Calcium and potassium currents in the Fucus egg

The electrical properties of unfertilized eggs of Fucus serratus L. were characterized using voltage clamp and current clamp with single electrodes. The plasma membrane of the unfertilized egg is excitable. Depolarizing the egg in current clamp induced a transient depolarizing voltage response, the amplitude of which was dependent on the presence of external Ca²⁺ or Ba²⁺ and was blocked by La³⁺. The repolarizing phase was blocked by tetraethylammonium ions. Repeated stimulation at frequencies greater than 0.5 Hz caused a transient loss of excitability. Voltage-clamp experiments revealed that an inward current with an activation threshold of -35 mV underlies the depolarizing phase of the voltage response. This current showed rapid activation and slow inactivation. The current was blocked by La³⁺ and could be carried by Ca²⁺ and Ba²⁺ but not by Sr²⁺ or Na⁺. Further depolarization to values more positive than-5 mV induced a slowly activating outward K⁺ current in addition to the inward current, which corresponded to the repolarizing phase of the voltage response. This K⁺ current showed little or no inactivation during stimulation and slow deactivation on return to the resting potential. Hyperpolarizing the egg elicited an inward current. On fertilization, the Fucus egg generates a depolarizing fertilization potential. Voltage-clamp experiments revealed an inward fertilization current underlying the fertilization potential. Within 15 min of fertilization a dramatic, irreversible increase in resting K⁺ permeability developed. The roles of the plasma-membrane channels in generation of the fertilization potential and egg activation are discussed.Abbreviations and Symbols ASW artificial seawater - SECC single-electrode current clamp - SEVC single-electrode voltage clamp - TEA tetraethylammonium - Vm membrane potential This work was supported by The Marine Biological Association U.K., Science and Engineering Research Council U.K. and The Royal Society of London.