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Departments of Physiology and Biochemistry and Biophysics, University of North Carolina, Chapel Hill, North Carolina 27599-7260
The effects of mono- and divalent ions on
Ca2+-gated cardiac muscle
Ca2+-release channel (ryanodine
receptor) activity were examined in [3H]ryanodine-binding
measurements. Ca2+ bound with the
highest apparent affinity to Ca2+
activation sites in choline chloride medium, followed by KCl, CsCl,
NaCl, and LiCl media. The apparent
Ca2+ binding affinities of
Ca2+ inactivation sites were lower
in choline chloride and CsCl media than in LiCl, NaCl, and KCl media.
Sr2+ activated the ryanodine
receptor with a lower efficacy than
Ca2+. Competition studies
indicated that Li+,
K+,
Mg2+, and
Ba2+ compete with
Ca2+ for
Ca2+ activation sites. In 0.125 M
KCl medium, the Ca2+ dependence of
[3H]ryanodine binding
was modified by 5 mM Mg2+ and 5 mM
,
-methyleneadenosine 5'-triphosphate (a nonhydrolyzable ATP
analog). The addition of 5 mM glutathione was without appreciable effect. Substitution of Cl
by 2-(N-morpholino)ethanesulfonic acid ion caused an
increase in the apparent Ca2+
affinity of the Ca2+ inactivation
sites, whereas an increase in KCl concentration had the opposite
effect. These results suggest that cardiac muscle ryanodine receptor
activity may be regulated by 1)
competitive binding of mono- and divalent cations to
Ca2+ activation sites,
2) binding of monovalent cations to
Ca2+ inactivation sites, and
3) binding of anions to anion
regulatory sites.
sarcoplasmic reticulum
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