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 accordin g to which changes in circulating concentrations of progesterone influ ence pulsatile secretion of LH and secretion of 17 beta-estradiol. Our working hypothesis was that changing the dose of progesterone would r esult in changes in frequency of LH pulses and secretion of 17 beta-es tradiol within 72 h. Five days after behavioral estrus, thirty-three c ows were randomly assigned to one of five groups: 1) control, no treat ment (CONT, n = 5); 2) treatment with two progesterone-releasing intra vaginal devices (PRIDs) for 11 days (2PRID, 5-6 ng/ml plasma progester one, 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); a nd 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 F-2 alpha on Days 1 and 2 (Day 0 = day of initiation of PRID treatments, fifth day of the estrous cycl e 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 c hanged on Day 8 in cows that were assigned to the 2-0.5PRID and 0.5-2P RID groups, and blood collections continued an additional 72 h to char acterize profiles of circulating concentrations of LH and 17 beta-estr adiol. 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 t hroughout the entire blood collection period than cows in the 2PRID an d CONT groups. An increase in the number of LH pulses was detected wit hin 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 i n the 0.5PRID group for the remainder of the period of blood collectio n, 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 concentrat ions of 17 beta-estradiol increased (p < 0.05) in cows shifted from th e 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 concurren tly affects frequency of pulsatile LH release and secretion of 17 beta -estradiol within 6-24 h.