Effect of Progesterone on the Release of Luteinizing Hormone Induced by a Synthetic Gonadotrophin-Releasing Factor in the Ewe

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Abstract
A synthetic decapeptide (Gn-RH) structurally identical to ‘ovine LH-releasing hormone’ was given by intra-carotid infusions (0, 0.5 and 1.5 µg/h for 3 h) to two groups of ewes late in the anoestrous season. One group received pretreatment with intra-venous progesterone (500 µg/h for 24 h) and the other no such treatment. Gn-RH was found to be a highly potent LH releasing agent capable of inducing ovulation. Physiological amounts of pro gesterone were incapable of preventing Gn-RH-induced pituitary LH release. It is con cluded that the blockade of LH release during the luteal phase or after progesterone administration may be due to steroid action at the level of the hypothalamus rather than at the pituitary.

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