Gonadotrophins and ovarian steroids in cattle. I. Pulsatile changes of concentrations in the jugular vein throughout the oestrous cycle.

Short-term secretion patterns of LH, FSH, progesterone and oestradiol-17 beta were evaluated throughout complete oestrous cycles of 6 heifers. Frequent blood samples (in 20-min intervals for 12 continuous h) were taken every 3-5 days from indwelling jugular catheters. There was a high incidence of concomitant LH and FSH pulses ranging from 72% at luteolysis to 83-100% during the luteal phase. Almost the same total number of LH and FSH pulses occurred during the early luteal phase (7.0 vs 7.4/12 h, respectively), however, there was an average of one additional FSH pulse in between the synchronous LH/FSH ones during the mid- and late luteal phase (6.9 FSH vs 3.4 LH pulses/12 h). Basal LH and FSH concentrations remained unchanged from the early until the late luteal period. During and after luteolysis frequency of LH and FSH release (14.5 vs 10.5 pulses/12 h) increased considerably as well as basal concentrations and magnitude of LH pulses. Secretion of both gonadotrophins persisted very frequently (13.3 LH and 10.7 FSH pulses/12 h) during pro-oestrus and oestrus when basal FSH concentrations and FSH pulse maxima approached a nadir. During the mid-luteal phase 45% of pulsatile progesterone occurred concomitantly with each coinciding LH/FSH pulse and 44% of pulsatile progesterone happened after additional single FSH pulses. Distinct short-term changes of oestradiol concentrations were not observed in the jugular vein but concentrations fluctuated randomly ranging from 2-6 pg/ml throughout the luteal period. Prior to and during heat mean concentrations of oestradiol were approximately 2-fold higher (P less than 0.05) than during the other periods of the cycle. It is concluded that the frequency of pulsatile LH release is modulated to a much greater extent than FSH by negative feedback of ovarian steroids. Some pulsatile progesterone secretion resulting from the stimulation of FSH (and LH) is still detectable in the jugular vein whereas of oestradiol-17 beta is not. The additional frequent monitoring of FSH might be more appropriate reflecting pituitary and hypothalamic function than only measuring LH.