Regulation of the RyR channel gating by Ca2+ and Mg2.

Ryanodine receptors (RyRs) are the Ca2+ release channels in the sarcoplasmic reticulum in striated muscle which play an important role in excitation-contraction coupling and cardiac pacemaking. Single channel recordings have revealed a wealth of information about ligand regulation of RyRs from mammalian skeletal and cardiac muscle (RyR1 and RyR2, respectively). RyR subunit has a Ca2+ activation site located in the luminal and cytoplasmic domains of the RyR. These sites synergistically feed into a common gating mechanism for channel activation by luminal and cytoplasmic Ca2+. RyRs also possess two inhibitory sites in their cytoplasmic domains with Ca2+ affinities of the order of 1 μM and 1 mM. Magnesium competes with Ca2+ at these sites to inhibit RyRs and this plays an important role in modulating their Ca2+-dependent activity in muscle. This review focuses on how these sites lead to RyR modulation by Ca2+ and Mg2+ and how these mechanisms control Ca2+ release in excitation-contraction coupling and cardiac pacemaking.