CA2-SENSITIVE CA2+ CHANNELS( CIRCULATIONS GENERATED BY CA2+)

We discuss here a mechanism for the formation of Ca2+ circulations oft en observed before the germination of fertilized Fucus eggs. From the observation that, in a range of external Ca2+ concentrations, fertiliz ed Fucus eggs germinate towards the low end of a Ca2+ gradient, it is deduced that the Ca2+ influx increases when the external Ca2+ concentr ation decreases. We show that this behaviour is associated to an insta bility of the cell state where the Ca2+ concentration is uniform in an d outside the egg, at the origin of the Ca2+ circulation. However, in order that the circulation can develop at a reasonable cell size, the sensitivity of the Ca2+ influx with the inner concentration must be la rge, in correct order of magnitude with respect to the measured influx variation, owing to the small dependence of the inner Ca2+ concentrat ion on the external one.