No abstract

SUMMARY Studies were performed to define the responses of plasma met‐enkephalin to various endocrine and pathological stimuli and to determine the relationship between plasma β‐endorphin and met‐enkephalin. During insulin‐induced hypoglycaemia ACTH, β‐LPH and β‐endorphin immunoreactivity rose in parallel, but plasma met‐enkephalin did not change significantly. Sephadex G75 chromatography of samples taken at the time of the peak response (45 min) confirmed the rise in both β‐LPH and β‐endorphin. During administration of dexamethasone, 0·5mg 6 hourly for 48 h, plasma cortisol and ACTH became undetectable at 24 h, and β‐LPH and β‐endorphin fell significantly by 24 h and were undetectable by 48 h; plasma met‐enkephalin, however, showed no significant change. Nine adrenalectomized patients with Cushing's disease and four patients with Addison's disease had elevated plasma ACTH, β‐LPH and β‐endorphin but normal plasma met‐enkephalin levels. Each of ten patients with renal failure had markedly elevated plasma met‐enkephalin immunoreactivity which co‐eluted with synthetic met‐enkephalin on BioGel P4 chromatography. Trypsin and carboxypeptidase‐B digestion of the P4 chromatography fractions generated met‐enkephalin immunoreactivity in earlier fractions, indicating the presence of a potential high molecular weight met‐enkephalin precursor. The results imply that different control mechanisms govern β‐endorphin and met‐enkephalin plasma levels and that the adrenal is not the only source of plasma met‐enkephalin. The elevated levels of met‐enkephalin in renal failure may be due to impaired clearance of met‐enkephalin or of its precursor or to increased met‐enkephalin production.

A chromatographic procedure has been developed for the characterization of ACTH- and lipotrophin- (LPH) related peptides in human plasma under acid-dissociating conditions to minimize artifacts of protein binding. The recovery and sensitivity of this method permits identification of ACTH at normal physiological levels in the circulation. Plasma profiles obtained from normal subjects and patients with pituitary dependent Cushing's disease, Addison's disease and Nelson's syndsrome showed only one significant peak of ACTH activity eluting in the position of purified native human 1--39 ACTH. However, the plasma profiles obtained from all the patients with the ectopic ACTH syndrome demonstrated a second peak of immunoreactive larger-molecular-weight ACTH and in some plasma samples this was the only form of ACTH observed. This larger-molecular-weight ACTH eluted midway between the void volume and 1--39 ACTH and co-eluted with a protein marker of molecular weight 22 000.

Basal and stimulated secretion of N-terminal pro-opiocortin (Pro-gamma-MSH), ACTH and LPH from seven pituitary and three ectopic ACTH secreting tumours have been studied in vitro using a perfused isolated cell system. The peptides were shown to be released concomitantly and in equimolar amounts. The pituitary tumours responded to stimulation with rat stalk median eminence extracts (SME) and synthetic AVP. However, peptide release from the ectopic tumours, although pulsatile, remained autonomous. Prior to surgery, gel-chromatographic profiles of plasma immunoreactive ACTH showed only one peak, which eluted in the position of 1-39 ACTH, in patients with the pituitary tumours, but there was a second peak of large molecular weight ACTH present in the plasma from those with the ectopic ACTH syndrome. This second form of ACTH could not be detected in any of the tumour cell column effluents. An eighth pituitary tumour was atypical, in its unusually large size, clinically aggressive nature and spectrum of peptide release. Although peptide release in response to stimulation with SME was similar to that observed with the other pituitary tumours, the chromatography of the plasma ACTH resembled the ectopic plasma pattern, showing two peaks of immunoreactivity.

Twenty-three patients with adrenocorticotrophichormone-(ACTH)-dependent Cushing's syndrome were subjected to selective venous catheterisation and sampling for ACTH on a total of 26 occasions. Out of 10 patients with pituitary-dependent disease, nine had raised ACTH concentrations in one or both high internal jugular vein samples. Eight patients had 11 proved sites of ectopic hormone production: of these, six were correctly identified by the sampling technique, and in four of them this was the only accurate method of localisation. The results of one catheterisation were misleading, and on 10 occasions they were inconclusive; five patients remained undiagnosed by any method. Overall, 15 of the 26 catheterisations provided diagnostically valuable information.Selective venous catheterisation and sampling for ACTH is effective in confirming a pituitary source of the hormone and may be valuable in locating the source of ectopic ACTH production in some cases.