INFLUENCE OF THYROTROPIN-RELEASING HORMONE (TRH) ON PLASMA PROGESTERONE AND PITUITARY HORMONE CONCENTRATIONS IN CATTLE

Four Friesian heifers 14–16 mo of age were injected through jugular catheters six times per day (0600, 0900, 1200, 1500, 1800 and 2100 h) with 100 μg thyrotropin-releasing hormone or saline (control). TRH and saline treatment continued for 14 days beginning 1 day after a luteolytic dose of prostaglandin (PGF2α, 30 mg) which was given during diestrus. Blood samples were collected via the catheters at 0900 and 2100 h each day for progesterone prolactin and LH determinations and before and 10, 20, 30, 40 and 60 min after the 0600 and 2100 h TRH injections on the 1st, 7th and 13th days of treatment for growth hormone (GH) and prolactin analysis. After 14 days, heifers were injected with 10 mg PGF2α and changes in blood progesterone were measured for 24 h. There were two treatment periods; hence, each animal was subjected to both TRH and saline injections. Results support the following conclusions: (1) Prolactin secretion in response to TRH did not diminish with time. (2) Plasma progesterone of TRH-treated animals was lower (P < 0.05) from approximately day 5 of the cycle (estrus = day 0) until corpora lutea were destroyed with PGF2α; this provides no support for a luteotrophic role for prolactin. (3) Corpora lutea of all animals regressed after 10 mg PGF2α, suggesting that TRH had not increased the resistance of the corpus luteum to the luteolytic effects of PGF2α.