Estradiol synergism and inhibition of insulin‐induced meiotic maturation in Rana pipiens oocytes in vitro

Abstract

AbstractEstradiol 17‐β (E2) was found to either inhibit or synergize Na‐insulin (Ins)‐induced meiotic maturation of Rana oocytes. Inhibition of Ins activity occurred in the presence of the follicular investments of the oocyte; synergism with Ins occurred in oocytes denuded of the follicle wall. Similarly, co‐incubation of E2 with frog pituitary homogenate (FPH) or pregnenolone (Pe) significantly decreased meiotic reinitiation as determined by germinal vesicle dissolution (GVD) in follicle‐enclosed oocytes. By contrast, E2 had no consistently significant effect on progesterone (P)‐induced meiosis in follicle‐enclosed oocytes. Furthermore, E2 had no significant effect, either inhibitory or synergistic, on Pe‐ or P‐induced GVD of denuded oocytes. Thus, of the meiotogens tested (Ins, P, Pe, FPH), all but P were consistently inhibited by E2 in the presence of the follicle wall. Na‐insulin was the only meiotogen tested (Ins, P, Pe) which was potentiated by E2 in denuded oocytes, However, when E2 and Ins were spatially separated on the surface of individual intact follicles, the result was synergism of Ins‐induced GVD rather than inhibition. These results suggest that Ins acts to induce GVD in the denuded oocyte through a mechanism distinct from that used by P (ie, Ins mechanism allows E2 synergism while the P mechanism does not). The E2 inhibitory effect on Ins‐induced GVD appears to be dependent upon simultaneous exposure of follicle wall tissue to mixtures of E2 and Ins. The synergistic effect of E2 on Ins‐induced GVD is dependent upon the simultaneous exposure of the oocyte surface to Ins and E2, either as a homogenous mixture in the case of denuded oocytes or as single substances at independent sites, for follicle‐enclosed oocytes.