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Effect of Mg2+ and ATP on depolarization-induced Ca2+ release in isolated triads
1Department of Physiology and Biophysics, Wright State University, Dayton, Ohio 45435, USA.
Abstract:
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.
Insights
Magnesium and ATP concentrations significantly impact calcium release in skeletal muscle. Higher MgATP levels inhibit release by activating Ca2+-ATPase, with varying Mg2+ concentrations affecting coupled and uncoupled ryanodine receptors differently.
Area of Science:
- Muscle physiology
- Calcium signaling
- Skeletal muscle energetics
Background:
- Skeletal muscle excitation-contraction coupling relies on calcium release.
- Ryanodine receptors (RyRs) are critical calcium channels in muscle.
- Triadic vesicles are key structures for regulating calcium release.
Purpose of the Study:
- To investigate the influence of varying free Mg2+ and ATP concentrations on depolarization-induced Ca2+ release.
- To differentiate the effects of Mg2+ and ATP on isolated vs. coupled ryanodine receptors within skeletal muscle triadic vesicles.
Main Methods:
- Simultaneous monitoring of Ca2+ release and ryanodine receptor activity.
- Experimental manipulation of free Mg2+ and ATP concentrations in isolated skeletal muscle triadic vesicles.
- Assessment of Ca2+ release kinetics under varying ionic conditions.
Main Results:
- Increased MgATP concentration inhibits Ca2+ release by stimulating Ca2+-ATPase.
- Half-maximal inhibition of coupled RyRs occurs at 1 mM Mg2+, while non-depolarized RyRs are inhibited at 75 microM Mg2+.
- Distinct Ca2+ release time constants suggest differences between triadic and isolated terminal cisternae RyRs.
Conclusions:
- Mg2+ and ATP concentrations are crucial modulators of skeletal muscle calcium release.
- Differential sensitivity of coupled and non-depolarized RyRs to Mg2+ highlights complex regulatory mechanisms.
- The findings provide insights into the distinct functional properties of RyRs within the triadic structure.