Endocrine and ovarian responses to exogenous estradiol-17 beta in 6-month-old heifers previously immunized against growth hormone-releasing factor.
We hypothesized that estradiol-17 beta (E2) was capable of stimulating an LH surge in 6-mo-old heifers immunized against growth hormone-releasing factor (GRFi). To test this hypothesis Angus x Simmental heifers were immunized against GRF (n = 11) or human serum albumin (HSAi) (n = 6) at 113 d of age. Heifers were injected i.m. with 500 micrograms of E2 at 172 d of age. To characterize the LH response, samples were collected at frequent intervals for 1 h before and 32 h after E2. To assess ovarian responses, real-time ultrasonography was performed from d 9 to 20, and serum progesterone was evaluated from d 6 to 25 after E2, then weekly for 13 wk. Exogenous E2 stimulated (P < .001) LH surges, preceded by negative E2 feedback that decreased serum LH, in all GRFi and HSAi heifers. During the negative feedback period, serum LH was greater (P < .001), and the change in LH from before to after E2 was lower (P < .05), in GRFi than in HSAi heifers. Puberty occurred in 7/17 heifers within 66 d after receiving E2. We conclude that GRFi does not impair the release of an E2-induced LH surge, but it does decrease hypothalamo-hypophyseal sensitivity to the inhibitory effect of E2.
Schoppee
P D
PD
Department of Animal Science, North Carolina State University, Raleigh, USA.
Armstrong
J D
JD
Harvey
R W
RW
Washburn
S P
SP
Felix
A
A
Campbell
R M
RM
D000818:Animals; D002417:Cattle / Q000378:metabolism / Q000502:physiology*; D004958:Estradiol / Q000097:blood / Q000494:pharmacology*; D005260:Female; D013007:Growth Hormone-Releasing Hormone / Q000276:immunology*; D007030:Hypothalamo-Hypophyseal System / Q000187:drug effects / Q000502:physiology; D007114:Immunization / Q000662:veterinary; D007986:Luteinizing Hormone / Q000097:blood*; D010053:Ovary / Q000187:drug effects* / Q000502:physiology; D011374:Progesterone / Q000097:blood*; D012709:Serum Albumin / Q000032:analysis; D012741:Sexual Maturation / Q000187:drug effects
D012709:Serum Albumin; D011374:Progesterone; D004958:Estradiol; D007986:Luteinizing Hormone; D013007:Growth Hormone-Releasing Hormone