MAGNESIUM INHIBITION OF RYANODINE-RECEPTOR CALCIUM CHANNELS - EVIDENCE FOR 2 INDEPENDENT MECHANISMS

The gating of ryanodine receptor calcium release channels (RyRs) depen ds on myoplasmic Ca2+ and Mg2+ concentrations. RyRs from skeletal and cardiac muscle are activated by mu M Ca2+ and inhibited by mM Ca2+ and Mg2+. Ca-45(2+) release from skeletal SR vesicles suggests two mechan isms for Mg2+-inhibition (Meissner, Darling & Eveleth, 1986, Biochemis try 25:236-244). The present study investigates the nature of these me chanisms using measurements of single-channel activity from cardiac- a nd skeletal RyRs incorporated into planar lipid bilayers. Our measurem ents of Mg2+- and Ca2+-dependent gating kinetics confirm that there ar e two mechanisms for Mg2+ inhibition (Type I and II inhibition) in ske letal and cardiac RyRs. The mechanisms operate concurrently, are indep endent and are associated with different parts of the channel protein. Mg2+ reduces P-o by competing with Ca2+ for the activation site (Type -I) or binding to more than one, and probably two low affinity inhibit ion sites which do not discriminate between Ca2+ and Mg2+ (Type-II). T he relative contributions of the two inhibition mechanisms to the tota l Mg2+ effect depend on cytoplasmic [Ca2+] in such a way that Mg2+ inh ibition has the properties of Types-I and II inhibition at low and hig h [Ca2+] respectively. Both mechanisms are equally important when [Ca2 + = 10 mu M in cardiac RyRs or 1 mu M in skeletal RyRs. We show that T ype-I inhibition is not the sole mechanism responsible for Mg2+ inhibi tion, as is often assumed, and we discuss the physiological implicatio ns of this finding.