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Department of Zoology, University of Washington, Seattle 98195, USA.

1. The development of Ca2+ and K+ currents was studied in ascidian muscle cells at twelve embryonic stages from gastrulation to the mature cell, a period of 24 h. A high degree of co-ordination occurs between the development of the inwardly rectifying K+ current (IK(IR)), which sets the resting potential, and Ca2+ and outward K+ currents, which determine action potential waveform. 2. At neurulation IK(IR), which had been present since fertilization, begins to decrease, reaching 12% of its previous density in 6 h. IK(IR) then immediately begins to increase again, reaching its previous density in another 6 h. 3. When IK(IR) begins to decrease, a high-threshold inactivating Ca2+ current and a slowly activating voltage-gated K+ current appear. 4. When IK(IR) returns to its previous density, two new currents appear: a sustained Ca2+ current with the same voltage dependence, but different conotoxin sensitivity than the inactivating Ca2+ current; and a Ca(2+)-dependent K+ current, which activates 8-10 times faster and at potentials 20-30 mV more negative than the voltage-dependent K+ current. 5. The transient downregulation of IK(IR) destabilizes the resting potential and causes spontaneous action potentials to occur. Because IK(IR) is absent when only a slowly activating high-threshold outward K+ current is present, these action potentials are long in duration. 6. The return of IK(IR) and the appearance of the rapidly activating Ca(2+)-dependent K+ current eventually terminate this activity. The action potentials of the mature cell occur only on stimulation, and are 10 times shorter in duration than those in the immature cell.

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