Development of sodium, calcium and potassium channels in the cleavage-arrested embryo of an ascidian.

1. The cleavage of the embryos of an ascidian, Halocynthia roretzi, was arrested with cytochalasin B (1 microgram/ml.) at the 16- or 64-cell stage. These cleavage-arrested embryos were still able to develop membrane excitability. 2. In the cleavage-arrested 64-cell embryos at the time when control embryos became hatched larvae, Ca spikes were evoked in the presumptive muscle blastomeres, and Na- and Ca-dependent action potentials were induced in some ectodermal blastomeres. 3. Membrane currents of the cleavage-arrested 16-cell embryos were recorded with the voltage-clamp technique and analysed as a function of developmental time at 15 degrees C. For this purpose, intact eggs, 4- and 8-cell embryos were also used. The cleavage-arrested embryos behaved electrically like single cells, due to tight electrical coupling between blastomeres. After the 25-hr stage decoupling occurred. 4. Both Na and Ca currents decreased during the initial 10 hr. Na current became less than one third and Ca current almost disappeared. At 17 hr both Na and Ca currents increased again. 5. The potential-dependence of the Na and Ca currents after 17 hr was similar to that in the egg, although substantial parallel shifts in the current-voltage relations were observed: a 5 mV positive shift for the Na current and a 15-20 mV negative shift for the Ca current. 6. Delayed (outward) K current developed gradually until 20 hr and then increased abruptly. The activation level for the delayed rectification was markedly negative (around -10 mV) in comparison with that of the egg (around + 100 mV). Anomalous (inward) K current, on the other hand, increased gradually without changes in the potential-dependence throughout development. 7. The results suggest that the differentiation of excitable membranes in the ascidian embryo does not involve changes in the properties of the individual channels, but rather changes in the numbers of various kinds of ion channels.