Calcium sparks in intact skeletal muscle fibers of the frog

Calcium sparks were studied in frog intact skeletal muscle fibers using a home-built confocal scanner whose point-spread function was estimated to be similar to0.21 mum in x and y and similar to0.51 mum in z. Observations were made at 17-20 degreesC 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, similar to4 ms; peak amplitude, similar to1 DeltaF/F (change in fluorescence divided by resting fluorescence); decay time constant, similar to5 ms; full duration at half maximum (FDHM), similar to6 ms; late offset, similar to0.01 DeltaF/F; full width at half maximum (FWHM), similar to1.0 mum; mass (calculated as amplitude x 1.206 x FWHM3), 1.3-1.9 mum(3). Although the rise time is similar to that measured previously in frog Cut fibers (5-6 ms; 17-23 degreesC), cut fiber sparks have a longer duration (FDHM, 9-15 ms), a wider extent (FWHM, 1.3-2.3 mum), 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.