Absence of Prolactin-Release Inhibiting Activity in Highly Purified LH-Releasing Factor

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Abstract
Highly purified pig LH [luteinizing hormone]-releasing factor (LRF) was tested for the ability to inhibit pituitary prolactln secretion in vivo (PIF activity). Cervical stimulation induced a significant depletion of pituitary prolactin content in estrous rats. When extract equivalent to 1 fragment of pig stalk median eminence [SME] was given before the stimulation, the depletion of pituitary prolactin was completely blocked. On the other hand, pretreatment with 50 ng of purified LRF, equivalent to 300 fragments of pig SME, failed to block the prolactin depletion following cervical stimulation. Pork LRF used in these studies raised plasma LH in ovariectomized rats pretreated with estrogen and progesterone in as small a dose as 0.05 [mu]g. One [mu]g LRF also induced ovulation in Nembutal-blocked cyclic rats. Thus, highly purified pork LRF was clearly devoid of PIF activity. This supports our previous conclusion that LRF is different from PIF.