Summary. All hormones were determined in blood samples collected simultaneously from the caudal vena cava and jugular vein at 20-min intervals for 12 h during the early (Day 4) and mid- (~Day 11) luteal phases of the oestrous cycle in 7 cows. Mean concentrations of oestradiol, progesterone and oxytocin were greater (P < 0·01) in the vena cava than in the jugular vein. Pulses of these hormones were also more easily identifiable in the vena cava. The frequency of LH pulses was higher (P < 0·01) during the early luteal than during the mid-luteal phase (8·0 versus 3·6 pulses/12 h). During both phases, 90–96% of all LH pulses were followed within 60 min by a pulse of oestradiol. Basal concentration and amplitude of oestradiol pulses were greater (P < 0·05) during the early than during the mid-luteal phase. The frequency of FSH pulses was similar to that of LH during the early luteal phase (8·5 and 8·0 pulses/12 h) but was greater (P < 0·01) than that of LH during the mid-luteal phase (6·3 versus 3·6 pulses/12 h). Thus, 41% more (P < 0·01) FSH pulses than LH pulses were observed during the mid-luteal phase. However, the separate FSH pulses were associated with low-amplitude short-duration pulses of LH as clarified by an additional study (in 3 cows) using 5-min sampling intervals: 90–100% of all LH/FSH pulses and separate FSH pulses were secreted either concomitantly with or followed by a pulse of progesterone. However, no separate FSH pulses were associated with a pulse of oestradiol. Basal concentration and amplitude of progesterone were greater (P < 0·01) during the mid-luteal than during the early luteal phase. The frequency of oxytocin pulses was similar to that of progesterone during the mid-luteal but not during the early luteal phase. During the mid-luteal phase 97% of all oxytocin pulses were associated with a pulse of progesterone. It is concluded that (1) separate FSH pulses are secreted in addition to parallel LH and FSH pulses during the mid-luteal phase; therefore, the frequency of secretion of LH may be modulated to a greater extent by ovarian steroids than is FSH pulse frequency; (2) pulses of progesterone are probably a result of stimulation by pulses of FSH and/or LH whereas pulses of oestradiol are caused by LH pulses; (3) ovarian oxytocin and progesterone are secreted concomitantly during the mid-luteal phase of the oestrous cycle.
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