Luminal Ca2+-regulated Mg2+ inhibition of skeletal RyRs reconstituted as isolated channels or coupled clusters.
In resting muscle, cytoplasmic Mg(2+) is a potent inhibitor of Ca(2+) 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 Ca(2+) sites (A-sites) thus preventing Ca(2+) activation. We investigate the effects of luminal and cytoplasmic Ca(2+) on Mg(2+) inhibition at the A-sites of skeletal RyRs (RyR1) in lipid bilayers, in the presence of ATP or modified by ryanodine or DIDS. Mg(2+) inhibits RyRs at the A-site in the absence of Ca(2+), indicating that Mg(2+) is an antagonist and does not simply prevent Ca(2+) activation. Cytoplasmic Ca(2+) and Cs(+) decreased Mg(2+) affinity by a competitive mechanism. We describe a novel mechanism for luminal Ca(2+) regulation of Ca(2+) release whereby increasing luminal [Ca(2+)] decreases the A-site affinity for cytoplasmic Mg(2+) by a noncompetitive, allosteric mechanism that is independent of Ca(2+) flow. Ryanodine increases the Ca(2+) sensitivity of the A-sites by 10-fold, which is insufficient to explain the level of activation seen in ryanodine-modified RyRs at nM Ca(2+), indicating that ryanodine activates independently of Ca(2+). We describe a model for ion binding at the A-sites that predicts that modulation of Mg(2+) inhibition by luminal Ca(2+) is a significant regulator of Ca(2+) release from the SR. We detected coupled gating of RyRs due to luminal Ca(2+) 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 Ca(2+) and cytoplasmic Mg(2+) did not compete at the A-sites of single open RyRs but did compete during multiple channel openings in rafts. Also, luminal Ca(2+) was a stronger activator of multiple openings than single openings. Thus it appears that RyRs are effectively "immune" to Ca(2+) emanating from their own pore but sensitive to Ca(2+) from neighboring channels.
Laver
Derek R
DR
School of Biomedical Sciences, University of Newcastle and Hunter Medical Research Institute, Callaghan, NSW 2308, Australia. derek.laver@newcastle.edu.au
O'Neill
Erin R
ER
Lamb
Graham D
GD
D000818:Animals; D002118:Calcium / Q000737:chemistry*; D015640:Ion Channel Gating; D008051:Lipid Bilayers / Q000737:chemistry*; D008274:Magnesium / Q000737:chemistry*; D008564:Membrane Potentials; D011817:Rabbits; D019837:Ryanodine Receptor Calcium Release Channel / Q000737:chemistry*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
D008051:Lipid Bilayers; D019837:Ryanodine Receptor Calcium Release Channel; D008274:Magnesium; D002118:Calcium