Effect of ADP on slow-twitch muscle fibres of the rat: implications for muscle fatigue |
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Authors: | W A Macdonald D G Stephenson |
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Institution: | Department of Zoology, La Trobe University, Bundoora, Victoria, 3083, Australia |
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Abstract: | Slow-twitch mechanically skinned fibres from rat soleus muscle were bathed in solutions mimicking the myoplasmic environment but containing different ADP] (0.1 μ m to 1.0 m m ). The effect of ADP on sarcoplasmic reticulum (SR) Ca2+-content was determined from the magnitude of caffeine-induced force responses, while temporal changes in SR Ca2+-content allowed determination of the effective rates of the SR Ca2+-pump and of the SR Ca2+-leak. The SR Ca2+-pump rate, estimated at pCa (−log10Ca2+]) 7.8, was reduced by 20% as the ADP] was increased from 0.1 to 40 μ m , with no further alteration when the ADP] was increased to 1.0 m m . The SR Ca2+-leak rate constant was not altered by increasing ADP] from 0.1 to 40 μ m , but was increased by 26% when the ADP] was elevated to 1.0 m m . This ADP-induced SR Ca2+-leak was insensitive to ruthenium red but was abolished by 2,5-di(tert-butyl)-1,4-hydroquinone (TBQ), indicating that the leak pathway is via the SR Ca2+-pump and not the SR Ca2+-release channel. The decrease in SR Ca2+-pump rate and SR Ca2+-leak rate when ADP] was increased led to a 40% decrease in SR Ca2+-loading capacity. Elevation of ADP] had only minor direct effects on the contractile apparatus of slow-twitch fibres. These results suggest that ADP has only limited depressing effects on the contractility of slow-twitch muscle fibres. This is in contrast to the marked effects of ADP on force responses in fast-twitch muscle fibres and may contribute to the fatigue-resistant nature of slow-twitch muscle fibres. |
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