Dehydroepiandrosterone (DHEA) is a steroid secreted by the adrenal cortex, with a characteristic, age-related, pattern of secretion. The decline of DHEA concentrations with age has led to the suggestion that old age represents a DHEA deficiency syndrome and that the effects of ageing can be counteracted by DHEA 'replacement therapy'. DHEA is increasingly being used in the USA, outside medical supervision, for its supposed anti-ageing effects. This commentary weighs the evidence for the existence of a DHEA deficiency syndrome and considers the value of DHEA 'replacement therapy'.
JP Hinson and PW Raven
JP Hinson, Puddefoot JR and S Kapas
Previous studies, by this group and others, have shown that vasoactive intestinal peptide (VIP) stimulates aldosterone secretion, and that the actions of VIP on aldosterone secretion by the rat adrenal cortex are blocked by beta adrenergic antagonists, suggesting that VIP may act by the local release of catecholamines. The present studies were designed to test this hypothesis further, by measuring catecholamine release by adrenal capsular tissue in response to VIP stimulation. Using intact capsular tissue it was found that VIP caused a dose-dependent increase in aldosterone secretion, with a concomitant increase in both adrenaline and noradrenaline release. The effects of VIP on aldosterone secretion were inhibited by atenolol, a beta1 adrenergic antagonist, but not by ICI-118,551, a beta2 adrenergic antagonist. Binding studies were carried out to investigate VIP receptors. It was found that adrenal zona glomerulosa tissue from control rats contained specific VIP binding sites (Bmax 853+/-101 fmol/mg protein; Kd 2.26+/-0.45 nmol/l). VIP binding was not displaced by ACTH, angiotensin II or by either of the beta adrenergic antagonists. The response to VIP in adrenals obtained from rats fed a low sodium diet was also investigated. Previous studies have found that adrenals from animals on a low sodium diet exhibit increased responsiveness to VIP. Specific VIP binding sites were identified, although the concentration or affinity of binding sites in the low sodium group was not significantly different from the controls. In the low sodium group VIP was found to increase catecholamine release to the same extent as in the control group, however, in contrast to the control group, the adrenal response to VIP was not altered by adrenergic antagonists in the low sodium group. These data provide strong support for the hypothesis that VIP acts by the local release of catecholamines in adrenal zona glomerulosa tissue in normal animals. It does not appear that VIP acts through the same mechanism in animals maintained on a low sodium diet. The mechanism by which VIP stimulates aldosterone in this group remains to be determined.
GP Vinson, R Teja, MM Ho, JP Hinson and Puddefoot JR
The tissue renin-angiotensin systems (RAS) may have specific roles that complement those of the systemic RAS. In the adrenal, the tissue RAS has been implicated in the regulation of glomerulosa tissue growth and function, and in mediating the response of the tissue to stimulation by ACTH and potassium ions. To examine the role of the rat adrenal tissue RAS in its response to angiotensin II stimulation, adrenals were incubated either as bisected glands or as separated capsular glands (largely glomerulosa) under control conditions, or in the presence of the angiotensin-converting enzyme inhibitor captopril, or of angiotensin II, or both. Captopril inhibited the two different tissue preparations in different ways. In the capsular gland it inhibited basal aldosterone output, but facilitated its response to angiotensin II. In the bisected gland, captopril inhibited the response of aldosterone to angiotensin II. Other data suggest that one way in which captopril functions is by preventing the conversion of fasciculata-generated 18-hydroxydeoxycorticosterone (18-OH-DOC) to aldosterone in the glomerulosa. Immunolocalisation of 18-OH-DOC in perfused rat adrenal confirms that one function of angiotensin II is to mobilise tissue-sequestered 18-OH-DOC. The results illustrate the importance of tissue RAS in the synthesis of aldosterone and the response to angiotensin II.
S Kapas, A Martinez, F Cuttitta and JP Hinson
This study was designed to investigate the synthesis and action of adrenomedullin in the rat adrenal gland. The results obtained from in situ hybridization and immunocytochemical studies suggest that adrenomedullin is synthesized not only in the medulla, but also within the zona glomerulosa of the rat adrenal cortex. Findings from in situ hybridization and binding studies also suggested that specific adrenomedullin receptors are expressed in the zona glomerulosa, and that low levels are present in the inner zones of the cortex. The Kd of the zona glomerulosa adrenomedullin receptor (5.5 nmol/l) suggests that it may respond to locally produced adrenomedullin rather than circulating concentrations of the peptide, which are in a lower range. It was found that adrenomedullin acted on zona glomerulosa cells in vitro to stimulate aldosterone release and cAMP formation, but in this tissue did not stimulate inositol phosphate turnover. The effect of adrenomedullin on aldosterone secretion was significantly attenuated by a protein kinase A inhibitor, suggesting that cAMP mediates the effects of adrenomedullin on aldosterone secretion. Adrenomedullin did not significantly affect the response of zona glomerulosa cells to stimulation by either ACTH or angiotensin II. Adrenomedullin did not affect the release of catecholamines, either adrenaline or noradrenaline, by intact adrenal capsular tissue. These data suggest that both adrenomedullin and its specific receptor are expressed in the rat adrenal zona glomerulosa, leading to the hypothesis that adrenomedullin may have an autocrine/paracrine role in the regulation of the rat adrenal zona glomerulosa.
PW Raven, S Kapas, M Carroll and JP Hinson
Stimulation of aldosterone by a serine protease, trypsin, was first reported in 1982, although the mechanism of this effect was unclear. Recently, a family of protease-activated receptors (PARs) has been described and four members of the family characterised and cloned, including the previously recognised thrombin receptor. This study investigated whether PARs mediate the action of trypsin on aldosterone secretion. Using intact rat adrenal capsular tissue, thrombin was found to increase aldosterone secretion, and the effects of trypsin on aldosterone secretion were confirmed. Both trypsin and thrombin were shown to activate phospholipase C, as measured by an increase in inositol triphosphate turnover by adrenal capsular tissue. It was also shown that U73122, a phospholipase C inhibitor, attenuated the aldosterone response to trypsin. These effects were consistent with the activation of a PAR. Northern blot analysis revealed the presence of mRNA encoding PAR-1, but not PARs-2, -3 or -4 in the adrenal capsule/zona glomerulosa. Messenger RNA encoding PAR-1 was increased by dietary sodium depletion, consistent with previous reports of an increased response to trypsin after sodium depletion. These data suggest that the actions of trypsin on aldosterone secretion are mediated by PAR-1.
LM Thomson, S Kapas, M Carroll and JP Hinson
Previous studies from our laboratory have reported that adrenomedullin is synthesised in rat zona glomerulosa cells. In the present studies, it was found that the human adrenocortical cell line H295R expresses the gene encoding adrenomedullin, and that immunoreactive adrenomedullin is released into the culture medium. Furthermore, it was found that secretion of adrenomedullin is regulated by angiotensin II and forskolin. Studies on the actions of adrenomedullin and calcitonin gene-related peptide (CGRP) revealed a stimulatory effect of adrenomedullin, but not of CGRP, on aldosterone and cortisol secretion. These data suggest that adrenomedullin is not acting by a CGRP receptor-mediated mechanism in the H295R cell line. Adrenomedullin was also found to increase cAMP production, suggesting that in the adrenal, as in other cell types, cAMP is a second messenger for adrenomedullin action. However, the effects of adrenomedullin were not fully mimicked by forskolin, possibly suggesting a role for an additional second messenger. The presence of mRNA encoding both the putative adrenomedullin receptors, L1 and calcitonin receptorlike receptor/receptor-associated modulatory protein 2 (CRLR/RAMP-2), was demonstrated in H295R cells, but RAMP-1 was not detected, suggesting that these cells do not express the CGRPI receptor CRLR/RAMP-1. Taken together, these data have demonstrated that adrenomedullin is synthesised and secreted by H295R cells. The observed rate of adrenomedullin synthesis suggests that this peptide exerts a paracrine/autocrine effect in this adrenocortical cell line, probably acting through a specific adrenomedullin receptor, to stimulate steroidogenesis and increase aldosterone synthase expression.