Mitochondrial adaptations in skeletal muscle cells in mammals exposed to gravitational unloading.

It is known that exposure to actual or simulated weightlessness is often accompanied by decreased muscle dynamic performance, and increased level of blood lactate accumulation. Decreased mitochondrial content found in fibers of the working muscles is considered to be one of the possible causes for those changes. Studies on oxidative potential of the muscle cell (i.e. capacity of the cell to oxidative energy production) under conditions of altered gravity have been carried out since late 70-ties. It was shown that the relatively short term spaceflight and hindlimb suspension induced significant decrease oxidative enzyme activities and mitochondrial volume density in rat fast muscle. However postural soleus muscle failed to exhibit similar changes, although the absolute mitochondrial content was found to be sufficiently lower after exposure to simulated microgravity. This phenomenon allowed to conclude that the pronounced soleus fiber atrophy masked the proportional absolute decrease in oxidative potential which failed to be revealed as subsequent changes in mitochondrial volume density and oxidative enzyme activity. It is also important, that biosatellite studies exposed considerable changes in mitochondria distribution pattern inside m. soleus fibers: volume density of mitochondria (and, correspondingly, activity of oxidative enzymes) increases (or does not change) in the center of fiber, and decreases at its periphery, in subsarcolemmal area. However the time course of mitochondrial alterations development (particularly during long-duration exposures to real or simulated microgravity) and some peculiarities of the mitochondria distribution were not described yet. Also, materials dealing with simultaneous time-course comparative analysis of mitochondrial characteristics and indices of physiological cost of submaximal exercise are very rare. The present paper is purposed to compare the data, obtained in several experimental studies, allowed to analyze the possible contribution of muscle mitochondria changes to changes in metabolic cost of submaximal exercise and the time-course dynamics of mitochondrial characteristics under conditions of actual or simulated gravitational unloading.

[Concentration of macroergic phosphates and oxidative potential of skeletal muscles]. / Kontsentratsiia makroergicheskikh fosfatov i okislitel'nyi potentsial skeletnykh myshts.

It is known that a long-duration decline of high-energy phosphate (HP) level in skeletal muscles, induced by administration of beta-guanidinpropionic acid (beta-GPA), is followed by an increase in mitochondrial enzyme activities (MEA). The same increase in MEA was observed in the course of physical exercise training. Under gravitational inloading decrease in MEA and increase in the level of high-energy phosphates occurred. If changes in (HP) level are believed to trigger the alterations in MEA, the increase in high-energy phosphate levels in muscles is to lead to a decline in MEA as well. The present work was purposed to reveal if changes in HP level under different contractile activity levels may be associated with changes in oxidative potential in the skeletal muscles.

[Ultrastructure of skeletal muscle fibers in monkeys after space flight]. / Ul'trastruktura skeletnykh myshechnykh volokon obez'ian posle kosmicheskogo poleta.

It is known that exposure of humans and animals to microgravity causes reduction in the cross-sected area of muscle fibers and muscle atrophy. These changes also involve ultrastructural alterations in muscle fibers. Therefore primates, that are physiologically close to humans, are to be examined to help a better understanding of the nature of these ultrastructural changes is muscles and muscle fibers. Although failed to find any relevant published data on the quantitative aspects of ultrastructural changes in muscle fibers of space-flown primates we believe that it is important to examine these aspects. The postflight study of monkey's m. soleus, and m. vastus lateralis did not reveal any significant changes in volume density of the myofibrillar apparatus. Mitochondria of m. soleus showed a distinct reduction in volume density, being more obvious in the subsarcolemmal zone than in the central one. Mitochondria of m. vastus lateralis showed a decrease (P > 0.05) in volume density. Following the flight, m. soleus and m. vastus lateralis of the monkeys showed a significant increase in the mean area of myofibrils, and a trend towards a decrease in the number of myofibrils per 100 micron 2. Besides, m. soleus showed a significant increase in the mean area of mitochondria, and a trend towards a decrease in the number of mitochondria per 100 micron 2. In m. vastus lateralis of the monkeys after space flight the number opf mitochondria tended to decrease and the mean area showed differential changes. It can be postulated that these phenomena may be associated with a reduction in the diffusion surface of mitochondria resulting from the diminished myofibrillar volume.

Ultrastructure of skeletal muscles of rhesus monkeys after spaceflight.

The ultrastructure of myofibrillar and mitochondrial apparatus of m. soleus and m. vastus lateralis of Bion 11 monkeys was investigated. In both muscles, the volume density and the mean area of myofibrils increased insignificantly and their number decreased; in m. soleus the mean area of mitochondria increased significantly and in m. vastus lateralis insignificantly.