Maximal and submaximal forces of slow fibers in human soleus after bed rest.

The effects of 2 and 4 mo of bed rest, with or without exercise countermeasures, on the contractile properties of slow fibers in the human soleus muscle were examined. Mean fiber diameters were 8 and 36% smaller after 2 and 4 mo of bed rest, respectively, than the pre-bed rest level. Maximum tetanic force (P(o)), maximum activated force (F(max)) per cross-sectional area (CSA), and the common-logarithm value of free Ca(2+) concentration required for half-maximal activation (pCa(50)) also decreased after 2 and 4 mo of bed rest. In contrast, maximum unloaded shortening velocity (V(o)) was increased after 2 and 4 mo of bed rest. After 1 mo of recovery, fiber diameters, P(o), F(max) per CSA (P > 0.05), and pCa(50) were increased and V(o) decreased toward pre-bed rest levels. Effects of knee extension/flexion exercise by wearing an anti-G Penguin suit for 10 h daily, and the effects of loading or unloading of the plantar flexors with (Penguin-1) or without (Penguin-2) placing the elastic loading elements of the suit, respectively, were investigated during ~2 mo of bed rest. In the Penguin-1 group, mean fiber diameter, P(o), F(max) per CSA, V(o), and pCa(50) were similar before and after bed rest. However, the responses of fiber size and contractile properties to bed rest were not prevented in the Penguin-2 group, although the degree of the changes was less than those induced by bed rest without any countermeasure. These results indicate that long-term bed rest results in reductions of fiber size, force-generation capacity, and Ca(2+) sensitivity, and enhancement of shortening velocity in slow fibers of the soleus. The data indicate that continuous mechanical loading on muscle, such as stretching of muscle, is an effective countermeasure for the prevention of muscular adaptations to gravitational unloading.

Histochemical responses of human soleus muscle fibers to long-term bedrest with or without countermeasures.

Effects of 2- or 4-month bedrest in -6 degrees head-down tilt position with or without countermeasures on the histochemical properties of fiber phenotype and cross-sectional area (CSA) were studied in human soleus. The CSAs in slow fibers decreased approximately 32% during 4-month bedrest. This reduction was normalized after 1-month recovery. Although the reduction of percent slow fibers was not significant statistically, the percent intermediate fibers was significantly elevated 4 months after bedrest. Such shift in fiber type was not normalized following 1-month recovery. Effects of wearing an anti-g Penguin suit which has a modest, but continuous resistance at the knee and ankle (Penguin-1) or with knee resistance without loading on the ankle (Penguin-2) for 10 consecutive hours daily were also investigated during approximately 2 months of bedrest. The subjects performed knee extension and flexion for the last 15 min of each hour while in a supine position in bed. Bedrest-induced fiber atrophy was prevented in the Penguin-1 group but not the Penguin-2 group. Transformation of fiber type was not prevented in either Penguin suit group. It is suggested that long-term bedrest causes an atrophy and a shift of fiber phenotype toward fast-twitch type in human soleus. Data also indicated that loading on the muscle is an effective countermeasure for prevention of fiber atrophy but not fiber-type transformation.

Myonuclear domain and myosin phenotype in human soleus after bed rest with or without loading.

After 2 or 4 mo of bed rest (6 degrees head-down tilt) and 1 mo of ambulation, there was a tendency toward a higher percentage of fibers expressing fast myosin heavy chain (MHC) isoforms and a de novo appearance of fibers coexpressing type I+IIa+IIx and IIa+IIx MHC in human soleus fibers. After 2 and 4 mo of bed rest, the mean size of type I fibers decreased by 12 (P > 0.05) and 39%, respectively. Because myonuclear number/mm of fiber length was unchanged, myonuclear domain was smaller after bed rest than before. The mean size and myonuclear domain of type I fibers were largest after 1 mo of recovery. The effects of wearing an antigravity device (Penguin suit), which had a modest but continuous resistance at the knee and ankle (Penguin-1) or knee resistance without loading on the ankle (Penguin-2), for 10 consecutive h/day were determined during 2 mo of bed rest. Mean fiber sizes in Penguin-1, but not Penguin-2, group were maintained at or above pre-bed-rest levels, whereas neither group showed phenotype changes. Myonuclear domain in type I fibers was larger in Penguin-1 and smaller in Penguin-2 group post- compared with pre-bed rest, indicating that a single daily 10-h bout of modest muscle loading can prevent bed-rest-induced soleus fiber atrophy but has minimal effect on myosin phenotype. The specific adaptive cellular strategies involved may be a function of the duration and magnitude of the adaptive stimulus as well as the immediate activity history of the fiber before the newly changed functional demands.

Role of patellar tendon on the population of muscle fiber types and the contractile properties of single glycerinated muscle fibers in quadriceps muscles of rats.

The role of the patellar tendon (PT) in the contractile properties of quadriceps muscle was investigated. After PT resection (partial or complete), PT reconstruction, or a sham operation, rats were forced to run on a rodent treadmill. The histochemical and mechanical characteristics of the vastus medialis (VM) and vastus lateralis (VL) muscles were examined 2, 4, and 8 weeks after treatment. There were no significant changes in either fiber type population or the contractile properties of glycerinated single muscle fibers from VM and VL 2 and 4 weeks after partial PT resection. Complete PT resection caused a decrease in the population of type I fibers in VM, while the running training after PT reconstruction increased the population of type I fibers in VM. For both the PT resection and PT reconstruction groups, the Hill coefficient, an indicator of the Ca2+ sensitivity of myofilaments, was calculated from the pCa-tension relationship of glycerinated single muscle fibers. In the PT reconstruction group, the Hill values for fibers isolated from VM were significantly decreased by running training, but the Hill values for VL were not. The effect of running training on the Ca2+ sensitivity of myofilaments in muscle fibers with PT reconstruction was different from that in muscle fibers without PT treatment. Both resection and partial resection of the PT in this animal model had heterogeneous effects on the contractile properties of quadriceps muscles; we presume that, clinically, we must give careful consideration to PT treatment.

Effects of spaceflight on enzyme activities and ultrastructure of fast-type skeletal muscles of rats.

The activity of succinate dehydrogenase in the medial gastrocnemius muscles of rats was significantly increased following a 2-week spaceflight (p < 0.05). That of phosphofructokinase in plantaris muscles was lowered after the flight (p < 0.05). The overall activities of both enzymes were effectively maintained during the flight. No structural alterations in the mitochondria or other organelles were observed in response to spaceflight. However, disordering of myofilament and central nucleus were often seen in the fast muscle during recovery after landing, but not immediately after landing. These observations indicate that spaceflight increases susceptibility to sarcomere damage and metabolic activity in a specific muscle during reloading.

Calcium movement of sarcoplasmic reticulum from hindlimb suspended muscle.

The function of the sarcoplasmic reticulum (SR) was examined in the slow soleus and fast extensor digitorum longus (EDL) muscles of rats submitted to 14 days of weightlessness produced by hindlimb suspension (HS). Ca2+ uptake, Ca2+ release and passive Ca2+ leakage through the SR membrane were investigated using a method of caffeine-induced contracture on the single mechanically skinned fibers. In the SR of suspended soleus muscles, the rate of Ca2+ uptake was higher than in the control muscles. However, there was no difference between the suspended and control muscles in the rate of Ca2+ uptake of the SR in EDL after HS. In soleus muscles, Ca2+ movements of the SR from the suspended muscle acquired the properties that were similar to those of the control fast muscle. The study of Ca2+ leakage showed that the velocity and amount of passive Ca2+ leakage from SR in soleus and EDL were apparently increased after HS. The results suggested that the functional properties of the SR membrane in slow and fast muscles were changed after HS.

A-band movement and junctional gap dissociation during caffeine-induced contracture of skeletal muscle fibers.

The action of caffeine on the myofibril profile of the sarcomere and the junctional gap of the intracellular membrane was studied ultrastructurally. When a high concentration of caffeine (5 mM) was applied, the movement of the A-band in sarcomeres dissected from the central portion of single muscle fibers was greater than that in sarcomeres dissected from the proximal and distal portions of the fiber. The amount of calcium released from the sarcoplasmic reticulum of the caffeine-treated muscle was greater in the terminal cisternae located on the narrow I band than in the wide I band, as determined by electron-probe analysis of ultrathin cryosections. The junctional gap was clearly expanded after caffeine treatment, resulting in an irreversible change in muscle contractility. These structural alterations may lead to the distinctive development of tension induced by caffeine. The expansion of the gap may also cause excitation-contraction uncoupling.