Elevated progesterone concentrations enhance prostaglandin F2alpha synthesis in dairy cows

The objective was to evaluate the influence of varying plasma progesterone (P(4)) concentrations throughout the luteal phase in dairy cows on PGF(2alpha) production (assessed as plasma concentrations of 13,14-dihydro-15-keto-PGF(2alpha); PGFM) following treatment with estradiol-17beta (E(2)) or oxytocin (OT). In all experiments, time of ovulations was synchronized with the OvSynch protocol and Day 0 corresponded to day of second GnRH injection. In Experiment 1, non-lactating dairy cows on Day 6 remained non-treated (n=9), received 20mg LH (n=7), or had ovarian follicles larger than 6mm aspirated (n=8). In Experiment 2, cows on Day 6 were untreated (n=9) or received 5000 IU hCG (n=10). In Experiments 1 and 2, all cows received 3mg E(2) on Day 17, and blood samples were collected every 30 min from 2h before to 10h after E(2). Experiment 3 was conducted in two periods, each from Days 0 to 17 of the estrous cycle. At the end of Period 1, animals switched treatments in a crossover arrangement. Animals in Group 2/8 (n=4) received 2 kg/d of concentrate in the first period and 8 kg/d in the second period. Animals in Group 8/2 (n=7) received the alternate sequence. Blood was collected daily for measurement of P(4) 4h after concentrate feeding. On Day 17, blood was collected from 1h before to 1h after a 100 IU OT injection. In Experiment 1, both plasma P(4) and release of PGF(2alpha) were similar between LH-treated and control cows (P>0.10). In Experiment 2, plasma P(4) was elevated to a greater extent on Day 17 in cows treated with hCG (P<0.05) and plasma PGFM was also greater in hCG-treated animals (treatment x time interaction; P<0.05). In Experiment 3, there was a group x period interaction (P<0.01) for plasma P(4), indicating that less concentrate feeding was associated with greater plasma P(4). Release of PGF(2alpha) in response to OT was greater for cows receiving less concentrate (group x period interaction; P<0.05). In conclusion, dairy cows with more elevated blood P(4) concentrations released more PGF(2alpha) in response to E(2) or OT.