Ca2+ sparks and embers of mammalian muscle. Properties of the sources.
Ca2+ sparks of membrane-permeabilized rat muscle cells were analyzed to derive properties of their sources. Most events identified in longitudinal confocal line scans looked like sparks, but 23% (1,000 out of 4,300) were followed by long-lasting embers. Some were preceded by embers, and 48 were "lone embers." Average spatial width was approximately 2 microm in the rat and 1.5 microm in frog events in analogous solutions. Amplitudes were 33% smaller and rise times 50% greater in the rat. Differences were highly significant. The greater spatial width was not a consequence of greater open time of the rat source, and was greatest at the shortest rise times, suggesting a wider Ca2+ source. In the rat, but not the frog, spark width was greater in scans transversal to the fiber axis. These features suggested that rat spark sources were elongated transversally. Ca2+ release was calculated in averages of sparks with long embers. Release current during the averaged ember started at 3 or 7 pA (depending on assumptions), whereas in lone embers it was 0.7 or 1.3 pA, which suggests that embers that trail sparks start with five open channels. Analysis of a spark with leading ember yielded a current ratio ranging from 37 to 160 in spark and ember, as if 37-160 channels opened in the spark. In simulations, 25-60 pA of Ca2+ current exiting a point source was required to reproduce frog sparks. 130 pA, exiting a cylindric source of 3 microm, qualitatively reproduced rat sparks. In conclusion, sparks of rat muscle require a greater current than frog sparks, exiting a source elongated transversally to the fiber axis, constituted by 35-260 channels. Not infrequently, a few of those remain open and produce the trailing ember.
Zhou
J
J
Department of Molecular Biophysics and Physiology, Rush University School of Medicine, 1750 W. Harrison St. Suite 1279JS, Chicago, IL 60612, USA.
Brum
G
G
Gonzalez
A
A
Launikonis
B S
BS
Stern
M D
MD
Rios
E
E
D000818:Animals; D001001:Anura; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D004594:Electrophysiology; D066298:In Vitro Techniques; D018482:Muscle, Skeletal / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology*
D019837:Ryanodine Receptor Calcium Release Channel; D002118:Calcium