Regulation Of Aldosterone Secretion Research Articles

β-2 Adrenergic Receptor Diplotype Defines a Subset of Salt-Sensitive Hypertension

Two genetic variants of the beta-2 adrenergic receptor, 46G>A and 79C>G, affect agonist-mediated receptor downregulation and vascular reactivity. We determined whether these variants were associated with hypertension, per se, blood pressure response to dietary sodium, 2 forms of salt-sensitive hypertension (low renin and nonmodulation), and the activity of the renin-angiotensin-aldosterone system. Included are 280 hypertensive and 65 normotensive white subjects who had the 2 beta-2 adrenergic receptor genotypes available. Of all subjects, 171 hypertensive and 48 normotensive subjects had complete data for intermediate phenotyping and blood pressure evaluation on high- and low-sodium balance. The beta-2 adrenergic receptor variants were not associated with hypertension per se. However, among hypertensive subjects, the change (from low to high sodium balance) in mean arterial pressure differed significantly by genotype and by diplotype. Compared with all of the other diplotypes combined, 46AA/79CC was associated with a greater change in blood pressure. Furthermore, this diplotype was associated with low-renin (LR) hypertension (identifying 32% of the LR hypertensives), higher plasma aldosterone, and lower plasma renin and serum potassium levels. In conclusion, the 46AA/79CC diplotype is associated with greater blood pressure response to dietary sodium and higher odds of LR hypertension. We propose that the mechanism for the observed association is inadequate suppression of aldosterone with salt intake, implicating the beta-2 adrenergic receptor in the regulation of aldosterone secretion. This hypothesis was confirmed in isolated glomerulosa cells, where beta-2 adrenergic receptor stimulation increased aldosterone secretion, whereas blockade reduced the stimulated aldosterone response. Importantly, this association could only be detected with an intermediate and not a distant phenotype.

Aldosterone and the Pathogenesis of Hypertension

Hypertension remains a major public health problem, affecting up to 20% of the adult population in Western societies. Despite progress in treatment, the rates of blood pressure control remain suboptimal. Hypertension is a heterogeneous disorder, and in the majority of cases, with so-called essential hypertension, no clear single identifiable cause is found. Syndromes of excessive mineralocorticoid production or activity are among the important causes of secondary hypertension. Aldosterone is the principal mineralocorticoid in humans, and primary aldosterone excess, when associated with an aldosterone secreting adenoma (Conns tumor), is amenable to surgical cure. Classically, patient with Conns tumor present with spontaneous hypokalemia and have a relative excess of aldosterone production with suppression of plasma levels of renin (a proxy for angiotensin II, the major trophic substance regulating aldosterone secretion). This combination of a high aldosterone and a low renin is however more commonly associated with nodular hyperplasia of the adrenal glands, a condition not improved by surgery and variably responsive to the effects of mineralocorticoid antagonists such as spironolactone. Although primary aldosteronism was previously considered to be rare, recent studies have reported prevalence rates of up to 20% among hypertensive patients. This reflects the increasing use of the plasma aldosterone concentration to renin activity ratio (ARR), rather than spontaneous hypokalemia, as a screening tool for aldosteronism. Many patients with high ARR have normokalemia and, although renin activity is low, the level of aldosterone is usually within the normal range. This group of patients may thus include those who were previously classified as having low-renin essential hypertension. Recent data suggest that disturbances in aldosterone metabolism and regulation may not be uncommon in patients with essential hypertension. Thus, relatively high serum aldosterone levels within the reference range in normotensive individuals are associated with a substantially increased risk of developing hypertension, highlighting the potential role for aldosterone in the etiology of essential hypertension. The present review addresses the physiology of aldosterone action and its role in the pathogenesis of hypertension.

Angiotensin II and Renal Tubular Ion Transport

Angiotensin II, a potent vasoconstrictor, also participates in the regulation of renal sodium and water excretion, not only via a myriad of effects on renal hemodynamics, glomerular filtration rate, and regulation of aldosterone secretion, but also via direct effects on renal tubule transport. In addition, angiotensin II stimulates H+ secretion and HCO3– reabsorption in both proximal and distal tubules and regulates H+-ATPase activity in intercalated cells of the collecting tubule. Different results regarding the effect of angiotensin II on bicarbonate reabsorption and proton secretion have been reported at the functional level, depending on the angiotensin II concentration and tubule segment studied. It is likely that interstitial angiotensin II is more important in regulating hemodynamic and transport functions than circulating angiotensin II. In proximal tubules, stimulation of bicarbonate reabsorption, Na+/H+-exchange, and Na+/HCO3– cotransport has been found using low concentrations (<10–9M), while inhibition of bicarbonate reabsorption has been documented using concentrations higher than 10–8M. Evidence for the regulation of H+-ATPase activity in vivo and in vitro by trafficking/exocytosis has been provided. An additional level of H+-ATPase regulation via protein synthesis may be important as well. Recently, we have shown that both aldosterone and angiotensin II provide such a mechanism of regulation in vivo at the level of the medullary collecting tubule. Interestingly, in this part of the nephron, the effects of aldosterone and angiotensin II are not sodium dependent, whereas in the cortical collecting duct, both aldosterone and angiotensin II, by contrast, affect H+ secretion by sodium-dependent mechanisms.

Structural determinants of agonist-induced signaling and regulation of the angiotensin AT 1 receptor

Angiotensin II (Ang II) regulates aldosterone secretion by stimulating inositol phosphate production and Ca 2+ signaling in adrenal glomerulosa cells via the G q-coupled AT 1 receptor, which is rapidly internalized upon agonist binding. Ang II also binds to the heptahelical AT 2 receptor, which neither activates inositol phosphate signaling nor undergoes receptor internalization. The differential behaviors of the AT 1 and AT 2 receptors were analyzed in chimeric angiotensin receptors created by swapping the second (IL2), the third (IL3) intracellular loops and/or the cytoplasmic tail (CT) between these receptors. When transiently expressed in COS-7 cells, the chimeric receptors showed only minor alterations in their ligand binding properties. Measurements of the internalization kinetics and inositol phosphate responses of chimeric AT 1A receptors indicated that the CT is required for normal receptor internalization, and IL2 is a determinant of G protein activation. In addition, the amino-terminal portion of IL3 is required for both receptor functions. However, only substitution of IL2 impaired Ang II-induced ERK activation, suggesting that alternative mechanisms are responsible for ERK activation in signaling-deficient mutant AT 1 receptors. Substitution of IL2, IL3, or CT of the AT 1A receptor into the AT 2 receptor sequence did not endow the latter with the ability to internalize or to mediate inositol phosphate signaling responses. These data suggest that the lack of receptor internalization and inositol phosphate signal generation by the AT 2 receptor is a consequence of its different activation mechanism, rather than the inability of its cytoplasmic domains to couple to intracellular effectors.

Primary aldosteronism, a common entity? the myth persists.

Primary aldosterone excess or hyperaldosteronism is an important cause of hypertension which, when associated with an aldosterone secreting adenoma, is amenable to surgical cure. The biochemical hallmarks of the condition are a relative excess of aldosterone production with suppression of plasma levels of renin (a proxy for angiotensin II, the major trophic substance regulating aldosterone secretion). This combination of a high aldosterone and a low renin is however more commonly associated with 'nodular hyperplasia' of the adrenal glands, a condition not improved by surgery and variably responsive to the effects of the mineralocorticoid antagonist, spironolactone. Until recently the prevalence of either form of secondary hypertension has been thought to be low such that few clinicians 'hunted' for it in the absence of hypokalaemia (the traditional clue for the syndrome). This view has been challenged, firstly by the realisation that no more than 50% of such patients will have a low plasma potassium and secondly by the assumption that a 'normal' plasma aldosterone is in fact inappropriately elevated if the renin level is low. A single measurement of the ratio of aldosterone to renin levels is claimed to be highly predictive of patients who will have primary aldosterone excess. This paper examines the logic behind such claims and presents evidence from the literature that an abnormal ratio is simply a different description of the low renin state and that such patients do not necessarily have mineralocorticoid hypertension. Most patients 'discovered' by this test will have what many call low-renin hypertension, a condition not amenable to specific therapy. Claims that they are peculiarly sensitive to the hypotensive effects of spironolactone have not been tested in controlled trials. The test would however be expected to pick up those individuals with true Conn's syndrome but such patients remain too few in number to justify widespread use of an expensive screening test.

The endothelin-aldosterone axis and cardiovascular diseases.

The results of the Randomized Aldactone Evaluation Study (RALES) and of several experimental studies have indicated that excess aldosterone detrimentally affects cardiovascular morbidity and mortality by acting through both classical and non-classical mineralocorticoid receptors. The effects mediated through classical mineralocorticoid receptors entail enhanced sodium and water reabsorption, potassium loss and hypokalaemia, congestion, increased vascular resistance and hypertension. Those occurring through non-classical mineralocorticoid receptors located on myofibroblasts comprise cardiac hypertrophy and fibrosis, which may be due to a direct effect of aldosterone on collagen synthesis. Data obtained in primary aldosteronism patients demonstrated left ventricular hypertrophy, as well as changes in left ventricular filling that can be accounted for by cardiac fibrosis. Available clinical data indicate that in a considerable proportion of congestive heart failure (CHF) patients treated with angiotensin converting enzyme (ACE) inhibitors, aldosterone secretion can escape from blockade of the renin-angiotensin system, thus suggesting that additional mechanisms, besides angiotensin II, can play an important role in the regulation of aldosterone secretion. Compelling evidence indicates that endothelin (ET)-1 is overtly increased in severe CHF and thus is a likely candidate for the aldosterone 'escape' phenomenon in CHF. Endothelin-1 is expressed in the adrenal cortex, together with its receptor subtypes A (ETA) and B (ETB), and directly stimulates aldosterone secretion in different species, in humans by acting via both ETA and ETB receptor subtypes. Moreover, we have recently found that the novel endothelin peptide ET-1 (1-31), by acting as an ETA agonist, can also be involved in the regulation of growth of the adrenal cortex, as well as in the pathogenesis of Conn's adenoma. In this paper, we review the findings suggesting a relationship between activation of the ET-1 system, enhanced aldosterone secretion and cardiac fibrosis and discuss the implications of endothelin antagonism for cardiovascular disease.

Expression and localization of human dopamine D2 and D4 receptor mRNA in the adrenal gland, aldosterone-producing adenoma, and pheochromocytoma.

Aldosterone secretion is evidently regulated by a dopaminergic inhibitory mechanism. Pharmacological characterization and autoradiographic studies revealed D2-like receptors in the adrenal cortex, especially in the zona glomerulosa. However, the subtype of the dopamine receptors involving this regulation has not been elucidated. To investigate which subtype of receptors expresses in the adrenal cortex, we examined the messages of D2-like receptors, D2, D3, and D4, by RT-PCR and in situ hybridization of adrenal glands and adrenal neoplasm. Both D2 and D4 receptors were expressed in normal adrenal glands, pheochromocytoma, and aldosterone-producing adenoma. However, the D2 receptors were not universally expressed, in contrast with the D4 receptors that were detected in all cases of aldosterone-producing adenoma and adrenal remnant. No D3 receptor message was detected by RT-PCR in any adrenal sample. Both D2 and D4 receptors were expressed in significant amounts in the adrenal medulla and pheochromocytoma. In the adrenal cortex, the expression of the D2 receptors was in the zona glomerulosa and zona reticularis, with no different signal intensities between the two zones. D4 receptors were mainly localized in the zona glomerulosa and, to a lesser extent, in the zona reticularis. Both receptors were expressed at low levels in the zona fasciculata. In aldosterone-producing adenoma, the expression of D2 and D4 was especially found in nonzona fasciculata-like cells. To elucidate which dopamine receptor regulates aldosterone secretion, the effects of specific D2 and D4 antagonists, raclopride and clozapine, respectively, were examined in cultured NCI-H295 cells. Dopamine further increased angiotensin II-induced aldosterone secretion by 20%. In the presence of 1 microM dopamine and angiotensin II, 10(-5)-10(-7) M clozapine decreased aldosterone levels by 40-55%. The decrease in aldosterone secretion by clozapine was completely reversed when raclopride was added simultaneously. These data suggest that dopamine exerts dual effects on aldosterone secretion in NCI-H295 cells. Activation of D4 receptors can increase aldosterone secretion, whereas an inhibitory effect is mediated via D2 receptors. In summary, we demonstrated the existence of both D2 and D4 receptors in the human adrenal gland and adrenal neoplasm. Both receptors play significant roles in the modulation of aldosterone secretion, but in opposite directions.

Aldosterone as a determinant of cardiovascular and renal dysfunction.

Aldosterone is a major regulator of extracellular fluid volume and the principal determinant of potassium metabolism 1,2,3,4,5. These effects are mediated by the binding of aldosterone to the mineralocorticoid receptor in target tissues, primarily the kidney. Volume is regulated through a direct effect on the collecting duct, where aldosterone promotes sodium retention and potassium excretion. The reabsorption of sodium ions produces a fall in the transmembrane potential, thus enhancing the flow of positive ions (such as potassium) out of the cell into the lumen. The reabsorbed sodium ions are transported out of the tubular epithellium into the renal interstitial fluid and from there into the renal capillary circulation. Three primary mechanisms control aldosterone release—the renin-angiotensin system, potassium, and adrenocorticotropic hormone. The renin-angiotensin system controls extracellular fluid volume via regulation of aldosterone secretion. In effect, the renin-angiotensin system keeps the circulating blood volume constant by causing aldosterone-induced sodium retention during volume deficiency and by decreasing aldosterone-dependent sodium retention when volume is ample. In recent years there has been a radical shift in our view of aldosterone's effects on the heart, the vasculature and the kidney6,7,8,9. Aldosterone's endocrine properties have taken on a broader perspective, involving non-classic actions in non-epithelial cells found in non-classic target tissues6, 10,11,12,13,14,15. The traditional concept, that aldosterone is synthesized only in the adrenal glomerulosa cell and acts almost exclusively on the kidney to modify sodium and potassium homoeostasis, needs to be expanded. There is increasing evidence that aldosterone can have an effect on vascular remodelling and collagen formation, and a non-genomic action to modify endothelial function. Among the most intriguing effects of aldosterone are its impact on fibrosis and activity associated with a cell surface receptor in certain target tissues, including endothelial cells 6, 7, 16,17,18,19. These actions contribute substantially to the pathophysiology of congestive heart failure (CHF), as well as progressive renal dysfunction. This new information has increased interest in the development of an antagonist to block aldosterone's effect, not just because of its diuretic action but primarily because of its potential cardiovascular and renal protective effects. In this review I consider the broad spectrum of non-genomic effects of aldosterone. It is becoming increasingly evident that these effects, occurring independently of haemodynamic factors, contribute to enhanced cardiovascular risk manifested by congestive heart failure and progressive renal disease. I also discuss the clinical trials with selective aldosterone receptor antagonists that are currently underway in patients with left ventricular hypertrophy, essential hypertension and systolic hypertension, and in those who have experienced myocardial infarction. Such trials will enhance our understanding of the role of aldosterone in the pathophysiology of cardiovascular disease. Also, selective aldosterone receptor antagonism holds promise for a reduction in cardiovascular and renal disease morbidity and mortality, and for enhancement of patient wellbeing.

Alpha1H T-type Ca2+ channel is the predominant subtype expressed in bovine and rat zona glomerulosa.

The low voltage-activated (T-type) Ca2+ channel has been implicated in the regulation of aldosterone secretion from the adrenal zona glomerulosa by extracellular K+ levels, angiotensin II, and ACTH. However, the identity of the specific subtype mediating this regulation has not been determined. We utilized in situ hybridization to examine the distribution of three newly cloned members of the T-type Ca2+ channel family, alpha1G, alpha1H, and alpha1I, in the rat and bovine adrenal gland. Substantial expression of only the mRNA transcript for the alpha1H-subunit was detected in the zona glomerulosa of both rat and bovine. A much weaker expression signal was detected for the alpha1H transcript in the zona fasciculata of bovine. Whole cell recordings of isolated bovine adrenal zona glomerulosa cells showed the native low voltage-activated current to be inhibited by NiCl2 with an IC50 of 6.4 +/- 0.2 microM. Because the alpha1H subtype exhibits similar NiCl2 sensitivity, we propose that the alpha1H subtype is the predominant T-type Ca2+ channel present in the adrenal zona glomerulosa.

Sample Requirements for Plasma Renin Activity and Immunoreactive Renin

Renin, an aspartic protease, is rate limiting in the generation of angiotensin (Ang) II, which is a key regulator in the maintenance of salt and water balance by regulating aldosterone secretion and blood pressure (1). Renin is synthesized and secreted into the blood exclusively by the juxtaglomerular cells of the kidney from prorenin, an inactive precursor, which is produced not only by the kidney, but by other tissues, including the adrenal glands, gonads, and the uteroplacental unit (2). Whereas the secretion of renin is tightly controlled, prorenin secretion is not and is ∼10-fold higher than renin (3). Although peripheral conversion of prorenin to renin is limited and does not typically occur in vivo, in vitro conversion of prorenin to renin can occur by cryoactivation, so sample handling is critical (4). Historically, samples for renin activity have been collected on ice, frozen, and assayed at 2–8 °C. Cryoactivation occurs when samples are exposed to refrigerated temperatures (4 °C) for extended periods of time (4)(5)(6)(7)(8). Because of the high concentration of prorenin relative to renin, cryoactivation can lead to a marked increase in renin concentration. Therefore, to avoid cryoactivation, samples should be collected and assayed at room temperature. Recently, IRMA assays have been developed to directly measure renin in the circulation. For many years, plasma renin activity (PRA) has been used …

Role of adrenal renin-angiotensin system in the control of aldosterone secretion in sodium-restricted rats.

This study examined the effect of the pharmacological manipulation of adrenal renin-angiotensin system (RAS) on aldosterone secretion from in situ perfused adrenals of rats kept on a normal diet and sodium restricted for 14 days. Neither the angiotensin-converting enzyme inhibitor captopril nor the nonselective angiotensin II receptor antagonist saralasin and the AT(1) receptor-selective antagonist losartan affected basal aldosterone output in normally fed rats. In contrast, they concentration dependently decreased aldosterone secretion in sodium-restricted animals, with maximal effective concentration ranging from 10(-7) to 10(-6) M. Captopril (10(-6) M), saralasin (10(-6) M), and losartan (10(-7) M) counteracted aldosterone response to 10 mM K(+) in sodium-restricted rats but not in normally fed animals. Collectively, these findings provide evidence that adrenal RAS plays a role in the regulation of aldosterone secretion, but only under conditions of prolonged stimulation of zona glomerulosa probably leading to overexpression of adrenal RAS.

Regulation of aldosterone secretion in patients with chronic congestive heart failure by endothelins

We studied acute (day 1) and long-term (day 14) effects of endothelin (ET) receptor blockade with the mixed ET A/B antagonist bosentan (1 g twice daily; n = 18) or placebo (n = 12) on plasma angiotensin II and aldosterone in 30 patients with symptomatic chronic heart failure taking angiotensin-converting enzyme inhibitors, diuretics, and digoxin. Hormones were determined before and 3 hours after morning doses of diuretics and digoxin and the double-blind study drug, respectively, on days 1 and 14. On day 1, angiotensin II increased from 16.1 ± 17.9 to 27.6 ± 5.6 ng/L (p <0.05) with bosentan and similarly with placebo (15.5 ± 9.3 and 36.0 ± 49.1 ng/L, p = 0.06) after the morning dose of diuretics and digoxin. Aldosterone tended to increase from 322 ± 239 to 362 ± 254 pmol/L (bosentan) and from 271 ± 70 to 297 ± 136 pmol/L (placebo). On day 14, before drug intake, angiotensin II was unchanged compared with day 1 in both groups. However, aldosterone was lower than on day 1 with bosentan (213 ± 124 vs 322 ± 239 pmol/L, p <0.05) and remained below baseline values 3 hours after drug intake, whereas it was unchanged with placebo. Thus, short-term ET A/B receptor antagonism decreases basal aldosterone secretion independently of angiotensin II, suggesting that ET participates in the regulation of aldosterone in patients already treated with angiotensin-converting enzyme inhibitors and diuretics.

Expression of prostaglandin E receptor EP4 subtype in rat adrenal zona glomerulosa: involvement in aldosterone release.

We examined the role of prostaglandin (PG) E receptors in the secretion of aldosterone. PGE2 is known to exert its various biological functions by binding to PGE receptors. There are four subtypes of PGE receptors, EP1, EP2, EP3, and EP4. Among the PGE receptors EP2 and EP4 subtypes are coupled to Gs protein and stimulate adenylyl cyclase. In this study, PGE2 caused a dose-dependent increase in aldosterone production from the rat adrenal zona glomerulosa cells in vitro accompanied with an increase in intracellular cAMP concentration. A specific agonist for EP2, butaprost, did not increase the cAMP production or the aldosterone release, suggesting the possibility that EP4 mediates the secretion of aldosterone by PGE2. Northern blot hybridization analysis disclosed that EP4 gene was expressed in the rat adrenal gland but that EP2 gene was not. In situ hybridization revealed that EP4 mRNA is present abundantly in the zona glomerulosa of rat adrenal gland. These findings suggest that the PGE2-EP4 system is involved in the regulation of aldosterone secretion from the rat adrenal gland.

Adrenal capillary endothelial cells stimulate aldosterone release through a protein that is distinct from endothelin.

We tested the possibility that bovine adrenal capillary endothelial cells (ECs) stimulate aldosterone secretion from bovine zona glomerulosa (ZG) cells by the release of a transferable factor. In coincubations of ZG cells and ECs in serum-free medium, aldosterone release was stimulated approximately 17-fold, and the stimulation was related to the concentration of ECs. The maximal stimulation by ECs was 75% of the maximal response to ACTH. In contrast, adrenal pericytes and fibroblasts were without effect. ECs incubated alone without ZG cells did not produce aldosterone. Conditioned medium from ECs (EC-CM), but not adrenal fibroblasts, stimulated aldosterone release approximately 3-fold. The stimulation increased with the concentration of EC-CM and the duration of conditioning time. Steroidogenic activity in EC-CM was abolished by pronase treatment, indicating that the active factor was a protein. However, the activity in EC-CM was distinct from that of endothelin-1 (ET-1), an endothelial peptide that also stimulates aldosterone secretion, as it was not blocked by the ET(B) receptor antagonist PD-145065, it did not alter [125I]ET-1 binding to ZG cells, and its release occurred before the release of ET-1. Neither ECs nor EC-CM stimulated the production of cortisol from zona fasciculata cells. The activity of EC-CM was not blocked by an angiotensin II AT1 receptor antagonist or a bradykinin B2 receptor antagonist. EC-CM stimulated increased intracellular calcium in fura-2-loaded ZG cells, but did not increase the production of cAMP. Using gel filtration, this peptide had an approximate molecular mass of 3000 Da and migrated earlier than ET-1. This study demonstrates that ECs in vitro alter steroidogenesis through the release of a transferable substance and suggests the existence of an endothelium-derived steroidogenic factor that is produced by adrenal capillary ECs. This endothelium-derived steroidogenic factor may function in the adrenal gland as a paracrine regulator of aldosterone secretion.

Low-Renin and Nonmodulating Essential Hypertension

Low-renin hypertension (LREH) accounts for an important subset of the hypertensive human population and is associated with salt-sensitivity and diuretic response and is also common in black patients. An interesting finding in LREH is the presence of normal plasma aldosterone levels.1 2 The ratio of plasma aldosterone to renin activity is used to screen for adrenal adenomas and hyperplasia3 and is usually >100 when plasma renin activity is expressed in ng/mL/hr and the aldosterone concentration is expressed in ng/dL. The plasma aldosterone/plasma renin activity ratios may be somewhat elevated in LREH (>30 but <100), which is especially relevant if the concurrent urinary sodium excretion exceeds 200 mEq/24 hours.4 It is not known what regulates aldosterone secretion in this setting or whether there is actual mineralocorticoid excess. Undue prior emphasis may have been placed on the continued suppression of plasma renin activity during severe salt restriction or volume depletion in LREH,1 and the suppression of plasma renin activity and basal aldosterone secretion5 in the elderly can also confound the analysis of LREH. Studies by Fisher et al2 focus on adrenal and pressor responsiveness to angiotensin II (Ang II) as a function of dietary salt intake in patients with LREH, normal-renin hypertension, and normal controls. Especially striking are the functional similarities between LREH and “ nonmodulating” essential hypertension with normal plasma renin activity (NMEH), including: (1) salt-sensitivity of the blood pressure; (2) blunted plasma aldosterone responses to Ang II infusion and upright posture after 5 days of rigid dietary sodium restriction; and (3) relatively low basal plasma aldosterone levels. These differences compared with normal controls and “modulating” hypertensive subjects disappeared when the dietary salt intake was increased to 200 mEq. The latter results are consistent with suppression of plasma Ang II activity during high-salt intake with …

Angiotensin II and aldosterone regulation

The circulating renin–angiotensin system is a major regulator of the secretion of the adrenocortical hormone, aldosterone. This renin–angiotensin aldosterone system is important in the control of salt and water balance and blood pressure. This review describes the historical background leading to the discovery of aldosterone in the 1950s and the recognition in the 1960s that angiotensin II was involved in its control. Although angiotensin II is important in the regulation of aldosterone secretion, its action is influenced by multiple other factors, especially potassium and atrial natriuretic peptide. In addition to the circulating renin–angiotensin system, a local renin–angiotensin system is present in the zona glomerulosa cell. This local system also appears to be involved in the regulation of aldosterone production. The mechanism by which angiotensin II stimulates the adrenal zona glomerulosa cell is described in some detail. Angiotensin II interacts with the angiotensin receptor (AT 1) membrane receptor that is coupled to cellular second messengers. Specific AT 1 receptor antagonists are now clinically used to block angiotensin II's action on various target organs, including the adrenal gland.

Angiotensin II stimulates both aldosterone secretion and DNA synthesis via type 1 but not type 2 receptors in bovine adrenocortical cells.

Angiotensin II (Ang II) type 2 receptor (AT2) has been shown to counteract the type 1 receptor (AT1)-mediated biological actions of Ang II in the cardiovascular system. The biological significance of AT2 receptor in the adrenals however remains unknown. In the present study, we investigated the roles of AT1 and AT2 receptor subtypes in the regulation of aldosterone secretion and DNA synthesis in bovine adrenocortical zona glomerulosa cells in vitro. Ang II (1 mumol/l)-stimulated aldosterone secretion was completely suppressed by AT1 antagonist CV-11974 but not affected by AT2 receptor antagonist PD-123319. Effects on DNA synthesis were investigated by determining the incorporation of BrdU into the nuclei of the cultured zona glomerulosa cells. Ang II (1 mumol/l)-stimulated DNA synthesis of the cells was also completely suppressed by CV-11974 but not by PD-123319. These results suggest that AT1 receptor but not AT2 receptor is the predominant receptor subtype which mediates the Ang II-stimulated aldosterone secretion and cell growth in bovine adrenocortical cells.

Gene expression and roles of angiotensin II type 1 and type 2 receptors in human adrenals.

Although stimulation of aldosterone secretion is one of the functions of angiotensin II, the gene expression and biological significance of the angiotensin II receptor subtypes, AT1 and AT2, in the human adrenal have not been characterized. We therefore investigated the transcription levels of the receptor subtype genes and their roles in regulation of steroid secretion by human adrenals. The expression of AT1 and AT2 receptor mRNA was assessed by reverse transcription-polymerase chain reaction followed by Southern blot analysis in normal adrenocortical tissues (n = 6) and a series of adrenal tumour tissues: aldosterone-producing adrenocortical adenoma (n=6), Cushing's syndrome (n = 6) and pheochromocytoma (n = 6). The role of the two receptor subtypes in steroid secretion in vitro was examined by incubating the tissue with angiotensin II(1 microM) with or without the selective AT1 antagonist CV-11974 (1 microM). Both AT1 and AT2 receptor mRNA transcripts were demonstrated in all of the human adrenal tissues tested. Angiotensin II-induced aldosterone secretion was suppressed 50% upon the addition of CV-11974. The selective AT2 agonist CGP-42112 increased aldosterone secretion by 55% over the control, which was not suppressed by CV-11974. Angiotensin II and CGP-42112 did not affect cortisol secretion. These results suggest that both AT2 and AT1 receptors may be involved in the regulation of aldosterone secretion and tumorigenesis of the human adrenals.

Interactions between ouabain, atrial natriuretic peptide, angiotensin-II and potassium: Effects on rat zona glomerulosa aldosterone production

The effects of ouabain, atrial natriuretic peptide, angiotensin-II and potassium on aldosterone production by collagenase dispersed rat zona glomerulosa cells were studied. A-II and 10 −4M ouabain-induced increases in aldosterone production was inhibited by 10 −9M ANP at all potassium concentrations examined. 10 −4M ouabain inhibited the A-II induced increase in aldosterone production at all potassium concentrations. The degree of this inhibition was smaller at higher potassium levels. Ouabain enhanced the inhibitory effect of ANP on A-II-induced aldosterone synthesis at all potassium concentrations. Interactions between A-II, ANP, ouabain and potassium may be of physiological significance in the regulation of aldosterone secretion.

Involvement of tyrosine phosphorylation and mapk activation in the mechanism of action of acth, angiotensin II and vasopressin

ACTH, Angiotensin II (Ang II) and Vasopressin (AVP) are among the well known regulators of aldosterone secretion and also have a trophic action on the adrenal gland. According to classic studies, Ang II and AVP activate phospholipase C (PLC), diacylglycerol (DAG) and inositol phosphate (InsPs) production whereas ACTH activates cAMP production. However, our data indicate that the three peptides are able to induce a time-dependent increase in the level of Tyr-phosphorylation of several proteins. Western Blot analysis indicates a biphasic activation of Tyr-phosphorylation by AVP, with a peak at 30 s and a second one at 15 min incubation. Ang II induced a rapid (2 min) and sustained activation of Tyr-phosphorylation, while ACTH induced a progressive time course with a plateau reached at 15 min. Ang II and AVP also increased phosphorylation of p42mapk and p44mapk, while ACTH did not affect MAPK activity. Moreover, pre-incubation of the cells with genistein (Tyr-kinase inhibitor) and PD 098059 (a MAPK inhibitor) did not affect InsPs production or aldosterone secretion induced by Ang II or AVP. These results suggest that the MAP-K pathway is involved in the control of cell growth rather than aldosterone secretion.