Calcium and related channels in fertilization and early development ofFucus

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Abstract
Unfertilized eggs ofFucus serratusare primed to respond rapidly to the fertilizing sperm. The unfertilized egg plasma membrane is excitable due to the presence of voltage-regulated Ca2+and K+channels. Sperm-egg interaction elicits a fertilization potential as the first observable fertilization event. It is speculated that sperm-gated Na+channels are responsible for the initial depolarization phase, leading to opening of Ca2+channels, allowing Ca2+influx and further depolarizing the membrane to the threshold for outward K+channels. K+efflux repolarizes the membrane and the zygote plasmalemma quickly becomes dominated by a large K+conductance. The involvement of Ca2+in axis formation and fixation is not clear. Ca2+carries a proportion of the inward current at the future rhizoid pole and asymmetric45Ca influx has been detected in polarizing zygotes. However, there is no requirement for external Ca2+in axis fixation. In contrast, Ca2+influx is required for expression of polarity and rhizoid growth. New developments in patch clamping can now enable localized areas of the plasma membrane in polarized cells to be studied. So far, both inward and outward single channel currents have been observed in the growing rhizoid tip, most probably carrying Cl-and K+respectively. These channels can be related to the currents identified by previous studies using the extracellular vibrating probe.