A MECHANISM ANTAGONIZING THE LUTEOTROPHIC ACTION OF EXOGENOUS PROLACTIN IN RATS

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Abstract
The luteotrophic action of injected prolactin appears to be antagonized by an endogenous luteolytic mechanism to such an extent that the functional life-span of the corpora lutea cannot be lengthened more than 3 days beyond that of the non-injected control. This type of antagonism was seen in both intact and hysterectomized pseudopregnant rats and in lactating rats deprived of their litters. The antagonism was not observed in hypophysectomized rats, in which prolactin injections maintained functional corpora lutea for at least 20 days after ovulation. These results suggest the existence of a hypophysial hormone capable of antagonizing endogenous as well as exogenous prolactin. Hysterectomy may affect the timing of the luteolytic mechanism, but the latter seems to operate effectively in the absence of uterine stimuli. Luteolysis is apparently inhibited during lactation, but removal of the suckling stimulus results in a prompt activation of the process. In control experiments with untreated lactating rats, litter removal 8 or 12 days post partum terminated luteal secretory function on Day 14 regardless of when the litters were removed. These results suggest that 7-day-old corpora lutea of lactation respond less quickly than 11-day-old corpora to the endogenous hormonal alterations resulting from litter removal.