Analysis of non‐linearity observed in the current—voltage relation of the tunicate embryo

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Abstract
1. In the gastrula of the tunicate (Halocynthia aurantium) the resting potential of the embryonic membrane was about −70 mV in both std ASW and Na‐free ASW. 2. The potential responses to depolarizing current in the early gastrula embryo showed distinct potential jumps from about −50 to about +40 mV during the application of current and from about 0 to resting level after its cessation thus forming a plateau at about 0 mV. Those jumps were not altered significantly by the removal of both Na and Ca ions from ASW except for the duration of the plateau after the cessation of current. 3. Blastomeres in an embryo were tightly coupled with each other electrotonically at the blastula and gastrula stages. The fact that the coupling ratio was nearly 1·0 made it reasonable to treat an embryo as one cell as far as electrical properties are concerned. 4. The I—V relation of the embryonic membrane consisted of an inward rectifying region, outward rectifying region and transitional zone between the two, resulting in a S‐shaped curve as in the egg cell membrane. But in the gastrula embryo, the transitional zone actually included the discontinuous part of the I—V curve (from about −50 to about +40 mV) due to the potential jump. 5. The ionic current (Ii) during the potential jump in Na‐free ASW was estimated from the slope of the potential response based on the assumption of constant capacitance during the membrane potential changes. The calculated Ii—V curve complemented the interrupted I—V curve smoothly and showed the existence of a negative resistance region from about −50 to about 0 mV in the steady‐state I—V relation. 6. The steady‐state I—V relation was altered significantly by changing the K concentration in ASW, and the existence of the negative resistance can be reasonably explained by the marked inward‐going rectification or anomalous rectification of K conductance. 7. The replacement of K with Rb in ASW produced the marked suppression of the inward‐going rectification of the embryonic membrane.