Effects of exogenous steroid hormones on the dominant follicle maintained by a Norgestomet implant in heifers

The maintenance of the dominant follicle during the follicular phase by a 9-d Norgestomet implant was used to study the effects of progesterone (P4), estradiol-17β (E2) and testosterone (T) on follicular turnover in heifers. From day 4 of Norgestomet treatment, heifers (n = 16) were injected daily for 4 d with corn oil (control), 150 mg P4, 10 mg E2 or 200 mg T (n = 4 per treatment). Follicular changes were monitored daily by ultrasonography. Plasma steroids and serum LH were measured by validated radioimmunoassays. In control heifers, dominant follicles were maintained during implantation and ovulated after implant removal. In both P4- and T-treated heifers, dominant follicles began to regress during the implant period and the dominant follicle from a new wave of follicles ovulated. In E2-treated heifers, dominant follicles regressed (3/4) and dominant follicles from the next wave ovulated. The dominant follicle became cystic in the fourth E2-treated heifer. Mean serum LH levels were suppressed (P < 0.01) from 0.55 ± 0.01 ng mL−1 (mean ± SEM) in control to 0.21 ± 0.02, 0.19 ± 0.02 and 0.43 ± 0.02 in P4-, E2- and T-treated heifers, respectively. The number of LH pulses in 8 h was reduced (P < 0.01) from 6.25 ± 0.75 in control to 2.00 ± 1.00 and 4.66 ± 0.35 in P4- and E2-treated heifers but not in T-treated heifers (6.33 ± 1.44). LH pulse amplitude (ng mL−1) was also suppressed (P < 0.01) in P4-treated (0.10 ± 0.04) and E2-treated (0.09 ± 0.02) heifers when compared with control heifers (0.41 ± 0.04) but not in T-treated heifers (0.42 ± 0.17). Exogenous P4 and E2 caused regression of the Norgestomet-maintained dominant follicle, and this effect was associated with altered serum LH profile. Injection of T caused atresia of the dominant follicle without altering the LH profile. Key words: Dominant follicle, atresia, heifers, steroids, LH, ultrasound, E2, estradiol-17β; P4, progesterone; T, testosterone