EXTRACELLULAR CA2(+)-DEPENDENT AND INDEPENDENT CALCIUM TRANSIENT IN FETAL MYOTUBES

Spatio-temporal changes in the intracellular calcium concentration [Ca2+](i) of dissociated mice myotubes from 14-day and 18-day-old fetuses were studied using digital imaging analysis of the Ca2+ indicator fura-2. Myotubes from 18-day-old fetuses displayed a transient [Ca2+](i) increase upon electrical stimulation either in nominally calcium-free external solution or in Krebs solution containing 100 mu M lanthanum. Thus, at this developmental stage, membrane depolarization appears to increase [Ca2+](i) by stimulating Ca2+ release from the sarcoplasmic reticulum independently of extracellular Ca2+ influx. Similarly, myotubes from 14-day-old fetuses also showed a calcium transient upon electrical stimulation in Krebs solution. However, in 46% of these myotubes the calcium transient was abolished when Ca2+ entry through calcium channels was suppressed.