Effects of oxytocin on follicular development and duration of the estrous cycle in heifers

Holstein heifers were used to study effects of exogenous administration of oxytocin on luteal function and ovarian follicular development. Twelve heif ers were monitored for 1 estrous cycle to confirm normal ovarian function. At the subsequent estrus, these animals were randomly assigned to 1 of 3 tr eatments: saline control, (Group 1, n = 4), oxytocin (Group 2, n = 4) and s aline pregnant (Group 3, n = 4). Group 2 received continuous infusion of ox ytocin (1.9 mg/d) from Days 14 to 26 after estrus, while Groups 1 and 3 rec eived saline infusion during the same period. Group 3 were artificially ins eminated at estrus. Daily blood samples were collected for oxytocin and pro gesterone assay. Ovarian follicles and corpus luteum (CL) development were monitored daily by transrectal ultrasonography until Day 32 after estrus. P lasma progesterone (P-4) concentrations prior to initiation of infusion wer e 7.6 +/- 1.3 ng/mL on Day 14. They then decreased to <1 ng/mL on Day 19 fo r Group 1 and on Day 28 for Group 2. The interestrous interval was longer ( P < 0.05) for heifers that received oxytocin infusion. During the infusion period Pq concentrations were not different (P > 0.05) between Group 2 and 3 but declined gradually from Day 20 in Group 2 despite the presence of hig h plasma oxytocin concentrations. Control heifers had 2 waves of follicular growth, with the second dominant follicle ovulating. Three of the 4 oxytoc in-infused animals had an additional wave, with the third dominant follicle ovulating. Oxytocin infusion had no effect on size of the ovulating follic le (P > 0.05) and the number of Class 1 follicles (3 to 5 mm, P > 0.1). Dif ferences in the number of Class 2 follicles (6 to 9 mm) among treatments on Days 15 to 22 after estrus were not detected (P > 0.1) except on Days 23 t o 26, when Group 2 had fewer follicles than Group 3 (P < 0.05).The results show that continuous infusion of oxytocin during normal luteolysis delays l uteal regression without inhibiting follicular development. (C) 2000 by Els evier Science Inc.