Relationships between FSH patterns and follicular dynamics and the temporal associations among hormones in natural and GnRH-induced gonadotropin surges in heifers.

Preovulatory LH and FSH surges and the subsequent periovulatory FSH surge were studied in heifers treated with a single injection of GnRH (100 microg, n = 6) or saline (n = 7). Blood samples were collected every hour from 6 h before treatment until 12 h after the largest follicle reached > or =8.5 mm (expected beginning of follicular deviation). The GnRH-induced preovulatory LH and FSH surges were higher at the peak and shorter in duration than in controls, but the area under the curve was not different between groups. The profiles of the preovulatory LH and FSH surges were similar within each treatment group, suggesting that the two surges involved a common GnRH-dependent mechanism. Concentrations of FSH in controls at the nadir before the preovulatory surge and at the beginning and end of the periovulatory surge were not significantly different among the three nadirs. A relationship between variability in the periovulatory FSH surge and number of 5.0 mm follicles was shown by lower FSH concentrations during 12-48 h after the beginning of the surge in heifers with more follicles (11.0 +/- 1.0 follicles (mean+/-s.e.m.) n = 7) than in heifers with fewer follicles (5.7 +/- 0.4, n = 6). This result was attributed to increased FSH suppression from increased numbers of follicles reaching 5.0 mm. Grouping of heifers into those with longer vs shorter intervals from a 4.5 mm to an 8.5 mm largest follicle did not disclose any relationship between length of the interval and FSH characteristics (e.g. profile of surge, area under curve, FSH concentrations at specific events). The hypothesis of a relationship between variation in the periovulatory FSH surge and variation in follicular dynamics was supported for the number of 5.0 mm follicles but not for the hour the largest follicle reached 8.5 mm. Thus, the expected time of follicle deviation was not altered by the extensive variation in the wave-stimulating FSH surge.