Effect of low [ATP] on depolarization-induced Ca2+ release in skeletal muscle fibres of the toad.
1. The effect of low [ATP] on depolarization-induced Ca2+ release from the sarcoplasmic reticulum (SR) was examined using mechanically skinned skeletal muscle fibres of the toad. The amount of Ca2+ released was determined by examining the extent of SR Ca2+ depletion following a single depolarization in the presence of specified total [ATP] and free [Mg2+] and 1 mM EGTA (< 10 nM Ca2+). 2. Reducing the total [ATP] from 8 to 2 mM did not significantly affect the extent of depolarization-induced Ca2+ release. When the total [ATP] was lowered to 0.5 mM, depolarization-induced Ca2+ release was markedly reduced. The additional presence of 1.5 mM AMP did not reverse this effect of low [ATP]. 3. At each total [ATP], elevation of the free [Mg2+] from the normal resting level of 1 mM to 3 mM, caused a similar reduction in depolarization-induced Ca2+ release. This effect was not due to the concomitant decrease in free [ATP] and occurred independently of the reduction in Ca2+ release observed at very low total [ATP]. 4. These results show myoplasmic [ATP] modulates depolarization-induced Ca2+ release. Thus, if the total [ATP] near the Ca2+ release channels becomes locally depleted in severely fatigued muscle fibres, it may contribute, in combination with an accompanying increase in free [Mg2+], to the reported reduction in Ca2+ release.
Owen
V J
VJ
School of Zoology, La Trobe University, Bundoora, Victoria, Australia.
Lamb
G D
GD
Stephenson
D G
DG
D000255:Adenosine Triphosphate / Q000494:pharmacology / Q000502:physiology*; D000818:Animals; D002024:Bufo marinus; D002118:Calcium / Q000378:metabolism*; D002491:Central Nervous System Agents / Q000494:pharmacology; D002794:Choline / Q000494:pharmacology; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000502:physiology; D008274:Magnesium / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D009467:Neuromuscular Depolarizing Agents / Q000494:pharmacology*; D012964:Sodium / Q000494:pharmacology
D002491:Central Nervous System Agents; D009467:Neuromuscular Depolarizing Agents; D000255:Adenosine Triphosphate; D012964:Sodium; D008274:Magnesium; D002794:Choline; D002118:Calcium