Effect of a nonsteroidal aromatase inhibitor on in vitro and in vivo secretion of estradiol and on the estrous cycle in ewes.

Three experiments were performed to study effects of decreased concentrations of estradiol-17β (E₂) on lifespan and function of ensuing ovine corpora lutea (CL). In experiment 1, 52 follicles were collected from 10 ewes and placed into individual culture with 0 or .01 μCi ³H-androstenedione (10 ng; ³H-A) and 0, 10^?11, 10^?9, 10^?7, or 10^?5 M of a nonsteroidal aromatase inhibitor, CGS16949A (CGS). Concentrations of E₂ secreted into the medium, and synthesis of estrogens as estimated by formation of ³H-water from ³H-A were decreased by 10^?5 and 10^?7 (P<.01), but not 10^?9 or 10^?11 M CGS. In experiment 2, luteolysis was induced in 24 ewes by injection of PGF₂ on days 5 to 10 of the estrous cycle (0 hr). Ewes received 0, 0.5, 1.0, 2.0 or 4.0 mg CGS per kg BW i.v. at ?12, 0, 12 and 24 hr, and an ovulatory dose of hCG at 36 hr. Jugular (P<.001) and vena caval (P<.001) concentrations of E₂ were decreased by CGS at all doses tested for 8 to 10 hr, but had returned to levels similar to control ewes by the time of the next injection. Concentrations of E₂ around the time of the LH surge were similar in control and treated ewes. During the subsequent luteal phase, concentrations of progesterone (P₄) were similar in control and treated ewes. Thus, transient decreases in E₂ during the follicular phase were not deleterious to the subsequent luteal phase. In experiment 3, luteolysis was induced in 18 ewes by injection of PGF₂ on days 6 or 7 (0 hr) of the estrous cycle. Ewes received 0 or 1 mg CGS per kg BW i.v. every 8 hr from 0 to 40 hr. Ovulation was induced with hCG at 36 hr. CGS reduced jugular (P<.001) and vena caval (P<.001) concentrations of E₂, prevented an endogenous surge of LH (P<.05) and increased (P<.001) concentrations of FSH. All ewes had ovulated a marked follicle by 72 hr, but onset of the luteal phase, as assessed by concentrations of P₄, was delayed (P<.01) in ewes receiving CGS. Delayed luteal phases were not solely attributable to the presence of new CL or to luteinization of follicular cysts. When data were aligned according to the day ewes were observed in estrus, profiles of P₄ did not differ with treatment. Therefore, normal luteal function ensued following estrus whether or not ewes re-ovulated. In conclusion, decreased secretion of E₂ by the preovulatory follicle was not involved in the ontogeny of CL of short lifespan or subnormal function. Instead, adequate production of E₂ or precisely timed E₂ secretion may be required during follicular development for subsequent functional luteinization.