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 approximately 0.21 microm in x and y and approximately 0.51 microm in z. Observations were made at 17-20 degrees 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, approximately 4 ms; peak amplitude, approximately 1 DeltaF/F (change in fluorescence divided by resting fluorescence); decay time constant, approximately 5 ms; full duration at half maximum (FDHM), approximately 6 ms; late offset, approximately 0.01 DeltaF/F; full width at half maximum (FWHM), approximately 1.0 microm; mass (calculated as amplitude x 1.206 x FWHM(3)), 1.3-1.9 microm(3). Although the rise time is similar to that measured previously in frog cut fibers (5-6 ms; 17-23 degrees C), cut fiber sparks have a longer duration (FDHM, 9-15 ms), a wider extent (FWHM, 1.3-2.3 microm), and a strikingly larger mass (by 3-10-fold). Possible explanations for the increase in mass in cut fibers are a reduction in the Ca(2+) buffering power of myoplasm in cut fibers and an increase in the flux of Ca(2+) during release.