Electrical excitability in the egg cell membrane of the tunicate.

1. With the purpose of studying the differentiation of an excitable membrane, the electrical properties of the tunicate egg, a mosaic egg, was examined by intracellular recording techniques. The species used were Halocynthia aurantium Pallas and H. roretzi Drashe.2. The membrane in the matured but unfertilized egg, the fertilized but uncleaved egg, and the cleaved egg showed the resting potential of +5 to -15 mV.3. Hyperpolarization beyond -60 mV elicited a regenerative response in the form of an ;off response' with a critical membrane potential of about -40 mV and with an over-shoot of 10-20 mV above the original resting potential.4. The removal of Na from standard artificial sea water and the enhancement of Ca in Na-free ASW revealed both Na and Ca components in the ;off response'.5. In both std ASW and Na-free ASW the I-V relation of the egg cell membrane showed marked non-linearity, forming an S-shaped curve. In the range of positive membrane potential, the embryonic membrane showed a moderate outward-going rectification; in the potential range below -60 mV there was a considerable inward-going rectification. So far as examined, the shape of the I-V relation was affected only by changes of K concentration in the ASW, but not by those of other cations.6. Specific capacity of the egg cell membrane was 1.0 muF/cm(2). The specific slope resistance below -70 mV was 130 kOmega cm(2) for the unicellular egg and 50 kOmega cm(2) for the two cell embryo.