The electrophysiology of the contractile protozoan Actinocoryne contractilis was studied with conventional intracellular recording techniques.

Resting membrane potential (−78 mV, s.d. = 8, N = 18) was dependent upon external K+. Rapid action potentials (overshoot up to 50 mV) were evoked either by mechanical stimulation or by current injection. Graded membrane depolarizations induced by graded mechanical stimuli correspond to receptor potentials.

The receptor potential was mainly Na+-dependent; the action potential was also mainly Na+-dependent, but involved a minor Ca2+-dependence. The two components of the action potential could be separated in Ca2+-free solution containing EGTA (1 mmol l−1), in low-Na+ solutions or by the addition of Co2+. The repolarizing phase of the action potential was sensitive to TEA ions and to 4-aminopyridine (4-AP).

Action potentials were followed in 10–20 ms by a rapid all-or-none contraction of the axopods and stalk. Contraction was blocked in Ca2+-free solution containing EGTA and by Co2+, which suggests a requirement of external Ca2+ for this event. Contraction was also abolished by 4-AP.

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