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ARTICLE |
Correspondence to Derek R. Laver: Derek.Laver{at}newcastle.edu.au
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 affinity 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.
© 2008 Laver and Honen This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.jgp.org/misc/terms.shtml). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 3.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/3.0/).
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