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1 School of Zoology, La Trobe University, Bundoora, Victoria 3083, Australia; and 2 Boston Biomedical Research Institute, Boston, Massachusetts 02114
In
skeletal muscle fibers, the intracellular loop between domains II and
III of the 
1-subunit of the dihydropyridine receptor (DHPR) may directly activate the adjacent Ca2+ release
channel in the sarcoplasmic reticulum. We examined the effects of
synthetic peptide segments of this loop on Ca2+ release in
mechanically skinned skeletal muscle fibers with functional excitation-contraction coupling. In rat fibers at physiological Mg2+ concentration ([Mg2+]; 1 mM), a
20-residue skeletal muscle DHPR peptide
[AS(20);
Thr671-Leu690; 30 µM], shown previously to
induce Ca2+ release in a triad preparation, caused only
small spontaneous force responses in ~40% of fibers, although it
potentiated responses to depolarization and caffeine in all fibers. The
COOH-terminal half of AS(20)
[AS(10)] induced much larger spontaneous
responses but also caused substantial inhibition of Ca2+
release to both depolarization and caffeine. Both peptides induced or
potentiated Ca2+ release even when the voltage sensors were
inactivated, indicating direct action on the Ca2+ release
channels. The corresponding 20-residue cardiac DHPR peptide [AC(20);
Thr793-Ala812] was ineffective, but its
COOH-terminal half [AC(10)] had effects similar to AS(20). In the presence of lower
[Mg2+] (0.2 mM), exposure to either
AS(20) or AC(10) (30 µM) induced substantial Ca2+ release. Peptide
CS (100 µM), a loop segment reported to inhibit Ca2+ release in triads, caused partial inhibition of
depolarization-induced Ca2+ release. In toad fibers, each
of the A peptides had effects similar to or greater than those in rat
fibers. These findings suggest that the A and C regions of the skeletal
DHPR II-III loop may have important roles in vivo.
excitation-contraction coupling; ryanodine receptor
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