IP3R Agonists Sensitize CICR
A sensitized CICR mechanism is critical for the persistence of [Ca2+]i oscillations in fertilized mammalian eggs, although the molecular mechanism(s) responsible for sensitizing the CICR is not known. Because SF stimulates production of IP3, increased [IP3]i may be responsible for sensitizing CICR. Our finding that injection of adenophostin A, which shares structural motifs with IP3, considerably enhanced Ca2+ release in response to injection of CaCl2 suggests that elevated [IP3]i alone might be capable of sensitizing CICR. Because adenophostin A is a nonhy-drolysable IP3R agonist, it is presumed that its injection resulted in steady intracellular levels of the agonist. Whether [IP3]i oscillates with each [Ca2+]i rise during fertilization is not known.
However, enhanced CICR responses are observed in fertilized eggs between [Ca2+]i rises, suggesting that if IP3 is persistently elevated, it could sensitize the IP3R to Ca2+ to produce [Ca2+]i oscillations. The suggestion that persistently elevated [IP3]i induced by the sperm or by SF may be the mechanism responsible for sensitizing CICR was recently strengthened by the finding that uncaging of IP3 at relatively low amounts for a prolonged period of time in mouse eggs induced fertilization-like oscillations and sensitized the CICR mechanism.
Our present findings demonstrate for the first time at the single cell level that pig SF stimulates production of IP3. These results now provide the basis for future work on whether the active component of SF is an unidentified testicular isoform of PLC or an activator of the egg PLCs.