Changing dose of progesterone results in sudden changes in frequency of luteinizing hormone pulses and secretion of 17 beta-estradiol in bovine females.

The aim of the present study was to elucidate the time course according to which changes in circulating concentrations of progesterone influence pulsatile secretion of LH and secretion of 17 beta-estradiol. Our working hypothesis was that changing the dose of progesterone would result in changes in frequency of LH pulses and secretion of 17 beta-estradiol within 72 h. Five days after behavioral estrus, thirty-three cows were randomly assigned to one of five groups: 1) control, no treatment (CONT, n = 5); 2) treatment with two progesterone-releasing intravaginal devices (PRIDs) for 11 days (2PRID, 5-6 ng/ml plasma progesterone, n = 7); 3) treatment with a 0.5 PRID for 11 days (0.5PRID, 1-2 ng/ml plasma progesterone, n = 7); 4) treatment with 2 PRIDs for 8 days followed by treatment with a 0.5 PRID for 3 days (2-0.5PRID, n = 7); and 5) treatment with a 0.5 PRID for 8 days followed by treatment with 2 PRIDs for 3 days (0.5-2PRID, n = 7). Cows subject to PRID treatments received injections of prostaglandin F2 alpha on Days 1 and 2 (Day 0 = day of initiation of PRID treatments, fifth day of the estrous cycle in CONT cows) to lyse the existing corpus luteum. Cows were bled for 12 h at 15-min intervals on Day 7.5 of the treatment period (twelfth day of the estrous cycle in CONT cows). The dose of progesterone was changed on Day 8 in cows that were assigned to the 2-0.5PRID and 0.5-2PRID groups, and blood collections continued an additional 72 h to characterize profiles of circulating concentrations of LH and 17 beta-estradiol. Cows treated with a 0.5 PRID had a greater (p < 0.05) number of LH pulses and higher (p < 0.05) concentrations of 17 beta-estradiol throughout the entire blood collection period than cows in the 2PRID and CONT groups. An increase in the number of LH pulses was detected within 6 h after the change from the high to the low dose of progesterone (2-0.5PRID), and frequency of LH pulses was similar to that of cows in the 0.5PRID group for the remainder of the period of blood collection. LH pulse frequency declined within 6 h after the shift from the low to the high dose of progesterone (0.5-2PRID) and was similar to that of cows in the 2PRID group by 12 h after the dose was changed. Within 6 h after the dose of progesterone was changed, circulating concentrations of 17 beta-estradiol increased (p < 0.05) in cows shifted from the high to low dose (2-0.5PRID) and declined (p < 0.05) after the dose of progesterone was changed from low to high (0.5-2PRID). We conclude that changing the circulating concentrations of progesterone concurrently affects frequency of pulsatile LH release and secretion of 17 beta-estradiol within 6-24 h.