Effect of Mg²⁺ and ATP on Depolarization-Induced Ca²⁺ Release in Isolated Triads

The effect of different free Mg2+ and ATP concentrations on depolarization-induced Ca2+ release in isolated skeletal muscle triadic vesicles was examined by simultaneously monitoring direct effects on ryanodine receptors from either isolated or coupled terminal cisternae. Free Mg2+ was increased to concentrations of 11-14 microM, 81 microM, 175-181 microM, and 1 mM while total ATP concentration was kept constant or MgATP concentration was kept constant. We observed the following. 1) Increasing MgATP reduces the measurable Ca2+ release from isolated vesicles by stimulating the Ca(2+)-ATPase in the terminal cisternae. 2) Half-maximal inhibition of functionally coupled ryanodine receptors during depolarization-induced Ca2+ release is observed at 1 mM Mg2+, whereas half-maximal inhibition of the nondepolarized ryanodine receptor is seen at 75 microM Mg2+ at the same free ATP and MgATP concentrations. 3) Two separate time constants for Ca2+ release were obtained for nondepolarized ryanodine receptors with free Mg2+ at 14 microM and free ATP at 6.1 mM; this may represent triadic ryanodine receptors vs. isolated terminal cisternae ryanodine receptors.