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This Article Full Text Full Text (PDF) All Versions of this Article: 573/1/187    most recent jphysiol.2006.105775v2 jphysiol.2006.105775v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Macdonald, W. A. Articles by Stephenson, D. G. Search for Related Content PubMed PubMed Citation Articles by Macdonald, W. A. Articles by Stephenson, D. G. Related Collections Skeletal Muscle and Exercise

¹ Department of Zoology, La Trobe University, Bundoora, Victoria, 3083, Australia

Slow-twitch mechanically skinned fibres from rat soleus muscle were bathed in solutions mimicking the myoplasmic environment but containing different [ADP] (0.1 µM to 1.0 mM). 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 (–log₁₀[Ca²⁺]) 7.8, was reduced by 20% as the [ADP] was increased from 0.1 to 40 µM, with no further alteration when the [ADP] was increased to 1.0 mM. The SR Ca²⁺-leak rate constant was not altered by increasing [ADP] from 0.1 to 40 µM, but was increased by 26% when the [ADP] was elevated to 1.0 mM. 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.

(Received 20 January 2006; accepted after revision 22 March 2006; first published online 23 March 2006)
Corresponding author W. A. Macdonald: Institute of Physiology and Biophysics, University of Aarhus, DK-8000, Århus C, Denmark. Email: wmd{at}fi.au.dk

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