Abstract
Summary
Peripheral LH levels are initially depressed and then elevated in ovariectomized heifers after injections of estrogen. The sites of the estrogen induction of this biphasic response were investigated. Plasma and pituitary LH levels, hypothalamic LH-RH levels and the in vitro response of pituitary tissue to hypothalamic extract (HE) were measured 3 and 18 hr after estrogen injection into ovariectomized heifers. LH levels were significantly depressed 3 hr after estrogen treatment. However, none of the other parameters were different from corresponding control group data at this time. Plasma LH was elevated, pituitary LH was depressed and the in vitro pituitary response to HE was significantly enhanced 18 hr after estrogen treatment. These data suggest that the initial, suppressive effect of estrogen is exerted at the hypothalamic or higher levels and that the LH-releasing effect of estrogen is at the pituitary level. The positive feedback effect of estrogen on the pituitary is exerted only after several hr exposure to estrogen and acts on the pituitary both by enhancing its response to LH-RH and by releasing stored LH.
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