Comparison of properties of calcium channels between the differentiated 1‐cell embryo and the egg cell of ascidians.

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Abstract
In the ascidians Halocynthia roretzi and H. aurantium the Ca channels in the differentiated embryo whose cleavage was arrested with cytochalasin B at the 1‐cell stage and in the unfertilized egg were studied using the voltage‐clamp technique. In the cleavage‐arrested 1‐cell embryo, which differentiates into a cell of epidermal type after culturing until the time of hatching of the control larvae, Ca channel and Ca‐induced K channel currents were observed upon depolarization of the membrane. Inward current through Ca channels in the embryo was analysed after suppressing Ca‐induced K current by intracellular injection of EGTA. Sr or Ba ions could substitute for Ca ions as the charge carrier through Ca channels both in the cleavage‐arrested embryo and in the egg. The selectivity ratios among these cations at their respective maximum inward currents were 1.0 (Ca):2.0 (Sr):4.5 (Ba) for the Ca channel in the embryo and 1.0 (Ca):1.9 (Sr):1.1 (Ba) for that in the egg. The time course of inactivation of Ca channels in Ca artificial sea water (ASW) was different from that in Sr or Ba ASW in the cleavage‐arrested embryo. Fast inactivation was observed only in Ca ASW, and slight and slow inactivation was seen in Ba or Sr solution. In the egg, Ca, Sr and Ba currents through Ca channels all showed a similar time course of inactivation. The time course and voltage dependence of inactivation in Ca ASW were studied by measuring Ca tail current at a constant potential level of ‐28 mV. In the cleavage‐arrested embryo the inactivation became slower and smaller in accordance with the decrease in inward Ca current when the potential level of the command pulse was increased in the positive direction from 10 to 80 mV. In the egg the time course of inactivation became faster when the potential level was similarly increased. The experimental results in (4) and (5) above suggest that the inactivation of the Ca channel in the cleavage‐arrested embryo was dependent on Ca inward current while that in the egg was potential dependent. The developmental changes of Ca channels from egg type to epidermal type were studied in the cleavage‐arrested 1‐cell embryo. The epidermal‐type Ca channels appeared at about 40 h after fertilization at 9 degrees C. The Ca channels in those blastomeres which differentiated to a cell of muscular type in the cleavage‐arrested 8‐ or 16‐cell embryo were studied after suppressing the outward current by tetraethylammonium and by intracellular injection of both Cs ions and EGTA.(ABSTRACT TRUNCATED AT 400 WORDS)