Adenophostin A Sensitizes CICR
Following fertilization or injection of SF, mammalian eggs become highly sensitized to the injection of CaCl2. This sensitized CICR mechanism is thought to be responsible for the persistence of oscillations in fertilized mammalian eggs. CICR in mammalian eggs appears to be mediated by the IP3R, although the molecular mechanism responsible for it is not known. Having demonstrated that SF stimulates IP3 production, IP3 alone may be the sensitizing stimulus. To test this hypothesis, we evaluated the effects of adenophostin A, an IP3R agonist, on long-term sensitization to CaCl2 injections. Adenophostin A was chosen because of its close structural homology to IP3, its high affinity for the IP3R, and its long half-life. As expected, unfertilized control eggs injected with CaCl2 (2.0 mM) alone showed a minor rise following the injection, and the amplitude of this rise reached a mean peak of 60 ± 41 nM (n = 6; Fig. 6A). No additional [Ca2+] rises were observed.
Injection of mouse eggs with 10 ^M adenophostin A induced [Ca2+]i oscillations that subsided after a variable amount of time (data not shown). After the adenophostin A-induced oscillations had significantly decreased in frequency or had ceased altogether, injection of CaCl2 (2.0 mM) caused an immediate rise that reached a mean peak [Ca2+]i of 409 ± 27 nM, which was significantly higher than the rise induced by CaCl2 injection in control eggs (P < 0.05; n = 14). Injection of CaCl2 reinitiated oscillations in some adenophostin A-injected eggs that had stopped oscillating (Fig. 6B; n = 4) or increased the frequency of spikes in currently oscillating eggs (n = 5). Therefore, agonists of the IP3R and in particular increased levels of [IP3]i induced by the sperm and SF may be sufficient to sensitize CICR and perpetuate [Ca2+]i oscillations.
FIG. 6. Adenophostin A sensitizes CICR. A) Injection of untreated eggs with CaCl2 (2.0 mM) causes only a slight [Ca2+] rise. B) Eggs injected with adenophostin A (Ad; 10 ^M) exhibit [Ca2+] oscillations. Following cessation of adenophostin A-induced oscillations, injection of CaCl2 (2.0 mM) induced large Ca2+ responses, and in many eggs, high-frequency oscillations were reinitiated. Arrows indicate times of injection of Ad or CaCl2, respectively.