Inhibition of creatine kinase reduces the rate of fatigue-induced decrease in tetanic [Ca2+]i in mouse skeletal muscle

1. Ca2+-phosphate (P-i) precipitation in the sarcoplasmic reticulum (SR) may cause reduced SR Ca2+ release in skeletal muscle fatigue. To study this, we inhibited the creatine kinase (CK) reaction with 2,4-dinitro-1-fluorobenzene (DNFB). The hypothesis was that with inhibition of CK, phosphocreatine would not break down to creatine and Pi. Therefore P-i transport into the SR would be limited and Ca2+-P-i precipitation would not occur. 2. Intact single fibres from a mouse foot muscle were fatigued by repeated short tetani under control conditions or after exposure to DNPB (10 muM). The free myoplasmic concentrations of Ca2+ ([Ca2+](i)) and Mg2+ ([Mg2+](i)) were measured with indo-1 and mag-indo-1, respectively. Changes in [Mg2+](i) were assumed to reflect alterations in myoplasmic ATP concentration. 3. During the first 10 fatiguing tetani, tetanic [Ca2+](i) increased both in control and after DNFB exposure. Thereafter tetanic [Ca2+](i) fell and the rate of fall was about fourfold lower after DNPB exposure compared with control. 4. Under control conditions, there was a good relationship between declining tetanic [Ca2+](i) and increasing [Mg2+](i) during the final part of fatiguing stimulation. This correlation was lost after DNFB exposure. 5. In conclusion, the present data fit with a model where Ca2+-P-i precipitation inhibits SR Ca2+ release in fatigue produced by repeated short tetani. Furthermore, the results suggest that the rate of P-i transport into the SR critically depends on the myoplasmic Mg2+/ATP concentration.