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Query: UMLS:C0015672 (fatigue)
 51,768 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

To directly assess the possible role of ADP in muscle fatigue, we have studied the effect of physiological MgADP levels on maximum Ca(2+)-activated isometric force and unloaded shortening velocity (Vus) of single skinned fiber segments from rabbit fast-twitch (psoas) and slow-twitch (soleus) muscles. MgADP concentration was changed in a controlled and well-buffered manner by varying creatine (Cr) in solutions, which also contained MgATP, phosphocreatine (PCr), and creatine kinase (CK). To quantify ADP as a function of Cr added, we determined the apparent equilibrium constant (K') of CK for the conditions of our experiments (pH 7.1, 3 mM Mg2+, 12 degrees C): K' = (sigma [Cr]. sigma [ATP])/(sigma [PCr]. sigma [ADP]) = 260 +/- 3 (SE). In this manner, ADP was altered essentially as occurs during stimulation in vivo but without the concomitant changes in pH and P(i), which affect force and Vus. As ADP (and Cr) was increased, force and Vus decreased in both fiber types; at the highest ADP level used, 200 microM, normalized force was 96.6 +/- 1.7% for psoas (n = 6) and 93.7 +/- 2.8% for soleus (n = 6), and Vus was 80.4 +/- 2.4% for psoas and 91.3 +/- 7.7% for soleus. Diffusion-reaction calculations indicated that radial gradients of metabolite concentrations within fibers could not explain the small effects of ADP on fiber mechanics, and experiments verified that metabolite levels were well buffered within fibers by the CK reaction. Exogenous CK was added to bathing solutions at 290 U/ml, threefold above that necessary to maintain Vus independent of CK concentration; in the absence of PCr and exogenous CK, at least a fourfold increased MgATP was necessary to maintain Vus at the control level. Adenylate kinase activity was not detectable; thus myofibrillar adenosine-triphosphatase and exogenous CK activities were the major determinants of nucleotide levels within activated cells. Cr alone (in absence of PCr and exogenous CK) also decreased force and Vus, presumably by a nonspecific mechanism. Over the physiological range, altered ADP had little or no effect on force or Vus in well-buffered conditions. It is therefore likely that other factors decrease force and Vus during muscular fatigue.
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PMID:Effect of physiological ADP concentrations on contraction of single skinned fibers from rabbit fast and slow muscles. 786 87

This study examined changes in contractile, biochemical, and histochemical properties of slow antigravity skeletal muscle after a 6-day spaceflight mission. Twelve male Sprague-Dawley rats were randomly divided into two groups: flight and ground-based control. Approximately 3 h after the landing, in situ contractile measurements were made on the soleus muscles of the flight animals. The control animals were studied 24 h later. The contractile measurements included force-velocity relationship, force-frequency relationship, and fatigability. Biochemical measurements focused on the myosin heavy chain (MHC) and myosin light chain profiles. Adenosine-triphosphatase histochemistry was performed to identify cross-sectional area of slow and fast muscle fibers and to determine the percent fiber type distribution. The force-velocity relationships of the flight muscles were altered such that maximal isometric tension (Po) was decreased by 24% and maximal shortening velocity was increased by 14% (P < 0.05). The force-frequency relationship of the flight muscles was shifted to the right of the control muscles. At the end of the 2-min fatigue test, the flight muscles generated only 34% of Po, whereas the control muscles generated 64% of Po. The flight muscles exhibited de novo expression of the type IIx MHC isoform as well as a slight decrease in the slow type I and fast type IIa MHC isoforms. Histochemical analyses of flight muscles demonstrated a small increase in the percentage of fast type II fibers and a greater atrophy of the slow type I fibers. The results demonstrate that contractile properties of slow antigravity skeletal muscle are sensitive to the microgravity environment and that changes begin to occur within the 1st wk. These changes were at least, in part, associated with changes in the amount and type of contractile protein expressed.
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PMID:Effect of spaceflight on skeletal muscle: mechanical properties and myosin isoform content of a slow muscle. 804 58