Effect of FSH infusion on follicle development in GnRH agonist-treated gilts

The aim was to investigate the effect of infusion of purified FSH alone on follicle development in hypogonadotrophic GnRH agonist-treated gilts. Large -White hybrid gilts (n = 12) were treated during the mid-luteal phase and a gain after 28 days (day 0) with a potent slow releasing GnRH agonist. On da y 3, seven gilts were infused for 168 h with 1.5 S1 units oFSH h(-1) (equiv alent to 1.5 units of bioactivity of NIH-FSH-S1 standard) and blood samples were collected. Ovaries were then recovered and all follicles greater than or equal to 1 mm in diameter were dissected and incubated for 2 h in 1 ml Eagle's minimum essential medium. The ovaries were recovered from the remai ning five GnRH agonist-treated gilts on day 10 and also from five cyclic gi lts during the late follicular phase (controls). Plasma FSH concentrations in GnRH agonist-treated gilts were lower (P < 0.01) than in follicular phas e controls, increased (P < 0.001) after 1 h of FSH infusion and reached a p lateau similar (P > 0.1) to that of controls after 8 h. Basal LH concentrat ions were similar (P > 0.1) between GnRH agonist-treated and control gilts and remained unchanged (P > 0.1) throughout the infusion period. GnRH agoni st treatment reduced (P < 0.01) basal oestradiol concentrations compared wi th control gilts. infusion with FSH alone increased (P < 0.001) plasma oest radiol concentrations after 96 h compared with those before infusion; when the animals were killed oestradiol concentrations were higher (P < 0.01) in GnRH agonist-treated gilts infused with FSH than in controls. This was als o apparent by vulval swelling and behavioural oestrus. There were more foll icles greater than or equal to 1 mm in diameter in the GnRH agonist-treated groups than in the controls (184, 153 and 86 per animal; P < 0.01). Infusi on with FSH increased the maximum follicle diameter (GnRH agonist: < 4 mm; FSH infused: < 12 mm; controls: < 10 mm) and tended to increase (P < 0.07) the mean number of follicles greater than or equal to 6 mm diameter per ani mal (FSH infused: 53; controls: 21). Total oestradiol production in vitro b y follicles greater than or equal to 1 mm was higher (P < 0.01) in GnRH ago nist-treated gilts infused with FSH and in follicular phase controls than i n animals treated with GnRH agonist alone. However, oestradiol and testoste rone secretion in vitro per follicle greater than or equal to 6 mm in diame ter was lower (P < 0.05) in FSH-infused animals than in controls. In summar y, although infusion of FSH alone stimulated the growth of multiple follicl es of preovulatory size in GnRH agonist-treated gilts, steroidogenic output by individual follicles was impaired.