Ca2+-activated ryanodine binding: mechanisms of sensitivity and intensity modulation by Mg2+, caffeine, and adenine nucleotides
The Ca2+-ryanodine receptor complex is a functional unit at the terminal cisternae (TC) of the sarcoplasmic reticulum (SR) whose proteins comprise the Ca2+ release channels which may be involved in excitation-contraction coupling. Ca2+, Mg2+, caffeine, and adenine nucleotides, but not inositol 1,4,5...
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Published in | Molecular pharmacology Vol. 31; no. 3; pp. 232 - 238 |
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Main Authors | , , |
Format | Journal Article |
Language | English |
Published |
Bethesda, MD
American Society for Pharmacology and Experimental Therapeutics
01.03.1987
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Subjects | |
Online Access | Get full text |
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Summary: | The Ca2+-ryanodine receptor complex is a functional unit at the terminal cisternae (TC) of the sarcoplasmic reticulum (SR)
whose proteins comprise the Ca2+ release channels which may be involved in excitation-contraction coupling. Ca2+, Mg2+, caffeine,
and adenine nucleotides, but not inositol 1,4,5-trisphosphate, may exert their inotropic effects on skeletal muscle SR by
direct allosteric modulation of the [3H]ryanodine-binding site. Micromolar Ca2+ is primarily responsible for activating [3H]ryanodine
binding by regulating receptor site density, affinity, and cooperativity. Mg2+ reduces the sensitivity to Ca2+ activation
by directly competing with Ca2+ for the activator site. However, inhibition by Mg2+ is overcome in the presence of beta,gamma-methyleneadenosine
5'-triphosphate (AMP-PCP; 1 mM) or caffeine (20 mM). Caffeine dramatically increases the affinity of the Ca2+ activator site
for Ca2+, whereas AMP-PCP or cAMP enhances the gating efficiency or the lifetime of the open state of the TC SR channel. A
kinetic model is proposed for four functional domains of the Ca2+-ryanodine receptor complex: the Ca2+-regulatory domain which
binds Ca2+ with microM affinity is primarily responsible for gating the Ca2+ channel of the TC SR in a cooperative manner,
and is inhibited by mM Mg2+ by direct competition for the activator site which appears to contain critical sulfhydryl groups;
a Ca2+-activate alkaloid binding domain in close proximity to the channel which binds ryanodine with nM affinity and rapidly
occludes upon complex formation; a domain which binds caffeine with low (greater than mM) affinity and directly influences
the sensitivity of the Ca2+-regulatory site; and a domain which binds adenine nucleotides with intermediate affinity (less
than mM), does not require phosphorylation, and intensifies the Ca2+ signal which triggers opening of the Ca2+-release channel. |
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Bibliography: | ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 0026-895X 1521-0111 |