Dimensions of calcium release domains in frog skeletal muscle fibers

The spatiotemporal properties of the Ca2+ release process in skeletal muscle fibers were determined using an improved confocal spot detection system. Muscle fibers were loaded with the low affinity fluorescent Ca2+ indicator OGB-5N and localized action potential-induced fluorescence signals were recorded from consecutive locations separated by 200 run within a single sarcomere. Three-dimensional reconstructions of the Ca2+ transients illustrate the existence of domains of increased fluorescence around Ca2+ release sites in the neighborhood of the T-tubules. We estimated the dimensions of these domains by drawing isochronal curves (DeltaF/F vs. spot position) and fitting Gaussian profiles to them. It was found that the earliest detectable full-width-at-half-maximum of these profiles was 0.77 +/- 0.25 mum and increased rapidly with time to 1.4 +/- 0.2 mum at their peak (18 degreesC). A brief, but statistically significant, delay of 0.8 +/- 0.42 ms was observed between the onset of the fluorescence transients at the Z- and M-lines. Our results are compatible with the possibility that, in response to AP stimulation, Ca2+ is not released exclusively from the junctional region of the sarcoplasmic reticulum, but from a broader expanse of the triadic region.