Temporal aspects of follicular growth and steroidogenesis in response to exogenous follicle-stimulating hormone administration during a superovulation regimen

Abstract

Angus heifers were allotted to either pituitary-derived follicle-stimulating hormone (FSH-P) or saline treatment groups in order to quantify temporal relationships among follicle growth and steroid hormone profiles. Heifers in the FSH-P treatment group received bi-daily injections of pituitary-derived follicle-stimulating hormone (28 mg over 4 days) and heifers designated as saline received bi-daily injections of saline. Plasma was collected 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 onset of treatments heifers received prostaglandin (PGF2a). FSH- treated heifers (n=6/time point) were terminated at 24, 48, 72, and 96 h following the onset of treatment; salinetreated heifers were terminated at 24 and 96 h (n=6/time point). Ovaries were collected, weighed, follicular number and size recorded and follicular fluid (FF) collected. Plasma concentration of progesterone (P) and estradiol (E2) and FF concentration of P, E2, estrone, testosterone, and androstenedione were determined by radioimmunoassays. Plasma concentration of E2 increased (P<0.03) by 35.5% within 36 h of initiation of FSH treatment. Plasma P decreased (P<0.0001) 53.6% by 12 h post-prostaglandin. E2 was the predominant FF steroid. FF E2 was greatest in follicles at 72 h after FSH treatment. FF E2 and plasma E2 were positively correlated (r=0.66; P<0.001 ). The number of large follicles (LF > I 0 mm diameter) increased over the course of the experiment. The total number of ovarian follicles 24 h after the start of FSH treatment was correlated (r=0.99; P<0.0001) with the number of small follicles (SF < 5mm). At 72 h after the onset of FSH treatment the number of medium follicles (MF = 6-9mm) was correlated with TF (r=0.97; P<0.0001). 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 1 96 of 206 follicles. The estrogen to progesterone ratio was used as an estimate of follicle viability. Eighty-five percent of the follicles were estimated to be viable (E:P ratio >1.0). The peak in E:A ratio in LF preceded by 24 h the peak concentration in FF E2 and plasma E2. In MF and SF the E:A ratio increased by 72 h. These results indicate that FSH's enhancement of ovarian follicle number was accompanied by increased: (1 ) activity of aromatase; and, (2) accumulation of FF E2, events which temporally preceded the increase in plasma concentration of E2.