Calcium transients in developing mouse skeletal muscle fibres

Ca2+ transients elicited by action potentials were measured using MagFluo-4, at 20-22 degrees C, in intact muscle fibres enzymatically dissociated from mice of different ages (7, 10, 15 and 42 days). The rise time of the transient (time from 10 to 90% of the peak) was 2.4 and 1.1 ms in fibres of 7- and 42-day-old mice, respectively. The decay of the transient was described by a double exponential function, with time constants of 1.8 and 16.4 ms in adult, and of 4.6 and 105 ms in 7-day-old animals. The fractional recovery of the transient peak amplitude after 10 ms, F-2(10)/F-1, determined using twin pulses, was 0.53 for adult fibres and ranged between 0.03 and 0.60 in fibres of 7-day-old animals This large variance may indicate differences in the extent of inactivation of Ca2+ release, possibly related to the difference in ryanodine receptor composition between young and old fibres. At the 7 and 10 day stages, fibres responded to Ca2+-free solutions with a larger decrease in the transient peak amplitude (25% versus 11% in adult fibres), possibly indicating a contribution of Ca2+ influx to the Ca2+ transient in younger animals. Cyclopiazonic acid (1 mu M), an inhibitor of the sarcoplasmic reticulum (SR) Ca2+-ATPase, abolished the Ca2+ transient decay in fibres of 7- and 10-day-old animals and significantly reduced its rate in older animals. Analysis of the transients with a Ca2+ removal model showed that the results are consistent with a larger relative contribution of the SR Ca2+ pump and a lower expression of myoplasmic Ca2+ buffers in fibres of young versus old animals.