Kinetic Differences Between Na Channels in the Egg and in the Neurally Differentiated Blastomere in the Tunicate

In the tunicate embryo, the cleavage can be arrested at any stage by treatment with cytochalasin B, and yet treated blastomeres can express differentiated phenotypes after a certain period of incubation. We took advantage of these large differentiated blastomeres, which were amenable to electrophysiological recordings, to compare Na channels in the egg cell and those in the neurally differentiated blastomere. The macroscopic Na current in the differentiated blastomere showed a marked slow-decaying component in addition to a fast one, whereas in the egg the fast-decaying component was predominant. Both the I-V relationship and the steady-state inactivation curve shifted about 8 mV in the positive direction in the neurally differentiated blastomere compared with those in the egg cell. Furthermore, single-channel current recordings revealed that Na channels reopened more frequently in the differentiated blastomere than in the egg cell. This characteristic of the channel corresponded well to the marked slow-decaying component of the macroscopic current in the differentiated blastomere. The single-channel conductance was similar in both types of cells. We conclude that the neurally differentiated blastomere expresses a Na channel that has properties of inactivation kinetics distinct from those of the egg-type Na channel and that no detectable egg-type channel coexists in the neurally differentiated blastomeres.