Multiple stores of calcium are released in the sea urchin egg during fertilization.

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Abstract
Fertilization initiates a transient increase in intracellular Ca2+ principally by Ca2+ release from intracellular stores. Possible multiple Ca2+ stores and multiple receptor regulation of the same store have been reported. Here we report the presence of at least two independent intracellular Ca2+ stores in the sea urchin egg, which are released during fertilization. Ca2+ release from one store is mediated by inositol 1,4,5-trisphosphate (IP3) and is sensitive to low molecular weight heparin. The other store is heparin insensitive and independent of IP3 regulation, but the regulatory factor remains unidentified. A transient increase in Ca2+ in heparin-loaded eggs is observed during fertilization, which suggests that IP3-independent Ca2+ release mediates the production of IP3 and release of the IP3-dependent store. Experiments presented here do not support the idea of sperm receptor coupling to inositol phospholipid hydrolysis through a GTP-binding protein mediating the fertilization response.