SERUM LUTEINIZING-HORMONE, 13,14-DIHYDRO-15-KETO-PROSTAGLANDIN-F2-ALPHA AND CORTISOL PROFILES DURING POSTPARTUM ANESTRUS IN BRAHMAN AND ANGUS COWS

Pluriparous suckled Brahman and Angus cows were utilized to evaluate t he effect of breed, day after calving and endogenous opioid peptides ( EOP) on hormonal profiles during postpartum anestrus. On Days 17 and 3 4 after calving, blood samples with and without heparin were collected at 15- and 30-min intervals, respectively, for a 7-h period via jugul ar cannula. Two hours after the start of blood sampling, cows of each breed were administered either 1 mg/kg iv naloxone or saline. Three ho urs later, all animals received 10 ng/kg iv GnRH. On Day 34 after calv ing cows received 0.2 IU/kg iv ACTH. Mean LH, basal LH and area under the LH curve increased (P < 0.01) from Day 17 to Day 34 after calving. Height of LH pulses increased (P < 0.05) by Day 34 after calving. Bra hman cows had higher (P < 0.05) mean LH, basal LH, LH pulse frequency and area under the LH curve than Angus cows. Naloxone increased postch allenge area under the LH curve in treated cows above that of control cows (P < 0.06). Naloxone also increased the postchallenge area under the LH curve above that of the prechallenge level (P < 0.01). No breed differences in the response to the naloxone challenge were observed. The LH response to naloxone challenge occurred earlier on Day 34 than on Day 17 after calving but the amount of LH released was similar betw een days. The GnRH-induced LH release was greater in Brahman than in A ngus cows (P < 0.04). Mean cortisol concentrations and area under the cortisol curve decreased (P < 0.05) between Day 17 and Day 34 after ca lving. Mean cortisol concentrations and area under the cortisol curve were lower (P < 0.01) in Brahman than in Angus cows. Cortisol secretio n after ACTH treatment was similar between Brahman and Angus cows. The cortisol response after ACTH challenge was positively correlated (r=0 .68; P < 0.001) to the prechallenge area under the cortisol curve. Und er optimal environmental conditions Brahman cows have a greater LH rel ease and their anterior hypophysis is more sensitive to GnRH challenge than the Angus cows.