The AT2 receptor-mediated stimulation of adrenal catecholamine release may potentiate the AT1 receptor-mediated aldosterone secretagogue action of angiotensin-II in rats

Sex differences in the renin-angiotensin-aldosterone system and its roles in hypertension, cardiovascular, and kidney diseases

Cardiovascular disease is a pathology that exhibits well-researched biological sex differences, making it possible for physicians to tailor preventative and therapeutic approaches for various diseases. Hypertension, which is defined as blood pressure greater than 130/80 mmHg, is the primary risk factor for developing coronary artery disease, stroke, and renal failure. Approximately 48% of American men and 43% of American women suffer from hypertension. Epidemiological data suggests that during reproductive years, women have much lower rates of hypertension than men. However, this protective effect disappears after the onset of menopause. Treatment-resistant hypertension affects approximately 10.3 million US adults and is unable to be controlled even after implementing ≥3 antihypertensives with complementary mechanisms. This indicates that other mechanisms responsible for modulating blood pressure are still unclear. Understanding the differences in genetic and hormonal mechanisms that lead to hypertension would allow for sex-specific treatment and an opportunity to improve patient outcomes. Therefore, this invited review will review and discuss recent advances in studying the sex-specific physiological mechanisms that affect the renin-angiotensin system and contribute to blood pressure control. It will also discuss research on sex differences in hypertension management, treatment, and outcomes.

Aldosterone breakthrough from a pharmacological perspective

Aldosterone (Aldo) breakthrough is a well-known phenomenon that occurs in patients with long-term renin-angiotensin aldosterone system (RAAS) blockade using inhibitors of renin or angiotensin converting enzyme or angiotensin II type 1 receptor blockers. The blockade of the mineralocorticoid receptor (MR), an Aldo binding receptor, is effective in managing patients with resistant hypertension, defined as uncontrollable blood pressure despite the concurrent use of three antihypertensive drugs. In other words, MR inhibitors are not used as first-line antihypertensive drugs in most guidelines for hypertension management. Aldo breakthrough puts hypertensive patients at higher risk of cardiovascular disease and worsens future outcomes. This review discusses Aldo secretion and the mechanism of Aldo breakthrough, dependent or independent of the RAAS, with consideration of the pharmacological aspects of this phenomenon, as well as hypothetical views.

The Interplay Between the Renin-Angiotensin-Aldosterone System and Parathyroid Hormone

The renin-angiotensin-aldosterone system (RAAS) is the regulatory system by which renin induces aldosterone production. Angiotensin II (Ang II) is the main effector substance of the RAAS. The RAAS regulates blood pressure and electrolyte balance by controlling blood volume and peripheral resistance. Excessive activation of the RAAS is an important factor in the onset of cardiovascular disease and the deterioration of this disease. The most common RAAS abnormality is primary aldosteronism (PA). Parathyroid hormone (PTH) is a peptide secreted by the main cells of the parathyroid gland, which promotes elevated blood calcium (Ca²⁺) levels and decreased blood phosphorus (Pi) levels. Excessive secretion of PTH can cause primary hyperparathyroidism (PHPT). Parathyroidism is highly prevalent in postmenopausal women and is often associated with secondary osteoporosis. PA and PHPT are common endocrine system diseases. However, studies have shown a link between the RAAS and PTH, indicating a positive relationship between them. In this review, we explore the complex bidirectional relationship between the RAAS and PTH. We also point out possible future treatment options for related diseases based on this relationship.

Renin angiotensin system molecules and nitric oxide local interactions in the adrenal gland of Trypanosoma cruzi infected rats

Chagas disease (CD) is a tropical zoonosis caused by the protozoan Trypanosoma cruzi. Severe autonomic dysfunction like reduced cardiac catecholamine-containing or acetylcholinesterase-positive innervation have been reported in CD. Renin-angiotensin system (RAS) seems to participate in the regulation of adrenal catecholamine secretion by adrenal medullary chromaffin cells, which might be dependent of nitric oxide (NO) pathways. To investigate the levels of RAS components in the adrenal gland during the acute infection with Y strain T. cruzi and in response to acute administration of an inhibitor of the enzyme NO synthase, L-NAME. Male Holtzman rats were inoculated intraperitoneally with Y strain T. cruzi and received L-NAME or tap water from one day before the infection until 13 or 17 days post-inoculation (dpi). The concentration of RAS molecules in the adrenal tissue was evaluated by ELISA immunoassay. Angiotensin converting enzyme 1 (ACE1) levels were significantly lower at 17 dpi when compared to 13 dpi. No significant differences were found compared with baseline, and no changes were detected in adrenal tissue levels of angiotensin converting enzyme 2 (ACE2), angiotensin II, or angiotensin-(1-7). Moreover, the treatment with L-NAME did not influence the levels of RAS components in adrenal tissue during the course of T. cruzi infection. We provided the first evidence that levels of RAS molecules change in the adrenal gland during acute phase of T. cruzi infection. Future studies are necessary to fully address the role of NO in RAS-associated adrenal gland function in CD.

Angiotensin II AT2 Receptors Contribute to Regulate the Sympathoadrenal and Hormonal Reaction to Stress Stimuli

Angiotensin II, through AT1 receptor stimulation, mediates multiple cardiovascular, metabolic, and behavioral functions including the response to stressors. Conversely, the function of Angiotensin II AT2 receptors has not been totally clarified. In adult rodents, AT2 receptor distribution is very limited but it is particularly high in the adrenal medulla. Recent results strongly indicate that AT2 receptors contribute to the regulation of the response to stress stimuli. This occurs in association with AT1 receptors, both receptor types reciprocally influencing their expression and therefore their function. AT2 receptors appear to influence the response to many types of stressors and in all components of the hypothalamic-pituitary-adrenal axis. The molecular mechanisms involved in AT2 receptor activation, the complex interactions with AT1 receptors, and additional factors participating in the control of AT2 receptor regulation and activity in response to stressors are only partially understood. Further research is necessary to close this knowledge gap and to clarify whether AT2 receptor activation may carry the potential of a major translational advance.

Renin angiotensin aldosterone inhibition in the treatment of cardiovascular disease

A collective century of discoveries establishes the importance of the renin angiotensin aldosterone system in maintaining blood pressure, fluid volume and electrolyte homeostasis via autocrine, paracrine and endocrine signaling. While research continues to yield new functions of angiotensin II and angiotensin-(1-7), the gap between basic research and clinical application of these new findings is widening. As data accumulates on the efficacy of angiotensin converting enzyme inhibitors and angiotensin II receptor blockers as drugs of fundamental importance in the treatment of cardiovascular and renal disorders, it is becoming apparent that the achieved clinical benefits is suboptimal and surprisingly no different than what can be achieved with other therapeutic interventions. We discuss this issue and summarize new pathways and mechanisms effecting the synthesis and actions of angiotensin II. The presence of renin-independent non-canonical pathways for angiotensin II production are largely unaffected by agents inhibiting renin angiotensin system activity. Hence, new efforts should be directed to develop drugs that can effectively block the synthesis and/or action of intracellular angiotensin II. Improved drug penetration into cardiac or renal sites of disease, inhibiting chymase the primary angiotensin II forming enzyme in the human heart, and/or inhibiting angiotensinogen synthesis would all be more effective strategies to inhibit the system. Additionally, given the role of angiotensin II in the maintenance of renal homeostatic mechanisms, any new inhibitor should possess greater selectivity of targeting pathogenic angiotensin II signaling processes and thereby limit inappropriate inhibition.

Antihypertensive effect of Ganjang (traditional Korean soy sauce) on Sprague-Dawley Rats

BACKGROUND/OBJECTIVES

Although Korean fermented foods contain large amounts of salt, which is known to exacerbate health problems, these foods still have beneficial effects such as anti-hypertension, anti-cancer, and anti-colitis properties. We hypothesized that ganjang may have different effects on blood pressure compared to same concentrations of salt.

MATERIALS/METHODS

Sprague-Dawley rats were divided into control (CT), NaCl (NC), and ganjang (GJ) groups and orally administered with 8% NaCl concentration for 9 weeks. The systolic blood pressure (SBP), serum chemistry, Na⁺ and K⁺ concentrations and renal gene expressions were measured.

RESULTS

The SBP was significantly increased in the NC group compared to the GJ and CT groups. In addition, the Na⁺ concentration in urine was higher in the GJ and NC groups than the CT group, but the urine volume was increased in the GJ group compared to the other groups. The serum renin levels were decreased in the GJ group compared to the CT group, while the serum aldosterone level was decreased in the GJ group relative to the NC group. The mRNA expression of the renin, angiotensin II type I receptor, and mineralocorticoid receptor were significantly lower in the GJ group compared to other groups. Furthermore, GJ group showed the lowest levels of genes for Na⁺ transporter in kidney cortex such as Na⁺/K⁺ ATPaseα1 (NKAα1), Na⁺/H⁺ exchanger 3 (NHE3), Na⁺/HCO₃^- co-exchanger (NBC), and carbonic anhydrases II (CAII).

CONCLUSIONS

The decreased SBP in the GJ could be due to decreased renin and aldosterone levels in serum and increased urinary volume and excretion of Na⁺ with its transporter gene alteration. Therefore, ganjang may have antihypertensive effect despite its high contents of salt.

Aldosterone Contributes to Sympathoexcitation in Renovascular Hypertension

BACKGROUND

Although angiotensin II (Ang II) is essential to the development of renovascular hypertension, aldosterone plays a role as well. Recent studies have demonstrated a cross-talk between Ang II type 1 and mineralocorticoid receptors in the brain and kidneys. However, the role of aldosterone in the autonomic and renal dysfunction of renovascular hypertension is not well understood.

AIM

The current study evaluated whether aldosterone contributes to cardiovascular and renal dysfunction in the 2 kidney-1 clip (2K1C) model.

METHODS

Mean arterial pressure (MAP) and baroreceptor reflex for control of the heart rate were evaluated in 2K1C treated or not treated with spironolactone (200mg/kg/day, 7 days). Tonic and reflex control of renal sympathetic nerve activity (rSNA) were assessed in urethane-anaesthetized rats. Plasma renin activity (PRA), kidney renin protein expression, renal injury, and central AT1 receptor protein expression were assessed.

RESULTS

Spiro reduced MAP (198±4 vs. 170±9mm Hg; P < 0.05), normalized rSNA (147±9 vs. 96±10 pps; P < 0.05), and increased renal baroreceptor reflex sensitivity in the 2K1C rats. Spiro reduced α-smooth muscle actin expression in the nonclipped kidney in the 2K1C group (5±0.6 vs. 1.1±0.2%; P < 0.05). There was no change in PRA; however, a decrease in renin protein expression in the nonclipped kidney was found in the 2K1C treated group (217±30 vs. 160±19%; P < 0.05). Spiro treatment decreased AT1 receptor in the central nervous system (CNS) only in 2K1C rats (138±10 vs. 84±12%; P < 0.05).

CONCLUSION

Aldosterone contributes to autonomic dysfunction and intrarenal injury in 2K1C, these effects are mediated by the CNS.

Angiotensin AT2 receptor stimulation inhibits activation of NADPH oxidase and ameliorates oxidative stress in rotenone model of Parkinson's disease in CATH.a cells

Oxidative stress has long been considered as a major contributing factor in the pathogenesis of Parkinson's disease (PD). The brain has an independent local renin-angiotensin system (RAS). Angiotensin II (Ang II) activates NADPH-dependent oxidases, which are a major source of superoxide and are upregulated in major aging-related diseases such as hypertension and neurodegenerative disease. In this study, we firstly examined whether CGP42112, an AT2 receptor (AT2R) agonist, may exert direct protective effects on the rotenone-induced CATH.a cell injury in vitro. We used CATH.a cell line to evaluate changes in cultured dopaminergic neuron levels of superoxide dismutase (SOD), glutathione (GSH) and reactive oxygen species (ROS). We also evaluated expression of NADPH oxidase, AT1 and AT2 receptors in treated with phosphate buffer saline (PBS), rotenone, Ang II, AT2R agonist CGP42112, or AT2R antagonist PD123319, alone and combined (n=6, each group). Quantitative reverse transcriptase PCR (qRT-PCR) and western blot were used to determine messenger RNA (mRNA) and protein levels of the AT1, AT2 receptors and NADPH oxidase. ROS generation was determined by the dichlorodihydrofluorescein diacetate fluorescent probe assay. The levels of SOD and GSH were measured by using available kits. In our study, CGP42112 (100nM) significantly reduced rotenone-induced oxidative stress and elevated the total SOD activity and GSH level. In addition, CGP42112 significantly increased AT2R expression and attenuated Ang II-induced NADPH oxidase activation, and these effects were completely abolished by the AT2R antagonist, PD123319 (1μM). Our results suggest that CGP42112 attenuates rotenone-induced oxidative stress in CATH.a neuron via activating AT2R and suppressing NADPH oxidase expression.

A new transgenic rat model overexpressing the angiotensin II type 2 receptor provides evidence for inhibition of cell proliferation in the outer adrenal cortex

This study aimed to elucidate the role of the AT(2) receptor (AT(2)R), which is expressed and upregulated in the adrenal zona glomerulosa (ZG) under conditions of increased aldosterone production. We developed a novel transgenic rat (TGR; TGRCXmAT(2)R) that overexpresses the AT(2)R in the adrenal gland, heart, kidney, brain, skeletal muscle, testes, lung, spleen, aorta, and vein. As a consequence the total angiotensin II (Ang II) binding sites increased 7.8-fold in the kidney, 25-fold in the heart, and twofold in the adrenals. The AT(2)R number amounted to 82-98% of total Ang II binding sites. In the ZG of TGRCXmAT(2)R, the AT(2)R density was elevated threefold relative to wild-type (WT) littermates, whereas AT(1)R density remained unchanged. TGRCXmAT(2)R rats were viable and exhibited normal reproduction, blood pressure, and kidney function. Notably, a slightly but significantly reduced body weight and a moderate increase in plasma urea were observed. With respect to adrenal function, 24-h urinary and plasma aldosterone concentrations were unaffected in TGRCXmAT(2)R at baseline. Three and 14 days of Ang II infusion (300 ng·min(-1)·kg(-1)) increased plasma aldosterone levels in WT and in TGR. These changes were completely abolished by the AT(1)R blocker losartan. Of note, glomerulosa cell proliferation, as indicated by the number of Ki-67-positive glomerulosa cells, was stimulated by Ang II in TGR and WT rats; however, this increase was significantly attenuated in TGR overexpressing the AT(2)R. In conclusion, AT(2)R in the adrenal ZG inhibits Ang II-induced cell proliferation but has no obvious lasting effect on the regulation of the aldosterone production at the investigated stages.

Angiotensin III stimulates aldosterone secretion from adrenal gland partially via angiotensin II type 2 receptor but not angiotensin II type 1 receptor

Angiotensin II (Ang II) and Ang III stimulate aldosterone secretion by adrenal glomerulosa, but the angiotensin receptor subtypes involved and the effects of Ang IV and Ang (1-7) are not clear. In vitro, different angiotensins were added to rat adrenal glomerulosa, and aldosterone concentration in the medium was measured. Ang II-induced aldosterone release was blocked (30.3 ± 7.1%) by an Ang II type 2 receptor (AT2R) antagonist, PD123319. Candesartan, an Ang II type 1 receptor (AT1R) antagonist, also blocked Ang II-induced aldosterone release (42.9 ± 4.8%). Coadministration of candesartan and PD123319 almost abolished the Ang II-induced aldosterone release. A selective AT2R agonist, CGP42112, was used to confirm the effects of AT2R. CGP42112 increased aldosterone secretion, which was almost completely inhibited by PD123319. In addition to Ang II, Ang III also induced aldosterone release, which was not blocked by candesartan. However, PD123319 blocked 22.4 ± 10.5% of the Ang III-induced aldosterone secretion. Ang IV and Ang (1-7) did not induce adrenal aldosterone secretion. In vivo, both Ang II and Ang III infusion increased plasma aldosterone concentration, but only Ang II elevated blood pressure. Ang IV and Ang (1-7) infusion did not affect blood pressure or aldosterone concentration. In conclusion, this report showed for the first time that AT2R partially mediates Ang III-induced aldosterone release, but not AT1R. Also, over 60% of Ang III-induced aldosterone release may be independent of both AT1R and AT2R. Ang III and AT2R signaling may have a role in the pathophysiology of aldosterone breakthrough.

The angiotensin II receptor type 2 polymorphism influences haemodynamic function and circulating RAS mediators in normotensive humans

BACKGROUND

The haemodynamic responses to angiotensin II type 1 (AT1) receptor blockade may be mediated in part by interactions between angiotensin II and the angiotensin II type 2 receptor (AT2R). An AT2R G1675A gene polymorphism has been described, but the functional effects of this polymorphism are unknown.

METHODS

Haemodynamic function, circulating renin-angiotensin system mediators and norepinephrine were measured in young healthy subjects at baseline and at 2 and 4 weeks after treatment with irbesartan. Subjects were divided into two groups on the basis of the AT2R G1675A gene polymorphism: GG subjects (n = 12) and AA/GA subjects (n = 22).

RESULTS

AA/AG subjects exhibited hypotensive and renal vasodilatory responses to irbesartan at 4 weeks, but GG subjects did not. In accord with haemodynamic effects, circulating aldosterone levels were suppressed in AA/AG, while circulating norepinephrine levels were augmented only in GG subjects. In contrast, increases in circulating renin, angiotensin II and plasma renin activity after irbesartan were exaggerated in AA/AG subjects.

CONCLUSIONS

The AT2R G1675A polymorphism is a determinant of haemodynamic responses to AT1 receptor blockade, an effect that may be due to influences on aldosterone escape.

Fetal adrenal gland alterations in a rat model of adverse intrauterine environment

By feeding a low-sodium diet to dams over the last third of gestation, we have developed an animal model of intrauterine growth restriction (IUGR). Given that fetal adrenal development and maturation occur during late gestation in rats, the aim of this study was to evaluate the expression of proteins and enzymes involved in steroidogenesis and catecholamine synthesis in adrenal glands from IUGR fetuses. A gene microarray was performed to investigate for alteration in the pathways participating in hormone production. Results show that increased aldosterone serum concentrations in IUGR fetuses were associated with higher mRNA adrenal levels of angiotensin II receptor type 1 (AT(1)R) and cytochrome P450 aldosterone synthase in response to decreased serum sodium content. Conversely, reduced serum corticosterone concentrations in these fetuses appear to result from alterations in gene expression involved in cholesterol metabolism, such as the augmented apolipoprotein E levels, and in steroidogenesis, like the decreased levels of cytochrome P45011beta-hydroxylase. Furthermore, increased AT(2)R expression and the presence of hypoxia and oxidative stress may, in turn, explain the higher adrenal mRNA levels of enzymes involved in catecholamine synthesis. Despite this increase, catecholamine adrenal content was reduced in males and was similar in females compared with sex-matched controls, suggesting higher catecholamine secretion. This could be associated with the induction of genes involved in inflammation-related, acute-phase response in IUGR fetuses. All of these alterations could have long-lasting health effects and may, hence, be implicated in the pathogenesis of increased blood pressure and cardiac hypertrophy observed in IUGR adult animals from this model.

Role of AT1 and AT2 receptor subtypes in salt-sensitive hypertension induced by sensory nerve degeneration

OBJECTIVE

To define the role of the type 1 angiotensin II (AT1) and type II (AT2) receptors in the development of salt-sensitive hypertension induced by sensory nerve degeneration.

DESIGN AND METHODS

Neonatal Wistar rats were given capsaicin 50 mg/kg s.c. on the first and second days of life. After weaning, male rats were divided into six groups and treated for 3 weeks with: control + high sodium diet (4%, CON-HS), capsaicin pretreatment + normal sodium diet (0.5%, CAP-NS), CAP-HS, CAP + HS + candesartan (10 mg/kg per day) (CAP-HS-CAN), CAP + HS + PD 123319 (30 mg/kg per day) (CAP-HS-PD), and capsaicin pretreatment + high sodium diet + candesartan + PD 123319 (CAP-HS-CAN-PD). Mean arterial pressure (MAP) was measured by carotid arterial catheterization. Urinary Na+ concentrations were determined by using a flame atomic absorption spectrophotometer. Levels of calcitonin gene-related peptide (CGRP) in dorsal root ganglia (DRG) and plasma renin activity (PRA) were determined by radioimmunoassay.

RESULTS

CGRP contents in DRG were decreased by capsaicin (P < 0.05). MAP was higher in CAP-HS rats compared with all the other groups (P < 0.05). The 24 h urine and sodium excretion increased when a high salt diet was given, but they were lower in CAP-HS and CAP-HS-CAN than in CON-HS (P < 0.05). PRA was suppressed in CON-HS and CAP-HS compared with CAP-NS, but it was higher in CAP-HS than in CON-HS (P < 0.05).

CONCLUSION

Insufficiently suppressed PRA by high salt intake may contribute to increased salt sensitivity and account for effectiveness of candesartan in lowering blood pressure in this model. Furthermore, PD 123319 attenuates the development of hypertension in salt-loaded rats neonatally treated with capsaicin, indicating that the AT2 receptor contributes to the increase in blood pressure.

Nitric oxide-dependent down-regulation of angiotensin II type 2 receptors during experimental sepsis

OBJECTIVE

The systemic renin-angiotensin system is highly activated during septic shock. This has focused interest in regulation of the adrenal angiotensin II type 2 receptor (AT2) as the target thought to mediate angiotensin II-induced adrenal catecholamine release during experimental sepsis in vivo. In addition, the influence of typical endogenous mediators of sepsis, such as proinflammatory cytokines and nitric oxide, on AT2 receptor expression should be investigated in vitro.

DESIGN

Prospective animal trial followed by a controlled cell culture study.

SETTING

Laboratory of the Department of Anesthesiology.

SUBJECTS

Male Sprague-Dawley rats weighing 200-250 g, PC12 cell line.

INTERVENTIONS

Rats were injected with lipopolysaccharide to stimulate Gram-negative sepsis or lipoteichoic acid to stimulate Gram-positive sepsis. AT2 receptor expression, abundance of the proinflammatory cytokines (interleukin-1beta, tumor necrosis factor-alpha, and interferon-gamma), and nitric oxide synthase II expression have been determined in the adrenal gland. Rat adrenal pheochromocytoma cells were incubated with these cytokines or with the nitric oxide donors sodium nitroprusside or S-nitroso-N-acetylpenicillamine to investigate the regulation of AT2 receptors during severe inflammation on a cellular level.

MEASUREMENTS AND MAIN RESULTS

In the adrenal gland, AT2 receptor expression was down-regulated in both models of sepsis, whereas tissue cytokine concentrations were elevated and nitric oxide synthase II expression was induced. Incubation of PC12 cells with proinflammatory cytokines resulted in a dose-dependent diminished expression of AT2 receptors, which was mimicked by incubation with nitric oxide donors. Blocking of cytokine-induced nitric oxide synthesis by co-incubation of PC12 cells with NG-nitro-l-arginine methyl ester prevented down-regulation of AT2 receptors.

CONCLUSIONS

These findings show that in our model of sepsis, the expression of AT2 receptors in the adrenal gland is down-regulated in a nitric oxide-dependent manner. Because AT2 receptors are thought to be involved in adrenal catecholamine secretion in a stimulatory fashion, the diminished expression of AT2 receptors could play an important role in the pathogenesis of septic shock via impaired angiotensin II-induced adrenal catecholamine release, despite a strong activation of the systemic renin-angiotensin system.

Activation of angiotensin II subtype 2 receptor induces catecholamine release in an extracellular Ca(2+)-dependent manner through a decrease of cyclic guanosine 3',5'-monophosphate production in cultured porcine adrenal medullary chromaffin Cells

We have previously demonstrated that CGP 42112 (AT(2) agonist > or =1 nM) markedly reduces catecholamine biosynthesis through AT(2), which is the major angiotensin II (AngII) receptor subtype in cultured porcine chromaffin cells. Also, we have shown that CGP 42112 (> or =1 nM) induces a reduction in cGMP production in these cells. The present study showed that AngII reduced cGMP production via AT(2) in a manner similar to that found with CGP 42112. AngII (1 nM) significantly increased catecholamine secretion from cultured porcine adrenal medullary chromaffin cells. The stimulation was significantly inhibited by PD 123319 (AT(2) antagonist). The stimulation was moderately, but significantly, attenuated by CV-11974 (AT(1) antagonist, > or =10 nM), suggesting an involvement of AT(1). Moreover, CGP 42112 (> or =10 nM) markedly increased catecholamine release from these cells. The stimulation by CGP 42112 was abolished by PD 123319, whereas CV-11974 had no effect, indicating that this response is also mediated by AT(2). We further examined whether extracellular Ca(2+) is involved in the stimulatory effect of AT(2) on catecholamine secretion. Removal of external Ca(2+) significantly suppressed either AngII plus CV-11974 (100 nM; which simulates specific AT(2) stimulation) or CGP 42112- induced catecholamine secretion. AngII plus CV-11974 or CGP 42112 caused a sustained increase in intracellular Ca(2+) ([Ca(2+)](i)), as determined in fura-2-loaded chromaffin cells in an extracellular Ca(2+)-dependent manner. In the presence of EGTA, the subsequent addition of AngII with CV-11974 and CGP 42112 did not cause any increase in [Ca(2+)](i) levels. Consistent with this finding, CGP 42112 (10 nM to 1 microM) did not alter inositol triphosphate (IP(3)) production, a messenger for mobilization of Ca(2+) from intracellular storage sites. In addition, the intracellular Ca(2+) chelator 1,2-bis(2-amino-phenoxy)ethane-N,N,N',N'- tetraacetic acid acetoxymethylester (BAPTA) did not affect CGP 42112-induced catecholamine release. We tested whether a decrease in cGMP was the cause of the stimulatory effect of AT(2) on catecholamine secretion. Pretreatment with 8-bromo-cGMP (1 mM) prevented the stimulatory effect of AngII plus CV-11974 and CGP 42112 on both catecholamine secretion and [Ca(2+)](i). When 8-bromo-cGMP was added after application of AngII plus CV-11974 or CGP 42112, [Ca(2+)](i) induced by these agents was gradually reduced toward the baseline values. Similarly, guanylin completely abolished the AngII- plus CV-11974-induced increase in both NE secretion and [Ca(2+)](i). The Ca(2+) channel blockers, nicardipine and omega-conotoxin G VIA, at 1 microM in both cases, were also effective in inhibiting AT(2) stimulation-induced secretion. On the other hand, neither T-type voltage-dependent Ca(2+) channel blockers, flunarizine, nor Ni(2+) affected catecholamine release caused by AT(2) stimulation. These findings demonstrate that AT(2) stimulation induces catecholamine secretion by mobilizing Ca(2+) through voltage-dependent Ca(2+) channels without affecting intracellular pools and that these effects could be mediated by a decrease in cGMP production.

Regulation of adrenal angiotensin receptor subtypes: a possible mechanism for sympathectomy-induced adrenal hypertrophy

OBJECTIVE

Previous studies indicate that the adrenal gland plays a compensatory role in the maintenance of blood pressure in chemically sympathectomized rats. However, the mechanisms responsible for compensatory adrenal responses are poorly understood. This study examined the regulation of adrenal growth and type 1 A, 1 B, and type 2 angiotensin II (Ang II) receptor (AT1A, AT1B and AT2) expression in the adrenal gland induced by sympathectomy.

METHODS

Five-week-old male Sprague-Dawley rats were treated with either guanethidine (50 mg/kg per day, intraperitoneally) or vehicle for 5 weeks. Norepinephrine and epinephrine levels in the atrium of the heart were measured by high-pressure liquid chromatography. Plasma renin activity was determined by radioimmunoassay. Adrenal AT1 and AT2 receptor density was determined by radioligand binding assay. Adrenal AT1A, AT1B and AT2 mRNA levels were determined by Northern blot analysis.

RESULTS

Norepinephrine and epinephrine levels in the atrium of the heart were decreased 86% (P < 0.0001) and 58% (P < 0.05) by guanethidine treatment, respectively. Plasma renin activity was decreased 71% (P< 0.001) in guanethidine-treated rats compared with vehicle. In contrast, the ratio of adrenal to body weight was increased 38% in guanethidine-treated rats compared with vehicle (P< 0.001). Adrenal AT1 and AT2 receptor density was increased by guanethidine treatment (P< 0.05). Adrenal mRNA levels for AT2 (P< 0.001) and AT1A (P< 0.01), but not AT1B (P>0.05), were increased in guanethidine-treated rats compared with vehicle (P< 0.01). There were positive correlations between adrenal weight and AT2 (r = 0.9, P< 0.001) and AT1A (r = 0.6, P< 0.05) but not AT1B (r = - 0.01, P > 0.05) expression.

CONCLUSIONS

Impairment of the sympathetic nervous system with guanethidine withdraws the normal stimulation of this system on the circulating renin-angiotensin system, but upregulates the expression of adrenal Ang II receptors. Increased expression of adrenal AT2 and AT1A receptors may play an important role in adaptive adrenal hypertrophy and hormonal responses to sympathectomy.

Functional involvement of angiotensin AT2 receptor in adrenal catecholamine secretion in vivo

The aim of the present study was to analyse modulations of adrenal catecholamine secretion from the adrenal gland of anesthetized dogs in response to locally administered angiotensin II (AngII) in the presence of either PD 123319 or CGP 42112, both of which are highly specific and selective ligands to angiotensin AT2 receptor. Plasma concentrations of epinephrine and norepinephrine in adrenal venous and aortic blood were quantified by a high performance liquid chromatography coupled with electrochemical detection (HPLC-EC) method. Adrenal venous blood flow was measured by gravimetry. Local administration of AngII (0.05 µg, 0.1 µM) to the left adrenal gland increased adrenal gland catecholamine output more than 30 times that found in nonstimulated states. Administration of either PD 123319 (0.085 µg (0.23 µM) to 8.5 µg (23 µM)) or CGP 42112 (0.005 µg (0.01 µM) to 5 µg (10 µM)) did not affect the basal catecholamine output significantly. The increase in adrenal catecholamine output in response to AngII was inhibited by ~80% following the largest dose of PD 123319. CGP 42112 significantly attenuated the catecholamine response to AngII by ~70%. PD 123319 and CGP 42112 were devoid of any agonist actions with respect to catecholamine output by the adrenal gland in vivo. Furthermore, both PD 123319 and CGP 42112 inhibited the increase in adrenal catecholamine secretion induced by local administration of AngII. The present study suggests that AT2 receptors play a role in mediating catecholamine secretion by the adrenal medulla in response to AngII receptor agonist administration in vivo.Key words: AT1 and AT2 subtypes, PD 123319, CGP 42112, AT2 antagonist, anesthetized dog.

Recent insight into therapy of congestive heart failure: focus on ACE inhibition and angiotensin-II antagonism

One possible intervention to interrupt the deleterious effects of the renin-angiotensin system is suppression of angiotensin II (Ang II) formation by inhibition of angiotensin-converting enzyme (ACE). However, ACE inhibition incompletely suppresses Ang II formation and also leads to accumulation of bradykinin. Angiotensin II type 1 (AT1) receptors are believed to promote the known deleterious effects of Ang II. Therefore, AT1 receptor antagonists have been recently introduced into therapy for hypertension and congestive heart failure (CHF). Although there are significant differences between the effects of AT1 receptor antagonists and ACE inhibitors including the unopposed stimulation of angiotensin II type 2 (AT2) receptors by AT1 receptor antagonists, the discussion of whether ACE inhibitors, AT1 receptor antagonists or the combination of both are superior in the pharmacotherapy of CHF is still largely theoretical. Accordingly, AT1 receptor antagonists are still investigational. Angiotensin-converting enzyme inhibitors remain first line therapy in patients with CHF due to systolic dysfunction. However, in patients not able to tolerate ACE inhibitor induced side effects, in particular cough, AT1 receptor antagonism is a good alternative. In clinical practice, emphasis should be placed on increasing the utilization of ACE inhibitors, as more than 50% of patients with CHF do not receive ACE inhibitors. In addition, the majority of those on ACE inhibitors receive doses lower than the dosage used in the large clinical trials. Although not yet completely proved, it is likely that high doses of ACE inhibition are superior to low doses with respect to prognosis and symptoms.