In some fields of steroid endocrinology, it has long been accepted that the potency of a hormone may depend not only on its secretion rate and rate of hepatic clearance but also on specific metabolic transformation at the site of the target cell. The crucial role of tissue 5α reductase activity on the action of testosterone is a spectacular example (Peterson, ImperatoMcGinley, Gautier & Sturla, 1977). Curiously—it is easy to be wise after the event—this line of thought seems rarely to have been exercized in explanations of corticosteroid action. Clearly, the severity of diseases of corticosteroid excess, of which Cushing's and Conn's syndromes are the best known examples, is strongly correlated with the secretion rate of cortisol and aldosterone respectively. However, in some forms of hypertension where deranged corticosteroid action might be expected to provide an acceptable explanation (e.g. increased mineralocorticoid secretion in low-renin essential hypertension), no abnormalities of secretion
Dopamine is capable of modulating zona glomerulosa function. Of this there now seems little doubt. However, whether varying dopamine levels in vivo forms the basis of a realistic normal physiological control mechanism for aldosterone secretion is far from clear. Reviewers have been cautious (Ganguly, 1984) or enthusiastic (Sowers, 1984) depending on the choice of evidence and the weight given to individual studies, but some resolution of the uncertainty is pressing since aberrations in this as yet unproven relationship have been suggested as basic abnormalities in a number of forms of hypertensive disease.
Evidence for and against the dopamine–aldosterone relationship has been obtained using dopamine itself and antagonists or agonists of its action in whole animals and in tissue preparations. Initial impetus for the dopamine hypothesis came from the observation that the dopamine agonist, bromocriptine, inhibited the response of aldosterone to frusemide-induced sodium loss (Edwards, Thorner, Miall et al. 1975) although
A. WILSON and R. FRASER
A method of estimating the peripheral plasma concentration of 11-deoxycorticosterone (DOC) in man by means of gas-liquid chromatography with electron capture detection is described. Purification requirements for samples and chromatographic media have been simplified by the use of a detector bypass valve which reduces the risk of detector contamination. For this reason the method is relatively short. There was no measurable blank using either plasma from an adrenalectomized subject or water. The normal range was found to lie between 4·1 and 17·2 ng/100 ml (mean 9·8). Dexamethasone administration to one subject resulted in a subnormal plasma DOC concentration.
J. COENEGRACHT and R. FRASER
1. A method is described of measuring the early thyroid 'clearance' by the technique of Berson, Yalow, Sorrentino & Roswit , with the results expressed as the percentage of extracellular fluid cleared per half hour. This measurement has satisfactorily segregated from a normal group: 10 out of 10 definitely and 11 out of 11 probably thyrotoxic, 11 out of 12 definitely myxoedematous and 1 out of 4 probably myxoedematous patients. The technique is relatively simple and rapid.
2. Measurement of the early thyroid clearance is closely correlated with the urinary 'T' index of thyroid uptake (for log values of each, r=0·90).
S. Harvey and R. A. Fraser
The refractoriness of guinea-pigs to the growth-promoting actions of exogenous GH has been suggested to be due to a deficiency or defect in tissue GH receptors or in GH-receptor gene expression. GH-receptor mRNA was, however, demonstrated by Northern blot analysis and by the polymerase chain reaction in extracts of guinea-pig liver, adipose tissue, brain, hypothalamus and pituitary gland. High-affinity, low-capacity binding sites for radio-labelled ovine GH were also demonstrated on the plasma membranes of guinea-pig liver and were similar to those in rat liver. These results demonstrate that the unresponsiveness of guinea-pigs to exogenous GH is not due to the absence of GH receptors.
Journal of Endocrinology (1992) 133, 357–362
S. Harvey and R. A. Fraser
'... by analogy to the situation with calcitonin, it appears worthwhile to look for PTH in the brain and for physiological and behavioral effects of the hormone in the central nervous system' (Gennari, 1988)
Parathyroid hormone (PTH)-like peptides, mRNA and degradative enzymes are present in hypophysiotropic regions of the hypothalamus, in which PTHbinding sites are located on neural membranes. Since exogenous PTH stimulates hypothalamic dopamine metabolism and the release of pituitary prolactin, PTH-like peptides in the hypothalamus may have neuroendocrine roles in the regulation of pituitary function. However, as PTH is produced peripherally and neurological disorders are symptomatic of hyperparathyroid disease states, parathyroidal PTH may also participate in the neuroendocrine control of the hypothalamo-pituitary axis.
PTH in the hypothalamo-pituitary axis
Unlike other peptides of the 'diffuse neuroendocrine system', PTH production was believed to be solely by the parathyroid gland (Rosenblatt et al. 1989), from which PTH
P. A. MASON and R. FRASER
A method for determining the plasma concentrations of six major corticosteroids, aldosterone, 18-hydroxy-11-deoxycorticosterone (18-OH-DOC), corticosterone, deoxycorticosterone (DOC), cortisol and 11-deoxycortisol using gas–liquid chromatography with electron capture detection is described. Esterification of suitable derivatives of these compounds with heptafluorobutyric anhydride (HFB) allowed detection of quantities of steroid, ranging from 0·3 pg for androstenetrione HFB (from cortisol) to 2·3 pg for corticosterone HFB. No detectable reagent blank was obtained for any compound when water was used instead of plasma and this was also the case when plasma from an adrenalectomized subject was analysed, with the exception of 18-OH-DOC where a reproducible but negligibly small blank occurred. Coefficients of variation for replicate determinations ranged from 8% for corticosterone to 17% for aldosterone. Concentrations in a series of normal human plasma samples were as follows: aldosterone, 4·0–18·0 ng/ 100 ml; 18-OH-DOC, 20–160 ng/ml; corticosterone, 0·08–0·80 μg/100 ml; DOC, 2·8–16·0 ng/100 ml; cortisol, 2·5–10·0 μg/100 ml;and 11-deoxycortisol, 40·0–400·0 ng/100 ml. When seven normal subjects were treated with dexamethasone, concentrations of DOC, cortisol and 11-deoxycortisol fell to below the limit of the normal range, those of 18-OH-DOC and corticosterone were at the lower end of the normal range while the concentration of aldosterone was not significantly affected.
I. Thomson, R. Fraser and C. J. Kenyon
We have previously reported that benzodiazepines inhibit microsomal steroid hydroxylases. We have now studied their effects at much lower drug concentrations and have also addressed the suggestion that benzodiazepines alter cellular calcium metabolism.
We investigated the in-vitro effects of midazolam on microsomal steroid hydroxylation by measuring basal and ACTH-stimulated cortisol and 17α-hydroxyprogesterone (17-OHP) synthesis. Threshold inhibition of basal cortisol production was achieved by 3·4 μmol midazolam/1 while ACTH-stimulated production required 13·6 μmol/l. This was accompanied by a biphasic response of 17-OHP production, rising to a maximum at 13·6 μmol midazolam/l for basal and 6·8 μmol midazolam/l for ACTH-stimulated synthesis suggesting a preferential inhibitory effect on 21-hydroxylase activity at < 6·8 μmol/l and additonal effects on 17α-hydroxylation at higher drug concentrations. This explains the inhibition of ACTH-stimulated cortisol synthesis by midazolam (50% inhibitor dose (IC50) 22 μmol/l). Using 21-deoxycortisol as substrate, we have demonstrated that midazolam is a competitive inhibitor of 21-hydroxylase (inhibitory constant (KI) 35 μmol/l).
Both midazolam and diazepam inhibited K+-stimulated aldosterone synthesis, with IC50 values of 1·2 μmol/l and 0·8 μmol/l respectively, which are far lower than those observed for ACTH-stimulated cortisol synthesis. With 11β-hydroxyprogesterone as substrate, the K I for the inhibition of aldosterone synthesis by midazolam was 54 μmol/l. Potassium stimulates aldosterone biosynthesis at least partly by changing intracellular free calcium levels. To investigate possible antagonistic effects of benzodiazepines on calcium metabolism, we measured 45Ca uptake in the presence of midazolam. Both basal (P < 0·01) and K+-stimulated 45Ca uptake (P < 0·05) were inhibited by the drug although the effects of K+ were not completely abolished. Comparison of the dose-dependent effects of midazolam on basal 45Ca uptake in cell suspensions prepared from different areas of the adrenal cortex indicated that zona glomerulosa cells are more sensitive to midazolam.
We confirm that benzodiazepines at low concentrations have a direct effect on microsomal steroid hydroxylase enzymes in vitro and postulate that the greater sensitivity to benzodiazepines of K+-stimulated aldosterone synthesis, when compared with either ACTH-stimulated cortisol synthesis or conversion of 21-deoxycortisol to cortisol, may be explained by additional effects of these drugs on plasma membrane calcium transport.
Journal of Endocrinology (1992) 135, 361–369
R.A. Fraser, K. Siminoski and S. Harvey
Specific hybridization of polyadenylated RNA, extracted from rat, rabbit and human pituitary glands with a 638 bp rabbit GH receptor (rGHR) cRNA was demonstrated by Northern analysis. In-situ hybridization of tissue sections with the probe demonstrated the localization of rGHR mRNA throughout the rat pituitary gland and its presence in the anterior lobe of the rabbit pituitary. Growth hormone binding sites on pituitary membranes were not, however, demonstrated by radioligand binding studies. Thus, although the GH receptor gene is expressed in pituitary tissue, functional GH receptors may not be inserted into pituitary plasma membranes.
W. R. Miller and H. M. Fraser
Cancers of the breast, endometrium and ovary can display endocrine sensitivity—and a proportion of such tumours regress when deprived of hormones (Hawkins & Miller, 1988). As a consequence, endocrine deprivation therapy is a major treatment modality, particularly in patients with breast cancer (Miller, 1990).
Given that endocrine factors also play a critical role in the aetiology of certain tumours (Preston-Martin, Pike, Ross et al. 1990), hormone manipulation might also prevent cancer. Initiatives attempting hormone-prevention of cancer have been given impetus by the availability of relatively non-toxic drugs such as luteinizing hormone-releasing hormone (LHRH) agonists and antioestrogens which block hormone action without the morbidity and irreversibility of surgical and radiological procedures. In this commentary we evaluate current thinking behind the use of hormone suppressant drugs as a method of prevention of cancer and conclude that such an approach is feasible but, in parallel with steroid contraception, there are important long-term consequences