Expression patterns of genes identified during an in situ hybridization screen in sea urchin.

Sea urchin eggs

The slow block to polyspermy in the sea urchin is mediated by the phosphatidyl inositol diphosphate (PIP₂) secondary messenger system. Activation of the binding receptors activates phospholipase C, which cleaves PIP₂ in the egg plasma membrane, releasing inositol triphosphate (IP₃) into the egg cell cytoplasm. IP₃ diffuses to the endoplasmic reticulum, where it opens calcium channels.

Humans use of IP3

Its main functions are to mobilize Ca²⁺ from storage organelles and to regulate cell proliferation and other cellular reactions.

In smooth muscle cells, for example, the increase in concentration of cytoplasmic calcium results in the contraction of the muscle cell.