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¹ School of Biomedical Sciences, University of Newcastle and Hunter Medical Research Institute, Callaghan, NSW 2308, Australia
² Department of Zoology, La Trobe University, Melbourne, Victoria 3086, Australia

Address correspondence to Derek Laver, School of Biomedical Sciences, University of Newcastle and Hunter Medical Research Institute, Callaghan, NSW 2308, Australia. Fax: 61-2-4921-7406; email: derek.laver{at}newcastle.edu.au

In resting muscle, cytoplasmic Mg²⁺ is a potent inhibitor of Ca²⁺ release from the sarcoplasmic reticulum (SR). It is thought to inhibit calcium release channels (RyRs) by binding both to low affinity, low specificity sites (I-sites) and to high affinity Ca²⁺ sites (A-sites) thus preventing Ca²⁺ activation. We investigate the effects of luminal and cytoplasmic Ca²⁺ on Mg²⁺ inhibition at the A-sites of skeletal RyRs (RyR1) in lipid bilayers, in the presence of ATP or modified by ryanodine or DIDS. Mg²⁺ inhibits RyRs at the A-site in the absence of Ca²⁺, indicating that Mg²⁺ is an antagonist and does not simply prevent Ca²⁺ activation. Cytoplasmic Ca²⁺ and Cs⁺ decreased Mg²⁺ affinity by a competitive mechanism. We describe a novel mechanism for luminal Ca²⁺ regulation of Ca²⁺ release whereby increasing luminal [Ca²⁺] decreases the A-site affinity for cytoplasmic Mg²⁺ by a noncompetitive, allosteric mechanism that is independent of Ca²⁺ flow. Ryanodine increases the Ca²⁺ sensitivity of the A-sites by 10-fold, which is insufficient to explain the level of activation seen in ryanodine-modified RyRs at nM Ca²⁺, indicating that ryanodine activates independently of Ca²⁺. We describe a model for ion binding at the A-sites that predicts that modulation of Mg²⁺ inhibition by luminal Ca²⁺ is a significant regulator of Ca²⁺ release from the SR. We detected coupled gating of RyRs due to luminal Ca²⁺ permeating one channel and activating neighboring channels. This indicated that the RyRs existed in stable close-packed rafts within the bilayer. We found that luminal Ca²⁺ and cytoplasmic Mg²⁺ did not compete at the A-sites of single open RyRs but did compete during multiple channel openings in rafts. Also, luminal Ca²⁺ was a stronger activator of multiple openings than single openings. Thus it appears that RyRs are effectively "immune" to Ca²⁺ emanating from their own pore but sensitive to Ca²⁺ from neighboring channels.

Key Words: ryanodine receptor • magnesium • calcium • skeletal muscle • lipid bilayer

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