermeasure the preload changes. Pressures in the pulmonary artery as well as in the wedge position increased significantly during leg up and HDT. However, changes were significantly more pronounced in the "leg up" position than during HDT. No changes were observed for arterial blood pressure, cardiac output, stroke volume and resistances. Nitroglycerin during HDT lowered blood pressure and pressures in the pulmonary artery and in PCW-position and reduced cardiac output significantly. Both approaches of volume loading of the heart induced significant changes and increases of preload. However, changes were more pronounced during the "leg up" position than during HDT. It is questioned whether HDT with -6 degrees is appropriate to truly reflect hemodynamic alterations during simulated weightlessness.

Hemodynamic effects of microgravity and their ground-based simulations

Hemodynamic effects of simulated microgravity were investigated, in various experiments, using radioactive isotopes, in which 40 healthy men, aged 35 to 42 years, took part. Blood shifts were evaluated qualitatively and quantitatively. Simulation studies included bedrest, head-down tilt (-5 degrees and -15 degrees), and vertical water immersion, it was found that none of the methods could entirely simulate hemodynamic effects of microgravity. Subjective sensations varied in a wide range. They cannot be used to identify reliably the effects of real and simulated microgravity. Renal fluid excretion in real and simulated microgravity was different in terms of volume and time. The experiments yielded data about the general pattern of circulation with blood displaced to the upper body.

Effects of suspension-induced osteopenia on the mechanical behaviour of mouse long bones

Whereas most studies of tail-suspension induced osteopenia have utilized rat femora, the present study investigated the effects of a 14 day tail-suspension on the mechanical behaviour of mice femora, tibiae and humeri. Force-deflection properties were obtained via three-point bending for long bones from suspended and control mice. Whole bone behaviour was characterized by converting the force-deflection values to stiffness, strength, ductility and energy parameters which were not normalized for specimen geometry. The effects of a systematic variation in the deflection rate over the range 0.1-10 mm min-1 were also evaluated. Statistical analysis indicated that the primary effect of the tail-suspension period was lowered bone mass which was manifested mechanically through lower values of the bone strength parameters. These effects were similar in the bones of both the fore and hind limbs. The results also demonstrated that the stiffness, ductility and energy characteristics were much less influenced by the tail-suspension. Whereas a significant dependence of the bone strength values upon deflection rate was observed for the femora and humeri, the other mechanical parameters were less sensitive. Based upon the nature of the physical and mechanical changes observed in the long bones following tail-suspension, the mouse appears to be a suitable animal model for the study of osteopenia.

Comparative effectiveness of a clinostat and a slow-turning lateral vessel at mimicking the ultrastructural effects of microgravity in plant cells

The object of this research was to determine how effectively the actions of a clinostat and a fluid-filled, slow-turning lateral vessel (STLV) mimic the ultrastructural effects of microgravity in plant cells. We accomplished this by qualitatively and quantitatively comparing the ultrastructures of cells grown on clinostats and in an STLV with those of cells grown at 1 g and in microgravity aboard the Space Shuttle Columbia. Columella cells of Brassica perviridis seedlings grown in microgravity and in an STLV have similar structures. Both contain significantly more lipid bodies, less starch, and fewer dictyosomes than columella cells of seedlings grown at 1 g. Cells of seedlings grown on clinostats have significantly different ultrastructures from those grown in microgravity or in an STLV, indicating that clinostats do not mimic microgravity at the ultrastructural level. The similar structures of columella cells of seedlings grown in an STLV and in microgravity suggest that an STLV effectively mimics microgravity at the ultrastructural level.

Does vector-free gravity simulate microgravity? Functional and morphologic attributes of clinorotated nerve and muscle grown in cell culture

Cocultured Xenopus neurons and myocytes were subjected to non-vectorial gravity by clinostat rotation to determine if microgravity, during space flights, may affect cell development and communications. Clinorotated cells showed changes consistent with the hypothesis that cell differentiation, in microgravity, is altered by interference with cytoskeleton-related mechanisms. We found: increases in the myocyte and its nuclear area, "fragmentation" of nucleoli, appearance of neuritic "aneurysms", decreased growth in the presence of "trophic" factors, and decreased yolk utilization. The effects were most notable at 1-10 rpm and depended on the onset and duration of rotation. Some parameters returned to near control values within 48 hrs after cessation of rotation. Cells from cultures rotated at higher speeds (>50 rpm) appeared comparable to controls. Compensation by centrifugal forces may account for this finding. Our data are consistent, in principle, with effects on other, flighted cells and suggest that "vector-free" gravity may simulate certain aspects of microgravity. The distribution of acetylcholine receptor aggregates, on myocytes, was also altered. This indicates that brain development, in microgravity, may also be affected.

Enhancement of phototropic response to a range of light doses in Triticum aestivum coleoptiles in clinostat-simulated microgravity

The phototropic dose-response relationship has been determined for Triticum aestivum cv. Broom coleoptiles growing on a purpose-built clinostat apparatus providing gravity compensation by rotation about a horizontal axis at 2 rev min-1. These data are compared with data sets obtained with the clinostat axis vertical and stationary, as a 1 g control, and rotating vertically to examine clinostat effects other than gravity compensation. Triticum at 1 g follows the well-established pattern of other cereal coleoptiles with a first positive curvature at low doses, followed by an indifferent response region, and a second positive response at progressively increasing doses. However, these response regions lie at higher dose levels than reported for Avena. There is no significant difference between the responses observed with the clinostat axis vertical in the rotating and stationary modes, but gravity compensation by horizontal rotation increases the magnitude of first and second positive curvatures some threefold at 100 min after stimulation. The indifferent response is replaced by a significant curvature towards the light source, but remains apparent as a reduced curvature response at these dose levels.

Cardiovascular and hormonal (aldosterone) responses in a rat model which mimics responses to weightlessness

Cardiovascular responses and fluid/electrolyte shifts seen during spaceflight have been attributed to cephalad redistribution of vascular fluid. The antiorthostatic (AO) rat (suspended, head-down tilt of 15-20 degrees) is used to model these responses. This study documents that elevated blood pressures in AO rats are sustained for periods of up to seven days, compared with presuspension values. Increased blood pressures in AO rats suggests a specific response to AO positioning, potentially relatable to a cephalad fluid shift. To assess a role for hormonal regulation of sodium excretion, serum aldosterone levels were measured. Circulating aldosterone concentrations were seen to increase approximately 100% during seven days of AO suspension concurrently with a pronounced natriuresis. These results suggest that aldosterone many not be involved in the long term regulation of increased Na+ excretion in AO animals. These studies continue to show the usefulness of models for the development of animal protocols for space flight.

Cardiovascular effects of simulated zero-gravity in humans

Head-down and head-up [correction of heat-up] tilted bedrest (5 degrees) and head out water immersion (HOWI) for 6 hr were compared.

PARAMETERS

Cardiac output (rebreathing method), blood pressure (arm cuff), forearm blood flow (venous occlusion plethysmography), total peripheral (TPR), and forearm vascular (FVR) resistances, Hct, Hb, relative plasma volume (PV) changes, and plasma catecholamines (single-isotope assay). During HOWI there was as expected a decrement in TPR, FVR, Mean arterial pressure (MAP, from 100 to 80 mmHg), Hct, and PV, and--as a new finding--catecholamines, which were 30-50% lower compared with both +5 and -5 degrees bedrest. During head down tilt, MAP was elevated (to 100-110 mmHg) and catecholamines did not fall, while TPR and EVR slowly decreased over 6 hr. HOWI is a stronger stimulus than -5 degrees bedrest, probably because HOWI elevates central venous pressure more markedly emptying the peripheral veins, while bedrest permits a distension of veins, which induces an increase in sympathetic nervous activity.

Hydroelectrolytic and hormonal modifications related to prolonged bedrest in antiorthostatic position

The effects of prolonged bedrest in antiorthostatic position (-4 degrees head down) on electrolyte balance were studied in 4 young volunteers. An increase was noted in sodium excretion during the first 4 days. Plasma renin activity and plasma aldosterone varied in parallel manner during the same period. Potassium balance and creatinine clearance were not significantly modified. In light of these data we feel that prolonged bedrest in antiorthostatic position constitutes an effective way to simulate on earth metabolic and hormonal modifications occurring in man under weightlessness conditions.

Early cardiovascular adaptation to zero gravity simulated by head-down tilt

The early cardiovascular adaptation to zero gravity, simulated by head-down tilt at 5 degrees, was studied in a series of 10 normal young men. The validity of the model was confirmed by comparing the results with data from Apollo and Skylab flights. Tilt produced a significant central fluid shift with a transient increase in central venous pressure, later followed by an increase in left ventricular size without changes in cardiac output, arterial pressure, or contractile state. The hemodynamic changes were transient with a nearly complete return to the control state within 6 hr. The adaptation included a diuresis and a decrease in blood volume, associated with ADH, renin and aldosterone inhibition.