1. 1.

    Action potentials (APs) have been recorded for the first time from the oocytes of an arthropod, the haematophagous insect Rhodnius prolixus. In saline containing 2mmol l−1 calcium, APs could be evoked by depolarizing the oocyte membrane from its resting potential of −50mV to a threshold between −35 and −40mV. The mean duration and overshoot of the APs were 2.6s and +16mV, respectively.

  2. 2.

    APs could not be evoked by depolarization of follicular epithelia that had been separated from the oocyte. It is concluded that the APs are generated at the oocyte cell membrane.

  3. 3.

    The overshoot of the APs was unaffected by the addition of tetrodotoxin (3 μmol l−1) or the removal of Na+ (choline replacement) from the bathing saline.

  4. 4.

    APs could be reversibly blocked by 1 mmol l−1 La3+ or 10 mmol l−1 Co2+. Verapamil at 50 μmol l−1 and l00 μmol l−1 reduced AP duration by 30% and 50%, respectively. The overshoot increased by 30 mV when bathing saline Ca2+ concentration was increased from 2 to 20 mmol l−1. These results suggested the occurrence of a Ca2+ influx during the rising phase of the AP.

  5. 5.

    Addition of Ba2+ increased the overshoot and duration of APs. In Ca2+-free saline, addition of 2 mmol l−1 Ba2+ resulted in the spontaneous production of a series of action potentials. The duration of APs was as long as 120 s in 20 mmol l−1 Ba2+ saline. It is suggested that Ba2+ may block a voltage-sensitive potassium conductance.

  6. 6.

    Possible functions of action potentials during fertilization and early development are discussed.

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