- Title
- Luminal Mg²⁺, a key factor controlling RYR2-mediated Ca²⁺ release: cytoplasmic and luminal regulation modeled in a tetrameric channel
- Creator
- Laver, Derek R.; Honen, Bonny N.
- Relation
- Journal of General Physiology Vol. 132, Issue 4, p. 429-446
- Publisher Link
- http://dx.doi.org/10.1085/jgp.200810001
- Publisher
- Rockefeller University Press
- Resource Type
- journal article
- Date
- 2008
- Description
- In cardiac muscle, intracellular Ca²⁺ and Mg²⁺ are potent regulators of calcium release from the sarcoplasmic reticulum (SR). It is well known that the free [Ca²⁺] in the SR ([Ca²⁺]L) stimulates the Ca²⁺ release channels (ryanodine receptor [RYR]2). However, little is known about the action of luminal Mg²⁺, which has not been regarded as an important regulator of Ca²⁺ release. The effects of luminal Ca²⁺ and Mg²⁺ on sheep RYR2 were measured in lipid bilayers. Cytoplasmic and luminal Ca²⁺ produced a synergistic increase in the opening rate of RYRs. A novel, high affinity inhibition of RYR2 by luminal Mg²⁺ was observed, pointing to an important physiological role for luminal Mg²⁺ in cardiac muscle. At diastolic [Ca²⁺]C, luminal Mg²⁺ inhibition was voltage independent, with Ki=45 μM at luminal [Ca²⁺] ([Ca²⁺]L) = 100 μM. Luminal and cytoplasmic Mg²⁺ inhibition was alleviated by increasing [Ca²⁺]L or [Ca²⁺]C. Ca²⁺ and Mg²⁺ on opposite sides of the bilayer exhibited competitive effects on RYRs, indicating that they can compete via the pore for common sites. The data were accurately fitted by a model based on a tetrameric RYR structure with four Ca²⁺-sensing mechanisms on each subunit: activating luminal L-site (40-μM affi nity for Mg²⁺ and Ca²⁺), cytoplasmic A-site (1.2 μM for Ca²⁺ and 60 μM for Mg²⁺), inactivating cytoplasmic I₁-site (~10 mM for Ca²⁺ and Mg²⁺), and I₂-site (1.2 μM for Ca²⁺). Activation of three or more subunits will cause channel opening. Mg²⁺ inhibition occurs primarily by Mg²⁺ displacing Ca²⁺ from the L- and A-sites, and Mg²⁺ fails to open the channel. The model predicts that under physiological conditions, SR load–dependent Ca²⁺ release (1) is mainly determined by Ca²⁺ displacement of Mg²⁺ from the L-site as SR loading increases, and (2) depends on the properties of both luminal and cytoplasmic activation mechanisms.
- Subject
- calcium release; skeletal muscle; Ca²⁺ release channel; magnesium deficiency; electron probe; sudden death; luminal Ca²⁺; sites
- Identifier
- http://hdl.handle.net/1959.13/39191
- Identifier
- uon:4426
- Identifier
- ISSN:0022-1295
- Language
- eng
- Full Text
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