[No influence of increased frequency on fatigability of tetanic contraction in rat atrophic soleus].
ABSTRACT The present study was performed to observe the time course and features of intermittent tetanic contractile function changes in soleus and extensor digitorum longus (EDL) muscles of tail-suspended rats. The optimal stimulating frequency, fatigability and time-dependent recovery after fatigue were measured in isolated muscle strips. The optimal stimulating frequency of soleus and EDL was 60 Hz and 120 Hz in control rats, respectively. It was not changed in 1-week unloaded soleus, but shifted to 80 Hz and 100 Hz in 2- and 4-week unloaded soleus, respectively. The maximal isometric tension (P(o)) of tetanic contraction at optimal stimulating frequency did not alter in 1- and 2-week unloaded soleus, but significantly decreased in 4-week unloaded soleus. After 5 min of fatigue, tetanic contractile tension of control soleus was decreased to 22.8% P(o), but significantly decreased to 10.4%, 10.0% and 11.6% P(o) in 1-, 2- and 4-week unloaded soleus, respectively. The tetanic contractile tension recovered to 98% P(o) in control soleus at the twentieth minute after fatigue, but only recovered to 79.0%, 83.6% and 78.5% P(o) in 1-, 2- and 4-week unloaded soleus. The optimal stimulating frequency, P(o), fatigability and time-dependent recovery of intermittent tetanic contraction were not altered in unloaded EDL compared with control. These results indicate that higher stimulating frequency can compensate the P(o) reduction in 1- and 2-week unloaded soleus, but not in 4-week unloaded soleus. The unloaded soleus, but not EDL, is more susceptible to fatigue than the synchronous controls. The unloaded soleus not only fatigues to a greater extent but also recovers significantly less than the control.
Full-textDOI: · Available from: Zhi-Bin Yu, Jan 30, 2014
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ABSTRACT: Spaceflight (SF) has been shown to cause skeletal muscle atrophy; a loss in force and power; and, in the first few weeks, a preferential atrophy of extensors over flexors. The atrophy primarily results from a reduced protein synthesis that is likely triggered by the removal of the antigravity load. Contractile proteins are lost out of proportion to other cellular proteins, and the actin thin filament is lost disproportionately to the myosin thick filament. The decline in contractile protein explains the decrease in force per cross-sectional area, whereas the thin-filament loss may explain the observed postflight increase in the maximal velocity of shortening in the type I and IIa fiber types. Importantly, the microgravity-induced decline in peak power is partially offset by the increased fiber velocity. Muscle velocity is further increased by the microgravity-induced expression of fast-type myosin isozymes in slow fibers (hybrid I/II fibers) and by the increased expression of fast type II fiber types. SF increases the susceptibility of skeletal muscle to damage, with the actual damage elicited during postflight reloading. Evidence in rats indicates that SF increases fatigability and reduces the capacity for fat oxidation in skeletal muscles. Future studies will be required to establish the cellular and molecular mechanisms of the SF-induced muscle atrophy and functional loss and to develop effective exercise countermeasures.Journal of Applied Physiology 09/2000; 89(2):823-39. · 3.43 Impact Factor
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ABSTRACT: Hindlimb unweighting (HU) causes upregulation of several muscle-specific genes responsible for the slow-to-fast transition in soleus skeletal muscle properties despite the profound muscle atrophy. The purpose of this study was to examine the expression of the fast and slow isoforms of the sarcoplasmic reticulum Ca(2+)-ATPase at the mRNA and protein level in the soleus muscle over a time course of HU and relate them to Ca(2+)-dependent ATPase activity and selected contractile properties. mRNA levels of the acetylcholine receptor (AChR) were measured to compare the signal of unweighting with denervation. Atrophy of the soleus muscles from tail-suspended rats was observed at all time points with muscle mass decreased by 52% at 28 days of HU (P < 0.05). Northern blot analysis showed the relative expression of the fast Ca2+ pump mRNA increased by 0, 250, 910, 1,340, and 4,050% over control levels at 1, 4, 7, 14, and 28 days of HU, respectively, whereas changes in slow mRNA were variable and modest in comparison. For the same time points, Western blot analysis showed relative expression of the fast Ca2+ pump protein increased by 30, 110, 320, 280, and 300% over control levels, whereas the slow-pump protein expression was unchanged except for a 75% decrease at 28 days of HU. Specific Ca(2+)-dependent ATPase activity was increased (P < 0.05) by 170% at 28 days of HU. Contractile properties measured in vitro at 14 and 28 days revealed time to peak tension and one-half relaxation time were shortened (P < 0.05) and a rightward shift in the tension-frequency curves in unloaded soleus muscles.(ABSTRACT TRUNCATED AT 250 WORDS)The American journal of physiology 05/1993; 264(5 Pt 1):C1308-15. · 3.28 Impact Factor
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ABSTRACT: 1. The effect of myoplasmic Mg2+ on Ca2+ release was examined in mechanically skinned skeletal muscle fibres, in which the normal voltage-sensor control of Ca2+ release is preserved. The voltage sensors could be activated by depolarizing the transverse tubular (T-) system by lowering the [K+] in the bathing solution. 2. Fibres spontaneously contracted when the free [Mg2+] was decreased from 1 to 0.05 mM, with no depolarization or change of total ATP, [Ca2+] or pH (pCa 6.7, 50 microM-EGTA). After such a 'low-Mg2+ response' the sarcoplasmic reticulum (SR) was depleted of Ca2+ and neither depolarization nor caffeine (2 mM) could induce a response, unless the [Mg2+] was raised and the SR reloaded with Ca2+. Exposure to 0.05 mM-Mg2+ at low [Ca2+] (2 mM-free EGTA, pCa greater than 8.7) also induced Ca2+ release and depleted the SR. 3. The response to low [Mg2+] was unaffected by inactivation of the voltage sensors, but was completely blocked by 2 microM-Ruthenium Red indicating that it involved Ca2+ efflux through the normal Ca2+ release channels. 4. In the absence of ATP (and creatine phosphate), complete removal of Mg2+ (i.e. no added Mg2+ with 1 mM-EDTA) did not induce Ca2+ release. Depolarization in the absence of Mg2+ and ATP also did not induce Ca2+ release. 5. Depolarization in 10 mM-Mg2+ (pCa 6.7, 50 microM-EGTA, 8 mM-total ATP) did not produce any response. In the presence of 1 mM-EGTA to chelate most of the released Ca2+, depolarizations in 10 mM-Mg2+ did not noticeably deplete the SR of Ca2+, whereas a single depolarization in 1 mM-Mg2+ (and 1 mM-EGTA) resulted in marked depletion. Depolarization in the presence of D600 and 10 mM-Mg2+ produced use-dependent 'paralysis', indicating that depolarization in 10 mM-Mg2+ did indeed activate the voltage sensors. 6. Depolarization in the presence of 10 mM-Mg2+ and 25 microM-ryanodine neither interfered with the normal voltage control of Ca2+ release nor caused depletion of the Ca2+ in the SR even after returning to 1 mM-Mg2+ for 1 min, indicating that few if any of the release channels had been opened by the depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)The Journal of Physiology 04/1991; 434(1):507-28. DOI:10.1113/jphysiol.1991.sp018483 · 4.54 Impact Factor