Abstract. Atrial natriuretic peptide (ANP) is a regulatory hormone widely expressed, along with its receptors, in organs and body tissues. ANP is well known to inhibit aldosterone secretion from mammalian adrenals, but its effect on glucocorticoid-hormone production is controversial. In vivo experiments showed that prolonged ANP administration raised the plasma concentration of cortisol in both normal and dexamethasone/captopril-treated guinea pigs (i.e. in animals with pharmacologically interrupted hypothalamic-pituitaryadrenal axis and renin-angiotensin system). ANP did not affect cortisol secretion from dispersed guinea pig zona fasciculatareticularis cells, but enhanced catecholamine release from adrenomedullary cells. ANP stimulated cortisol output from guinea pig adrenal slices containing medullary chromaffin tissue, and the ß-adrenoceptor antagonist l-alprenolol blocked this effect. The conclusion is drawn that ANP, when the structural integrity of the adrenal gland is preserved, is able to enhance glucocorticoid secretion in guinea pigs, through an indirect mechanism involving the rise in the catecholamine release, which in turn, acting in a paracrine manner, stimulate secretion of inner adrenocortical cells. IntroductionAtrial natriuretic (ANP) is the first member of a family of peptides originally discovered in the late 1970s in the secretory granules of atrial myocytes, and including brain natriuretic peptide and C-type natriuretic peptide (reviewed in ref. 1). ANP acts via two G protein-coupled receptors, named A and B, that are coupled to guanylate cyclase (reviewed in refs. 2,3). Subsequent studies showed that ANP and its receptors are present in several extra-atrial tissues, among which are heart ventricles, blood vessels, brain, lungs, kidneys and endocrine glands.Mammalian adrenal zona glomerulosa possesses A and B subtypes of ANP receptors, and many lines of evidence indicate that ANP, via the cyclic-GMP pathway, inhibits either basal or agonist-stimulated aldosterone secretion (reviewed in refs. 4,5). The possible effects of ANP on the zona fasciculata and glucocorticoid secretion have been far less investigated. Although ANF receptors have been demonstrated in the rat zona fasciculata (6), most studies did not report any effect of ANP on glucocorticoid secretion in this species (1,4). However, findings suggest that ANP is able to lower basal and especially ACTH-stimulated glucocorticoid production from cultured human and cow zona fasciculata cells (7,11). The A subtype of ANP receptors is expressed also in the adrenal medulla (12,13), and the bulk of evidence indicates that ANP inhibits catecholamine release (14-17). However, ANP has been more recently reported to induce tyrosine hydroxylase mRNA expression and to raise catecholamine content in rat pheochromocytoma PC12 cells, via the guanylate cyclase-dependent cascade (18).Therefore, it seemed worthwhile to investigate the in vivo and in vitro effects of ANP on glucocorticoid and catecholamine secretion from the guinea-pig adrenal gland. Gu...

Insulin-like growth factor (IGF)-I is a ubiquitously synthesized peptide that, along with IGF-II, acts via the IGF-R type I receptor. IGF-I and its receptor are expressed in the adrenal gland of humans and bovines, the secretion of which they seem to stimulate. As in humans and cows, the main glucocorticoid hormone secreted by guinea-pig adrenals is cortisol, and hence we have studied the adrenocortical effects of IGF-I in this species. In vivo experiments showed that prolonged IGF-I administration raised the plasma concentration of cortisol in both normal and dexamethasone/captopriltreated guinea pigs, thereby ruling out the possibility that IGF-I may act by activating the hypothalamic-pituitaryadrenal axis and the renin-angiotensin system. In vitro experiments demonstrated that IGF-I enhanced basal, but not maximally agonist [ACTH and angiotensin-II (Ang-II)]stimulated, cortisol secretion from freshly dispersed guinea-pig inner adrenocortical cells. The IGF-I immuno-neutralization suppressed the IGF-I secretagogue effect, without altering the cortisol response to both ACTH and Ang-II. IGF-I raised cyclic-AMP and inositol triphosphate release from dispersed guinea-pig cells, and the effect was reversed by the adenylate cyclase inhibitor SQ-22536 and the phospholipase-C (PLC)