227 EFFECTS OF A SINGLE DOSE OF A NONSTEROIDAL AROMATASE INHIBITOR ON OVARIAN FUNCTION IN CATTLE

Abstract

Many countries have banned the use of estrogenic products in farm animals. Nonsteroidal aromatase inhibitors prevent the body from producing its own estrogen. The effects on the ovary are thought to be a result of suppression of estrogen-producing follicles and a rebound in endogenous levels of FSH through the removal of the negative feedback effect of estradiol. An experiment was designed to determine the effects of a nonsteroidal aromatase inhibitor, letrozole, on ovarian function in cattle. The specific objective was to test the hypothesis that letrozole will arrest growth of the dominant follicle, resulting in emergence of a new follicular wave at a predictable interval post-treatment. Beef heifers were assigned randomly to 4 groups and given phosphate-buffered saline [control; (n = 10)] or letrozole at a dose of 500 (n = 9), 250 (n = 10), or 125 (n = 10) μgkg^-1 i.v. 4 days after follicular ablation (≈ 2.5 days after wave emergence, at the time dominant follicle selection is manifest). Blood samples were collected and ovarian structures were monitored daily by transrectal ultrasonography. Analysis of variance for repeated measures, one-way ANOVA, paired t-test, and 2-sample t-test were applied to the analysis of the data. The diameter profile of the dominant follicle was larger in heifers treated with letrozole than in control heifers (P < 0.05). The intervals from treatment to new wave emergence and from treatment to onset of regression of the extant dominant follicle were longer (P < 0.05) in heifers treated with letrozole than in controls, although variances in the intervals were not different. A small but significant reduction in circulating estradiol concentration was observed, and plasma LH concentrations were higher (P < 0.05) in letrozole-treated heifers than in controls. Lower plasma concentrations of FSH in letrozole-treated heifers than in controls (P < 0.03) was interpreted as an indirect effect resulting from prolonged follicular dominance. In summary, a single dose of letrozole did not induce regression of the extant dominant follicle, nor did it directly affect FSH release. Conversely, letrozole extended the lifespan of the dominant follicle, in association with increased endogenous levels of LH, thereby delaying the next FSH surge and subsequent follicular wave emergence. Results suggest that letrozole has potential as a nonsteroidal method of controlling ovarian function in cattle but further studies are needed to clarify dosage and timing of treatment.

Research supported by the Natural Sciences and Engineering Research Council of Canada and Bioniche Life Science Inc.