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

Spatio-temporal changes in the intracellular calcium concentration [Ca 2+](i) of dissociated mice myotubes from 14-day and 18-day-old fetuses were studied using digital imaging analysis of the Ca2+ indicator fur a-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 r eticulum independently of extracellular Ca2+ influx. Similarly, myotub es from 14-day-old fetuses also showed a calcium transient upon electr ical stimulation in Krebs solution. However, in 46% of these myotubes the calcium transient was abolished when Ca2+ entry through calcium ch annels was suppressed.