This chapter provides an overview of ultrastructural and cellular mechanisms in myocardial deconditioning in weightlessness. The chapter reviews the tail-suspended animals in the context of the Cosmos 2044 project, which indicate that for all the myocardial parameters, the tail-suspended animals show similar, but much smaller changes than the flight animals. This indicates that while ground-based model experiments are valuable, flight experiments must be carried out to verify the results obtained from the former and to aid in planning future experiments. The data reviewed in the chapter suggests that further experiments involving both flight- and tail-suspended animals are required. One of the candidates for the causative agent of heart changes—namely, the fluid volume and pressure shifts that are known to occur in weightlessness—can be simulated by changing the position of primates and human beings from upright to supine, or by inducing a fluid shift in rats by tail suspension. If the structural changes in the heart can be demonstrated in situations where body muscular activity is held at the same level as in the controls then it can be assumed that the fluid pressure and volume changes in the cardiovascular system are likely to be the causative agent.

Spermatogonial cell loss has been observed in rats flown on Space Lab 3, Cosmos 1887, Cosmos 2044 and in mice following irradiation with X-ray or with high energy (HZE) particle beams. Spermatogonial loss is determined by cell counting in maturation stage 6 seminiferous [correction of seminferous] tubules. With the exception of Iron, laboratory irradiation experiments (with mice) revealed a similar pattern of spermatogonial loss proportional to the radiation dose at levels less than 0.1 Gy. Helium and Argon irradiation resulted in a 5% loss of spermatogonia after only 0.01 Gy exposure. However, significant spermatogonial loss (45%) occured at this radiation level with Iron particle beams. The loss of spermatogonia during each space flight was less than 10% when compared to control (non-flight) animals. This loss, although small, was significant. Although radiation may be a contributing factor in the loss of spermatogonia during space flight, exposure levels, as determined by dosimetry, were not significant to account for the total cell loss observed.

The left ventricle of hearts from rats flown on the Cosmos 1887 biosatellite for 12.5 days was compared to the same tissue of synchronous and vivarium control animals maintained in a ground based laboratory. The volume density of the mitochondria in the myocardium of the space-flown animals was statistically less (p equal less than 0.01) than that of the synchronous or vivarium control rats. Exposure to microgravity resulted in a certain degree of myocardial degeneration manifested in mitochondrial changes and accumulation of myeloid bodies. Generalized myofibrillar edema was also observed.

Testes from rats flown on Cosmos 1887 for twelve and a half days were compared to basal control, synchronous control and vivarium maintained rats. When the mean weights of flight testes, normalized for weight/100 gms, were compared to the vivarium controls they were 6.7 percent lighter. Although the flight testes were lighter than the synchronous, the difference is not significant. Counts of spermatogonial cells from 5 animals in each group revealed a 4 percent decrease in flight compared to vivarium controls. In both cases the t-Test significance was less than 0.02. The serum testosterone levels of all animals (flight, synchronous and vivarium) were significantly below the basal controls.

Morphological changes were observed in the left ventricle of rat heart tissue from animals flown on the Cosmos 1887 biosatellite for 12.5 days. These tissues were compared to the synchronous and vivarium control hearts. While many normal myofibrils were observed, others exhibited ultrastructural alterations, i.e., damaged and irregular-shaped mitochondria and generalized myofibrillar edema. Analysis of variance (ANOVA) of the volume density data revealed a statistically significant increase in glycogen and a significant decrease in mitochondria compared to the synchronous and vivarium controls. Point counting indicated an increase in lipid and myeloid bodies and a decrease in microtubules, but these changes were not statistically significant. In addition, the flight animals exhibited some patchy loss of protofibrils (actin and myosin filaments) and some abnormal supercontracted myofibrils that were not seen in the controls. This study was undertaken to gain insight into the mechanistic aspects of cardiac changes in both animals and human beings as a consequence of space travel (1). Cardiac hypotrophy and fluid shifts have been observed after actual or simulated weightlessness and raise concerns about the functioning of the heart and circulatory system during and after travel in space (2-4).

Testes from rats flown on Cosmos 1887 were compared with vivarium control and synchronous control samples. The mean weights of flight testes, normalized for weight per 100 g, were 6.4% less when compared with the vivarium controls. Counts of spermatogonia from tissue sections (seminiferous tubules in maturation stage 6) from five animals in each group revealed 4% fewer spermatogonia in flight testes compared with synchronous controls and 11% fewer spermatogonia in flight samples compared with vivarium controls.

Space flight, with its unique environmental constraints such as immobilization, decreased and increased pressures, and radiation, is known to affect testicular morphology and spermatogenesis. Selye, summarized the manifestations of physiological response to nonspecific stress and he pointed out that atrophy of the gonads always occurred. Reports of data collected from two dogs flown in space for 22 days (Cosmos 110) indicate that there was an increase of 30 to 70% atypical spermatozoa when compared to ground based controls. Seventy-five days after the flight the abnormalities had decreased to the high normal value of 30% and mating of these dogs after this period produced normal offspring, suggesting complete recovery. Effects of immobilization and increased gravity were investigated by spinning rats and mice at 2x g for 8-9 weeks. A decrease in testicular weight was noted in spun animals when compared to controls. Immobilization has been show to cause arrest of spermatogenesis in Macaca meminstrins.

NASA plans to have a space station operating in 1994 and is considering a 30 month Mars flight. These plans call for exposure to microgravity for longer periods of time than space travelers have endured to date. Vascular deconditioning is known to occur during space flight and during simulated weightlessness. The degree of deconditioning for these extended flights and the amount of possible reversibility is unknown. If a sudden demanding burden should be placed on the astronaut after prolonged deconditioning, there could be serious consequences. Exercise has been tried with limited success. What is needed is a counter measure to deconditioning. Calcium channel blockers are known to protect the heart during the recovery phase after heart attacks by regulating the calcium influx, thus protecting the cell and mitochondria from calcium overload. Sudden demands also increase blood flow, mimicking the post attack reperfusion, and could be serious for a deconditioned heart. We have found nifedipine, a calcium channel blocker, to be a promising drug for prevention of structural changes during simulated weightlessness.

Muscle loss during spaceflight remains a medical concern in prolonged space travel. We have studied changes in cyclic AMP-dependent protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37; cAPK) levels in heart muscle after simulated weightlessness and after actual microgravity on Space Lab-3. Biochemical measurements were made in the soluble and the particulate fraction of disrupted muscle tissue, but did not identify the exact subcellular location of this enzyme. In order to establish localization of the protein kinase in the muscle cells of the rat, monoclonal antibodies to the regulatory (R) subunits of the type II isoenzyme were prepared and employed in a protein A-gold immunolabeling procedure.Both rat striated and heart muscle were fixed in 2.5% glutaraldehyde (GA), cacodylate buffer, for 1 hour. Similar samples were fixed in Triple Fix (T.F.) in cacodylate buffer for one week. Osmium was not used.

Space Lab 3 (SL-3) was flown on Shuttle Challenger providing an opportunity to measure the effect of spaceflight on rat testes. Cannon developed the idea that organisms react to unfavorable conditions with highly integrated metabolic activities. Selye summarized the manifestations of physiological response to nonspecific stress and he pointed out that atrophy of the gonads always occurred. Many papers have been published showing the effects of social interaction, crowding, peck order and confinement. Flickinger showed delayed testicular development in subordinate roosters influenced by group numbers, social rank and social status. Christian reported increasing population size in mice resulted in adrenal hypertrophy, inhibition of reproductive maturation and loss of reproductive function in adults. Sex organ weights also declined. Two male dogs were flown on Cosmos 110 for 22 days. Fedorova reported an increase of 30 to 70% atypical spermatozoa consisting of tail curling and/or the absence of a tail.

Eight month old male C57BL6 mice were exposed without anesthesia to whole-body irradiation in circular holders. The mice were tested for behavioral decrements after 0.5 and 50 rads of Fe particle irradiation at 6 and 12 months post irradiation to obtain long term results. A standard maze was used and the animals were timed for completion thereof. A string test also was administered to the mice, testing their ability to grasp and move along a string to safety. The results from animals exposed to 50 rads were significantly different from [correction of fron] control results to p = < .001 in both systems of testing. The hippocampus (believed to be the location of environmental interaction in the brain) and the retina were examined for ultrastructural changes. The ultrastructural changes were similar to those we found in our Cosmos 782, 936 and in our Argon experiments. The mouse data indicate that iron particles were able to induce long term changes in the central nervous system which lead to behavioral impairment.

Quantization of the testicular spermatogonial population reduction in six rats is performed 12 hours after their return from seven days aboard Space Lab-3. The observed 7.1 percent organ weight loss, and 7.5 percent stage six spermatogonial cell population reduction in comparison with control rats correlate very well. Accurate dosimetry was not conducted on board, but radiation can not be considered the primary cause of the observed change. The decrease in protein kinase in the heart of these rats indicates that stress from adapting to weightlessness, the final jet flight, or other sources, is an important factor.

The effects of microgravity on cardiac ultrastructure and cyclic AMP metabolism in tissues of rats flown on Spacelab 3 are reported. Light and electron microscope studies of cell structure, measurements of low and high Km phosphodiesterase activity, cyclic AMP-dependent protein kinase activity, and regulatory subunit compartmentation show significant deviations in flight animals when compared to ground controls. The results indicate that some changes have occurred in cellular responses associated with catecholamine receptor interactions and intracellular signal processing.

High-energy (HZE) cosmic ray particles in space present an environmental hazard, especially to long-duration high altitude space travel. The energy released when these particles slow down and stop in living tissue causes deleterious biological effects, including death of cells. In order to study and quantitate the effects of this radiation, a number of cell systems could be used. The spermatogonial cell population in mouse testes has a rapidly dividing cell population, can be examined three days after irradiation, and is one of the most sensitive tissues to radiation. Therefore, it is ideally suited for quantitative studies of radiation effects. Previous work has shown that the various types of spermatogonial cells differ in their radiation sensitivity and that this can be very easily evaluated with the light/electron microscope. Using the system of staging established by Oak-berg and Huckins, the mixed or single cell population can be evaluated. Our work, using this method, has revealed cell sensitivity at doses as low as one rad. In these experiments, the animals were sacrificed three days post-irradiation because this is the time when maximum cell loss can be viewed. However, we are now reporting long term results of HZE particles using iron irradiation.

It has been well documented that the spermatogonial cell population demonstrates a heterogeneous response to irradiation (1,5). Our previous work (3,4,5) has shown that when one observes cross sections of irradiated testis tissue at spermatogonial stage VI, the survival curve is different from that typical for a single cell response. Since we score all surviving spermatogonia in our cell counts, the results actually represent a response of several cell types. The plotted data as shown in Figure 1 shows a steeper slope at the beginning of the curve, a somewhat different slope between 20 and 80 rads, and a new increase at 80 rads. We have interpreted this to represent several cell types with different levels of sensitivity (5). This interpretation is consistent with the known increase in radiosensitivity of spermatogonia as they proceed through the cycle from stage I to stage VI. In this report, we will correlate the response of a single spermatogonial cell type with HZE particle irradiation dose and compare it to results using x rays.