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Journal of Experimental Biology 122,177-192 (1986)
Published by Company of Biologists 1986

Sodium-Calcium Action Potential Associated with Contraction in the Heliozoan Actinocoryne Contractilis

COLETTE FEBVRE-CHEVALIER ¹, ANDRÉ BILBAUT ², QUENTIN BONE ³, and JEAN FEBVRE ¹

¹ Laboratoire de Protistologie Marine, Station Zoologique CEROV 06230, Villefranche-sur-Mer, France
² Laboratoire d'Histologie et Biologie Tissulaire, Université Claude Bernard, 69621, Villeurbanne Cedex, France
³ The Marine Laboratory, Citadel Hill, Plymouth, PL1 2PB, UK

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.

Key words: heliozoan, generator and action potentials, contraction

Accepted on November 11, 1985

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