title: Magnesium inhibition of ryanodine-receptor calcium channels: evidence for two independent mechanisms.
abstract: The gating of ryanodine receptor calcium release channels (RyRs) depends on myoplasmic Ca2+ and Mg2+ concentrations. RyRs from skeletal and cardiac muscle are activated by microm Ca2+ and inhibited by mm Ca2+ and Mg2+. 45Ca2+ release from skeletal SR vesicles suggests two mechanisms for Mg2+-inhibition (Meissner, Darling & Eveleth, 1986, Biochemistry 25:236-244). The present study investigates the nature of these mechanisms using measurements of single-channel activity from cardiac- and skeletal RyRs incorporated into planar lipid bilayers. Our measurements of Mg2+- and Ca2+-dependent gating kinetics confirm that there are two mechanisms for Mg2+ inhibition (Type I and II inhibition) in skeletal and cardiac RyRs. The mechanisms operate concurrently, are independent and are associated with different parts of the channel protein. Mg2+ reduces Po by competing with Ca2+ for the activation site (Type-I) or binding to more than one, and probably two low affinity inhibition sites which do not discriminate between Ca2+ and Mg2+ (Type-II). The relative contributions of the two inhibition mechanisms to the total Mg2+ effect depend on cytoplasmic [Ca2+] in such a way that Mg2+ inhibition has the properties of Types-I and II inhibition at low and high [Ca2+] respectively. Both mechanisms are equally important when [Ca2+] = 10 microm in cardiac RyRs or 1 microm in skeletal RyRs. We show that Type-I inhibition is not the sole mechanism responsible for Mg2+ inhibition, as is often assumed, and we discuss the physiological implications of this finding.
authors.affiliation: Muscle Research Group, Division of Neuroscience, John Curtin School of Medical Research, Australian National University, GPO Box 334, Canberra, ACT 2601, Australia.
authors.forename: D R
authors.initials: DR
authors.lastname: Laver
authors.forename: T M
authors.initials: TM
authors.lastname: Baynes
authors.forename: A F
authors.initials: AF
authors.lastname: Dulhunty
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects* / Q000378:metabolism; D015640:Ion Channel Gating / Q000187:drug effects*; D017136:Ion Transport / Q000187:drug effects; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D008954:Models, Biological; D009124:Muscle Proteins / Q000187:drug effects* / Q000378:metabolism; D018482:Muscle, Skeletal / Q000737:chemistry; D009206:Myocardium / Q000737:chemistry; D019837:Ryanodine Receptor Calcium Release Channel; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism
chemical_list: D015220:Calcium Channels; D009124:Muscle Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D008274:Magnesium; D002118:Calcium