Tension- and afferent input-associated responses of neuromuscular system of rats to hindlimb unloading and/or tenotomy.

Responses of electromyogram (EMG) in soleus muscle and both afferent and efferent neurograms at the fifth lumbar (L(5)) segmental level of spinal cord were investigated during acute and chronic unloading induced by hindlimb suspension and/or tenotomy in adult rats. The soleus EMG and afferent neurogram decreased 88 and 37%, respectively, relative to those at quadrupedal posture on the floor after acute hindlimb suspension that causes passive shortening of soleus due to ankle plantarflexion. However, the afferent neurogram (P < 0.

Role of gravitational loading in the development of rat soleus muscle fibers.

Effects of hindlimb unloading during the first 3 months after birth on the development of soleus muscle fibers were studied in rats. The mean absolute weigh and cross-sectional area of whole soleus muscle in the unloaded rats were -1/3 and 1/4 of those in the controls, respectively. But the unloading did not affect the lengths of muscle, at 90 degrees of ankle joint angle, and of muscle fibers sampled from tendon to tendon, and the total sarcomere number.

Heat stress facilitates stretch-induced hypertrophy of cultured muscle cells.

Increased mechanical stress induced by stretch is an important growth stimulus in skeletal muscle. Heat shock proteins (HSPs) are an important family of endogenous, protective proteins. HSP90 and HSP70 families show elevated levels under beat stress.

Effects of hindlimb suspension on soleus muscle fibers and their spinal motoneurons in Wistar Hannover rats.

Cross-sectional areas and succinate dehydrogenase (SDH) activities of soleus muscle fibers and their spinal motoneurons in male Wistar Hannover rats were determined after 16 days of hindlimb suspension. A decreased percentage of type I fibers and an increased percentage of type I+II fibers were observed after hindlimb suspension. Cross-sectional areas of all types of fibers were smaller in the hindlimb suspended than control rats.

Responses of Hoffman-reflex in human soleus to gravity and/or fluid shift.

Responses of Hoffman (H)-reflex in human soleus to changes in the levels of gravity, activities of skeletal muscles, and/or fluid distribution of lower limbs during the parabolic flight of a jet airplane and/or using a tilting table were studied. The time interval between the electrical stimulation and the appearance of either M- or H-wave and the amplitude of M-wave were not influenced by the changes in gravity and fluid distribution levels. However, the H-wave amplitude was increased when the subjects were exposed to microgravity (microgravity-G).

Effects of running exercise during recovery from hindlimb unloading on soleus muscle fibers and their spinal motoneurons in rats.

The effects of hindlimb unloading and recovery with or without running exercise on morphological and metabolic properties of soleus muscle fibers and their spinal motoneurons in rats were investigated. Ten-week-old rats were hindlimb suspended for 2 weeks and thereafter were rehabilitated with or without voluntary running exercise for 2 weeks. A decreased percentage of type I fibers and atrophy of all types of fibers were observed after hindlimb unloading.

Changes of bone morphology in response to hindlimb suspension of rats.

Effects of chronic hindlimb suspension or exposure to 2-G from postnatal day 4 to month 3 followed by ambulation recovery on the floor on the morphology of hindlimb bones were investigated in rats. The dorsi-flexion of the ankle was inhibited in the suspended group and such phenomena were not recovered at all. The mean weight and length of femur, tibia, and fibula were less than the cage controls at the end of suspension, but gradually increased during recovery.

Effects of gravitational loading on rat soleus muscle fibers following hindlimb suspension.

Effects of 16 days of hindlimb suspension and 16 days of ambulation recovery at 1-G or 2-G environment on the characteristics of soleus muscle fibers were studied in male Wistar Hannover rats. The mean cross-sectional area and myonuclear number in isolated single fibers at the termination of suspension were approximately 30% and 25% of the controls, respectively. Satellite cells were distributed evenly throughout the fiber length in the control.

Growth-related changes in cell body size and succinate dehydrogenase activity of spinal motoneurons innervating the rat soleus muscle.

Cell body sizes and oxidative enzyme (succinate dehydrogenase) activities of spinal motoneurons innervating the soleus muscle were determined in rats ranging in postnatal age from 3 to 13 weeks. The soleus motoneurons were labeled by a retrograde neuronal tracer, nuclear yellow. The mean cell body sizes of motoneurons increased from 3 to 7 weeks of age, while the mean succinate dehydrogenase activities of motoneurons decreased from 3 to 7 weeks of age.

Effects of nine weeks of unloading on neuromuscular activities in adult rats.

Effects of 9-week hindlimb suspension and 8-week recovery on locomotor performance and electromyogram (EMG) activities of soleus (Sol), plantaris (Pl), lateral gastrocnemius (LG), and tibialis anterior (TA), were studied in adult rats. Hyperextension of knee and ankle joints, noted after nine weeks of suspension, did not recover during 8-week ambulation. Growth of Sol was fully inhibited by suspension and did not recover completely within 8 weeks of ambulation.

Succinate dehydrogenase activity in rat dorsolateral ventral horn motoneurons at L6 after spaceflight and recovery.

Succinate dehydrogenase (SDH) activity levels of motoneurons in the rostral, middle, and caudal portions of the dorsolateral region of the ventral horn of the 6th lumbar (L6) segment of the rat spinal cord were determined after 14 days of spaceflight and after 9 days of recovery on Earth. The mean SDH activity of motoneurons with cell body sizes between 500 and 800 micrometers2 located in the rostral portion of the L6 segment was lower in spaceflight than in age-matched control rats. The decrease in motoneuron SDH activity persisted for at least 9 days of recovery on Earth.

Upregulation of HSP72 in reloading rat soleus muscle after prolonged hindlimb unloading.

To clarify the changes of heat shock protein (HSP) 72 in the rat soleus muscle after hind-limb unloading (HU) and during reloading, 7-week-old male Wistar rats were hind-limb-suspended for 9 weeks, thereafter ambulatory recovery was permitted for 8 weeks. The body and absolute soleus weights were significantly lower in the HU than in the age-matched control group after HU and during reloading. The soleus weight relative to body weight was also significantly lower in the HU than in the age-matched control group at the end of 9 weeks of suspension, but returned to the control level after 2 weeks of reloading.

Differential responses of HSPs to heat stress in slow and fast regions of rat gastrocnemius muscle.

In a recent study, we showed that the rat slow soleus and fast plantaris muscles exhibited different time courses for the response of specific heat shock proteins (HSPs) after 1 h of heat stress. We hypothesized that these differential responses were related, in part, to the varying fiber type composition of these muscles. To further test this hypothesis, we now have determined the responses of Hsp60, Hsp72, and Hsc73 during the 60 h following exposure to a single bout of heat stress in the deep (relatively high percentage of slow fibers) and superficial regions (only fast fibers) of the adult rat gastrocnemius muscle.

Effects of heat stress and mechanical stretch on protein expression in cultured skeletal muscle cells.

Effects of heat stress, mechanical stretching or a combination of both on the expression of heat shock proteins (HSPs) and total protein level were studied in a culture system. Rat skeletal muscle cells (L6) were cultured on flexible-bottomed culture plates. They were subjected to one of the four following conditions: (1) 97 h incubation at 37 degrees C, (2) 1 h incubation at 41 degrees C followed by 96 h incubation at 37 degrees C, (3) 1 h incubation at 37 degrees C followed by 96 h cyclic stretching (18% of initial length, 2-s stretch and 4-s release) at 37 degrees C or (4) 1 h incubation at 41 degrees C followed by 96 h cyclic stretching at 37 degrees C.

Effects of chronic hindlimb suspension on landing performance in response to head-down drop in rats.

Effects of hindlimb unloading and reloading on the patterns of landing and posture adjustment in response to head-down drop from a height of approximately 30 cm were investigated in rats. Seven weeks old male Wistar rats were hindlimb-unloaded by tail suspension for 9 consecutive weeks. Motor tests were performed immediately after the termination of suspension and recovery patterns were checked during 8 weeks of ambulation recovery.

Inhibition of small-conductance Cl- channels by the interleukin-1beta-stimulated production of superoxide in rabbit gastric parietal cells.

We have shown previously that the G protein-coupled production of superoxide anion (O2-) leads to closure of small-conductance Cl- channels (0.3-0.4 pS) in the basolateral membrane of rabbit parietal cells.

Responses of motor and sensory neurons of rodents to spaceflight.

Spinal motoneurons innervating skeletal muscles comprised predominantly of high oxidative fibers, i.e. slow oxidative and fast oxidative glycolytic, have higher oxidative enzyme activities than motoneurons innervating skeletal muscles comprised primarily of low oxidative fibers, i.

Dependence of normal development of skeletal muscle in neonatal rats on load bearing.

Antigravity function plays an important role in determining the morphological and physiological properties of the neuromuscular system. Inhibition of the normal development of the neuromuscular system is induced by hindlimb unloading during the neonatal period in rats. However, the role of gravitational loading on the development of skeletal muscle in rats is not well understood.

Effects of 9 weeks hindlimb suspension on neuromuscular activity patterns in rat.

It is well known that gravitational unloading induces muscle atrophy associated with a shift of fiber type in slow-twitch muscle. Ishihara et al. (1996 & 1997) reported that 2 weeks of spaceflight caused a decrease in succinate dehydrogenase activities of ventral horn and dorsal root ganglion neurons in rats.

Effects of 9 weeks of hindlimb unloading on motor performances in adult rats.

It has been reported that abnormal steps associated with an ankle hyper-extension during walking were observed in adult rats after 2 weeks of hindlimb suspension (Canu and Falempin, 1997 & 1998). But such phenomena were normalized after 7 days of reambulation recovery. Canu and Falempin (1996) suggested that the spinal cord has a capacity to generate a well-organized pattern of locomotion even after a period of muscle disuse.

Air-righting responses to chronic hindlimb suspension and ambulation recovery in adult rats.

Laouris et al. (1990) reported that the development of air-righting in growing new-born rats was clearly suppressed when their neck and/or back rotations were prevented. Thus, it is suggested that the neck and back muscles play an important role for a quick righting in response to drop from supine position.

Role of afferent input in muscle atrophy.

It is well known that soleus muscle of rat atrophies following spaceflight or hindlimb suspension (Ohira et al., 1992). It is, further, reported that the electromyogram (EMG) of soleus muscle disappears immediately in response to unloading by exposure to actual micro-g environment (Kawano et al.

Motoneuron and sensory neuron plasticity to varying neuromuscular activity levels.

The size and phenotypic properties of the neural and muscular elements of the neuromuscular unit are matched under normal conditions. When subjected to chronic decreases or increases in neuromuscular activity, however, the adaptations in these properties are much more limited in the neural compared with the muscular elements.

Gravitational unloading effects on muscle fiber size, phenotype and myonuclear number.

The effects of gravitational unloading with or without intact neural activity and/or tension development on myosin heavy chain (MHC) composition, cross-sectional area (CSA), number of myonuclei, and myonuclear domain (cytoplasmic volume per myonucleus ratio) in single fibers of both slow and fast muscles of rat hindlimbs are reviewed briefly. The atrophic response to unloading is generally graded as follows: slow extensors > fast extensors > fast flexors. Reduction of CSA is usually greater in the most predominant fiber type of that muscle.