Effect of ADP on slow-twitch muscle fibres of the rat: implications for muscle fatigue

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.