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Kwanyee Leung, Vasantha Padmanabhan, E. M. Convey, R. E. Short, and R. B. Staigmiller

Summary. Changes in the ability of Gn-RH to induce gonadotrophin release with time after synchronization of oestrus was determined in 4 groups of 6 cows each. Cows were given Gn-RH at 40-min intervals for 6 h beginning at − 24,0,18 or 36 h (time 0 = removal of progestagen implant). Changes in concentration (ng/ml) of serum LH after Gn-RH averaged 2·9, 6·2, 6·4 and 33·4, whereas serum FSH averaged 25·7, 35·8, 35·8 and 97·3. Thus the responsiveness of the pituitary to Gn-RH had increased by 36 h after implant removal. Other groups of cows subjected to the same synchronization scheme were slaughtered at 0 h, 24 h or at various times after onset of oestrous behaviour. Gn-RH binding to crude pituitary membrane preparations was assessed. There was no apparent change in the affinity constant of Gn-RH-binding sites with time after synchronization. The number of Gn-RH-binding sites remained unchanged until the period of oestrus when a significant decline with time was detected. We conclude that the increase in pituitary responsiveness to Gn-RH that occurs before the preovulatory gonadotrophin surge was not directly associated with changes in number or affinity of pituitary Gn-RH-binding sites in crude pituitary membrane preparations.

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K. Leung, V. Padmanabhan, L. J. Spicer, H. A. Tucker, R. E. Short, and E. M. Convey

Summary. Thirty primiparous suckling beef cows were slaughtered on Day 7, 14, 28,42 or 56 after parturition. Some had resumed oestrous cyclicity by the time they were slaughtered on Days 42 and 56. Amongst acyclic cows between Days 7 and 42, pituitary LH concentrations and basal and GnRH-induced release of LH from pituitary explants doubled. Pituitary FSH concentration and basal release in FSH increased only by 15–20%, while GnRH-induced release of FSH in vitro was unchanged. During post-partum anoestrus, overall mean concentrations of serum FSH did not change, whereas overall mean concentrations and pulse amplitudes of serum LH increased. Numbers and affinity constants of GnRH-binding sites in pituitary glands remained constant during the post-partum period studied. We conclude that, under these experimental conditions, numbers and affinity constants of GnRH-binding sites in the pituitary gland of post-partum beef cows do not limit the ability of the anterior pituitary gland to release gonadotrophins.