Involvement of multiple intracellular release channels in calcium
sparks of skeletal muscle.
Proc. Natl. Acad. Sci. U. S. A., 97 (8):
4380--4385 (April 2000)DOI: 10.1073/pnas.070056497
Abstract
In many types of muscle, intracellular Ca$^2+$ release for contraction
consists of brief Ca$^2+$ sparks. Whether these result from the
opening of one or many channels in the sarcoplasmic reticulum is
not known. Examining massive numbers of sparks from frog skeletal
muscle and evaluating their Ca$^2+$ release current, we provide
evidence that they are generated by multiple channels. A mode is
demonstrated in the distribution of spark rise times in the presence
of the channel activator caffeine. This finding contradicts expectations
for single channels evolving reversibly, but not for channels in
a group, which collectively could give rise to a stereotyped spark.
The release channel agonists imperatoxin A, ryanodine, and bastadin
10 elicit fluorescence events that start with a spark, then decay
to steady levels roughly proportional to the unitary conductances
of 35\%, 50\%, and 100\% that the agonists, respectively, promote
in bilayer experiments. This correspondence indicates that the steady
phase is produced by one open channel. Calculated Ca$^2+$ release
current decays 10- to 20-fold from spark to steady phase, which requires
that six or more channels be open during the spark.
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