SPONTANEOUS CALCIUM TRANSIENTS REGULATE MYOFIBRILLOGENESIS IN EMBRYONIC XENOPUS MYOCYTES

Spontaneous transient elevations of intracellular calcium (Ca-i(2+)) h ave functional roles in the development of Xenopus spinal neurons. How ever, less is known about the roles of elevations of Ca-i(2+) in the d ifferentiation of other cell types. We have examined Xenopus myocytes as a first step in determining if Ca-i(2+) transients are a more gener al feature of differentiation in excitable cells. We find that culture d myocytes, like neurons, exhibit spontaneous Ca-i(2+) transients duri ng an early developmental period. These transients average 1.4 min in duration and occur at an average frequency of 6/hr in cultures contain ing myocytes and neurons. Culture conditions can influence transient p roduction; for example, myocyte-enriched cultures have a lower inciden ee of transient-producing cells. Transients persist in 0-Ca2+ medium, indicating that they arise from intracellular stores, Caffeine-sensiti ve Ca2+ stores are present in these cells, and depletion or block, of these stores eliminates transient production. To determine if transien ts play a functional role during development, we blocked their product ion with intracellular BAPTA, a rapid Ca2+ chelator. Cellular differen tiation is significantly inhibited only when BAPTA is applied early in development, during the period of transient production, while later B APTA treatments have no effect. Blocking transient production severely perturbed myofibril organization and sarcomere assembly. However, oth er aspects of myocyte differentiation were not affected by transient b lockade, indicating that not all myogenic differentiation programs are regulated in this manner. Our results suggest that spontaneous Ca-i(2 +) transients play a role in cytoskeletal organization during myofibri llogenesis. (C) 1996 Academic Press, Inc.