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 b y a 9-d Norgestomet implant was used to study the effects of progester one (P4), estradiol-17 beta (E2) and testosterone (T) on follicular tu rnover in heifers. From day 4 of Norgestomet treatment, heifers (n = 1 6) 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 monit ored daily by ultrasonography. Plasma steroids and serum LH were measu red by validated radioimmunoassays. In control heifers, dominant folli cles were maintained during implantation and ovulated after implant re moval. 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. T he dominant follicle became cystic in the fourth E2-treated heifer. Me an 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 +/- 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-treat ed (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. Injecti on of T caused atresia of the dominant follicle without altering the L H profile.