1. 1.

    An electrophysiological study was made of the giant, non-coupled salivary gland cells of the leech Haementeria ghilianii (de Filippi, 1849).

  2. 2.

    Resting membrane potential (−40 mV to −80 mV) was primarily dependent on K+, with a small contribution from a Na+ conductance and an electrogenic Na+ pump. Resting Cl permeability was low.

  3. 3.

    The cells generated overshooting action potentials (70–110 mV, 100–400 ms) which appeared to be mediated exclusively by Ca2+ because they were unaffected by removal of external Na+ and were blocked by 5 mmoll−1 Co2+.

  4. 4.

    Removal of external Ca2+ and addition of 1 mmoll−1 EGTA produced spontaneous action potentials of reduced amplitude (peaking at about 0 mV) and greatly increased duration [typically tens of seconds but sometimes resulting in sustained depolarizations (plateau potentials) extending up to 30min or more]. Action potential amplitude was then dependent on external Na+ concentration, and action potentials were abolished by removal of Na+. The responses were locked by 5 mmoll−1 Co2+, indicating that they were produced by Na+ flowing through Ca2+ channels.

  5. 5.

    Addition of micromolar concentrations of Ca2+ to Ca2+ free saline decreased spike duration and amplitude, suggesting a competition between Na+ and Ca2+.

  6. 6.

    An electrogenic Na+ pump was activated by removal of Ca2+, presumably as result of the influx of Na+ during spiking; this produced large increases in membrane potential which occurred spontaneously or when Ca2+ was reintroduced.

  7. 7.

    In normal saline, spike overshoot and duration were increased when the temperature was lowered by 10°C, whereas in Ca2+-free solution, they were reduced by this change. This suggests that the Ca2+ channel may be differentially affected by cooling, depending on the presence or absence of Ca+

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