Calcium Buffer Injections Arrest Fucoid Egg Development by Suppressing Calcium Gradients

The polarity of fucoid eggs is fixed when rhizoidal growth starts. A steady flow of calcium ions into the incipient tip is thought to establish a high calcium zone, which is needed for tip formation and outgrowth. To test this model, we injected six different BAPTA-type calcium buffers into Pelvetia eggs at about 3 to 7 h before outgrowth normally starts. Critical final cell concentration of each buffer proves to block outgrowth (as well as cell division) for up to two weeks. The critical inhibitory concentration falls as the calcium dissociation constant, K_D, rises (and the buffers weaken) in the series of five buffers from 5,5' dimethyl-bapta (K_D 0.4 µM) to 4,4' difluoro-bapta (K_D = 5 µM); then finally rises again with the weakest buffer tested, 5,5' methylnitro-bapta (K_D = 60 µM). Thus calcium buffers with a K_D of about 5 µM prove most effective in blocking development. The effects of injecting a buffer do not depend upon the amount of coinjected calcium. To analyze these results, we imagine that each buffer acts to facilitate free calcium diffusion and thus suppresses gradient formation. This model leads to an exact equation and prediction of the concentrations of various buffers needed to inhibit development. The data fit this equation rather well if it is assumed that the free calcium concentration in the incipient tip is normally kept at about 10 µM and thus far above the general cytosolic level.