41
Regulation of preovulatory estradiol and its impacts throughout the bovine estrous cycle.
oocyte maturation, sperm transport, uterine environment, and embryo survival. Furthermore, cattle with
elevated preovulatory estradiol (HighE2) concentrations prior to fixed time AI had greater conception
rates and pregnancy maintenance compared to cattle that did not experience a rise in estradiol (LowE2).
However, the mechanisms by which estradiol exerts these effects is not well-characterized. HighE2
animals had faster follicular growth rates and upregulation of the entire steroidogenic pathway compared
to LowE2 animals. When LowE2 animals were supplemented with physiological pulses of GnRH, they
had similar LH pulse frequency to HighE2 animals and an increase in circulating estradiol. When a
physiological dose of GnRH was administered at CIDR removal during a synchronization protocol estrus
expression, interval to estrus, and conception rates to AI were improved. Furthermore, relative abundance
of GnRH-I and II within bovine antral follicles was increased in follicles with low concentrations of
estradiol compared to follicles with elevated estradiol suggesting GnRH-I and II may act as local
regulators of estradiol production. Estradiol can also have a direct impact on the uterus, on d 6 post-AI,
HighE2 heifers had increased accessory sperm numbers, and produced embryos that were more advanced
and had improved quality compared to LowE2 heifers; however, there was no difference in embryo
recovery rates. When embryos were transferred to ovariectomized estradiol supplemented cows, cows in
the control treatment lost 75% of existing pregnancies, while cows receiving estradiol lost only 35% of
existing pregnancies, during the period between maternal recognition of pregnancy and embryo
attachment. But following a fixed-time AI protocol, there were no differences in conceptus survival
based on recovery rates, IFNT concentrations, and apoptosis of the trophectoderm on d 16 after AI
between HighE2 and LowE2 animals. However, glucose transporter expression in the endometrium, and
also glucose and protein concentration in ULF was influenced by preovulatory estradiol concentrations
and conceptus presence. In conclusion, increased preovulatory concentrations of estradiol are mediated
through an upregulation of the steroidogenic pathway, and HighE2 animals had increased sperm transport
and improved embryo quality, but not increased embryo survival to maternal recognition of pregnancy.
Thus, increased pregnancy success among HighE2 animals must be regulated by differences in conceptus
survival between d 16 and d 29 of gestation.