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Spontaneous activity regulates calcium‐dependent K+ current expression in developing ascidian muscle

1 In embryonic ascidian muscle, outward K+ currents develop in two stages: the initial expression of a slowly activating, voltage‐gated K+ current (IKv) near the time of neurulation is followed about 6 h later by a rapidly activating calcium‐activated K+ current (IK(Ca)). During this 6 h interval, inward Ca2+ currents (ICa) appear and the inward rectifier (IK(IR)), the sole resting conductance, is transiently downregulated. These events predict a period of spontaneous activity. The following experiments were designed to test this prediction and to examine the relevance of spontaneous activity for muscle cell development. 2 By recording activity in cell‐attached patches, we have found that muscle cells generate spontaneous action potentials during this 6 h window of time when IK(IR) is downregulated and outward K+ currents are slow. Action potentials occur at a mean frequency of 13.9 min−1. 3 When activity is blocked by the transient application of the Ca2+ channel blocker Cd2+, IK(Ca) fails to develop. This disruption is specific for IK(Ca): IK(IR) and ICa develop normally in activity‐blocked cells. Application of Cd2+ either before or after the window of activity has no effect. 4 The reappearance of IK(IR) and the development of IK(Ca) and the mature form of ICa are all prevented by transcription blockers, with a sensitive period corresponding to the period of activity. 5 These data show that, although the expression of three channel types depends on transcription during the period of spontaneous activity, only the development of IK(Ca) depends on activity.