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Journal of Experimental Biology 138,431-453 (1988)
Published by Company of Biologists 1988

Calcium-Dependent Action Potentials in Giant Salivary Gland Cells of the Leech: Haementeria Ghilianii: Calcium Removal Induces Dependence on Sodium

WERNER A. WUTTKE ¹ and MICHAEL S. BERRY ¹

¹ Biomedical and Physiological Research Group, School of Biological Sciences, University College of Swansea, Singleton Park, Swansea SA2 8PP, Wales, UK

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

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

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. 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⁺

Key words: calcium action potential, leech, salivary gland

Accepted on March 23, 1988