EFFECT OF LOW [ATP] ON DEPOLARIZATION-INDUCED CA2-MUSCLE FIBERS OF THE TOAD( RELEASE IN SKELETAL)

1. The effect of low [ATP] on depolarization-induced Ca2+ release from the sarcoplasmic reticulum (SR) was examined using mechanically skinn ed skeletal muscle fibres of the toad. The amount of Ca2+ released was determined by examining the extent of SR Ca2+ depletion following a s ingle 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-in duced Ca2+ release. When the total [ATP] was lowered to 0.5 mM, depola rization-induced Ca2+ release was markedly reduced. The additional pre sence of 1.5 mM AMP did not reverse this effect of low [ATP]. 3. At ea ch total [ATP], elevation of the free [Mg2+] from the normal resting l evel of 1 mM to 3 mM, caused a similar reduction in depolarization-ind uced Ca2+ release. This effect was not due to the concomitant decrease in free [ATP] and occurred independently of the reduction in Ca2+ rel ease observed at very low total [ATP]. 4. These results show myoplasmi c [ATP] modulates depolarization-induced Ca2+ release. Thus, if the to tal [ATP] near the Ca2+ release channels becomes locally depleted in s everely fatigued muscle fibres, it may contribute, in combination with an accompanying increase in free [Mg2+], to the reported reduction in Ca2+ release.