Gonadotrophins and ovarian steroids in cattle.

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Abstract
Pulsatile LH and FSH release from the pituitary gland of 8 post-partum dairy cows was assessed in series of frequent bleedings (every 20 min for 12 h) performed in about weekly intervals from day 4 through day 32 post partum (pp). The frequency of short-term secretion of LH increased during individually varying time periods (1.5–5 weeks) from about 1 pulse/4 h to about 1 pulse/1.25 h prior to the re-establishment of luteal function. FSH pulses (average frequency 1 pulse/2 h) were always released concomitantly with LH, but extra FSH pulses occurred in between. Interpulse intervals of LH and FSH (80 ±4 vs 82 ± 7 min) merged prior to the resumption of cyclicity. LH and FSH pulse frequencies remained slightly slower during this first periovulatory period pp than during pro-oestrus and oestrus of regular cycles; also, with one exception, no visible signs of oestrus were expressed. Despite low progesterone concentrations during the first luteal phase there was strong negative feedback mainly on LH, thus considerably prolonging the intervals between consecutive LH pulses (up to 620 min) whereas FSH release remained much faster (up to 270 min interpulse interval). LH interpulse intervals tended to be longer during short compared to normal length cycles post partum (420 ± 107 vs 285 ± 51 min) in the presence of only slightly altered FSH release (154 ± 37 vs 107 ± 7 min). As the pituitary gland is able to secrete already early pp frequent pulsatile FSH contrary to LH, a selective suppression of LH including a maximal sensitivity to the negative feedback action even of low ovarian steroid concentrations and a lack of the positive feedback mechanism during an initial period have to be postulated. The ovarian-uterine unit seems to play a major role in the timing of the resumption of unimpaired reproductive functions post-partum.