Pituitary responsiveness to synthetic LH-RH and pituitary LH content at various reproductive stages in the sheep

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Abstract
In sheep the basal concentration of LH [luteinizing hormone] in jugular vein plasma was significantly higher during the first 50 days of gestation than in late pregnancy or at parturition. The pituitary response to a single i.v. injection of 200 .mu.g synthetic LH-RH [luteinizing hormone releasing hormone] was determined at different stages of gestation and compared with that of anestrous and cyclic sheep. Pituitary response to LH-RH decreased progressively with advancing gestation: by 56 days after mating the response had declined to 35% and by parturition to 14% of the value in anestrous sheep. The pituitary response to LH-RH increased after parturition and the pattern of recovery differed in non-lactating and lactating sheep. By 63 days post partum the response to LH-RH in non-lactating and lactating animals had returned to values similar to those in sheep during anestrus and sheep during the luteal phase of the estrous cycle. A decrease in pituitary responsiveness during pregnancy was associated with a decrease in pituitary content of LH. The quantity of LH released in response to a standard injection of LH-RH was linearly related to pituitary LH content.