No abstract

Endothelins (ET) are a family of vasoactive peptides that act via two subtypes of receptors, named ETA and ETB. ET-1 binds to both ETA and ETB, whereas the isopeptide ET-3 preferentially binds to ETB. The localization of ETA and ETB receptors in the rat adrenal gland and their involvement in the adrenal secretagogue effect of ETs has been studied in vitro. Autoradiographic assessment of the selective displacement of [125I]ET-1, [125I]ET-3 and [125I]BQ-3020 (an ETB agonist) by BQ-123 or BQ-788 (specific antagonists of ETA and ETB, respectively) indicates that the zona glomerulosa and adrenal medulla possess both ETA and ETB, whereas the zona fasciculata/reticularis is exclusively provided with ETB. ET-1, ET-3 and BQ-3020 enhance aldosterone and corticosterone secretion by dispersed cells of the zona glomerulosa and zona fasciculata/reticularis, respectively. BQ-123 does not affect the secretagogue action of these three agonists, whereas BQ-788 completely annuls it. ET-1 induces a marked rise in catecholamine release by fragments of the adrenal medulla, and both BQ-123 and BQ-788 partially reverse this effect. ET-3 and BQ-3020 elicit a catecholamine release that is less intense than that produced by ET-1; this response is unaffected by BQ-123 and abolished by BQ-788. Thus, in the rat, the corticosteroid secretagogue effect of ETs seems to be exclusively mediated by the ETB receptor subtype, and the catecholamine secretagogue action by both ETA and ETB. The functional relevance of ETA receptors present in the zona glomerulosa remains to be investigated.

No abstract

Adrenomedullin (ADM) and proadrenomedullin N-terminal 20 peptide (PAMP) are two hypotensive peptides, contained in adrenal medulla, which are able to inhibit aldosterone secretion from zona glomerulosa. In this study we have compared the effects of the two peptides on the production of post-pregnenolone steroids by dispersed rat zona glomerulosa cells. ADM and PAMP did not alter basal steroid secretion. Conversely, they inhibited angiotensin-II (1O'9 M)-stimulated 18-hydroxy-11-deoxycorticosterone, corticosterone, 18-hydroxycorticosterone and aldosterone production, without affecting progesterone and lldeoxycorticosterone secretion. PAMP was significantly more effective than ADM, their minimal effective concentrations being lO"°M and 10*-10"? M, respectively. ADM, but not PAMP markedly lowered either basal and agonist-enhanced release of 11-dehydrocorticosterone. Collectively, these findings allow us to draw the following conclusions: (i) ADM and PAMP electively inhibit 1118-hydroxylase and aldosterone synthase, PAMP being probably to be considered the proadrenomedullin-derived physiological inhibitor of mineralocorticoid secretion in rats; and (ii) ADM, but not PAMP, also exerts a clear-cut inhibitory action on 1116-hydroxysteroid dehydrogenase, the enzyme that converts corticosterone to its inactive form.Adrenomedullin (ADM) and proadrenomedullin Nterminal 20 peptide (PAMP) are two hypotensive peptides, which are produced as a part of a 185-amino acid prohormone called preproadrenomedullin (for review, see 14, 15). Both peptides are expressed in adrenal medulla (5, 6, 17, 22), and specific receptors for ADM and PAMP have been demonstrated in the adrenal cortex, and especially in zona glomerulosa (4, 14). Hence, ADM and PAMP have been included in that group of peptides, secreted from medullary chromaffin cells, which may modulate the function of adrenocortical cells in a paracrine manner (13, 14).Accordingly, compelling evidence indicates that 2Correspondence to: G. G. Nussdorfer at the above address. Tel: (+39)49-827-2317; Fax: (+39)49-827-2319; E-mail: ggnanat@ipdunidx.unipd.it ADM and PAMP are both able to specifically inhibit angiotensin-II (ANG-II)-stimulated aldosterone (ALDO) secretion from dispersed rat and human zona glomerulosa cells, probably by impairing Ca" influx (1, 2, 9, 24). There are also indications that ADM, but not PAMP effect is mediated by the subtype 1 of calcitonin gene-related peptide receptors (1, 2, 8, 9). The inhibitory action of ADM and PAMP appears to be electively addressed against zona glomerulosa and mineralocorticoid secretion and to play a potentially important role in the regulation of fiuid and electrolyte homeostasis (18), but the locus at which the two peptides impair steroid synthesis is not known. It, therefore, seemed worthwhile to investigate the effects of ADM and PAMP on the response to ANG-II of the main post-pregnenolone steroid hormones released from dispersed rat zona glomerulosa cells.

Rat adrenocortical cells are almost completely dependent upon the continuous supply of cholesterol derived from serum lipoproteins. However, a prolonged (5-day) administration of 4-aminopyrazolo-pyrimidine (4-APP), a potent hypocholesterolaemic drug, though provoking a notable decrease in the intra-adrenal concentration of esterified and free cholesterol, did not significantly affect basal plasma level of corticosterone. Morphometry showed a conspicuous hypertrophy of zona fasciculata cells, coupled with a striking proliferation of smooth endoplasmic reticulum (SER) and peroxisomes and with a profound lipid-droplet depletion. The secretory response of zona fasciculata cells to ACTH was still present, but reduced by half with respect to control rats. The simultaneous administration of mevinolin, an inhibitor of cholesterol synthesis, to 4-APP-treated rats caused an additional drop in the intracellular content of free cholesterol and notably lowered basal plasma corticosterone concentration. Mevinolin magnified the 4-APP-induced zona fasciculata cell hypertrophy, as well as SER and peroxisome proliferation. The secretory response to ACTH was completely suppressed. These data are compatible with the view that the morphological changes, which rat zona fasciculata cells undergo during prolonged hypocholesterolaemia, are the expression of the activation of the endogenous cholesterol synthesis. This compensatory response, enabling zona fasciculata cells to maintain a normal basal rate of hormonal output and to respond (though less efficiently) to their main physiological stimulus, seems to be completely independent of any activation of the hypothalamo-hyphophyseal axis, since dexamethasone/ACTH treated rats were used. The hypothesis is advanced that the mechanism underlying this response may involve the decrease of the intracellular free-cholesterol pool.