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Original Article |
baylor{at}mail.med.upenn.edu
Calcium sparks were studied in frog intact skeletal muscle fibers using a home-built confocal scanner whose point-spread function was estimated to be
0.21 µm in x and y and 0.51 µm in z. Observations were made at 17–20°C on fibers from Rana pipiens and Rana temporaria. Fibers were studied in two external solutions: normal Ringer's ([K+] = 2.5 mM; estimated membrane potential, –80 to –90 mV) and elevated [K+] Ringer's (most frequently, [K+] = 13 mM; estimated membrane potential, –60 to –65 mV). The frequency of sparks was 0.04–0.05 sarcomere–1 s–1 in normal Ringer's; the frequency increased approximately tenfold in 13 mM [K+] Ringer's. Spark properties in each solution were similar for the two species; they were also similar when scanned in the x and the y directions. From fits of standard functional forms to the temporal and spatial profiles of the sparks, the following mean values were estimated for the morphological parameters: rise time, 4 ms; peak amplitude, 1 
F/F (change in fluorescence divided by resting fluorescence); decay time constant, 5 ms; full duration at half maximum (FDHM), 6 ms; late offset, 0.01 F/F; full width at half maximum (FWHM), 1.0 µm; mass (calculated as amplitude x 1.206 x FWHM3), 1.3–1.9 µm3. Although the rise time is similar to that measured previously in frog cut fibers (5–6 ms; 17–23°C), cut fiber sparks have a longer duration (FDHM, 9–15 ms), a wider extent (FWHM, 1.3–2.3 µm), and a strikingly larger mass (by 3–10-fold). Possible explanations for the increase in mass in cut fibers are a reduction in the Ca2+ buffering power of myoplasm in cut fibers and an increase in the flux of Ca2+ during release.
Key Words: ryanodine receptors • fluo-3 • confocal microscopy • excitation-contraction coupling • frog muscle
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