Abstract
1. The prompt rise in the blood level of ACTH provoked in rats by such mild stresses as paw stimulation of epinephrine injection was significantly blocked by pretreatment of the animals with a small dose of Pitressin, while that following unilateral adrenalectomy was moderately inhibited. Pitressin, however, failed to suppress the ACTH release following such severe stress as laparotomy or immobilization.
2. ACTH once released into the general circulation seems to be potentiated by pretreatment of the animals with Pitressin.
3. The inhibitory effect of Pitressin is due to vasopressin itself, and not to its contamination with other substances contained in posterior lobe extracts.
4. In chronically stressed rats, the prompt rise of blood ACTH level was not observed after paw stimulation or unilateral adrenalectomy.The sensitivity of the adrenal gland to exogenous ACTH remained unchanged in these rats.
5. Posterior-lobectomy abolished the adrenal response to epinephrine injection. The resting level of blood ACTH remained high 3-4 days after the removal of posterior pituitary gland. Paw stimulation did not produce further elevation of the level of blood ACTH in these rats.
The authors are indebted to Dr. A. BRODISH for his help in the performance of blood ACTH assay. We wish to thank Prof. V. DUVIGNEAUD for his generous gift of the synthetic lysine vasopressin employed in this experiment.This study was supported by U. S. P. H. S. Grant A-256
