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Circulating FSH isoform patterns during recurrent increases in FSH throughout the oestrous cycle of heifers

Blood samples were collected from heifers (n = 6; 450 ± 7.7 kg) 2–4 times a day during the first and second follicular waves, and during the gonadotrophin surge to determine whether changes in circulating FSH isoforms occur during cyclic quantitative changes in FSH throughout the oestrous cycle. Serum was assayed for FSH, LH, oestradiol and progesterone. Selected samples collected during the first (samples 1–3) and second (samples 4–6) postovulatory recurrent FSH increase and during the subsequent gonadotrophin surge (samples 7 and 8) were analysed for FSH isoforms by chromatofocusing. No change (P > 0.05) in isoform profile occurred during the first or second recurrent FSH increase, when oestradiol concentrations were 0.6 ± 0.07 and 0.6 ± 0.09 pg ml^–1 and progesterone concentrations were 0.03 ±0.01 and 2.4 ± 0.19 ng ml^–1, respectively. The percentage of FSH eluting in the pH range 7.4–7.0 increased (P < 0.05) from 14.2 ± 2.2 during the luteal phase (samples 1–6) to 20.2 ± 2.3 (sample 7) and 31.4 ± 3.4% (sample 8) during the preovulatory gonadotrophin surge, while oestradiol concentrations were higher (P < 0.05; 4.9 ± 0.39 pg ml^–1) than in the luteal phase of the cycle. In summary, FSH isoform patterns did not change during the cyclic quantitative changes in FSH associated with emergence of the first or second follicular wave. However, during the gonadotrophin surge, in association with increased oestradiol concentrations, an increase in the amount of less acidic isoforms of FSH was observed. Therefore, qualitative changes in FSH are not important in the physiological regulation of follicle turnover during the luteal phase of the oestrous cycle of heifers.