TEMPORAL ASPECTS OF OVARIAN FOLLICULAR-GROWTH AND STEROIDOGENESIS FOLLOWING EXOGENOUS FOLLICLE-STIMULATING-HORMONE IN ANGUS HEIFERS

Ultrasonography and endocrine assay techniques were used to monitor st ructural and hormonal alterations made by the ovary in response to the biological actions of pituitary-derived follicle-stimulating hormone (FSH-P). Angus heifers (n = 36) were allotted to receive injections (t wice per day) of either FSH-P (up to a total of 28 mg over a maximum o f 4 days beginning on Day 10 of a synchronized estrous cycle) or salin e in order to quantify temporal relationships among follicle growth an d steroid hormone profiles. Transrectal ultrasonography was utilized a t 12-h intervals to monitor and record follicle growth. Plasma was col lected every 12 h for the first 48 h of the experiment and then every 6 h for the remainder of the experiment. At 48 and 60 h after the onse t of treatments, prostaglandin F2 alpha (PGF2 alpha; 25 mg) was admini stered (i.m.). FSH-treated heifers (n = 6 at each time) were terminate d at 24, 48, 72 and 96 h following the onset of treatment Saline-treat ed heifers were terminated at 24 and 96 h (n = 6 at each time). After ovaries were obtained, follicular number and size were recorded and fo llicular fluid (FF) was collected. Plasma concentration of progesteron e (P) and estradiol (E2) and FF concentration of P, E2, estrone, testo sterone and androstenedione were determined by radioimmunoassays. Plas ma concentration of E2 increased (P < 0.05) within 36 h of initiation of FSH treatment. Plasma P decreased (P < 0.0001) by 12 h post-PGF2 al pha. Ultrasonographic examination revealed a significant decrease in t he number of small follicles by 48 h, whereas the number of medium fol licles increased (P < 0.05) by 60 h after the initiation of FSH treatm ent. The number of large follicles (LF greater than or equal to 10 mm diameter) increased(P < 0.01) over the course of the experiment. The t otal number of ovarian follicles (TF) 24 h after the start of FSH trea tment was correlated (r = 0.99; P < 0.0001) with the number of small f ollicles (SF less than or equal to 5mm). At 72 h after the onset of FS H treatment, the number of medium follicles (i.e. 6-9 mm) was correlat ed with TF (r = 0.97; P < 0.0001). Estradiol was the predominant FF st eroid. Follicular fluid E2 was greatest in follicles at 72 h after FSH treatment. Follicular fluid E2 and plasma E2 were positively correlat ed (r = 0.66; P < 0.001). Follicular aromatase activity was estimated by evaluating the ratio of FF estrogens (E) to androgens (A). Elevated aromatase activity (E:A ratio > 1.0) was detected in 196 of 206 folli cles. The estrogen to progesterone ratio was used as an estimate of fo llicle viability. Eighty-five percent of the follicles were estimated to be viable (E:P ratio > 1.0). The peak E:A ratio in LF preceded by 2 4 h the peak concentration in FF E2 and plasma E2. In MF and SF the E: A ratio increased by 72 h. Enhancement of ovarian follicular growth (i .e. increased number and size of follicles; increased steroidogenesis) by exogenous, pituitary-derived FSH is characterized by (1) increased activity of aromatase, and (2) accumulation of FF E2, events which te mporally preceded the increase in plasma concentration of E2. These ob servations will aid efforts to incorporate recombinant bovine FSH and somatotropin in an effort to develop more predictable superstimulation and ovulation induction protocols.