Summary. Blood samples were collected every 15 min for 6 h during the follicular (1 day before oestrus), and early (Days + 1 to + 3), mid- (Days + 4 to + 8), and full (Days + 9 to +14) luteal phases of the oestrous cycle. Serum concentrations of immunoactive LH were measured by radioimmunoassay. The biological activity of serum LH was determined by an in-vitro bioassay that uses LH-induced testosterone production from mouse interstitial cells as an endpoint. Only ovine and bovine LH and hCG had appreciable activity in this bioassay. The temporal pattern of secretion of bioactive LH paralleled the secretory pattern of immunoactive LH at all stages of the ovine oestrous cycle. However, the secretory pattern itself varied regularly through the oestrous cycle. The frequency of secretory excursions of LH was highest during the follicular phase (6·2 ± 0·9 pulses/6 h) and was progressively reduced through the luteal phase (1·1 ± 0·1 pulses/6 h during full luteal phase). Conversely, amplitude of secretory excursions of immunoactive LH was low during the follicular phase (0·79 ± 0·08 ng/ml) and significantly (P < 0·05) increased during the mid- and full luteal phases (1·49 ± 0·10 and 2·37 ± 0·20 ng/ml, respectively). The biopotency of LH (bioactive LH/immunoactive LH) at the peak of secretory excursions was 1·00 ± 0·03 during the follicular phase, 0·66 ± 0·02 during the mid-luteal phase, and 1·18 ± 0·19 during full luteal activity. The biopotency of LH was markedly reduced during the preovulatory surge of gonadotrophin. During the full luteal phase each pulse of LH secretion was associated with an abrupt increase in secretion of progesterone. These results indicate that both the quantitative and qualitative character of serum LH varies through the oestrous cycle of the sheep.
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