CHANGES IN PLASMA LEVELS OF PROLACTIN AND LH INDUCED BY LUTEOLYTIC OR LUTEOTROPHIC TREATMENT IN INTACT CYCLING SHEEP AFTER SECTION OF THE PITUITARY STALK

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ABSTRACT Oestradiol has been shown to cause either premature luteal regression or prolonged luteal maintenance depending on whether it is administered early or late in the oestrous cycle. Although these effects are not observable in the absence of the uterus, it was considered desirable to examine the possibility that the pituitary gland as well as the uterus may be involved in this phenomenon. Accordingly, plasma LH and prolactin concentrations were studied in sheep treated with oestradiol from day 3 (luteotrophic action) or day 11 (luteolytic action) with and without pituitary stalk section. Plasma LH fell below the limit of detection (0.3 ng/ml) in the plasma of sheep after pituitary stalk section, whether or not oestradiol had been administered. In the intact ewe, oestradiol given from day 3 resulted in luteal maintenance associated with a significant increase in the secretion of plasma LH and prolactin. However, similar treatment in the stalk sectioned ewe also resulted in luteal maintenance without any major increase in gonadotrophin secretion. On the other hand, when oestradiol was given from day 11, there was no evidence either from the intact or the stalk sectioned sheep that the pituitary contributed to the luteolytic action of oestradiol may possibly have a pituitary component, the luteolytic action is probably mediated solely by the oestrogenic action on the uterus.