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

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Abstract
The cleavage of the embryos of an ascidian, H. roretzi, was arrested with cytochalasin B (1 .mu.g/ml) at the 16- or 64-cell stage. These cleavage-arrested embryos were still able to develop membrane excitability. 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. Membrane currents of the cleavage-arrested 16-cell embryos were recorded with the voltage-clamp technique and analyzed as a function of developmental time at 15.degree. C. For this purpose, intact eggs, 4- and 8-cell embryos were used. The cleavage-arrested embryos behaved electrically like single cells, due to tight electrical coupling between blastomeres. After the 25 h stage decoupling occurred. Both Na and Ca currents decreased during the initial 10 h Na current became < 1/3 and Ca current almost disappeared. At 17 h both Na and Ca currents increased again. The potential-dependence of the Na and Ca currents after 17 h 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. Delayed (outward) K current developed gradually until 20 h 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 increased gradually without changes in the potential-dependence throughout development. Probably, 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.