Reduced external calcium or sodium stimulates calcium influx in Pelvetia eggs

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Abstract
The effect of external calcium and sodium ion concentrations on the calcium fluxes on the Pelvetia fastigiata De Toni egg was measured. Decreasing external [Ca2+] greatly increased the permeability of the eggs to Ca2+; at 1 mM external Ca2+ this permeability was 60 times as great as it was at the normal [Ca2+] of 10 mM. Lowering the external [Na+] also increased Ca2+ influx; at 2 mM Na+, the Ca2+ influx was 2–3 times as great as it was at the normal [Na+] if choline was used as a Na+ substitute. Lithium was less effective as a Na+ substitute in increasing Ca2+ influx. The extra Ca2+ influx in low [Na+] seemed to be dependent on internal [Na+]. The Ca2+ efflux increased transiently and then declined in low Na+ media.