Preovulatory LH and FSH profiles of aluteal cycles

Abstract

Interovulatory intervals are significantly shortened in aluteal cycles (Coffman et al, Theriogenology. 2014;82:1241-45). We hypothesized that the early return to estrus and ovulation would result from a marked elevation in gonadotropins. Therefore, the aim of this study was to characterize the LH and FSH blood profiles and follicle development of cycling mares during the preovulatory period of aluteal cycles. Eight cycling mares were studied during one control (luteal) cycle (CTRL) and one aluteal cycle (AL), arranged in a crossover design. With the aid of serial ultrasound exams, mares detected in estrus and with ≥ 35 mm follicles were treated with deslorelin acetate (1.8 mg intramuscularly). At detected ovulation (Day 0), mares were randomly allotted to a CTRL or an AL cycle as previously described: administration of 500 mcg of cloprostenol twice daily on Days 0, 1, and 2 and once daily on Days 3 and 4. Blood samples for RIA determination of plasma LH and FSH concentrations were collected daily. Areas under the curve (AUC) for concentrations of plasma LH and FSH from samples collected starting six days preceding ovulation (Day 0) were calculated with the linear interpolation trapezoidal method and compared between groups using a t-test. Interovulatory intervals (IOI) were compared using a paired t-test and a Sign test was used to compare the number of dominant follicles (≥ 28 mm), ovulations and ovulationfailures. The significance level was set as p ≤ 0.05 and a trend as 0.05 < p ≤ 0.1. Total AUC concentrations (± SEM) of LH for AL and CTRL groups were 41.57 ± 8.72 ng·day·mL-1 and 16.69 ± 4.17 ng·day·mL-1, respectively (p = 0.012) and FSH for AL and CTRL groups were 68.35 ± 11.44 ng·day·mL-1 and 45.60 ± 5.69 ng·day·mL-1, respectively (p = 0.079). Mean (± SD) IOI of AL cycles was significantly shorter than that of CTRL cycles (12.88 ± 1.35 days vs. 21.38 ± 1.30 days, p < 0.0001). A significant number of mares (5/8, 62.5%) presented an increase in the number (median ± IQR) of dominant follicles during AL cycles (2.5 ± 1.75 vs. 1.5 ± 1; p = 0.025). In contrast, no significant difference in the median (± IQR) number of ovulations was observed between AL and CTRL groups (1 ± 0 vs. 1 ± 0.75, respectively; p = 0.563). While all AL and CTRL cycles were ovulatory, there was a trend for AL mares to have remaining presumptive preovulatory follicles failing to ovulate (3/8 versus 0/8, respectively; p = 0.083). In conclusion, aluteal cycles were characterized by elevated concentrations of blood LH and FSH during the preovulatory period. These elevated gonadotropins were likely associated with the increase in the number of dominant follicles that became presumptive preovulatory follicles in aluteal cycles.