THE EFFECT OF DISUSE ON FAST AND SLOW SKELETAL MUSCLE

It is well established that the motor activity pattern regulates physiological and molecular processes in mammalian skeletal muscle. Adaptations to disuse have been extensively studied, but observations of contractile function have yielded conflicting results. Much of this has been due to the manner in which the muscles were studied. The most significant shortcomings of previous investigations include the use of heterogeneous muscles and a variety of immobilization techniques. Because of the differences known to exist between the various muscle fiber types and their specific adaptations to increased muscular activity, it was very important to make a comprehensive evaluation of muscle properties in both fast and slow skeletal muscles and their adaptations to disuse. The present study was therefore undertaken to characterize the disuse-mediated alterations in both isometric and isotonic contractile properties and selected biochemical parameters in the slow, type I, soleus (SOL); the fast, type IIA and B, extensor digitorum longus (EDL); and the fast, type IIB, superficial vastus lateralis (SVL) muscles. Judging from the present findings, it is apparent that the tonically active SOL atrophies to a greater extent than the fast EDL and SVL following hindlimb casting. The preferential effect is evident in both morphological and contractile properties. It is important, however, to point out that despite preferential type I atrophy, the present data fail to substantiate fiber type switching or the notion that fast muscles are somehow unaffected by disuse. Despite elevated catabolic processes, impaired contractile function at rest, and an increased dependence on glycolysis, contractile properties of atrophied muscles, both slow and fast, differed little from controls in their pattern of change with continuous contractile activity. The present results also suggest that skeletal muscle is a highly mutable tissue with the ability to completely recover from extensive atrophy. Adaptations incurred by chronic immobilization as well as those observed during remobilization proceed in a fiber-type specific manner and emphasize the importance of motor activity on the functional integrity of skeletal muscle

A Comparison of Sarcoplasmic Reticulum Function in Fast and Slow Skeletal Muscle Using Crude Homogenate and Isolated Vesicles

Sarcoplasmic reticulum vesicles (FSR) were isolated from relatively homogeneous muscle samples representative of type I, IIA, and IIB fibers and Ca2+ uptake (Vmax and total capacity) determined. Crude homogenates of these same fiber populations were also assayed for Ca2+ uptake and the results compared to the FSR values. Both techniques produced qualitatively similar results and demonstrated distinct fiber type differences in both the rate and extent of Ca2+ uptake. The results obtained support the contention that the crude homogenate technique accurately reflects the activity of the sarcoplasmic reticulum

Hindlimb Immobilization: Length-tension and Contractile Properties of Skeletal Muscle

The effect of hindlimb immobilization (IM) on the contractile properties of fast and slow skeletal muscle was studied in rats following various periods of IM ranging from 1 to 42 days; muscle atrophy, muscle, fiber, and sarcomere length, and the length-tension characteristics were determined after 42 days of IM. The slow-twitch soleus (SOL), the fast-twitch extensor digitorum longus (EDL), and the fast-twitch superficial region of the vastus lateralis (SVL) all showed rapid atrophy following the onset of IM, reaching a new reduced steady-state weight by day 21. After 42 days of IM the passive tension (g) and active twitch tension (g/cm2) plotted vs. muscle length (cm) were shifted to the left for the slow-twitch SOL, indicating a decreased extensibility compared with control muscles. The peak tetanic tension of the slow SOL declined to 47% of the control level of 2,893 ± 125 g/cm2, whereas the fast EDL maintained 72% of its initial force of 4,392 ± 229 g/cm2, and the fast SVL was unaltered by IM. Peak twitch tension and peak rate of tension development and decline fell rapidly in the slow SOL while remaining relatively unaltered in the fast-twitch muscles. Surprisingly, maximal isotonic shortening velocity was elevated in both fast and slow muscles with IM. These results indicate that IM produces muscle atrophy in fast as well as slow skeletal muscle and, in addition, causes fiber type-specific changes in the contractile properties

Effect of Disuse on Sarcoplasmic Reticulum in Fast and Slow Skeletal Muscle

The effect of 6 wk of hindlimb immobilization on rat skeletal muscle sarcoplasmic reticulum (SR) was determined in the slow-twitch, type I soleus (SOL), the fast-twitch, type IIA deep region of the vastus lateralis (DVL), and the fast-twitch, type IIB superficial region of the vastus lateralis (SVL). Immobilization produced a significant decline in the Ca2+ uptake rate (Vmax) of SR vesicles from the slow SOL (0.930 ± 0.116 to 0.365 ± 0.071 µmol Ca2+ · mg-1 ·min-1), while the SR Vmax increased in the fast SVL (2.763 ± 0.133 to 5.209 ± 0.687) and was unaltered in the DVL. Vesicles from the fast SVL and DVL also exhibited a higher total Ca2+ uptake capacity following immobilization. An evaluation of the time course of the immobilization-mediated effect revealed an increased Ca2+ uptake capacity in all three samples after 1 wk. In the SOL total Ca2+ uptake returned to control level after 2 wk, while in the fast-twitch muscles the higher capacities were maintained. The Ca2+-stimulated SR ATPase activity was not altered in any of the muscles studies, although the total SR ATPase activity increased twofold in the slow SOL

A Comparison of Rat Myosin from Fast and Slow Skeletal Muscle and the Effect of Disuse

Certain enzymatic and structural features of myosin, purified from rat skeletal muscles representative of the fast twitch glycolytic (type IIb), the fast twitch oxidative (type IIa), and the slow twitch oxidative (type I) fiber, were determined and the results were compared with the measured contractile properties. Good correlation was found between the shortening velocities and Ca2+-activated ATPase activity for each fiber type. Fast twitch white (type IIb) and mixed fast twitch red (type IIa/IIb) muscles could not be distinguished physiologically and showed three identical isomyosins (FM1, FM2, and FM3) by nondenaturing electrophoresis. The relative abundance of fast twitch light chains (LC) in rat white muscle (type IIb) is comparable with other mammalian fast twitch muscles, except for the reduced amount of LC3f in the rat. The low level of LC3f in the rat is corroborated by correlation between light chain distribution and the ratio of fast myosin isomyosins. Fast twitch red type IIa and fast twitch white type IIb muscles have similar myosin ATPase activities and maximal shortening velocity, but differ in terms of isomyosin profile and in the percentage of light chains and light chain stoichiometry. Short term hind limb immobilization caused prolongation of contraction time and one-half relaxation time in the fast twitch muscles and a reduction of these contractile properties in slow twitch soleus. Furthermore, the increased maximum shortening velocity in the immobilized soleus could be correlated with increased Ca2+-ATPase, but no change was observed in the enzymatic activity of the fast twitch muscles. No alteration in light chain distribution with disuse was observed in any of the fiber types, The myosin from slow twitch soleus could be distinguished from fast twitch myosins on the basis of the pattern of peptides generated by proteolysis of the heavy chains. Six weeks of hind limb immobilization resulted in both an increased ATPase activity and an altered heavy chain primary structure in the slow twitch soleus muscle

Recovery Time Course in Contractile Function of Fast and Slow Skeletal Muscle after Hindlimb Immobilization

Contractile properties were evaluated in rats remobilized after 6 wk of hindlimb casting to evaluate the regenerative capacity of fast and slow skeletal muscles. Contractile parameters were determined in vitro (22ºC) in the type I soleus (SOL), type IIA and IIB extensor digitorum longus (EDL), and the type IIB superficial vastus lateralis (SVL). Immobilization (IM) shortened the SOL isometric twitch duration after which contraction time and half-relaxation time required 4 and 7 days to recover, respectively. In contrast, IM prolonged the twitch in the EDL and SVL and recovery required 14 and 7 days, respectively. Peak tetanic tension (g/cm2) fell in the SOL and EDL with IM and full recovery required 28 days. In this regard, the SVL remained unaltered. Rates of tension development and decline remained essentially unaltered in the fast muscles after IM but fell in the SOL, requiring 14 days to fully recover. Maximal shortening velocity, which had been elevated in all three muscles by IM, recovered rapidly. The present results demonstrate that both fast and slow muscle have the ability to completely recover from 6 weeks of IM

Acid Phosphatase and Protease Activities in Immobilized Rat Skeletal Muscles

The effect of hind-limb immobilization on selected lysosomal enzyme activities was studied in rat hind-limb muscles composed primarily of type I, IIA, or IIB fibers. Following immobilization, acid protease and acid phosphatase both exhibited significant (P \u3c 0.05) increases in their activity per unit weight in all three fiber types. Acid phosphatase activity increased at day 14 of immobilization in the three muscles and returned to control levels by day 21. Acid protease activity also changed biphasically, displaying a higher and earlier rise than acid phosphatase. The pattern of change in acid protease, but not acid phosphatase, closely parallels observed muscle wasting. The present data therefore demonstrate enhanced proteolytic capacity of all three fiber types early during muscular atrophy. In addition, the data suggest a dependence of basal hydrolytic and proteolytic activities and their adaptive response to immobilization on muscle fiber composition

Effect of Hindlimb Immobilization on the Fatigability of Skeletal Muscle

The soleus (SOL) and extensor digitorum longus (EDL) muscles of the rat were studied in situ (33.5ºC) after 6 wk of disuse atrophy produced by hindlimb immobilization (IM). IM resulted in depressed peak twitch (Pt) and tetanic (P0) tension as well as a decreased rate of tension development (+dP/dt) and decline (-dP/dt) in the slow-twitch SOL. The fast-twitch EDL was affected to a lesser extent, because only Po (g·cm-2) and Po,-dP/dt (g·cm-2·ms-1) were depressed after IM. Twitch duration, as measured by contraction time and one-half relaxation time, was shortened in the SOL and prolonged in the EDL. In both the fast and slow muscles 30 min of electrical stimulation resulted in a significant decline in Po. Relative to the prefatigued Po, the atrophied and control muscles showed a similar fatigue pattern. This occurred in spite of lower ATP and glycogen concentrations and higher lactate levels in the atrophied muscles. Our results indicate significant impairment of contractile function following IM in fast and slow muscles with preferential alterations in the slow SOL. In addition, despite a lower resting capacity and an increased dependence on glycolysis, contractile properties of atrophied muscles differed little from control muscles in their pattern of response to 30 min of electrical stimulation

Effect of Thyrotoxicosis on Sarcoplasmic Reticulum in Rat Skeletal Muscle

The effect of thyrotoxicosis on the capacity of fragmented sarcoplasmic reticulum (FSR) and crude homogenate (CH) to sequester Ca2+ was determined in rat muscle for the slow-twitch type I soleus (SOL), the fast-twitch type IIA deep region of the vastus lateralis (DVL),and the fast-twitch type IIB superficial region of the vastus lateralis (SVL). The maximal rate of Ca2+ uptake (Vmax) and Km were determined in both the CH and FSR preparations, and the total Ca2+ uptake capacity of the CH was determined. In the slow SOL, thyrotoxicosis increased the Vmax (8.20 ± 0.96 vs. 15.70 ± 0.92 µmol Ca2+ · g wet muscle-1 · min-1) and the total Ca2+uptake (17.62 ± 1.30 vs. 27.13 ± 2.16 µmol Ca2+ · g wet muscle-1) of the CH preparation. Thyrotoxicosis increased the FSR yield 2.3-fold in the slow-twitch SOL; however, the kinetic characteristics (Vmax and Km) of these vesicles were not altered. Thyrotoxicosis had no effect on the CH and FSR preparations in either the type IIA or type IIB sample. These results can be explained by a thyroid hormone-mediated increase in the quantity of the sarcoplasmic reticulum in type I muscle and suggest no effect on the hormone on the qualitative nature of the Ca2+-enzyme interaction

Muscle Fatigue with Prolonged Exercise: Contractile and Biochemical Alterations

Alterations in the contractile and biochemical properties of fast and slow skeletal muscle were studied in rats following a prolonged swim to exhaustion. The exercise produced glycogen depletion (less than 1 mg/g tissue) in muscles representative of all three fiber types; the isometric contractile properties were altered in the 84% type I soleus (SOL) and the 60% type IIa extensor digitorium longus (EDL) but not in the 100% type IIb superficial region of the vastus lateralis (SVL). Peak tetanic tension (Po) and the rate of tension development and decline all decreased after prolonged exercise in both the SOL and the EDL. The maximal isotonic shortening velocity was highly correlated with the myofibrillar ATPase activity, and both were relatively resistant to fatigue. Furthermore, the Ca2+ sensitivity of the myofibrils was unaffected by exercise in both fast and slow muscle. The Ca2+ uptake capacity of the sarcoplasmic reticulum (SR) was reduced in both the SOL and the fast-twitch type IIa deep region of the vastus lateralis, whereas the SR ATPase activity was unchanged. Our findings provide evidence that prolonged exercise produces alterations in contractile and biochemical properties of type I and IIa but not type IIb fibers and that muscle fatigue as measured by a decline in Po is not necessarily correlated with glycogen depletion