A calcium-dependent action potential of long duration in the chorionated egg of the locust, Locusta migratoria

Electrical excitability has been demonstrated for the first time in a chorionated insect egg. Depolarization of the locust egg evokes and action potential with a number of unusual features: (1) The duration of the first action potential from an egg bathed in 2 mM Ca saline exceeds 100 s. (2) Excitability is restricted to a particular period of egg development, after chorionation but before oviposition. The input resistance of eggs is reduced 5-fold after oviposition, and oviposited eggs are generally inexcitable in the absence of barium. (3) The rising phase of the action potential consists of a fast (150 mV/s) component which is calcium sensitive, and a slow (2 mV/s) component which results from a time-dependent reduction in outward rectification at depolarized potentials. The action potential plateau results from a persistent Ca2+ current and a reduction in outward rectification. (4) The duration of action potentials evoked at 5–7 min intervals declines progressively with little change in overshoot. (5) Calcium channels are apparently localized to the region of the oölemma near the micropyle. (6) Excitability of chorionated eggs is unaffected by external tetraethylammonium, which increases the excitability of eggs prior to chorionation.Possible functions of the action potential are discussed.