Acute follicular response to FSH in heifers downregulated long term with a GnRH agonist and with suppressed ovarian follicular growth

The objectives of this study were to (1) ascertain ovarian follicular status in heifers where the pituitary gonadotrope cells producing LH and FSH were downregulated by long-term treatment with a GnRH agonist and (2) characterize the acute superstimulation of follicular growth in response to FSH in heifers where the pituitary was downregulated with a GnRH agonist. At the start of the study, heifers (3 year old) were implanted with GnRH agonist (n = 20) or received no treatment (control, n = 5). After 6 months, follicular growth in heifers implanted with GnRH agonist was restricted to early antral follicles (2–4 mm). At this time, these heifers were allocated to four groups and follicular development was superstimulated with FSH as follows: group 1 (n = 5) and group 2 (n = 5), a conventional FSH protocol with injections over 4 days (Days 0–3) with Group 2 receiving two times the normal dose of FSH; group 3 (n = 5), a single injection of FSH in saline on Day 0; group 4 (n = 5), a single injection of FSH in slow-release polyvinylpyrrolidone on Day 0. Follicular growth in the control heifers (n = 5) was superstimulated using the conventional 4-day FSH protocol. On Day 5, heifers in groups 1 and 2 had similar (P > 0.05) numbers of follicles in the size categories 6 to 7 mm (12.8 ± 3.0 follicles) and 8 to 9 mm (6.5 ± 1.0) as control heifers (6–7 mm, 14.6 ± 2.2; 8–9 mm, 6.2 ± 2.2) and six of the former heifers had follicles 10 mm (3.2 ± 1.4). Also on Day 5, follicular growth for heifers in group 3 was restricted to 4 to 5 mm (11.6 ± 3.0) and 6 to 7 mm (7.6 ± 3.7), whereas four out of five heifers in group 4 had follicles of 6 to 7 mm (8.2 ± 2.3) and three heifers had follicles of 8 to 9 mm (4.6 ± 2.2) and 10 mm (2.2 ± 0.9). Injection of exogenous LH on Day 5 induced ovulation in 9 out of 10 heifers in groups 1 and 2, no heifers in group 3, and 2 of 5 heifers in group 4. Plasma concentrations of progesterone 6 days after ovulation were the same (P > 0.05) for heifers in groups 1 and 2 (8.9 ± 0.9 ng/mL) and control heifers (10.0 ± 0.9 ng/mL). This study has shown that heifers treated chronically with a GnRH agonist have suppressed ovarian follicular growth but are able to respond to acute superstimulation with FSH. Furthermore, follicles in heifers treated with a GnRH agonist undergo morphologically normal growth in response to FSH and can ovulate and develop into a CL. The long-term GnRH agonist–treated heifer provides a practical model for repeated ovarian follicular superstimulation, multiple ovulation, and embryo transfer, without the need to control stage of the estrous cycle.