Experimental Studies on ACTH Secretion Mechanism

Abstract

Increasing evidences are showing the importance of the hypothalamus for the regulation of ACTH secretion from the pituitary. In this report the author has tried to investigate the functional pattern of the Hypothalamo-Pituitary-Adrenal system of the rat, which is expected to operate under various noxious conditions, adrenalectomy with and without prednisolone treatment, furthermore ACTH administration. For the follow up study of the Hypothalamo-Pituitary-Adrenal system special attention was paid for the analytical study of the individual organ in the system.
For this purpose direct assay of Corticotrophin Releasing Activity (CRA) in the Median Eminence (M.E.) and also direct assay of the plasma and pituitary ACTH-like activity and simultaneous determination of plasma and adrenal corticosterone level were carried out. CRA of the crude M.E. extract (M.E.E.) prepared with cold glacial acetic acid following the description of Vernikos-Danellis was determined by the in vitro assay technique, which was described by Tsuji-Nakai (Folia Endocrinologica Japonica 43 : 957, 1968)
This method is based on the ACTH assay technique after Tsuji-Yasui, of which, specificity, high sensitivity and reproduce-ability experienced carefully checking.
Three halves of rat pituitary were placed in one flask and incubated for 60 minutes at first (pre-incubation) and then M.E.E. described above was added to the incubation medium.
After final incubation for 60 minutes ACTH secreted in the medium was determined after Tsuji-Yasui's bioassay method of ACTH.
As control experiment it was checked, if the crude M.E.E. used in this experiment has any ACTH-like activity from the view point of corticoidogenic action (Folia Endocrinologica Japonica, 41 : 643, 1965).
It was observed, that extra large dosis of M.E.E., such as corresponding to 20 M.E.E. and also 150 mu lysine-8-vasopressin checked with in vitro assay exhibited ACTH-like activity, but small dosis of M.E.E. such as corresponding to 2M.E.E. as is usually used for the in vitro CRA assay system exhibited no ACTH-like activity in itself.
Therefore the author has chosen the quantitative fluctuation of CRA in 2M.E.E. as for the parameter for the qualitative evaluation of CRA in given hypothalamus.
The rat were exposured to various noxious stimuli, such as 1.) exposure to ether vapour for one minute, 2.) laparotomy, 3.) intermittent bell alarming for 3 min. duration, 4.) intravenous histamine, and 5.) subcutaneous epinephrine administration.
The quantitative changes of CRA after these stimuli were followed up along with the simultaneous investigation of other parameters of Hypothalamo-Pituitary-Adrenal function.
The results are summarized as follows :
1) CRA showed elevated activity already 2.5 or 5 minutes after various kinds of stimuli. Other parameters of Pituitary-Adrenal function, particularly ACTH-like activity of pituitary and plasma, showed increased value following the primary elevation of CRA.
The conclusion drawn was that, the hypothalamus is the final common pathway of various kinds of noxious stimuli and the very change of CRA in the median eminence of hypothalamus seems to regulate the Hypothalamo-Pituitary-Adrenal function.
2) As far as this experiment is concerned lysine-8-vasopressin seems to act directly upon hypothalamus and to release CRF.
But from the result of control experiment a partially direct effect upon pituitary as a sort of CRF for itself could not be excluded.
3) Comparing studies of the ACTH secretion during 2.5 min., 7 min., and 15 min., following various kinds of stimuli disclosed the presence of different pattern of ACTH se-creation following different kinds of stimuli.
These results strongly point out the importance of direct investigation of ACTH-like activity of plasma, not of the indirect follow up of pituitary function through plasma steroid level.