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  • 1
    Ca2+-phosphate (Pi) 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 Pi transport into the SR would be limited and Ca2+-Pi 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 DNFB (10 μM). 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 DNFB 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+-Pi precipitation inhibits SR Ca2+ release in fatigue produced by repeated short tetani. Furthermore, the results suggest that the rate of Pi transport into the SR critically depends on the myoplasmic Mg2+/ATP concentration.