Release of Growth Hormone by TRH in Intact Rats or in Intact or Hypophysectomized Rats bearing a Heterotopic Pituitary

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Abstract
Thyrotropin-releasing hormone (TRH) was administered i.v. using urethane anesthesia to either intact female and male controls or intact (IT) or hypophysectomized (HT) rats of both sexes bearing an anterior pituitary (AP) graft under the kidney capsule. TRH at all the doses used (0.15, 0.6 and 1.2 .mu.g) elicited a clear GH [growth hormone] rise in both IT and HT rats, while only at the highest does (1.2 .mu.g) was it effective in intact controls. Qualitative and quantitative EM studies performed on the in situ and ectopic pituitaries from IT female rats demonstrated that TRH stimulated exocytosis from the ectopic pituitary at all doses used, whereas only the highest TRH dose increased the frequency of exocytosis from the pituitary in situ. The similar responsiveness to TRH of the ectopic pituitary irrespective of the presence of an in situ AP, rules out the possibility that peripheral hormonal factors, e.g., lack of estrogens or thyroid hormones, may play a crucial role in the preferential GH response to TRH present in HT rats.