The signal transduction pathway(s) utilized by the sperm or SF to signal [Ca2+]i oscillations in mammalian eggs is not fully elucidated. Our results obtained by injection of SF into mouse eggs and Xenopus oocytes show 1) that incubation of SF or eggs with U73122, an inhibitor of the PI pathway, blocks the generation of [Ca2+]i oscillations, 2) that injection of SF induces a significant increase in [IP3]i, 3) that PLC71, PLC72, and PLC84 and/or its splice variants from the sperm are not likely to be the active component(s) of boar SF, and 4) that injecting adenosphostin A, an IP3R agonist, results in a sensitized CICR mechanism(s). We conclude that SF initiates and sustains [Ca2+]i rises in mammalian eggs by persistently activating the PI pathway.
SF Stimulates the PI Pathway to Induce [Ca2+]i Oscillations
Fertilization-induced [Ca2+]i oscillations in mouse eggs have been reported to be blocked, in a dose-dependent manner, by incubation of eggs with the aminosteroid U73122. In our studies, incubation of mouse eggs with 20 U73122 also blocked the oscillations triggered by injection of SF and this concentration was similar to that required to block fertilization-associated oscillations. Similarly, preincubation of sea urchin eggs with 20 ^M U73122 completely blocked the sperm-induced [Ca2+]i rise . U73122 (30 ^M) was also effective in inhibiting SF-induced [Ca2+] oscillations in mouse eggs if SF, rather than eggs, was preincubated with the inhibitor prior to its injection.