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

1. The sarcoplasmic reticulum (SR) Ca2+ content (expressed in terms of endo genous SR Ca2+ content under physiologically resting conditions and measure d 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+ c ontent) were determined in mechanically skinned skeletal muscle fibres of t he rat at different [ADP] (< 0.10 muM to 1.04 mM). 2. The estimated SR Ca2+ pump rate at 200 nM Ca2+ did not change when [ADP] increased from below 0.10 muM to 10 muM but decreased by about 30% when [A DP] increased from 10 ar to 1.04 mM. 3. The rate constant of SR Ca2+ leak increased markedly with rising [ADP] w hen [Ca2+] in solution was 200 nM (apparent dissociation constant K-d(ADP) = 64 +/- 27 muM). Decreasing the [Ca2+] in solution from 200 nM to ( 10 nM significantly increased the leak rate constant at all [ADP]. The SR Ca2+ le ak rate constant could be significantly reduced by blocking the SR Ca2+ pum p with 2,5-di(tert-butyl)-1,4-hydroquinone (TBQ). 4. The decrease in the SR Ca2+ pump rate and the increase in the rate const ant of SR Ca2+ leak when the [ADP] increased from < 0.10 <mu>M, to 1.04 mM caused a 4.4-fold decrease in SR Ca2+ loading ability at 200 nM Ca25. The results can be fully explained by a mechanism whereby the presence o f ADP causes a marked increase in the ADP-sensitive fraction of the phospho rylated pump protein, which can act as a Ca2+-Ca2+ exchanger and demonstrat es that ADI? is an important modulator of SR function in skeletal muscle.