Magnesium inhibition of ryanodine-receptor calcium channels: evidence for two independent mechanisms

J Membr Biol. 1997 Apr 1;156(3):213-29. doi: 10.1007/s002329900202.

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Calcium / metabolism*
  • Calcium / pharmacology
  • Calcium Channels / drug effects*
  • Calcium Channels / metabolism
  • Ion Channel Gating / drug effects*
  • Ion Transport / drug effects
  • Kinetics
  • Magnesium / pharmacology*
  • Models, Biological
  • Muscle Proteins / drug effects*
  • Muscle Proteins / metabolism
  • Muscle, Skeletal / chemistry
  • Myocardium / chemistry
  • Ryanodine Receptor Calcium Release Channel
  • Sarcoplasmic Reticulum / drug effects
  • Sarcoplasmic Reticulum / metabolism

Substances

  • Calcium Channels
  • Muscle Proteins
  • Ryanodine Receptor Calcium Release Channel
  • Magnesium
  • Calcium