Effects of ATP and related compounds on the Ca-induced Ca release mechanism of theXenopus SR

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Abstract
The effects of ATP and related compounds on the Ca2+ release mechanism of the sarcoplasmic reticulum (SR) was studied by using skinned skeletal muscle fibers ofXenopus laevis. ATP evoked marked Ca2+ release at very low level of Mg2+. β, γ-Methylene analogue of ATP was almost as effective as ATP, which suggests Ca2+ release evoked by ATP is elicited without ATP hydrolysis. ADP and AMP also evoked Ca2+ release from the SR, but the effect of them became gradually weaker than that of ATP as the number of phosphates decreased. CTP, UTP and ITP were less potent than ATP. Adenosine also evoked more effective Ca2+ release than inosine. The compounds with adenine base, therefore, seem to elicit more potent Ca2+ release than those which have the same number of phosphates but do not consist of adenine base. AMP and Ca2+ ion evoked Ca2+ release synergistically, and the Ca2+ release responses evoked by ATP and related compounds showed the same pharmacological characteristics as Ca-induced Ca release. So, these Ca2+ release responses are construed as the manifestation of the same mechanism as Ca-induced Ca release. Effective concentration range of ATP and the effect of pyrophosphate on Ca2+ release evoked by ATP suggest that neither the high affinity ATP catalytic site of (Ca2++Mg2+) ATPase of the SR nor the low affinity ATP binding site, reported by Dupont (1977), is implicated in the enhancement of these Ca2+ release responses from the SR.

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