Changes in holding and ion‐channel currents during activation of an ascidian egg under voltage clamp

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Abstract
1. The unfertilized egg of an ascidian, Halocynthia roretzi, was activated by the divalent ionophore A23187 in natural or artificial sea water (nSW or ASW) or by an external solution containing a high concentration of Ca ions (high‐Ca ASW) under voltage‐clamp condition.2. Activation current began with an abrupt increase in the holding current and decayed relatively slowly with a common time course in various ASWs. Activation current was both Na‐ and Ca‐dependent. The peak time, Tp, and the peak amplitude, D, of the activation current in nSW at 15 °C and ‐90 mV were 27 ± 4 sec and ‐1·50 ± 0·47 nA, respectively.3. The currents through Na, Ca and anomalous K channels were evoked by test pulses with constant intervals in nSW and high‐Sr, high‐Ca and high‐K ASWs. Na‐channel current was enhanced during activation. In contrast, Ca‐channel current decreased. In high‐Ca, Na‐free ASW the Ca current through Na channels increased while the Ca current through Ca channels decreased. The time for the maximum of Na current, Tmax, was (7·2 ± 1·5) × 10 sec at 15 °C and index R, the ratio of the maximum amplitude to the amplitude before activation, was 2·31 ± 0·18 in nSW. The time for the minimum of Ca‐channel current, Tmin, was about 70 sec, being almost the same as Tmax of Na current.4. The current through anomalous K channels increased initially and decreased later with a time lag behind the decrease in Ca‐channel current.5. Both Tp of activation current and Tmax of Na current were reduced by raising the temperature. Q10 for Tp and Tmax was 2·2 and 2·3, respectively.6. When the egg was activated in ASW containing scorpion toxin, the Na current through normal channels increased strongly while the Na current through toxinmodified channels increased less markedly.7. There was no significant change in the total membrane capacity before and during activation.