Role of exogenous estrogen in initiation of estrus and induction of an LH surge

Abstract Text: Among cattle the Luteinizing Hormone (LH) surge that causes ovulation occurs shortly after the onset of a spontaneous estrus.  In addition an injection of 100 mg of Gonadotropin Releasing Hormone (GnRH) can induce an LH surge capable of inducing ovulation.  We hypothesized that different preovulatory estradiol profiles would result in different ovulatory LH surges, and that an injection of GnRH (100 mg) would induce a secondary LH surge among cows that exhibited standing estrus prior to the GnRH injection.  In order to establish the importance of estradiol on initiating an LH surge, ovariectomized multiparous cows (n=26) received estradiol cypionate (ECP), estradiol benzoate (EB) or no treatment (CON) to mimic a preovulatory period.  Prior to treatment, all cows received a progesterone-releasing device (CIDR) for 7 d, 25 mg injection of prostaglandin-F2α (PGF) at CIDR removal (d -2), and an injection of GnRH (100 µg; d 0 h 0) 2 d later to mimic the follicular phase.  Utilizing a 3x3 Latin Square design, cows received either ECP 36 h before GnRH injection, EB 12 h before GnRH injection, or no estradiol (CON).  Blood samples were collected every 4 h from h -36 to GnRH and every 30 min from GnRH until h 2.5.  Peak LH concentration and interval from GnRH to peak LH was analyzed using the GLM procedure in SAS.  Circulating concentrations of LH were analyzed as repeated measures using the MIXED procedure in SAS.  There was an effect of treatment on peak concentration of LH with EB treated cows having a greater (P < 0.01) peak LH concentrations (20.5±3.0 ng/mL) than ECP (9.9±3.0 ng/mL) or CON (10.6±3.1 ng/mL).  In addition, there was a difference in the interval from GnRH to peak LH concentration with ECP treated cows reaching peak LH concentrations 638.3±41.8 min before GnRH and EB and CON cows reaching peak LH concentrations after GnRH (83.3±41.8 and 35.3±43.0 min, respectively).  There was an effect of treatment, time, and a treatment by time interaction (P < 0.01) on circulating concentrations of LH, with ECP treated cows having increased concentrations of LH at hour -16, -12, -8, and -4 compared to EB and CON.  However, EB had greater concentrations of LH than ECP at 30, 60, 90, 120, and 150 min after GnRH and CON having greater concentrations of LH at 30 and 60 min after GnRH compared to ECP.  In summary, exogenous estradiol influenced timing and peak concentrations of an LH surge.

Keywords: estradiol, Luteinizing Hormone , Gonadotropin Releasing Hormone