CALCIUM TRANSIENTS AND CALCIUM HOMEOSTASIS IN ADULT-MOUSE FAST-TWITCHSKELETAL-MUSCLE FIBERS IN CULTURE

Skeletal muscle fibers enzymatically dissociated from adult mouse flex or digitorum brevis muscles were maintained in culture for up to 8 day s. After various times in culture, fibers were loaded with fura 2, and Ca2+ transients for trains of1, 5, and 10 action potentials (100 Hz) triggered by external electrical stimulation were calculated from fluo rescence ratio records corrected for noninstantaneous reaction of fura 2 with Ca2+. The decay rate constants of Ca2+ transients decreased wi th increasing stimulation duration, indicating a slowing of the Ca2+-r emoval properties with increased stimulation duration. After 6 days in culture, Ca2+ decay rate constants decreased dramatically for all sti mulation durations and the differences in decay rate constants among 1 , 5, and 10 pulses became smaller. Intracellular parvalbumin content m easured by single-fiber immunofluorescence decreased with time in cult ure in parallel with the decrease in the decay rate constant of Ca2+ t ransients. Our results suggest that there is a correlation between par valbumin content and the decay rate constant of the Ca2+ transient.