Myoplasmic Mg2+ concentration in Xenopus muscle fibres at rest, during fatigue and during metabolic blockade.
Intracellular free Mg2+ concentration ([Mg2+]i) was measured in isolated single fibres of Xenopus muscle using the fluorescent Mg2+ indicator furaptra. In resting muscle the [Mg2+]i was 1.7 mM in a Mg(2+)-free Ringer solution. There was no significant change in [Mg2+]i over 2 h in Mg(2+)-free Ringer solution. Elevating extracellular [Mg2+] to 40 mM for 5 min caused a small rise (0.13 mM) in [Mg2+]i. There was no detectable rise in [Mg2+]i after 5 min in Na(+)-free Ringer solution. These results suggest that the membrane is relatively impermeable to Mg2+ and that there was no detectable Na(+)-Mg2+ exchange over 5 min. When muscle fibres were fatigued by repeated tetani continued until force declined to about 40% of control, [Mg2+]i showed characteristic changes. During the early period of fatigue when force first showed a small decline and then became almost stable, [Mg2+]i was unchanged; during the final period of fatigue when force declined more rapidly, [Mg2+]i increased by 0.8 mM. Recovery of [Mg2+]i took about 30 min. Recovery of force was complex: tetanic force first declined (post-contractile depression) and then slowly recovered to control. Since the minimum force occurred at about the time when [Mg2+]i had recovered, it seems unlikely that post-contractile depression is caused by elevated [Mg2+]i. Rigor, produced by inhibiting oxidative phosphorylation and glycolysis, was associated with a larger increase (1.6 mM) in [Mg2+]i than fatigue. The rise in [Mg2+]i during fatigue and metabolic blockade could be explained as release of Mg2+ normally bound to ATP. A model of the metabolic changes and the resulting increase in [Mg2+]i explains our results reasonably well.
Westerblad
H
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Department of Physiology, University of Sydney, NSW, Australia.
Allen
D G
DG
D000255:Adenosine Triphosphate / Q000378:metabolism; D000331:Aequorin; D000818:Animals; D002118:Calcium / Q000378:metabolism; D005260:Female; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism*; D009119:Muscle Contraction / Q000502:physiology; D009132:Muscles / Q000378:metabolism*; D014981:Xenopus
D000331:Aequorin; D000255:Adenosine Triphosphate; D008274:Magnesium; D002118:Calcium