Effects of ADP on sarcoplasmic reticulum function in mechanically skinned skeletal muscle fibres of the rat

摘要

class="enumerated" style="list-style-type:decimal">The sarcoplasmic reticulum (SR) Ca²⁺ content (expressed in terms of endogenous SR Ca²⁺ content under physiologically resting conditions and measured from caffeine-induced force responses) and the effective rates of the SR Ca²⁺ pump and SR Ca²⁺ leak (measured from the temporal changes in SR Ca²⁺ content) were determined in mechanically skinned skeletal muscle fibres of the rat at different [ADP] (< 0.10 μm to 1.04 mm).The estimated SR Ca²⁺ pump rate at 200 nm Ca²⁺ did not change when [ADP] increased from below 0.10 μm to 10 μm but decreased by about 30 % when [ADP] increased from 10 μm to 1.04 mm.The rate constant of SR Ca²⁺ leak increased markedly with rising [ADP] when [Ca²⁺] in solution was 200 nm (apparent dissociation constant Kd^ADP= 64 ± 27 μm). Decreasing the [Ca²⁺] in solution from 200 nm to < 10 nm significantly increased the leak rate constant at all [ADP]. The SR Ca²⁺ leak rate constant could be significantly reduced by blocking the SR Ca²⁺ pump with 2,5-di(tert-butyl)-1,4-hydroquinone (TBQ).The decrease in the SR Ca²⁺ pump rate and the increase in the rate constant of SR Ca²⁺ leak when the [ADP] increased from < 0.10 μm to 1.04 mm caused a 4.4-fold decrease in SR Ca²⁺ loading ability at 200 nm Ca²⁺.The results can be fully explained by a mechanism whereby the presence of ADP causes a marked increase in the ADP-sensitive fraction of the phosphorylated pump protein, which can act as a Ca²⁺-Ca²⁺ exchanger and demonstrates that ADP is an important modulator of SR function in skeletal muscle.