Several undesirable effects have been associated with the use of U73122; thus, the specificity of this inhibitor as a PI inhibitor has been questioned. At the concentrations used in our study, U73122 did not induce an increase in basal [Ca2+]i levels. Moreover, the finding that it did not inhibit [Ca2+]i oscillations induced by injection of IP3 reveals that the Ca2+ release mechanisms in these eggs were functional. Furthermore, U73122, at concentrations similar to those used in this study, inhibited the activity of PLC7 isolated from fertilized Xenopus oocytes and blocked fertilization-induced IP3 production and Ca2+ release in these oocytes.
In previous studies in nonmammalian eggs, fertilization was accompanied by an increase in the turnover of polyphosphoinositides and [IP3]i. In these studies, [IP3]i concentrations were determined in groups of asynchronous eggs, so the exact relation between the increased [IP3]i and fertilization could not be determined. Recently, using an IP3 mass assay quantification procedure, addition of SF into sea urchin homogenates induced IP3 production. However, cellular homogenates do not always reproduce the events occurring in intact cells. For these reasons, it was important to determine whether [IP3]i increased in intact single cells following microinjection of SF. In the present study, we demonstrated that injection of SF induces a large increase in [IP3]i in single Xenopus oocytes. Further, the increases in [IP3]i induced by SF were similar to those induced by addition of lysophosphatidic acid, an agonist that activates a G-protein-coupled receptor in these oocytes. Thus, the finding that boar SF is capable of initiating normal PI activation in Xenopus oocytes reflects the degree of conservation of the SF active component(s). We did not determine whether SF induces IP3 production in mouse eggs. However, SF-induced activation in mouse eggs has been accompanied by progressive downregulation of the IP3R. Degradation of the IP3R was also observed in fertilized eggs, but it was not detected in eggs activated by agents that did not stimulate the PI pathway. Degradation of the IP3R in somatic cells has been demonstrated to be exclusively associated with IP3 production. Thus, these results suggest that fertilization and SF also persistently stimulate the PI pathway in mammalian eggs.