Characterisation of an adrenal zona glomerulosa-stimulating component of posterior pituitary extracts as alpha-MSH

Pathophysiology of melanocortin receptors and their accessory proteins

The melanocortin receptors (MCRs) and their accessory proteins (MRAPs) are involved in regulation of a diverse range of endocrine pathways. Genetic variants of these components result in phenotypic variation and disease. The MC1R is expressed in skin and variants in the MC1R gene are associated with ginger hair color. The MC2R mediates the action of ACTH in the adrenal gland to stimulate glucocorticoid production and MC2R mutations result in familial glucocorticoid deficiency (FGD). MC3R and MC4R are involved in metabolic regulation and their gene variants are associated with severe pediatric obesity, whereas the function of MC5R remains to be fully elucidated. MRAPs have been shown to modulate the function of MCRs and genetic variants in MRAPs are associated with diseases including FGD type 2 and potentially early onset obesity. This review provides an insight into recent advances in MCRs and MRAPs physiology, focusing on the disorders associated with their dysfunction.

Functional Zonation of the Adult Mammalian Adrenal Cortex

The standard model of adrenocortical zonation holds that the three main zones, glomerulosa, fasciculata, and reticularis each have a distinct function, producing mineralocorticoids (in fact just aldosterone), glucocorticoids, and androgens respectively. Moreover, each zone has its specific mechanism of regulation, though ACTH has actions throughout. Finally, the cells of the cortex originate from a stem cell population in the outer cortex or capsule, and migrate centripetally, changing their phenotype as they progress through the zones. Recent progress in understanding the development of the gland and the distribution of steroidogenic enzymes, trophic hormone receptors, and other factors suggests that this model needs refinement. Firstly, proliferation can take place throughout the gland, and although the stem cells are certainly located in the periphery, zonal replenishment can take place within zones. Perhaps more importantly, neither the distribution of enzymes nor receptors suggest that the individual zones are necessarily autonomous in their production of steroid. This is particularly true of the glomerulosa, which does not seem to have the full suite of enzymes required for aldosterone biosynthesis. Nor, in the rat anyway, does it express MC2R to account for the response of aldosterone to ACTH. It is known that in development, recruitment of stem cells is stimulated by signals from within the glomerulosa. Furthermore, throughout the cortex local regulatory factors, including cytokines, catecholamines and the tissue renin-angiotensin system, modify and refine the effects of the systemic trophic factors. In these and other ways it more and more appears that the functions of the gland should be viewed as an integrated whole, greater than the sum of its component parts.

Generation of a novel monoclonal antibody that recognizes the alpha (α)-amidated isoform of a valine residue

Background

Alpha (α)-amidation of peptides is a mechanism required for the conversion of prohormones into functional peptide sequences that display biological activities, receptor recognition and signal transduction on target cells. Alpha (α)-amidation occurs in almost all species and amino acids identified in nature. C-terminal valine amide neuropeptides constitute the smallest group of functional peptide compounds identified in neurosecretory structures in vertebrate and invertebrate species.

Methods

The α-amidated isoform of valine residue (Val-CONH₂) was conjugated to KLH-protein carrier and used to immunize mice. Hyperimmune animals displaying high titers of valine amide antisera were used to generate stable hybridoma-secreting mAbs. Three productive hybridoma (P15A4, P17C11, and P18C5) were tested against peptides antigens containing both the C-terminal α-amidated (–CONH₂) and free α-carboxylic acid (−COO⁻) isovariant of the valine residue.

Results

P18C5 mAb displayed the highest specificity and selectivity against C-terminal valine amidated peptide antigens in different immunoassays. P18C5 mAb-immunoreactivity exhibited a wide distribution along the neuroaxis of the rat brain, particularly in brain areas that did not cross-match with the neuronal distribution of known valine amide neuropeptides (α-MSH, adrenorphin, secretin, UCN1-2). These brain regions varied in the relative amount of putative novel valine amide peptide immunoreactive material (nmol/μg protein) estimated through a fmol-sensitive solid-phase radioimmunoassay (RIA) raised for P18C5 mAb.

Conclusions

Our results demonstrate the versatility of a single mAb able to differentiate between two structural subdomains of a single amino acid. This mAb offers a wide spectrum of potential applications in research and medicine, whose uses may extend from a biological reagent (used to detect valine amidated peptide substances in fluids and tissues) to a detoxifying reagent (used to neutralize exogenous toxic amide peptide compounds) or as a specific immunoreagent in immunotherapy settings (used to reduce tumor growth and tumorigenesis) among many others.

Melanocortin receptor accessory proteins in adrenal disease and obesity

Melanocortin receptor accessory proteins (MRAPs) are regulators of the melanocortin receptor family. MRAP is an essential accessory factor for the functional expression of the MC2R/ACTH receptor. The importance of MRAP in adrenal gland physiology is demonstrated by the clinical condition familial glucocorticoid deficiency type 2. The role of its paralog melanocortin-2-receptor accessory protein 2 (MRAP2), which is predominantly expressed in the hypothalamus including the paraventricular nucleus, has recently been linked to mammalian obesity. Whole body deletion and targeted brain specific deletion of the Mrap2 gene result in severe obesity in mice. Interestingly, Mrap2 complete knockout (KO) mice have increased body weight without detectable changes to food intake or energy expenditure. Rare heterozygous variants of MRAP2 have been found in humans with severe, early-onset obesity. In vitro data have shown that Mrap2 interaction with the melanocortin-4-receptor (Mc4r) affects receptor signaling. However, the mechanism by which Mrap2 regulates body weight in vivo is not fully understood and differences between the phenotypes of Mrap2 and Mc4r KO mice may point toward Mc4r independent mechanisms.

Melanocortin receptor accessory proteins in adrenal gland physiology and beyond

The melanocortin receptor (MCR) family consists of five G-protein-coupled receptors (MC1R-MC5R) with diverse physiological roles. MC1R controls pigmentation, MC2R is a critical component of the hypothalamic-pituitary-adrenal axis, MC3R and MC4R have a vital role in energy homeostasis and MC5R is involved in exocrine function. The melanocortin receptor accessory protein (MRAP) and its paralogue MRAP2 are small single-pass transmembrane proteins that have been shown to regulate MCR expression and function. In the adrenal gland, MRAP is an essential accessory factor for the functional expression of the MC2R/ACTH receptor. The importance of MRAP in adrenal gland physiology is demonstrated by the clinical condition familial glucocorticoid deficiency, where inactivating MRAP mutations account for ∼20% of cases. MRAP is highly expressed in both the zona fasciculata and the undifferentiated zone. Expression in the undifferentiated zone suggests that MRAP could also be important in adrenal cell differentiation and/or maintenance. In contrast, the role of adrenal MRAP2, which is highly expressed in the foetal gland, is unclear. The expression of MRAPs outside the adrenal gland is suggestive of a wider physiological purpose, beyond MC2R-mediated adrenal steroidogenesis. In vitro, MRAPs have been shown to reduce surface expression and signalling of all the other MCRs (MC1,3,4,5R). MRAP2 is predominantly expressed in the hypothalamus, a site that also expresses a high level of MC3R and MC4R. This raises the intriguing possibility of a CNS role for the MRAPs.

Genetic modifications of mouse proopiomelanocortin peptide processing

Pro-opiomelanocortin (POMC) is a prohormone which undergoes extensive tissue and cell specific post-translational processing producing a number of active peptides with diverse biological roles ranging from control of adrenal function to pigmentation to the regulation of feeding. One approach to unraveling the complexities of the POMC system is to engineer mouse mutants which lack specific POMC peptides. We describe here the design, generation, validation, and preliminary analysis of one such partial POMC mutant specifically lacking α-MSH. In contrast to POMC null mutant mice, mice lacking α-MSH in the presence of all other POMC peptides maintain adrenal structures and produce corticosterone comparable to wildtype littermates; however, they still have decreased levels of aldosterone, as found in POMC null mutant mice. Our findings demonstrate that α-MSH is not needed for maintenance of adrenal structure or for corticosterone production, but is needed for aldosterone production. These data demonstrate that mouse strains generated with precise genetic modifications of POMC peptide processing can answer questions about POMC peptide function. Further analysis of this and additional strains of mice with modified POMC peptide processing patterns will open up a novel avenue for studying the roles of individual POMC peptides.

Effects of melanocortins on adrenal gland physiology

The melanocortin-2-receptor (MC(2) receptor), also known as the ACTH receptor, is a critical component of the hypothalamic-pituitary-adrenal axis. The importance of MC(2) receptor in adrenal physiology is exemplified by the condition familial glucocorticoid deficiency, a potentially fatal disease characterised by isolated cortisol deficiency. MC(2)receptor mutations cause ~25% of cases. The discovery of a MC(2) receptor accessory protein MRAP, mutations of which account for ~15%-20% of familial glucocorticoid deficiency, has provided insight into MC(2) receptor trafficking and signalling. MRAP is essential for the functional expression of MC(2) receptor. MRAP2, a novel homolog of MRAP, can also facilitate MC(2) receptor cell surface expression and function. Like MRAP, MRAP2 is a small transmembrane domain glycoprotein capable of homodimerising. In addition, MRAP/MRAP2 can heterodimerise. The presence of MRAP2 adrenal expression suggests a possible role for MRAP2 in adrenal physiology, which has yet to be elucidated. Importantly, new data shows that the MRAPs can interact with all the other melanocortin receptors (MC(1,3,4,5) receptor). In contrast to MC(2) receptor, this interaction results in reduced melanocortin receptor surface expression and signalling. MRAP2 is predominantly expressed in brain. Hypothalamic expression has been demonstrated for both MRAP and MRAP2. The ability of MRAPs to modulate different members of the melanocortin receptor family in a bidirectional manner is intriguing. Furthermore, central nervous system expression of MRAPs points to a role beyond MC(2) receptor mediated adrenal steroidogenesis.

Biological role of the pars intermedia in lower vertebrates

The most obvious function of the pars intermedia in lower vertebrates is the secretion of melanocyte-stimulating hormone (MSH) for the purpose of pigmentary control. In some amphibia, elasmobranchs and teleosts, the histological study of the pars intermedia, the radioimmunoassay of pituitary and plasma MSH and the effects of hypophysectomy and of MSH injection suggest that the activity of the pars intermedia is regulated to satisfy the needs of cryptic colour change. MSH secretion is associated with dispersion of melanin granules and with melanogenesis. However, in other teleost species, both the evidence from pituitary cytology and the failure to respond to MSH injection suggest that pigmentary change is not regulated by changes in the plasma titres of MSH. Results discussed here indicate that MSH alone may be inadequate for pigmentary control. Evidence for non-pigmentary functions of the pars intermedia is circumstantial and fragmentary. It is based on cytological observations of altered pars intermedia activity under certain conditions, and on observations of physiological changes that accompany increased melanotropic activity. Such function include effects of plasmas titres of cortisol in teleosts, resistance to adrenaline-induced hyperglycaemia in toads, and effects on neural activity in fish and amphibia. Evidence for pars intermedia involvement in osmoregulation is briefly discussed.

Nature and control of peptide release from the pars intermedia

Pro-opiocortin, the precursor of ACTH, LPH and gamma-MSH, is biosynthesized in both the cells of the pars intermedia and the corticotrophs of the pars distalis. In the pars distalis its processing does not vary significantly from species to species whereas in the pars intermedia large differences occur. The release of ACTH, beta-LPH and pro-gamma-MSH from the corticotrophs is under common positive control by hypothalamic corticotropin-releasing factor (CRF) and the nature of the peptides remains unchanged when they are secreted. The release of all five pars intermedia peptides that we have measured in vitro appears to be under tonic dopaminergic inhibition. The secreted peptides have also been identified chromatographically. The lack of unequivocal physiological function in the periphery, the diversity of the pars intermedia peptides and this common control mechanism tend to preclude a simple endocrine role for the pars intermedia. The neural effects of MSH and endorphin are well documented and specific neuronal uptake therefore cannot by dismissed. The absence of pars intermedia in the adult human pituitary suggests that such a site of synthesis of these peptides plays a minor role in learning and behaviour in a species (such as Homo sapiens) that has a highly evolved intelligence and may, instead, need to synthesize the peptides only in the brain.

Fine structure and cytochemistry of the mammalian pars intermedia

Various cytophysiological aspects of the pars intermedia of the pituitary are discussed. Cells containing melanocyte-stimulating hormone (MSH) have been studied under normal and experimental conditions. They react to variations in ionic equilibrium, but without any clear correlation with natraemia and osmotic blood pressure. The MSH-cell stimulation in hypernatraemic mice, which is not inhibited by bromocriptine, seems more specific than the stimulation in hyponatraemic mice, which is blocked by bromocriptine. The existence of a corticotropic cell system has been clearly demonstrated in the mouse (where it is particularly obvious), in the rat and in the cat but its significance is not clear. Although very poorly vascularized, the pars intermedia is rapidly invaded by tracer protein (horseradish peroxidase) injected either intravenously or intracerebroventricularly. The hypophysial cleft rapidly stores the tracer which can be resorbed by macrophagic epithelial cells lying free in the colloid contained in the cleft. Horseradish peroxidase lingers in the pars intermedia but is rapidly eliminated from the other hypophysial lobes after intraventricular (third ventricle) injection. Diffuse innervation of the pars intermedia applies to both glandular (MSH and ACTH) and non-glandular (epithelial and stellate) cells. While aminergic innervation of the pars intermedia is obvious, cholinergic innervation has not been demonstrated ultrastructurally or histochemically. Peptidergic fibres only occasionally penetrate marginal areas of the pars intermedia and seldom establish synaptic contacts with glandular cells. A specific relationship might exist between the pars intermedia and oxytocin fibres in view of the marginal distribution of the latter in the neural lobe. Numerous stellate cells of the pars intermedia react intensely with antiserum to gliofibrillar acid protein, indicating their astrocyte nature, which reinforces the idea of an analogy between the folliculo-stellate system of the hypophysis and the glial cells.

Mechanisms of adrenocorticotropin-induced activation of extracellularly regulated kinase 1/2 mitogen-activated protein kinase in the human H295R adrenal cell line

The role of ACTH in stimulating or inhibiting growth of adrenal cells has been a subject of some controversy. Reports that ACTH may stimulate ERK/MAPK in Y1 cells have suggested a role for cAMP in this process. In attempting to extend this work, the ACTH responses in the human H295R cell line have been studied. This cell line makes only a very modest cAMP response to ACTH, yet the ERK1/2 response is highly reproducible and immediate but not prolonged. It is minimally reduced by the protein kinase A inhibitor, H89, but unaffected by protein kinase C and calcium inhibitors. Inhibition of epidermal growth factor receptor or other tyrosine kinase receptor transactivation was without effect, as was inhibition of c-Src activity or c-Src phosphorylation. The most effective inhibitor of this pathway was dansylcadaverine, an inhibitor of receptor internalization. These findings imply that ACTH-induced ERK1/2 activation in H295R cells is dependent on a mechanism distinct from that by which most G protein-coupled receptors activate ERK1/2 but that nevertheless seems to depend on receptor internalization.

Pro-opiomelanocortin peptides and the adrenal gland

The adrenal gland regulates a number of essential biological functions through production of steroids and catecholamines. Pro-opiomelanocortin (POMC)-derived peptides have been implicated in all aspects of generating, maintaining, and functioning of the adrenal glands. An appreciation for the roles of POMC-derived peptides with respect to the adrenal has been gained from experiments in vitro, and in vivo in different animal models which surgically, pharmacologically, or genetically decrease or increase the amount of POMC peptides available. We recently produced a mouse model with a deletion of the entire coding region of the POMC gene, thus lacking all POMC-derived peptides, from all sources, and at all times. Here we will summarize and discuss the results of traditional in vivo studies on the role of POMC peptides in adrenal development, maintenance, and function in the context of findings in a mouse model genetically lacking all POMC-derived peptides.

Development, maintenance, and function of the adrenal gland in early postnatal proopiomelanocortin-null mutant mice

Adult mouse mutants homozygous for an engineered proopiomelanocortin (POMC)-null allele lack macroscopically distinct adrenal glands and circulating adrenal hormones. To understand the basis for this adrenal defect, we compared the development of adrenal primordia in POMC-null mice and littermate controls. POMC-null mutant mice are born with adrenal glands that are morphologically indistinguishable from those of their wild-type littermates. However, in mutants adrenal cells fail to proliferate postnatally and adrenals atrophy until they have disappeared macroscopically in the adult. While present, mutant adrenals are differentiated as evidenced by the presence of enzymes for the final steps in the synthesis of corticosterone, aldosterone, and catecholamines. However, in contrast to adrenals of wild-type littermates, adrenals of POMC-null mutants do not produce corticosterone, not even in response to acute stimulation with exogenous ACTH. They do produce aldosterone; however, it is produced at reduced levels correlating with adrenal size. Transplantation of POMC-null mutant adrenals to adrenalectomized wild-type littermates results in adrenals with normal morphology and production of both corticosterone and aldosterone. These findings demonstrate that POMC peptides are not required for prenatal adrenal development and that POMC peptides in addition to ACTH are required for postnatal proliferation and maintenance of adrenal structures capable of producing both glucocorticoids and mineralocorticoids.

Adrenocortical zonation and ACTH

The clear morphological distinction between the cells of the different adrenocortical zones has attracted speculation and experiment to interpret their functions and the ways in which they are regulated. Considerable data have been produced in recent years that has benefited a fuller understanding of the processes of steroidogenesis and of cell proliferation at the molecular level. This now enables the reexamination of earlier concepts. It is evident that there is considerable species variation, and this article, dealing mainly with the rat, reaches conclusions that do not necessarily apply to other mammals. In the rat adrenal, however, the evidence suggests that the greatest differences between the functions of the zones are between the glomerulosa and the fasciculata. Here the sometimes all-or-nothing demarcation in their complement of components associated with steroidogenesis or with cell proliferation suggests a stark division of labor. In this model the fasciculata is the main engine of steroid hormone output and the glomerulosa is the site of cell proliferation, recruitment, and differentiation. Regulating these functions are angiotensin II and other paracrine components that modulate and maintain the glomerulosa, and ACTH, that maintains the fasciculata, and recruits new fasciculata cells by transformation of proliferating glomerulosa cells. Grafted onto this mostly vegetative function of the glomerulosa is CYP11B2, limited to just a fraction of the outer glomerulosa in rats on a normal laboratory diet and generating aldosterone (and 18-hydroxycorticosterone) from precursors whose origin is not, from the evidence summarized here, very clear, but may include the fasciculata, directly or indirectly. The biosynthesis of aldosterone in the rat certainly requires reinterpretation.

Expression of melanin-concentrating hormone receptor messenger ribonucleic acid in tumor tissues of pheochromocytoma, ganglioneuroblastoma, and neuroblastoma

Expression of melanin-concentrating hormone (MCH) receptor messenger ribonucleic acid (mRNA) was studied by RT-PCR and Northern blot analysis in human brain; pituitary; adrenal glands; tumor tissues of adrenal tumors, ganglioneuroblastomas, and neuroblastomas; and various cultured tumor cell lines. RT-PCR analysis showed that MCH receptor mRNA was widely expressed in brain tissues, pituitary, normal portions of adrenal glands (cortex and medulla), tumor tissues of adrenocortical tumors (12 of 13 cases), pheochromocytoma (all 7 cases), ganglioneuroblastoma (1 case), neuroblastoma (all 5 cases), and various cultured tumor cell lines (6 of 7 cell lines), including 2 neuroblastoma cell lines. Northern blot analysis showed the expression of MCH receptor mRNA ( approximately 2.4 kb) only in the tumor tissues of 5 pheochromocytomas, 1 ganglioneuroblastoma, and 4 neuroblastomas, indicating that the expression levels of MCH receptor mRNA are much higher in these tumors than in the other tissues. These findings raised the possibility that MCH or MCH-like peptides may be related to the pathophysiology of these neural crest-derived tumors.

Lipoproteins, lipid droplets, lysosomes, and adrenocortical steroid hormone synthesis: morphological studies

Recent studies concerning cellular cholesterol homeostasis suggest that there is a relationship between the serum lipoproteins (low density and high density lipoproteins: LDL and HDL), the intracellular storage of cholesterol (lipid droplets), lysosomes, and the steroidogenic activity of adrenocortical cells. This review surveys the current knowledge on cholesterol import from LDL/HDL by adrenocortical cells, its regulation, and the participation of lipid droplets and lysosomes in this process. The possible role of adrenocortical cell microvilli in the uptake of LDL/HDL is discussed. Under certain physiological, experimental, and pathological circumstances lysosomes accumulate unesterified and/or esterified cholesterol in the form of lipid-lysosome complexes. As suggested by the data presented in this review, lipid-lysosome complexes appear to be involved in cholesterol homeostasis, via altering lipid compartmentalization. Since previous reports do not clearly demonstrate a positive correlation between the volume of lipid- and lysosome-compartments and the rate of steroid hormone synthesis [for review, see Nussdorfer (1986) Int. Rev. Cytol., 98:1-405], the objective of this review is to provide a better understanding of the interactions of plasma lipoproteins, lipid droplets, lysosomes, and steroidogenesis.

Effects of pro-opiomelanocortin peptides on adrenocortical steroidogenesis

Whilst studying the effects of pro-opiomelanocortin (POMC) peptides on adrenocortical steroidogenesis we showed that alpha-melanocyte-stimulating hormone (alpha-MSH) has a specific glomerulotropic effect and potentiates both the mineralocorticotropic and glucocorticotropic effects of ACTH. The fragments ACTH-(4-10) and ACTH-(11-13) are responsible for the glomerulotropic effect of alpha-MSH. beta-Endorphin enhances, inhibits or has no effect on corticosteroidogenesis, depending on the dose and on the functional state of the adrenocortical cells (saturation of the receptors). beta-Endorphin antagonizes the effect of alpha-MSH on aldosterone production. It is suggested that the modulatory effects of POMC peptides on adrenocortical steroidogenesis may be of physiological significance.

Interaction of ACTH, beta-endorphin and alpha-melanocyte stimulating hormone in relation to the corticosteroid production of isolated rat adrenocortical zona fasciculata and zona glomerulosa cells

The combined effects of ACTH, beta-endorphin (beta-EP) and alpha-MSH were studied on the corticosteroidogenesis of isolated rat adrenocortical zona fasciculata and zona glomerulosa cells. beta-EP potentiated the effects of ACTH and alpha-MSH on the zona fasciculata corticosterone production but inhibited those on the zona glomerulosa aldosterone production. beta-EP did not affect the combined action of 4 x 10(-11) M ACTH and 5 x 10(-9) M alpha-MSH on the zona fasciculata or the zona glomerulosa cells, but it inhibited the stimulatory action of the combination of 1.6 x 10(-10) M ACTH and 10(-9) M alpha-MSH on the zona glomerulosa aldosterone production. An interaction of ACTH, beta-EP and alpha-MSH in relation to the zona fasciculata and zona glomerulosa corticosteroid production was found.

Proopiomelanocortin-derived peptides, phosphoinositides, cAMP, and aldosterone secretion

Since the intracellular messengers of various proopiomelanocortin-derived peptides remain ambiguous at best, we have investigated the possible involvement of phosphoinositide metabolism in aldosterone secretion evoked by alpha-MSH, beta-LPH, as well as ACTH in rat and calf adrenal glomerulosa cells. We have also examined the cAMP responses in the adrenal glomerulosa cells to alpha-MSH comparing it with those of ACTH. Our results showed that neither alpha-MSH, beta-LPH, nor ACTH increased inositol triphosphate (IP3) or other inositol phosphates in adrenal glomerulosa cells while increasing aldosterone secretion from the same cells. Angiotensin II, known to cause hydrolysis of the phosphoinositides, increased IP3 in these adrenal cells in a dose-dependent manner. Both ACTH and alpha-MSH raised the cAMP levels in the calf adrenal glomerulosa cells, although the magnitude of the increase of cAMP in response to ACTH was greater. These findings suggest that IP3 as a mediator of alpha-MSH- and beta-LPH-induced aldosterone secretion is not likely and other mediator(s) may be involved.

Evidence for a cold-induced aldosterone stimulation in the rat

This study was undertaken to determine the secretion of aldosterone by male Long-Evans rats acclimated for six weeks to moderate cold (15 C), in comparison with rats maintained at thermo-neutral temperature (28 C). The following determinations were made: corticosteroids in plasma and adrenals, PRA, and hydromineral balance. Cold acclimation highly increased the plasma and adrenal levels of aldosterone and corticosterone. The cold stimulation of aldosterone was induced neither by the renin-angiotensin system, nor by alterations of hydromineral balance: PRA, plasma sodium and potassium concentrations, blood hematocrit, and hydromineral balance at 15 C and 28 C did not differ. Moreover this stimulation was induced neither by ACTH, nor by any other hypophyseal factors, since plasma aldosterone levels remained high in hypophysectomized rats. This study provides evidence of an aldosterone stimulation which appeared during moderate cold acclimation; the origin of this stimulation must be investigated.

Increased production of alpha-melanocyte-stimulating hormone in the pituitary gland of patients with untreated Addison's disease

There is evidence that peptides related to alpha-melanocyte-stimulating hormone (alpha-MSH) are involved in regulating the zona glomerulosa of the adrenal cortex in certain species. We have investigated the amount of immunoreactive (IR)-alpha-MSH in the human pituitary gland of patients suffering from Addison's disease. We show increased numbers of cells containing demonstrable IR-alpha-MSH in the anterior lobe in these patients. Using an antiserum with specificity for the acetylated N-terminus of alpha-MSH we suggest that the major form present is desacetyl-alpha-MSH. These findings are in keeping with a role for anterior lobe derived desacetyl-alpha-MSH in the regulation of the human adrenal cortex.

Effects of POMC-derived peptides on aldosterone secretion in vivo

1. To investigate a role for peptides derived from the precursor molecule pro-opiomelanocortin (POMC) on the control of aldosterone secretion (ASR), alpha-, beta-, gamma 1-, and gamma 2-melanocyte stimulating hormone (MSH), corticotropin-like intermediate lobe peptide (CLIP) or beta-endorphin were infused into the adrenal arterial supply of sheep with an adrenal cervical autotransplant. 2. None of the peptides had any significant effect on aldosterone secretion rate in Na replete, unstressed, conscious animals. In contrast, ACTH-stimulated ASR approximately twofold. 3. POMC-derived peptides other than ACTH appear to have little or no effect on the short-term control of aldosterone secretion in vivo, although a role in control and modulation of adrenal function over the longer term cannot be discounted.

Central dopaminergic regulation of aldosterone secretion in sheep

Central dopaminergic mechanisms involved in the regulation of plasma aldosterone concentration were investigated in 16 conscious sheep following Na depletion with intramuscularly administered furosemide. Intracerebroventricular infusion of dopamine (20 micrograms/min) decreased plasma aldosterone significantly to 52 +/- 8% of basal level and increased plasma renin activity (PRA) significantly to 172 +/- 25% of basal level in this animal model. In addition, intracerebroventricular infusion of the dopamine antagonist metoclopramide (20 micrograms/min) in artificial cerebrospinal fluid vehicle significantly increased aldosterone levels to 144 +/- 14% of basal level and decreased PRA to 62 +/- 5% of basal value. Neither intracerebroventricular infusion of the vehicle nor intravenous infusions of metoclopramide or dopamine at the same doses changed aldosterone or PRA levels. Intracerebroventricular bolus injections of metoclopramide (20 micrograms/kg in 0.4 ml of vehicle) were also effective, increasing aldosterone levels to 266 +/- 22% of basal level and decreasing PRA to 70 +/- 12% of basal level. Intravenous bolus injections of the same dose of metoclopramide were ineffective. Dopamine was infused intracerebroventricularly into two uniadrenalectomized sheep with the remaining adrenal transplanted to the neck. Aldosterone levels were decreased to 49 +/- 10% of basal level, and PRA was increased to 157 +/- 10% of basal value. None of the infusions or injections changed arterial or intracranial pressure, or plasma K, Na, and cortisol levels. These data indicate that endogenous or exogenous dopamine may act on central dopamine receptors to decrease plasma aldosterone concentration by an unknown humoral mechanism. The known aldosterone regulators, plasma Na, K, angiotensin II, and adrenocorticotropic hormone, are not involved in the regulation.

Acute effects of alpha-MSH on the rat zona glomerulosa in vivo

alpha-MSH acutely enhanced the plasma concentration of aldosterone (but not that of corticosterone) in the rat, with a maximal response at a dose of 100 micrograms/kg. This dose of alpha-MSH increased the blood level o aldosterone and the activity of 11 beta-hydroxylase and 18-hydroxylase of capsular adrenals in rats infused for 24 h with dexamethasone, dexamethasone plus ACTH, or captopril plus angiotensin II, but not in animals treated with captopril alone. The plasma concentration of corticosterone and the activity of 11 beta-hydroxylase in the inner adrenal layers were not changed. These findings indicate that alpha-MSH is specifically involved in the acute stimulation of the late steps of the secretory activity of the rat zona glomerulosa, and that this action of alpha-MSH requires a normal level of circulating angiotensin II.

Non-ACTH components of adult human pituitary extracts which stimulate adrenal steroidogenesis

Human pituitary extracts were fractionated by chromatography on Sephadex G-50 and G-25, and low molecular weight components were further separated by HPLC. Eluates were tested for their activity in stimulating steroidogenesis in suspensions of rat adrenal capsule (largely zona glomerulosa) and inner zone (fasciculata/reticularis) cells. Several biologically active components were reproducibly isolated. Three stimulated glomerulosa cells specifically, and one of these was tentatively identified by HPLC and RIA criteria as desacetyl-alpha-MSH. Alpha-MSH was not detected. One component stimulated both cell types but two others stimulated inner zone cells, and were without effect on glomerulosa cells: this type of activity has not previously been described, and is not associated with any peptide derived from pro-opiomelanocortin which has so far been tested. The data suggest that, in addition to corticotrophin, further pituitary peptides may be involved in the control of adrenocortical function.

Effects of chronic administration of a methionine-enkephalin analogue on the zona glomerulosa of the rat adrenal cortex

The effects of D-ala2-met-enkephalinamide (DALA) on the zona glomerulosa of dexamethasone-ACTH-treated rats were investigated by coupled radioimmunologic and morphometric techniques. Short-term DALA administration provoked a significant increase in the aldosterone plasma level along with a notable lipid droplet depletion in zona glomerulosa cells. Long-term DALA treatment induced a striking hypertrophy of zona glomerulosa cells and a further rise in the blood concentration of aldosterone. These findings seem to indicate that DALA is involved not only in the acute enhancement of aldosterone output but also in the stimulation of the growth and steroidogenic capacity of rat adrenal zona glomerulosa.

Impaired aldosterone secretion from dispersed adrenal capsular cells of chronically alpha-MSH-treated rats

To investigate the chronic effects of alpha-melanocyte-stimulating hormone (alpha-MSH) on aldosterone secretion, synthetic alpha-MSH (8 micrograms/day) was infused in Sprague-Dawley rats by miniosmotic pumps for 5 days. Saline was infused in equivalent volume for 5 days using the same type of pumps in the control group of rats. Aldosterone secretion from the capsular cells of the two groups was examined in the basal state and in response to various stimuli of aldosterone secretion. Aldosterone secretion in vitro from the alpha-MSH-treated rats was significantly impaired in response to all stimuli tested including cyclic AMP, suggesting an intracellular defect in aldosterone synthesis in that group. These results are similar to those observed after chronic adrenocorticotropin administration.

The relationship between tissue preparation and function; methods for the study of control of aldosterone secretion: a review

The study of the control of aldosterone synthesis and secretion by the rat adrenal gland has over the past thirty years involved the application of many different in vivo and in vitro techniques. In this review the relationship between the data that each of these methods has produced is compared. There are striking differences in overall steroid production rates, and in the qualitative nature of the steroid profile which the various methods produce. In particular, aldosterone is secreted at higher rates in vivo, and when whole tissue preparations are used in vitro, than in incubations of isolated glomerulosa cells. In addition, while corticosterone is a major product of glomerulosa tissue in vitro, the available evidence suggests that it is not a major glomerulosa product in vivo.

Central serotonergic stimulation of aldosterone secretion

Serotonin stimulates aldosterone secretion both in vitro and in vivo, and serotonin antagonism decreases plasma aldosterone levels in patients with idiopathic aldosteronism. This study was designed to assess the effects of the serotonin precursor, 5-hydroxytryptophan (5HTP), upon aldosterone secretion in man, and to determine whether stimulatory effects of 5HTP are mediated through the central nervous system. Oral 5HTP, administered as a single 200-mg dose, increased plasma aldosterone levels from 4.7 +/- 0.6 to 13.3 +/- 2.8 ng/dl in dexamethasone-pretreated, normal volunteers. Peripheral inhibition of decarboxylation of 5HTP, achieved by pretreatment with carboxydopa, 25 mg three times daily for 3 d, significantly increased the stimulatory effects of 5HTP on aldosterone levels (P less than 0.001). No change in aldosterone levels occurred in subjects who received placebo after pretreatment with dexamethasone and carboxydopa. Increased aldosterone was not accompanied by increases in plasma levels of renin activity, potassium, or ACTH. Plasma levels of 5HTP were markedly increased by carboxydopa pretreatment, but peak plasma levels of serotonin were not significantly altered. Four patients with idiopathic aldosteronism all had an increase in plasma aldosterone levels after 5HTP administration, whereas the response in four patients with aldosterone-producing adenoma was variable. Incubation of isolated human and rat adrenal glomerulosa cells with serotonin resulted in increased aldosterone secretion by both sets of cells, whereas 5HTP was ineffective in stimulating aldosterone secretion in vitro. We conclude that central serotonergic pathways are involved in the stimulation of aldosterone induced by administration of 5HTP. This mechanism may be an important etiologic factor in the hypersecretion of aldosterone that occurs in patients with idiopathic aldosteronism.

Structure-activity studies with ACTH/alpha-MSH fragments on corticosteroid secretion of isolated zona glomerulosa and fasciculata cells

The steroidogenic action of ACTH/alpha-MSH fragments was studied on isolated zona glomerulosa and zona fasciculata cells dispersed by collagenase. ACTH-(4-7), ACTH-(6-10), ACTH-(4-10) and ACTH-(11-13) stimulated corticosterone production of the zona fasciculata and aldosterone production of the zona glomerulosa cells. ACTH-(7-10) was ineffective. ACTH-(4-7) appeared to be the most potent peptide of the tested fragments. None of the fragments affected the steroidogenic action of ACTH-(1-39). It is suggested that similar to the melanotropic effect of alpha-MSH two 'message' sequences for adrenocortical stimulation exist in the alpha-MSH part of the ACTH molecule.

Plasma immunoreactive gamma melanotropin in patients with idiopathic hyperaldosteronism, aldosterone-producing adenomas, and essential hypertension

A non-ACTH aldosterone-stimulating factor(s) has been implicated in the pathogenesis of idiopathic hyperaldosteronism (IHA). Although this factor has not been fully characterized, some evidence suggests that it may be related to a pro-gamma-melanotropin (pro-gamma-MSH), derived from the NH2-terminal region of pro-opiomelanocortin. In the present study, plasma immunoreactive (IR-) gamma-MSH levels at 0800 h in patients with IHA were evaluated (90 +/- 17 fmol/ml; range: 13-173 fmol/ml) and found to be significantly higher (P less than 0.05) than those in subjects with aldosterone-producing adenomas (33 +/- 8 fmol/ml), essential hypertension (33 +/- 6 fmol/ml), and normotensive controls (19 +/- 2 fmol/ml). Seven of nine IHA subjects had circulating IR-gamma-MSH levels above the normal range (greater than 35 fmol/ml). In plasmas sampled at 1200 h, IR-gamma-MSH was significantly higher in patients with IHA (95 +/- 26 fmol/ml) and adenomas (63 +/- 23 fmol/ml), as compared with essential hypertensives (31 +/- 6 fmol/ml) and normotensives (19 +/- 3 fmol/ml). Mean plasma IR-ACTH, plasma cortisol, and urinary cortisol levels did not differ significantly between any of these groups. In order to evaluate the effect of a pro-gamma-MSH in vitro, adrenal adenoma tissue was obtained from two patients, one with elevated IR-gamma-MSH (61 fmol/ml) and a second with low IR-gamma-MSH (12 fmol/ml). Aldosterone secretion by dispersed adenoma cells from the former, but not the latter, underwent a fourfold dose-dependent (10(-14)-10(-9) M) increase in response to human Lys-gamma 3-MSH. These data suggest that a pro-gamma-MSH may be implicated as a pathogenic factor in a subset of patients with primary aldosteronism, particularly among those differentially diagnosed as having IHA.

Responses of rat adrenal glomerulosa and inner zone cells to synthetic ACTH analogs and proopiomelanocortin-derived peptides

A comparison of the responsiveness of isolated rat adrenal decapsular and glomerulosa cells to corticotrophin 1-39 (ACTH 1-39), synthetic ACTH analogs (characterized by a shorter amino acid chain length, the substitution of certain amino acids in the natural sequence by other amino acid residues, the replacement of the C-terminal carboxyl group by an amide), and proopiomelanocortin-derived peptides was performed by measuring corticosterone and aldosterone production, respectively. The potencies of the synthetic ACTH analogs correlated closely with the length of the peptides, similarly in both zones. No activity was observed with the proopiomelanocortin-derived peptides in either zone, with the exception of beta-LPH and alpha-MSH.

The effects of ACTH, serotonin, K+ and angiotensin analogues on 32P incorporation into phospholipids of the rat adrenal cortex: basis for an assay method using zona glomerulosa cells

The effects of various concentrations of serotonin, ACTH, K+, angiotensin II (AII), angiotensin III (AIII) and [Sar1]angiotensin II (SAII) on steroidogenesis and the incorporation of 32P (after preincubation to near equilibrium with the ATP pool) into phosphatidylinositol (PI), phosphatidic acid (PA) and phosphatidylcholine (PC) in a preparation of capsular cells from rat adrenals, consisting of 95% zona glomerulosa (z.g.) and 5% zona fasciculata plus reticularis (z.f.r.) cells, were investigated. Serotonin and ACTH stimulated steroidogenesis in the usual manner but had little or no effect on 32P incorporation into any of the three phospholipids. However, AII, AIII and SAII stimulated steroidogenesis and also 32P incorporation into PA and PI (maximally to about 280% of control values) but not into PC. These results taken together with other data on effects on the cAMP output and Ca2+ fluxes of z.g. cells suggest that stimulation by ACTH and serotonin is mediated by cAMP as second messenger. However, the angiotensins probably act through Ca2+, with associated changes in phospholipid metabolism. The 32P incorporation into PA as a function of lg concentration of AII was linear and showed a reasonable index of precision (0.36 +/- 0.03, eight experiments, 0.23 +/- 0.02 for a further eight experiments) and correlation with steroidogenesis. The corresponding incorporation into PI showed a maximum effect and a much poorer index of precision (1.02 +/- 0.30 (4.69 +/- 3.7] over the same full range of AII concentration used. The effects of AIII and SAII showed similar characteristics for 32P incorporation into both PA and PI, but, as for stimulation of steroidogenesis, at higher concentrations for AIII than for AII. The effects of different doses of AII, AIII and ACTH on the corticosterone output and 32P incorporation into PA, PI and PC of a preparation of cells, consisting of more than 98% z.f.r. cells, from rat decapsulated adrenals were also studied. ACTH, at low doses, which nevertheless markedly stimulated corticosterone output, had a small (maximally to about 125% of control values) but significant effect on 32P incorporation into PA, PI and PC. The maximum effect was usually at about 10(-10) M ACTH and was not significant at 10(-8) M.

alpha-MSH analogues and adrenal zona glomerulosa function

Recent studies on the control of adrenal zona glomerulosa function and aldosterone secretion have focussed attention on the role of MSH-like peptides. In particular, at low concentrations, alpha-MSH has a specific stimulatory effect on rat adrenal glomerulosa cells. The synthesis of alpha-MSH analogues which have potent and prolonged effects on melanocyte systems offers new methods of examining the specificity of this response. Two peptides were tested in which potential for a beta-turn configuration was stabilised. These were: [Nle4, D-Phe7]-alpha-MSH and the cyclic [Cys4, Cys10]-alpha-MSH. In contrast to their effects on melanocyte systems, only [Cys4, Cys10]-alpha-MSH stimulated glomerulosa cells, and it was equipotent with alpha-MSH, while [Nle4, D-Phe7]-alpha-MSH and shorter fragments had no effect when added alone. [Nle4, D-Phe7]-alpha-MSH, however, augmented the response of cells already maximally stimulated with alpha-MSH and in this respect its actions resembled those of gamma-MSH and related peptides. The augmentation produced by [Nle4, D-Phe7]-alpha-MSH and gamma 3-MSH was not additive when the two peptides were added together with alpha-MSH. The results suggest that the specificity of the alpha-MSH receptors in rat adrenal glomerulosa cells and the peptide structure-function relationships in this system are different from those described for melanocytes.

Idiopathic hyperaldosteronism. A possible role for aldosterone-stimulating factor

To test the hypothesis that idiopathic hyperaldosteronism is secondary to increased adrenal stimulation by aldosterone-stimulating factor, we measured the latter in seven patients with idiopathic hyperaldosteronism and in four patients who had undergone surgical removal of an aldosterone-producing adenoma. In the patients with hyperaldosteronism, plasma aldosterone concentrations (mean +/- 1 S.E.) were 38 +/- 10 and 78 +/- 19 ng per deciliter in the supine and upright position, respectively (P less than 0.01). Supine plasma aldosterone-stimulating factor was 81 +/- 5 ng per deciliter in 15 normal subjects and 185 +/- 10 (P less than 0.01) in the patients with idiopathic hyperaldosteronism. After removal of an aldosterone-producing adenoma, plasma aldosterone-stimulating factor was normal. The supine value in each patient with idiopathic hyperaldosteronism was above the normal range (61 to 91 ng per deciliter) and increased to 290 +/- 59 ng per deciliter after four hours of upright posture. Twenty-four hour urinary excretion of aldosterone-stimulating factor was 424 +/- 35 ng (normal, 145 +/- 3; P less than 0.01) by affinity chromatography and high-pressure liquid chromatography, and it was not suppressed after two days of treatment with dexamethasone (0.5 mg orally every six hours). At the end of 48 hours, plasma concentrations were 248 +/- 40 ng per deciliter. Plasma cortisol and ACTH concentrations were under 2 micrograms per deciliter and under 40 pg per milliliter, respectively. We conclude that increased secretion of aldosterone-stimulating factor may be the cause of idiopathic hyperaldosteronism.

Alpha-melanocyte stimulating hormone increases plasma levels of glucagon and insulin in rabbits

Intravenous injections of 25 and 2.5 micrograms alpha-melanocyte stimulating hormone (alpha-MSH) increased plasma levels of glucagon, insulin and free fatty acids in fasted and fed rabbits. 45 micrograms beta-melanocyte stimulating hormone (beta-MSH) had similar effects, whereas 22 micrograms gamma-2-melanocyte stimulating hormone (gamma-MSH) was inactive. The alpha-MSH-induced increases in the plasma levels of glucagon, insulin and free fatty acids were not inhibited by alpha- or beta-adrenergic blocking drugs. The alpha-MSH-induced increases in the plasma levels of insulin were, however, augmented by phentolamine (an alpha-adrenergic receptor blocking drug). The plasma levels of glucose were increased by 25 micrograms alpha-MSH in fed rabbits, only, and were decreased by alpha-MSH during alpha-receptor blockade. The acute in vivo effects of alpha-MSH and beta-MSH on the plasma levels of glucagon, insulin and free fatty acids were rather similar to those previously reported for corticotropin (ACTH). It is possible that the 4-10 ACTH sequence, present in alpha-MSH, beta-MSH and ACTH, but not in gamma-MSH, is a message sequence for the observed effects. However, ORG 2766, a 4-9 ACTH analogue, was inactive. The mechanism by which alpha-MSH increased the plasma levels of glucagon and insulin in rabbits remains to be determined. It is possible, that the effects were mediated by both a central nervous action and a direct action on the endocrine pancreas.

Effects of corticotropin-releasing factor (CRF) on aldosterone and 18-hydroxycorticosterone secretion

To determine the effect of CRF administration on in vivo mineralocorticoid release, we performed experiments in rats and in man. In man, plasma aldosterone and 18-hydroxycorticosterone levels increased following i.v. CRF injection. In sitting subjects, the response was greater than in supine subjects, indicating an influence of posture. In the rat, CRF-induced increases in corticosteroid levels were dose-related, the largest response being for corticosterone, followed by 18-hydroxycorticosterone and then by aldosterone. Thus synthetic ovine CRF stimulates not only glucocorticoid but also mineralocorticoid secretion from the adrenals.

Vasopressin: a potent growth factor in adrenal glomerulosa cells in culture

Previous studies have shown that vasopressin stimulates the mitotic activity in adrenal zona glomerulosa cells in intact as well as in hypophysectomized rats. (Payet and Isler, Cell and Tissue Res. 172, 1976; Payet and Lehoux, J. steroid Biochem. 12, 1980). We now report that this effect is direct and specific, since vasopressin stimulates the mitotic activity of rat adrenal zona glomerulosa cells in primary cultures. These cells were prepared by dissociation with collagenase in the culture medium MEM-d-Valine. Isolated cells were placed in 3.5 diameter petri dishes in MEM-d-valine medium containing 15% fetal calf serum and antibiotics for two days and 5% fetal calf serum for subsequent cultures. The medium was changed at 24 hr intervals. The hormones were added 3 days after the culture was started. The mitogenic effect of vasopressin was found to be dependent both on time and hormone concentrations. Vasopressin (10(-11) M) stimulated thymidine incorporation 4.8 +/- 0.6-fold after 2 days of treatment and 5.3 +/- 1.6-fold after 8 days. When ACTH (10(-11) M) was added together with vasopressin (10(-11) M) the mitogenic effect was enhanced at 6.5 +/- 1.9-fold after 2 days and 12.9 +/- 6.9-fold after 8 days of treatment. The aldosterone and corticosterone outputs were also stimulated by the combined presence of vasopressin and ACTH in the incubation medium; a maximal effect was observed between 6 and 8 days of treatment. Vasopressin (10(-11) M) + ACTH (10(-11) M) stimulated the aldosterone output 7-fold and that of corticosterone by 18-fold. When added alone, vasopressin, as well as ACTH alone had only a small effect on the aldosterone output. However, ACTH alone stimulated the corticosterone output 10-fold. In conclusion, vasopressin is an important and specific growth factor of the adrenal zona glomerulosa cells. In addition, together with ACTH vasopressin stimulates the aldosterone and corticosterone output both in vivo and in vitro in primary cell cultures.

Inhibition of corticotropin release in vitro by dexamethasone, aldosterone and spironolactone

Dexamethasone, aldosterone and spironolactone inhibited the release of immunoreactive corticotropin (ACTH) from primary culture of the rat anterior pituitary cells. The steroids inhibited only the ACTH release stimulated by Pitressin and not the basal ACTH release by non-stimulated cells. On a molar basis, aldosterone appears to be the most efficient inhibitor of ACTH release while the effect of spironolactone is similar to the effect of dexamethasone. Simultaneous incubation with aldosterone and spironolactone inhibited the ACTH release to the same extent as spironolactone alone. This indicates that aldosterone's effect on ACTH release is also inhibited by spironolactone at the pituitary level.

Melanocyte stimulating hormone and the inhibition of sexual behaviour in the female rat

When administered to ovariectomized females, primed with oestradiol benzoate (OB) and progesterone, alpha-MSH increased lordosis behaviour in those rats that showed a low level of receptivity but in rats that were receptive it had an inhibitory effect. alpha-Melanocyte stimulating hormone also inhibited proceptive behaviour in these females. Luteinizing hormone releasing hormone (LHRH), on the other hand, failed to have a similar inhibiting effect although it did stimulate lordosis. While adrenalectomy failed to alter the stimulatory effect of alpha-MSH on lordosis it completely prevented the inhibitory actions of alpha-MSH on both lordosis and proceptive behaviour. Thus, while alpha-MSH may stimulate lordosis through a central action its inhibitory effects on lordosis and proceptive behaviour are probably mediated via the adrenal glands. Although alpha-MSH may stimulate the secretion of an adrenal steroid that blocks the facilitatory action of progesterone on female sexual behaviour the present results suggest that this inhibitory effect could be mediated by a catecholamine.

alpha-MSH and zona glomerulosa function in the rat

The responses of rat adrenal zona glomerulosa cells to stimulation by alpha-MSH and ACTH and related peptides have been studied. The major findings were that: (1) alpha-MSH stimulated corticosterone production in glomerulosa cells from from normal animals at concentrations of about 10(-10) mol/l, but other steroids, including aldosterone, were not significantly stimulated until levels of 10(-7) mol/l were used. Peptide structure-function relationships showed that in the adrenal cortex, in contrast with other systems, ACTH 4-10 had no effect and did not block the response of glomerulosa cells to alpha-MSH, bisacetyl Ser 1-alpha-MSH, (nor-valine-12)-alpha-MSH, and ACTH 1-13 amide were equipotent with alpha-MSH, while alpha-MSH 1-10 had activity but was considerably less potent. alpha-MSH 6-13, 7-13, 8-13 and lys-11-acetyl-alpha-MSH were all inactive. N-formyl-N-epsilon-benzyloxycarbonyl alpha-MSH stimulated only at 10(-6) mol/l. (2) Normalised alpha-MSH dose-response curves for aldosterone production in glomerulosa cells from normal rats, and corticosterone in inner zone cells were coincident. In glomerulosa cells, prior sodium depletion shifts the dose-response curve for aldosterone to the left, indicating a more sensitive response, and for corticosterone to the right. Bromocriptine treatment (which depresses the level of alpha-MSH in circulating plasma) and metoclopramide (which enhances it) respectively increased and decreased the sensitivity of the response of corticosterone to alpha-MSH in subsequently incubated glomerulosa cells, but had no effect on aldosterone. (3) In contrast, normalised ACTH stimulated dose-response curves for glomerulosa corticosterone and aldosterone, and for fasciculata corticosterone production were all coincident, and were unaffected by sodium depletion, or by metoclopramide or bromocriptine pretreatment. (4) Cyclic-AMP production by glomerulosa cells was stimulated by alpha-MSH only at levels of in excess of 10(-5) mol/l, five orders of magnitude greater than required to produce significant corticosterone stimulation. Under cyclic-AMP stimulation, the normalised responses of glomerulosa corticosterone and aldosterone, and of inner zone corticosterone were all coincident. The data suggest that alpha-MSH at low concentrations (less than 10(-7) mol/l) interacts with a glomerulosa cell receptor which is distinct from the ACTH receptor but interacts with the ACTH receptor at concentrations greater than 10(-'5) mol/l. Corticosterone production is stimulated by alpha-MSH in cells from normal animals at concentrations within the normal range for circulating plasma (approximately 3 X 10(-10) mol/l), while aldosterone is stimulated by similar concentrations of alpha-MSH in cells from sodium depleted animals. The effects of sodium depletion are not modulated through changes in plasma alpha-MSH levels. At low concentrations alpha-MSH stimulation of glomerulosa cells is unlikely to be modulated by cyclic-AMP as second messenger.

Glucocorticoid and mineralocorticoid effects on adrenocorticotropin and beta-endorphin in the adrenalectomized rat

Immunoreactive ACTH (ir-ACTH) and immunoreactive beta-endorphin (ir-betaEP) were determined in plasma, anterior pituitary, neuro-intermediate lobe, and hypothalamus of sham-adrenalectomized rats, and adrenalectomized rats given six daily injections of vehicle (oil), dexamethasone, 9alpha-fluorocortisol or deoxycorticosterone. 6 d after adrenalectomy, anterior pituitary ir-ACTH and ir-betaEP were double, and plasma levels approximately fivefold those in controls. Adrenalectomy did not alter hypothalamic levels of either peptide, or ir-betaEP in neuro-intermediate lobe, in which tissue ir-ACTH was below detection limit at routine dilutions. Dexamethasone (0.2-200 mug/d) concurrently suppressed plasma ir-ACTH and ir-betaEP, with a near maximal effect at 20 mug, and a half-maximal effect between 2 and 6 mug; similar dose-response characteristics were found for thymolysis. Step-wise increases in anterior pituitary content of both peptides were found, with no change in hypothalamic levels of either peptide, or neuro-intermediate lobe ir-betaEP. 9alpha-fluorocortisol (0.2-200 mug/d) produced plasma, anterior pituitary, and hypothalamic effects equivalent to dexamethasone, but with one-tenth the potency. Unlike dexamethasone, higher doses of 9alpha-fluorocortisol significantly elevated neuro-intermediate lobe ir-betaEP. Deoxycorticosterone (2-2,000 mug/d) produced no significant changes in plasma, anterior pituitary or hypothalamic levels of either peptide; like 9alpha-fluorocortisol, doses of >60 mug/d significantly elevated neuro-intermediate lobe ir-betaEP. Whereas ir-ACTH and ir-betaEP synthesis in and release from the anterior pituitary are under complex negative feedback glucocorticoid control, there exists a mineralocorticoid-specific effect on neuro-intermediate lobe content of ir-betaEP.