Comparison of Ca(2+) sparks produced independently by two ryanodine receptor isoforms (type 1 or type 3).
The molecular determinants of a Ca(2+) spark, those events that determine the sudden opening and closing of a small number of ryanodine receptor (RyR) channels limiting Ca(2+) release to a few milliseconds, are unknown. As a first step we investigated which of two RyR isoforms present in mammalian embryonic skeletal muscle, RyR type 1(RyR-1) or RyR type 3 (RyR-3) has the ability to generate Ca(2+) sparks. Their separate contributions were investigated in intercostal muscle cells of RyR-1 null and RyR-3 null mouse embryos. A comparison of Ca(2+) spark parameters of RyR-1 null versus RyR-3 null cells measured at rest with fluo-3 showed that neither the peak fluorescence intensity (DeltaF/F(o) = 1.25 +/- 0.7 vs. 1.55 +/- 0.6), spatial width at half-max intensity (FWHM = 2.7 +/- 1.2 vs. 2.6 +/- 0.6 microm), nor the duration at half-max intensity (FTHM = 45 +/- 49 vs. 43 +/- 25 ms) was significantly different. Sensitivity to caffeine (0.1 mM) was remarkably different, with sparks in RyR-1 null myotubes becoming brighter and longer in duration, whereas those in RyR-3 null cells remained unchanged. Controls performed in double RyR-1/RyR-3 null cells obtained by mice breeding showed that sparks were not observed in the absence of both isoforms in >150 cells imaged. In conclusion, 1) RyR-1 and RyR-3 appear to be the only intracellular Ca(2+) channels that participate in Ca(2+) spark activity in embryonic skeletal muscle; 2) except in their responsiveness to caffeine, both isoforms have the ability to produce Ca(2+) sparks with nearly identical properties, so it is rather unlikely that a single RyR isoform, when others are also present, would be responsible for Ca(2+) sparks; and 3) because RyR-1 null cells are excitation-contraction (EC) uncoupled and RyR-3 null cells exhibit a normal phenotype, Ca(2+) sparks result from the inherent activity of small clusters of RyRs regardless of the participation of these RyRs in EC coupling.
Conklin
M W
MW
Department of Physiology, University of Wisconsin, Madison, Wisconsin 53706, USA.
Ahern
C A
CA
Vallejo
P
P
Sorrentino
V
V
Takeshima
H
H
Coronado
R
R
D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002110:Caffeine / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D007366:Intercostal Muscles / Q000166:cytology / Q000196:embryology / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D018613:Microscopy, Confocal; D008954:Models, Biological; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*
D020033:Protein Isoforms; D019837:Ryanodine Receptor Calcium Release Channel; D002110:Caffeine