Micro-injection of inositol 1,3,4,5-tetrakisphosphate activates sea urchin eggs by a mechanism dependent on external Ca2+.

Micro-injection of submicromolar concentrations of inositol 1,3,4,5-tetrakisphosphate caused a raising of the fertilization envelope in eggs of the sea urchin Lytechinus variegatus. This effect was dependent both on the presence of extracellular Ca2+ and on co-injection with a Ca2+-mobilizing compound, inositol 2,4,5-trisphosphate. Inositol 1,3,4,5-tetrakisphosphate was the most potent compound tested in this assay; removal of the 3- or 5-phosphates or randomization of the phosphates in the inositol ring decreased its potency. These results show that inositol 1,3,4,5-tetrakisphosphate is an intracellular second messenger, and suggest that its function is to control cellular Ca2+ homoeostasis at the plasma membrane.