Search

Two prominent theories to explain the physiological effects of microgravity relate to the cascade of changes associated with the cephalic shifts of fluids and the absence of tissue deformation forces. One-g experiments for humans used bed rest and the head-down tilt (HDT) method, while animal experiments have been conducted using the tail-suspended, head-down, and hindlimbs non-weightbearing model. Because of the success of the HDT approach with rats to simulate the gravitational effects on the musculoskeletal system exhibited by humans, the same model has been used to study the effects of gravity on the cardiopulmonary systems of humans and other vertebrates. Results to date indicate the model is effective in producing comparable changes associated with blood volume, erythropoiesis, cardiac mass, baroreceptor responsiveness, carbohydrate metabolism, post-flight VO2max, and post-flight cardiac output during exercise. Inherent with these results is the potential of the model to be useful in investigating responsible mechanisms. The suspension model has promise in understanding the capillary blood PO2 changes in space as well as the arterial PO2 changes in subjects participating in a HDT experiment. However, whether the model can provide insights on the up-or-down regulation of adrenoreceptors remains to be determined, and many investigators believe the HDT approach should not be followed to study gravitational influences on pulmonary function in either humans or animals. It was concluded that the tail-suspended animal model had sufficient merit to study in-flight and post-flight human physiological responses and mechanisms.

Since 1961, there have been more than 165 flights involving several hundred individuals who have remained in a space environment from 15 min to more than a year. In addition, plans exist for humans to explore, colonize, and remain in microgravity for 1000 d or more. This symposium will address the current state of knowledge in select aspects associated with the cardiovascular, fluid and electrolytes, musculoskeletal, and the neuroendocrine and immune systems. The authors will focus on responses, mechanisms, and the appropriate countermeasures to minimize or prevent the physiological and biochemical consequences of a microgravity environment. Since exercise is frequently cited as a generic countermeasure, this topic will be covered in greater detail. Models for simulated microgravity conditions will be discussed in subsequent manuscripts, as will future directions for ground-based research.

Despite the fact that the first human was in space during 1961 and individuals have existed in a microgravity environment for more than a year, there are limited spaceflight data available on the responses of the neuroendocrine and immune systems. Because of mutual interactions between these respective integrative systems, it is inappropriate to assume that the responses of one have no impact on functions of the other. Blood and plasma volume consistently decrease with spaceflight; hence, blood endocrine and immune constituents will be modified by both gravitational and measurement influences. The majority of the in-flight data relates to endocrine responses that influence fluids and electrolytes during the first month in space. Adrenocorticotropin (ACTH), aldosterone, and anti-diuretic hormone (ADH) appear to be elevated with little change in the atrial natriuretic peptides (ANP). Flight results longer than 60 d show increased ADH variability with elevations in angiotensin and cortisol. Although post-flight results are influenced by reentry and recovery events, ACTH and ADH appear to be consistently elevated with variable results being reported for the other hormones. Limited in-flight data on insulin and growth hormone levels suggest they are not elevated to counteract the loss in muscle mass. Post-flight results from short- and long-term flights indicate that thyroxine and insulin are increased while growth hormone exhibits minimal change. In-flight parathyroid hormone (PTH) levels are variable for several weeks after which they remain elevated. Post-flight PTH was increased on missions that lasted either 7 or 237 d, whereas calcitonin concentrations were increased after 1 wk but decreased after longer flights. Leukocytes are elevated in flights of various durations because of an increase in neutrophils. The majority of post-flights data indicates immunoglobulin concentrations are not significantly changed from pre-flight measurements. However, the numbers of T-lymphocytes and natural killer cells are decreased with post-flight conditions. Of the lymphokines, interleukin-2 production, lymphocyte responsiveness, and the activity of natural killer cells are consistently reduced post-flight. Limited head-down tilt (HDT) data suggest it is an effective simulation model for microgravity investigations. Neuroendocrine and pharmacological countermeasures are virtually nonexistent and should become high priority items for future research. Although exercise has the potential to be an effective countermeasure for various neuroendocrine-immune responses in microgravity, this concept must be tested before flights to Mars are scheduled.

To study the influences of 16 wk of endurance training on the reflex regulation of resting blood pressure, nontrained (NT) and trained (T) female hypertensive rats (SHR) were subjected to conditions of lower body negative pressure (LBNP). Measurements of muscle cytochrome oxidase activity and run time to exhaustion indicated that the animals were endurance trained. The rats (NT = 6, T = 7) were tranquilized with 300-600 micrograms.kg-1 diazepam (IV) before heart rates and blood pressures were measured over a range of 2.5-10.0 mm Hg of negative pressure. When subjected to conditions of LBNP, the reflex tachycardia of the T group was greater than the NT at the lower (-2.5 and -5.0 mm Hg) negative pressures. Although arterial pressure declines were similar in both groups, the T group experienced significantly less of a decline in central venous pressure than the NT animals. When chlorisondamine was used as a ganglionic blocker (2.5 mg.kg-1, IV), the fall in CVP at 10 mm Hg negative pressure was greater for the NT group while the fall in the initial systemic arterial pressure was more for the T group. From these results we concluded that training had altered the interaction between cardiopulmonary and arterial baroreflexes in these hypertensive rats and a nonneural component had been altered such as cardiac function.

This study determined the validity of previously published or newly derived equations to predict fat-free body mass (FFB) in high school wrestlers from the midwestern United States. Five laboratories participated in the data-pooling study (total sample of 860 subjects). Measures included body composition by underwater weighing and anthropometric measurements of body mass, stature, and selected circumferences, diameters, and skinfolds. Cross-validation of selected equations to predict FFB revealed the lowest levels of error from the equations of Lohman, Thorland et al., Katch and McArdle, and Behnke and Wilmore. Modification of the constants in these equations or generation of new equations did not substantially reduce prediction error. Overall, total error for these top equations ranged from 2.44 to 2.59 kg. However, based on observed trends, this error was of lower magnitude with the younger and lighter subjects and of higher magnitude with the older and heavier subjects. We conclude that these equations could be used singularly or collectively to determine FFB, and a minimal weight could then be derived and assigned to a scholastic wrestler.

The responses of non-trained and endurance-trained rats to conditions of lower body negative pressure (LBNP) was evaluated in normotensive, borderline hypertensive, and genetic hypertensive groups, as well as in sub-groups subjected to conditions of ventilation with 100% oxygen, systematic hemorrhaging, or sino-aortic denervations. Compared to their non-trained controls, normotensive trained rats exhibited significantly greater and faster falls in arterial blood pressure. This finding suggested a change in baroreceptor sensitivity. Related, but not statistically significant trends were observed with the hypertensive groups. Borderline hypertensive rats (DOCA injections) did not demonstrate any of these differences. Measurements of blood changes during the LBNP procedure and the effects of inspiring 100% oxygen indicated that the aortic and carotid chemoreceptors were not responsible for this training effect. After baroreceptor denervation, the group differences were abolished. In addition, the training effects were generally absent when hemorrhaging was performed, a result suggesting a difference in compliance. We have concluded from these results that endurance training will be associated with greater decreases in arterial blood pressure during LBNP than will be experienced by non-trained populations. However, the responsible mechanisms are unclear and will require further investigation.

Previous research by Tcheng and Tipton developed two prediction equations appropriate for the estimation of minimal weight for high school wrestlers. The purpose of this investigation was to cross-validate these equations using densitometric estimates of minimal weight. Skeletal dimension measurements and hydrostatic weighing were completed on 220 Iowa high school wrestlers at various times in and out of season. High concurrent validity (r = 0.93) and small residual errors (less than 0.33 kg) were observed when compared to densitometric estimates. Regression weights were of similar magnitude to those from the previous study. Two new prediction models were tested. Model II enhanced the multiple R over previous equations to 0.962 and decreased the SEE by 25% (0.55 kg). The equation was: Minimal Weight (lbs) = 0.49 x Current Weight (lbs) + 1.65 x Height (inches) + 1.81 x Chest Diameter (cm) + 6.70 x Right Wrist Diameter (cm) + 1.35 x Chest Depth (cm) - 156.56. It was concluded that the Tcheng-Tipton equations or the new models could be used as valid indicators of a minimal weight for scholastic wrestlers.

The combined influence of diabetes and moderate treadmill exercise training on select metabolic and cardiovascular parameters was investigated with mature male Sprague-Dawley rats assigned to either control diabetic or diabetic groups receiving exogenous insulin. Experimental diabetes was induced with streptozotocin (80 mg.kg-1, i.v.) and verified by blood glucose concentrations greater than 16 mmol. The animals were designated as control, insulin-injected (5 U.kg-1, twice daily), or saline-injected (twice daily), and assigned to either non-trained or trained sub-groups. Insulin treatment partially restored the measured physiological functions to within normal limits. All animals were trained at 60 to 70% maximal oxygen consumption for 9 wk and exhibited higher maximal oxygen consumption values and cytochrome oxidase activity of the soleus muscles. Diabetes caused lower (P less than 0.05) reductions in resting heart rate but training-induced bradycardia did not occur in any group. Heart rate response to atropine sulfate (1 mg.kg-1, atrial choline acetyltransferase activity, atrial acetylcholine concentration, and quinuclidinyl benzilate binding was measured to evaluate changes in the parasympathetic nervous system. Atropine-induced cardiac acceleration was most pronounced in control and least effective in diabetic animals. Endurance training had no meaningful influence on this response to cholinergic inhibition. Quinuclidinyl benzilate binding for the diabetic and the diabetic groups receiving insulin revealed no change in receptor number, receptor affinity, or training effects. These findings indicated that 9 wk of exercise training improves the aerobic capability of insulin-deficient rats without changing cardiovascular characteristics associated with the parasympathetic nervous system.

The launching of the shuttle in 1981 initiated a new era in the space age. In spite of the more than 20 yr of experience and research on the anatomical and physiological effects of weightlessness, problems remain. The resolution of these problems requires countermeasures, of which exercise deserves to be considered. The uncertainty concerning the importance of exercise has evolved, in part, because of the limited number of subjects studied, the paucity of controlled experimental designs, the inability to follow standardized routines in a space environment, and the lack of specificity in the exercises prescribed. Exercise has the potential to be an effective countermeasure for the decreases in bone density, fluid volumes, muscle mass, muscular strength, orthostatic tolerance, cardiovascular deconditioning, and submaximal exercise performances that occur in a O-gravity environment if aerobic training is minimized, maximum isometric and power-type exercises are emphasized, and circuit-training principles utilized. Because the majority of future space flights will last 21 d or less, the majority of future studies on the role of exercise should concentrate on that time period.