Graduate Thesis Or Dissertation
 

Gonadotropin releasing hormone-induced secretion of luteinizing hormone during the milk ejection reflex in the postpartum beef cow

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  • A study was conducted to determine the effect of the milk ejection reflex on exogenous gonadotropin releasing hormone (GnRH)- induced release of luteinizing hormone (LH) after short term calf removal. Twenty-four postpartum multiparous beef cows were assigned 3 randomly to groups arranged in a 2[superscript 3] factorial design to be either suckled (S) or nonsuckled (NS) and treated with saline (C) or GnRH. Calves were isolated from their dams for 4 hours on days 1 and 14 postpartum. At the end of 4 hours, calves were reunited with their dams in the S+C and S+GnRH groups while calves of dams in the NS+C and NS+GnRH groups remained separated an additional 2 hours. Cows were injected intravenously with saline or 200 ug GnRH following the 4 hour isolation period; 5 minutes after calves had begun suckling. Sera from jugular blood samples collected 15 minutes prior to the end of the 4 hour isolation period, immediately prior to injection (0 hour) and at 15 minute intervals thereafter for 120 minutes were analyzed for LH. Serum concentrations of LH in control cows did not differ due to suckling or stage of the postpartum period and averaged 2.3 + .1 ng/ml. Pituitary response to GnRH was determined by computing the quantity of LH released in 120 minutes. Quantity of LH released (ng • m1[superscript -1] • min[superscript -1] ) in response to GnRH on day 14 was greater (P<.001) than that released on day 1 in both suckled and nonsuckled cows (S+GnRH, 37.1 ± 3.9 vs 18.3 ± 5.0; NS+GnRH, 34.7 ± 5.9 vs 14.5 ± 1.1). However, GnRH-induced release of LH did not differ between suckled and nonsuckled cows on either day 1 or 14 postpartum. These data indicate that response of the bovine pituitary to GnRH during the postpartum period is not influenced by the act of suckling but is enhanced with time after parturition.
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