Studies on Dose Response Relationship of Luteinizing Hormone-Releasing Hormone (Lh-Rh) in Sheep

Abstract

Three ewes, three wethers and one ram were used to study responses to LH-RH purified from porcine hypothalami. The levels of serum LH after LH-RH administration in the ewes were compared during diestrus. Each animal received a dose of 1, 3, 9 and 27 μg LH-RH intracarotidly. Jugular blood samples were collected at intervals before and after the LH-RH injection and serum LH was measured by a double antibody radio-immunoassay. At all dosages of LH-RH studied, each animal exhibited a significant increase in serum LH 2.5 min. after LH-RH administration. The maximal responses in the ewes and ram appeared to be obtained with 3 μg of LH-RH which increased mean serum LH levels 7 to 14 ng/ml over pretreatment levels. Higher levels of LH could not be induced with greater doses (9 or 27 μg) of LH-RH in the ewes or the ram. However, mean serum LH levels were increased by 160 ng/ml in the three wethers after administration of 27 fxg LH-RH. This demonstrates that the pituitary is more responsive to LH-RH in the castrate than in the intact sheep. In the ewes the maximum levels of serum LH detected after administration of LH-RH are well below reported preovulatory levels of LH. This refutes the assumption that LH-RH is solely responsible for LH release prior to ovulation and suggests that the pituitary responsiveness to LH-RH may also be an important factor in the rise of LH prior to ovulation. Copyright © 1970. American Society of Animal Science. Copyright 1970 by American Society of Animal Science