Nitric oxide inhibits aldosterone synthesis by a guanylyl cyclase-independent effect

Effects of Beraprost with or without NOS Inhibition on Plasma Aldosterone and Hemodynamics in Healthy Cats

OBJECTIVES

The aim of the study was to evaluate the hemodynamic and RA system effects of the oral administration of the clinical dose of beraprost for feline CKD in healthy cats, and also to examine whether NOS inhibition reversed them.

METHODS

A placebo-controlled pharmacological sequential design study was carried out to assess the plasma aldosterone and renin concentrations (PAC and PRC), blood pressure, heart rate, and exploratorily to estimate renal plasma flow (RPF) and renal vascular resistance (RVR) with simplified methods.

RESULTS

Beraprost reduced PAC when compared to the placebo (p < 0.05); this was reversed when NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME) was added to the beraprost treatment (p < 0.01). No differences in the PRC or hemodynamic parameters were detected between beraprost and the placebo. The correlation ratios (η2) showed opposite relationships between beraprost and the added L-NAME effects on PAC, mean blood pressure (MBP), heart rate, estimated RPF (p < 0.001), estimated RVR (p < 0.01), and PRC (p < 0.05).

CONCLUSIONS

In healthy cats, the clinical dose of beraprost suppresses PAC, which can be reversed by the inhibition of NOS.

Aldosterone and aldosterone synthase inhibitors in cardiorenal disease

Modulation of the renin-angiotensin-aldosterone system is a foundation of therapy for cardiovascular and kidney diseases. Excess aldosterone plays an important role in cardiovascular disease, contributing to inflammation, fibrosis, and dysfunction in the heart, kidneys, and vasculature through both genomic and mineralocorticoid receptor (MR)-mediated as well as nongenomic mechanisms. MR antagonists have been a key therapy for attenuating the pathologic effects of aldosterone but are associated with some side effects and may not always adequately attenuate the nongenomic effects of aldosterone. Aldosterone is primarily synthesized by the CYP11B2 aldosterone synthase enzyme, which is very similar in structure to other enzymes involved in steroid biosynthesis including CYP11B1, a key enzyme involved in glucocorticoid production. Lack of specificity for CYP11B2, off-target effects on the hypothalamic-pituitary-adrenal axis, and counterproductive increased levels of bioactive steroid intermediates such as 11-deoxycorticosterone have posed challenges in the development of early aldosterone synthase inhibitors such as osilodrostat. In early-phase clinical trials, newer aldosterone synthase inhibitors demonstrated promise in lowering blood pressure in patients with treatment-resistant and uncontrolled hypertension. It is therefore plausible that these agents offer protection in other disease states including heart failure or chronic kidney disease. Further clinical evaluation will be needed to clarify the role of aldosterone synthase inhibitors, a promising class of agents that represent a potentially major therapeutic advance.

Inhibition of endogenous ouabain by atrial natriuretic peptide is a guanylyl cyclase independent effect

Background

Endogenous ouabain (EO) and atrial natriuretic peptide (ANP) are important in regulation of sodium and fluid balance. There is indirect evidence that ANP may be involved in the regulation of endogenous cardenolides.

Methods

H295R are human adrenocortical cells known to release EO. Cells were treated with ANP at physiologic concentrations or vehicle (0.1% DMSO), with or without guanylyl cyclase inhibitor 1,2,4 oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). Cyclic guanosine monophosphate (cGMP), the intracellular second messenger of ANP, was measured by a chemiluminescent immunoassay and EO was measured by radioimmunoassay of C18 extracted samples.

Results

EO secretion is inhibited by ANP treatment, with the most prolonged inhibition (90 min vs ≤ 60 min) occurring at physiologic ANP concentrations (50 pg/mL). Inhibition of guanylyl cyclase with ODQ, also reduces EO secretion. The inhibitory effects on EO release in response to cotreatment with ANP and ODQ appeared to be additive.

Conclusions

ANP inhibits basal EO secretion, and it is unlikely that this is mediated through ANP-A or ANP-B receptors (the most common natriuretic peptide receptors) or their cGMP second messenger; the underlying mechanisms involved are not revealed in the current studies. The role of ANP in the control of EO synthesis and secretion in vivo requires further investigation.

Adrenal Corticosteroid Perturbation by the Endocrine Disruptor BDE-47 in a Human Adrenocortical Cell Line and Male Rats

Polybrominated diphenyl ethers (PBDEs) have been previously shown to alter various endocrine biosynthetic pathways. Growing epidemiological evidence suggests that PBDEs alter cardiovascular function. The goal of this study was to examine the effects of BDE-47 on adrenal corticosteroid pathways that play vital roles in cardiovascular homeostasis and pathophysiology. The effect of BDE-47 on aldosterone and cortisol secretion was characterized in a human adrenocortical cell line. HAC15 cells were exposed to various concentrations of BDE-47 (1 nM to 100 μM). Cell viability, corticosteroid secretion, gene expression of enzymes involved in corticosteroid synthesis, and metabolic activity was examined. Additionally, Sprague Dawley male rats were orally exposed to BDE-47 (10 or 100 µg/kg), 5 days per week for 16 weeks. Organ weights and plasma corticosteroid levels were measured. In HAC15 cells, basal and stimulated aldosterone and cortisol secretion was significantly increased by BDE-47. Gene expression of several enzymes involved in corticosteroid synthesis and mitochondrial metabolism also increased. In Sprague Dawley rats, adrenal but not heart, kidney, or liver weights, were significantly increased in BDE-47 treatment groups. Plasma corticosterone levels were significantly increased in the 100 µg BDE-47/kg treatment group. No change in plasma aldosterone levels were observed with BDE-47 exposure. These data indicate that BDE-47 disrupts the regulation of corticosteroid secretion and provides further evidence that PBDEs are potential endocrine disruptors. Future studies will determine the underlying molecular mechanism of altered corticosteroid production and examine whether these alterations result in underlying cardiovascular disease in our rodent model of 16-week BDE-47 exposure.

Activation of Ovarian Taste Receptors Inhibits Progesterone Production Potentially via NO/cGMP and Apoptotic Signaling

Taste receptors are not only expressed in the taste buds, but also in other nongustatory tissues, including the reproductive system. Taste receptors can be activated by various tastants, thereby exerting relatively physiologic functions. The aim of this study was to investigate the effects and potential mechanisms underlying ovarian taste receptor activation on progesterone production using saccharin sodium as the receptor agonist in a pseudopregnant rat model. Taste 1 receptor member 2 (TAS1R2) and taste 2 receptor member 31 (TAS2R31) were demonstrated to be abundantly expressed in the corpora lutea of rats, and intraperitoneal injection of saccharin sodium can activate both of them and initiate their downstream signaling cascades. The activation of these ovarian taste receptors promoted nitric oxide (NO) production via endothelial nitric oxide synthase (eNOS). NO production then increased ovarian cyclic guanosine 3',5'-monophosphate (cGMP) levels, which, in turn, decreased ovarian cyclic adenosine 3',5'-monophosphate levels. In addition, the activation of ovarian taste receptors induced apoptosis, possibly through NO and mitogen-activated protein kinase signaling. As a result, the activation of ovarian taste receptors reduced the protein expression of steroidogenesis-related factors, causing the inhibition of ovarian progesterone production. In summary, our data suggest that the activation of ovarian taste receptors inhibits progesterone production in pseudopregnant rats, potentially via NO/cGMP and apoptotic signaling.

Effect of Short-Term Sunlight Exposure on Blood Pressure and Pulse Rate in Vitamin D3-Insufficient, Prehypertensive Patients: A Pilot Study

INTRODUCTION

This study was conducted to evaluate the effect of short-term sunlight exposure on blood pressure (BP) and pulse rate (PR) in vitamin D3-insufficient, prehypertensive patients.

METHODS

Twenty prehypertensive male participants were prospectively enrolled in this pilot study. BP and PR were measured using 24-hour ambulatory BP monitoring and endocrine biomarkers were assessed.

RESULTS

Sunlight exposure decreased 24-hour systolic BP (SBP), diastolic BP (DBP), and PR (SBP: 132.6 mm Hg to 129.3 mm Hg, DBP: 77.6 mm Hg to 75.7 mm Hg, and PR: 76.1 bpm to 71.3 bpm, p values: 0.0011, 0.0012, and <0.0001, respectively). The decrement patterns of SBP, DBP, and PR during nighttime (SBP: 123.5 mm Hg to 117.9 mm Hg, DBP: 72.2 mm Hg to 68.0 mm Hg, and PR: 68.2 bpm to 59.1 bpm, p values: 0.0015, 0.0003, and <0.0001, respectively) were more profound compared between daytime and nighttime. Blood levels of 25-hydroxyvitamin D3 were significantly increased (p = 0.0001) but aldosterone levels were significantly decreased (p = 0.0014) after sunlight exposure. In addition, an inverse relationship between 25-hydroxyvitamin D3 and aldosterone levels was observed (R = -0.4709, p = 0.0419).

DISCUSSION/CONCLUSION

The pilot study gives promising results that it is worthwhile to evaluate short-term sunlight exposure as a potentially effective approach in decreasing BP and PR in 25-hydroxyvitamin D3-insufficient prehypertensive patients in a larger trial with a control group.

Physiological relevance of nitric oxide in ovarian functions: An overview

Nitric oxide (NO, nitrogen monoxide), a short-lived, free radical carrying an unpaired electron, is one of the smallest molecules synthesized in the biological system. In addition to its role in angiogenesis, neuronal function and inflammatory response, NO has wide-spread significance in regulation of ovarian function in vertebrates. Based on tissue-specific expression, three different nitric oxide synthase (NOS) isoforms, neuronal (nNOS) or NOS1, inducible (iNOS) or NOS2 and endothelial (eNOS) or NOS3 have been identified. While expression of both inducible (iNOS) and constitutive NOS (eNOS) isoforms varies considerably in the ovary at various stages of follicular growth and development, selective binding of NO with proteins containing heme moieties have significant influence on ovarian steroidogenesis. Besides, NO modulation of ovulatory response suggests physiological significance of NO/NOS system in mammalian ovary. Compared to the duality of NO action on follicular development, steroidogenesis and meiotic maturation in mammalian models, participation of NO/NOS system in teleost ovary is less investigated. Genes encoding nos1 and nos2 have been identified in fish; however, presence of nos3 is still ambiguous. Interestingly, two distinct nos2 genes, nos2a and nos2b in zebrafish, possibly arose through whole genome duplication. Differential expression of major NOS isoforms in catfish ovary, NO inhibition of meiosis resumption in Anabas testudineus follicle-enclosed oocytes and NO/sGC/cGMP modulation of oocyte maturation in zebrafish are some of the recent advancements. The present overview is an update on the advancements made and shortfalls still remaining in NO/NOS modulation of intercellular communication in teleost vis-à-vis mammalian ovary.

Obligatory Metabolism of Angiotensin II to Angiotensin III for Zona Glomerulosa Cell-Mediated Relaxations of Bovine Adrenal Cortical Arteries

Hyperaldosteronism is associated with hypertension, cardiac hypertrophy, and congestive heart failure. Steroidogenic factors facilitate aldosterone secretion by increasing adrenal blood flow. Angiotensin (Ang) II decreases adrenal vascular tone through release of zona glomerulosa (ZG) cell-derived vasodilatory eicosanoids. However, ZG cell-mediated relaxation of bovine adrenal cortical arteries to Ang II is not altered by angiotensin type 1 or 2 receptor antagonists. Because traditional Ang II receptors do not mediate these vasorelaxations to Ang II, we investigated the role of Ang II metabolites. Ang III was identified by liquid chromatography-mass spectrometry as the primary ZG cell metabolite of Ang II. Ang III stimulated ZG cell-mediated relaxation of adrenal arteries with greater potency than did Ang II. Furthermore, ZG cell-mediated relaxations of adrenal arteries by Ang II were attenuated by aminopeptidase inhibition, and Ang III-stimulated relaxations persisted. Ang IV had little effect compared with Ang II. Moreover, ZG cell-mediated relaxations of adrenal arteries by Ang II were attenuated by an Ang III antagonist but not by an Ang (1-7) antagonist. In contrast, Ang II and Ang III were equipotent in stimulating aldosterone secretion from ZG cells and were unaffected by aminopeptidase inhibition. Additionally, aspartyl and leucyl aminopeptidases, which convert Ang II to Ang III, are the primary peptidase expressed in ZG cells. This was confirmed by enzyme activity. These data indicate that intra-adrenal metabolism of Ang II to Ang III is required for ZG cell-mediated relaxations of adrenal arteries but not aldosterone secretion. These studies have defined an important role of Ang III in the adrenal gland.

Integrative and comparative reproductive biology: From alligators to xenobiotics

Dr. Louis J. Guillette Jr. thought of himself as a reproductive biologist. However, his interest in reproductive biology transcended organ systems, life history stages, species, and environmental contexts. His integrative and collaborative nature led to diverse and fascinating research projects conducted all over the world. He doesn't leave us with a single legacy. Instead, he entrusts us with several. The purpose of this review is to highlight those legacies, in both breadth and diversity, and to illustrate Dr. Guillette's grand contributions to the field of reproductive biology. He has challenged the field to reconsider how we think about our data, championed development of novel and innovative techniques to measure endocrine function, helped define the field of endocrine disruption, and lead projects to characterize new endocrine disrupting chemicals. He significantly influenced our understanding of evolution, and took bold and important steps to translate all that he has learned into advances in human reproductive health. We hope that after reading this manuscript our audience will appreciate and continue Dr. Guillette's practice of open-minded and passionate collaboration to understand the basic mechanisms driving reproductive physiology and to ultimately apply those findings to protect and improve wildlife and human health.

Upregulation of Neuronal Nitric Oxide Synthase mRNA and Protein in Adrenal Medulla of Water-deprived Rats

Experiments were performed to investigate whether adrenal neuronal nitric oxide synthase (nNOS) mRNA and protein expression are responsive to alterations in body volume. Using an RT-PCR technique, the relative quantities of nNOS mRNA as well as the tyrosine hydroxylase and phenylethanolamine N-methyltransferase mRNA in the adrenals of water-deprived rats significantly increased from 12 hr to 4 days. In situ hybridization and immunohistochemical study showed that water deprivation activated nNOS mRNA and protein expression in the adrenal medulla. Four days after water deprivation, nNOS protein expression determined by Western blot significantly increased in the adrenal gland. Our results are the first to demonstrate that nNOS syntheses in the adrenal medulla are markedly increased in water-deprived rats. This study also indicates that the upregulation of nNOS synthesis of the adrenal medulla is associated with the activation of adrenal medullary function in the face of volume depletion.

Fetal endocrine and metabolic adaptations to hypoxia: the role of the hypothalamic-pituitary-adrenal axis

In utero, hypoxia is a significant yet common stress that perturbs homeostasis and can occur due to preeclampsia, preterm labor, maternal smoking, heart or lung disease, obesity, and high altitude. The fetus has the extraordinary capacity to respond to stress during development. This is mediated in part by the hypothalamic-pituitary-adrenal (HPA) axis and more recently explored changes in perirenal adipose tissue (PAT) in response to hypoxia. Obvious ethical considerations limit studies of the human fetus, and fetal studies in the rodent model are limited due to size considerations and major differences in developmental landmarks. The sheep is a common model that has been used extensively to study the effects of both acute and chronic hypoxia on fetal development. In response to high-altitude-induced, moderate long-term hypoxia (LTH), both the HPA axis and PAT adapt to preserve normal fetal growth and development while allowing for responses to acute stress. Although these adaptations appear beneficial during fetal development, they may become deleterious postnatally and into adulthood. The goal of this review is to examine the role of the HPA axis in the convergence of endocrine and metabolic adaptive responses to hypoxia in the fetus.

Crosstalk between nitric oxide synthases and cyclooxygenase 2 in the adrenal cortex of rats under lipopolysaccharide treatment

The effect of lipopolysaccharide on the modulation of steroid production by adrenal cells has been recently acknowledged. The purpose of this study was to determine the in vivo effects of LPS on adrenal cyclooxygenase 2 (COX-2) expression, analyze its crosstalk with the nitric oxide synthase (NOS) system, and assess its involvement on the modulation of glucocorticoid production. Male Wistar rats were injected with LPS and with specific inhibitors for NOS and COX activities. PGE2 and corticosterone levels were determined by RIA. Protein levels were analyzed by immunoprecipitation and western blotting. Transfection assays were performed in murine adrenocortical Y1 cells. Results show that LPS treatment increases PGE2 production and COX-2 protein levels in the rat adrenal cortex. Systemic inhibition of COX-2 blunted the glucocorticoid response to ACTH, as well as the increase in NOS activity and the NOS-2 expression levels induced by LPS. Conversely, NOS inhibition prevented the LPS-dependent increase in PGE2 production, COX-2 protein levels, and the nitrotyrosine modification of COX-2 protein. Treatment of adrenocortical cells with a NO-donor significantly potentiated the LPS-dependent increase in NFκB activity and COX-2 expression levels. In conclusion, our results show a significant crosstalk between COX-2 and NOS in the adrenal cortex upon LPS stimulation, in which each activity has a positive impact on the other. In particular, as both the activities differently affect adrenal steroid production, we hypothesize that this kind of fine modulation enables the gland to adjust steroidogenesis to prevent either an excessive or an insufficient response to the endotoxin challenge.

Involvement of PI3K/Akt and p38 MAPK in the induction of COX-2 expression by bacterial lipopolysaccharide in murine adrenocortical cells

Previous studies from our laboratory demonstrated the involvement of COX-2 in the stimulation of steroid production by LPS in murine adrenocortical Y1 cells, as well as in the adrenal cortex of male Wistar rats. In this paper we analyzed signaling pathways involved in the induction of this key regulatory enzyme in adrenocortical cells and demonstrated that LPS triggers an increase in COX-2 mRNA levels by mechanisms involving the stimulation of reactive oxygen species (ROS) generation and the activation of p38 MAPK and Akt, in addition to the previously demonstrated increase in NFκB activity. In this sense we showed that: (1) inhibition of p38 MAPK or PI3K/Akt (pharmacological or molecular) prevented the increase in COX-2 protein levels by LPS, (2) LPS induced p38 MAPK and Akt phosphorylation, (3) antioxidant treatment blocked the effect of LPS on p38 MAPK phosphorylation and in COX-2 protein levels, (4) PI3K inhibition with LY294002 prevented p38 MAPK phosphorylation and, (5) the activity of an NFκB reporter was decreased by p38 MAPK or PI3K inhibition. These results suggest that activation of both p38 MAPK and PI3K/Akt pathways promote the stimulation of NFκB activity and that PI3K/Akt activity might regulate both p38 MAPK and NFκB signaling pathways. In summary, in this study we showed that in adrenal cells, LPS induces COX-2 expression by activating p38 MAPK and PI3K/Akt signaling pathways and that both pathways converge in the modulation of NFκB transcriptional activity.

Adrenocortical and adipose responses to high-altitude-induced, long-term hypoxia in the ovine fetus

By late gestation, the maturing hypothalamo-pituitary-adrenal (HPA) axis aids the fetus in responding to stress. Hypoxia represents a significant threat to the fetus accompanying situations such as preeclampsia, smoking, high altitude, and preterm labor. We developed a model of high-altitude (3,820 m), long-term hypoxia (LTH) in pregnant sheep. We describe the impact of LTH on the fetal HPA axis at the level of the hypothalamic paraventricular nucleus (PVN), anterior pituitary corticotrope, and adrenal cortex. At the PVN and anterior pituitary, the responses to LTH are consistent with hypoxia being a potent activator of the HPA axis and potentially maladaptive, while the adrenocortical response to LTH appears to be primarily adaptive. We discuss mechanisms involved in the delicate balance between these seemingly opposing responses that preserve the normal ontogenic rise in fetal plasma cortisol essential for organ maturation and in this species, birth. Further, we examine the response to, and ramifications of, an acute secondary stressor in the LTH fetus. We provide an integrative model on the potential role of adipose in modulating these responses to LTH. Integration of these adaptive responses to LTH plays a key role in promoting normal fetal growth and development under conditions of a chronic stress.

Influence of nitric oxide on in vitro growth, survival, steroidogenesis, and apoptosis of follicle stimulating hormone stimulated buffalo (Bubalus bubalis) preantral follicles

Effect of sodium nitroprusside (SNP), a nitric oxide (NO) donor, on in vitro survival, growth, steroidogenesis, and apoptosis of buffalo preantral follicles (PFs) was investigated. PFs (200~250 µm) were isolated by micro-dissection and cultured in 0 (control), 10^-3, 10^-5, 10^-7, and 10^-9 M SNP. To examine the reversible effect of SNP, PFs were cultured with 10^-5 M SNP + 1 mM N^ω-nitro-L-arginine methyl ester (L-NAME) or 1.0 µg hemoglobin (Hb). The results showed that greater concentrations of SNP (10^-3, 10^-5, 10^-7 M) inhibited (p < 0.05) FSH-induced survival, growth, antrum formation, estradiol production, and oocyte apoptosis in a dose-dependent manner. However, a lower dose of SNP (10^-9 M) significantly stimulated (p < 0.05) the survival, growth, antrum formation, follicular oocyte maturation, and stimulated progesterone secretion compared to the control. A combination of SNP + L-NAME promoted the inhibitor effect of SNP while a SNP + Hb combination reversed this effect. Nitrate and nitrite concentrations in the culture medium increased (p < 0.05) in a dose-dependent manner according to SNP concentration in the culture medium. At higher concentrations, SNP had a cytotoxic effect leading to follicular oocyte apoptosis whereas lower concentrations have stimulatory effects. In conclusion, NO exerts a dual effect on its development of buffalo PFs depending on the concentration in the culture medium.

Angiotensin II regulates adrenal vascular tone through zona glomerulosa cell-derived EETs and DHETs

Elevated concentrations of aldosterone are associated with several cardiovascular diseases. Angiotensin II (Ang II) increases aldosterone secretion and adrenal blood flow. This concurrent increase in steroidogenesis and adrenal blood flow is not understood. We investigated the role of zona glomerulosa (ZG) cells in the regulation of vascular tone of bovine adrenal cortical arteries by Ang II. ZG cells enhanced endothelium-dependent relaxations to Ang II. The ZG cell-dependent relaxations to Ang II were unchanged by removing the endothelium-dependent response to Ang II. These ZG cell-mediated relaxations were ablated by cytochrome P450 inhibition, epoxyeicosatrienoic acid (EET) antagonism, and potassium channel blockade. Analysis of ZG cell EET production by liquid chromatography/mass spectrometry demonstrated an increase in EETs and dihydroxyeicosatrienoic acids with Ang II stimulation. These EETs and dihydroxyeicosatrienoic acids produced similar concentration-dependent relaxations of adrenal arteries, which were attenuated by EET antagonism. Whole-cell potassium currents of adrenal artery smooth muscle cells were increased by Ang II stimulation in the presence of ZG cells but decreased in the absence of ZG cells. This increase in potassium current was abolished by iberiotoxin. Similarly, 14,15-EET induced concentration-dependent increases in potassium current, which was abolished by iberiotoxin. ZG cell aldosterone release was not directly altered by EETs. These data suggest that Ang II stimulates ZG cells to release EETs and dihydroxyeicosatrienoic acids, resulting in potassium channel activation and relaxation of adrenal arteries. This provides a mechanism by which Ang II concurrently increases adrenal blood flow and steroidogenesis.

Nitric oxide inhibits ACTH-induced cortisol production in near-term, long-term hypoxic ovine fetal adrenocortical cells

We previously reported that in the sheep fetus, long-term hypoxia (LTH) resulted in elevated basal plasma adrenocorticotropic hormone (ACTH(1- 39)) whereas the cortisol levels were not different from normoxic controls. We also showed that LTH enhances endothelial nitric oxide synthase (eNOS) expression in the fetal adrenal. This study was designed to determine the effect of NO on cortisol production in adrenocortical cells from LTH fetal sheep. Ewes were maintained at high altitude (3820 m) from ∼40 days' gestation (dG) to near term. Between 138 and 141 dG, fetal adrenal glands were collected from LTH and age-matched normoxic control fetuses. Adrenal cortical cells were pretreated with sodium nitroprusside (SNP), nitro-L-arginine methyl ester (L-NAME), L-arginine, or diethyleneamine NO (DETA-NO) and then challenged with 10 nmol/L ACTH. Cortisol responses were compared after 1 hour. Adrenocorticotropic hormone -induced cortisol secretion was significantly higher in LTH versus control (P < .01). Enhancement of NO with L-arginine resulted in a significant reduction of ACTH-mediated cortisol production in the LTH group. DETA-NO also caused a significant decrease in ACTH-mediated cortisol production (P < .05). Inhibition of NOS with L-NAME significantly increased cortisol production in the LTH group (P < .05 compared to ACTH alone), whereas the effect on the control group was not significant. Nitric oxide synthase activity was significantly higher in the LTH group compared to control, but this difference was eliminated following ACTH treatment. These data indicate that LTH enhances adrenal cortical sensitivity to the inhibitory effects of NO on cortisol production. Nitric oxide may, therefore, play an important role in regulating ACTH-induced cortisol production in the LTH fetal adrenal.

Interactions of NADPH oxidase, renin-angiotensin-aldosterone system and reactive oxygen species in mequindox-mediated aldosterone secretion in Wistar rats

High doses of mequindox (MEQ) are associated with oxidative stress and pathological toxicity in the kidney. In this study, we demonstrated long term effects of MEQ on intra- or extra-adrenal renin-angiotensin-aldosterone system (RAAS) in vivo. RAAS plays a major role in aldosterone secretion. High doses of MEQ in the diet for 180 days in male rats led to inhibition of intra- and extra-adrenal RAAS, concident with down-regulation of Na(+)/K(+)-ATPase (NAKA) and mineralocorticoid receptor (MR), the downstream of aldosterone action. Significant changes of malondialdehyde (MDA), reduced glutathione (GSH), and superoxide dismutase (SOD) in kidney were also observed in the high doses (110, 275mg/kg) groups. The mRNA levels of most subunits of NADPH oxidase were significantly upregulated at low doses (25-110mg/kg) but the upregulation was diminished at higher doses in both kidney and adrenal gland, indicating a complicated and contradictory effect of MEQ on NADPH. These results highlight the complex interactions of drug metabolism, RAAS, NADPH oxidase and oxidative stress in response to MEQ-induced tissue toxicity and aldosterone secretion.

Systemic nitric oxide clamping in normal humans guided by total peripheral resistance

AIM

We wanted to stabilize the availability of nitric oxide (NO) at levels compatible with normal systemic haemodynamics to provide a model for studies of complex regulations in the absence of changes in NO levels.

METHODS

Normal volunteers (23-28 years) were infused i.v. with the nitric oxide synthase (NOS) inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME) at 0.5 mg kg(-1) h(-1). One hour later, the NO donor sodium nitroprusside (SNP) was co-infused in doses eliminating the haemodynamic effects of l-NAME. Haemodynamic measurements included blood pressure (MABP) and cardiac output (CO) by impedance cardiography.

RESULTS

l-NAME increased MABP and total peripheral resistance (TPR, 1.02 + or - 0.05 to 1.36 + or - 0.07 mmHg s mL(-1), mean + or - SEM, P < 0.001). With SNP, TPR fell to a stable value slightly below control (0.92 + or - 0.05 mmHg s mL(-1), P < 0.05). CO decreased with l-NAME (5.8 + or - 0.3 to 4.7 + or - 0.3 L min(-1), P < 0.01) and returned to control when SNP was added (6.0 + or - 0.3 L min(-1)). A decrease in plasma noradrenaline (42%, P < 0.01) during l-NAME administration was completely reversed by SNP. Plasma renin activity decreased during l-NAME administration and returned towards normal after addition of SNP. In contrast, plasma aldosterone was increased by l-NAME and remained elevated.

CONCLUSIONS

Concomitant NOS inhibition and NO donor administration can be adjusted to maintain TPR at control level for hours. This approach may be useful in protocols in which stabilization of the peripheral supply of NO is required. However, the dissociation between renin and aldosterone secretion needs further investigation.

Long-term hypoxia increases endothelial nitric oxide synthase expression in the ovine fetal adrenal

This study was designed to test the hypothesis that fetal adrenal nitric oxide synthase (NOS) is elevated in response to long-term hypoxia (LTH). Pregnant ewes were maintained at high altitude (3820 m) for approximately the last 100 days of gestation. Between days 138 and 141 of gestation, adrenal glands were collected from LTH fetuses and age-matched normoxic controls. Quantitative real-time polymerase chain reaction (qRT-PCR) and Western analysis were used to quantify NOS expression, and NOS distribution was examined by immunohistochemistry and double-staining immunofluorescence for endothelial NOS (eNOS) and 17alpha-hydroxylase (CYP17). Neuronal NOS (nNOS) was expressed at very low levels and with no differences between groups. Expression of eNOS was significantly greater in the LTH group compared with control. Neuronal NOS was distributed throughout the cortex while the greatest density of eNOS was observed in the zona fasciculata/reticularis area and eNOS co-localized with CYP17. We conclude that LTH enhances eNOS expression in the inner adrenal cortex which may play a role in regulation of cortisol biosynthesis in the LTH fetus.

The Janus effect: two faces of aldosterone

Inhibition of the nitric oxide pathway by N(omega)-nitro-l-arginine methyl ester (l-NAME) is well known to produce hypertension and proteinuria, but the mechanisms are less straightforward. Prolonged administration of mineralocorticoids mimics the pathological findings produced by l-NAME. Ikeda and colleagues provide a clue to the mechanism by showing that exposure to l-NAME increases plasma aldosterone 50-fold, and that spironolactone markedly attenuates the renal changes. Thus, chronic l-NAME exposure may turn out to be a model of mineralocorticoid excess.

Spironolactone suppresses inflammation and prevents L-NAME-induced renal injury in rats

Chronic inhibition of nitric oxide synthase by N(omega)-nitro- L-arginine methyl ester (L-NAME) causes progressive renal injury with systemic hypertension and interstitial macrophage infiltration. We have previously shown that there is local activation of the renin-angiotensin-aldosterone system in the renal cortex as a major pathogenic feature of macrophage infiltration. In this study, we measured the effects of the aldosterone antagonist, spironolactone, on renal injury in L-NAME-treated male Wistar rats. After 12 weeks of L-NAME-treatment, rats had increased systolic blood pressure, urinary protein excretion, and serum creatinine and histological analysis showed glomerulosclerosis, interstitial fibrosis, and macrophage infiltration. Treatment with spironolactone significantly prevented these renal changes, whereas treatment with hydralazine had no effect. The cortical expression of osteopontin was significantly elevated in L-NAME-treated rats, and expression of its mRNA significantly correlated with the number of infiltrating macrophages and degree of interstitial fibrosis. Spironolactone treatment markedly suppressed osteopontin expression. Our results suggest that reduced nitric oxide bioavailability caused renal inflammation and fibrosis through an aldosterone receptor-dependent mechanism associated with osteopontin expression independent of its systemic hemodynamic effects.

Inhibition of Akt pathway phosphorylation as a mechanism in the pathogenesis of functional intestinal obstruction in carcinomatosis peritonei

BACKGROUND AND OBJECTIVES

The purpose of this study was to confirm our hypothesis that the development of functional intestinal obstruction in carcinomatosis peritonei (CP) is related to cytokine-mediated inhibition of the Akt pathway and to investigate the phenomenon of relative adrenal insufficiency in CP.

METHODS

Human adrenocortical cells (NCI-H295R) were treated with serum derived from eight cancer patients who had intestinal obstruction and functional adrenal insufficiency. Serum from three normal healthy subjects and three who had CP but without intestinal obstruction or adrenal insufficiency were used as controls. The differential effects of serum on the treated cells were studied using Western blot analysis. Cortisol production of these treated cells was assayed with cortisol ELISA kits.

RESULTS

Phosphorylation of Akt at Ser473 and Ser308 in cells was significantly reduced when treated with serum from patients with intestinal obstruction but not controls. Phosphorylation of PDK1 at Ser241, mTOR downstream targets like p70S6 at Thr421/Ser424 and Thr389, and lastly 4EBP-1 at Ser70 a downstream target of p70S6 was reduced by approximately 50%, 40%, and 70%, respectively. There was enhanced phosphorylation of elF4E an initiating factor in protein translation in cells treated with patient serum compared to controls. Cortisol synthesis was stimulated upon treatment with patient serum but not with control serum.

CONCLUSION

Inhibition of Akt phosphorylation is a mechanism that could play a major role in the development of intestinal obstruction in carcinomatosis peritonei. The identification of the mediating cytokines will lead to the development of cogent targeted therapeutic strategies.

Nitric oxide-independent stimulation of soluble guanylate cyclase with BAY 41-2272 in cardiovascular disease

The nitric oxide (NO)-soluble guanylate cyclase (sGC)-cyclic 3',5'-guanosine monophosphate (cGMP) pathway plays an important role in cardiovascular regulation by promoting vasodilation and inhibiting vascular smooth muscle cell growth, platelet aggregation, and leukocyte adhesion. In pathophysiological states with endothelial dysfunction this signaling pathway is impaired. Activation of sGC has traditionally been achieved with nitrovasodilators; however, these drugs are associated with the development of tolerance and potentially deleterious cGMP-independent actions. In this review the actions of BAY 41-2272, the prototype of a new class of NO-independent sGC stimulators, in cardiovascular disease models is discussed. BAY 41-2272 binds to a regulatory site on the alpha-subunit of sGC and stimulates the enzyme synergistically with NO. BAY 41-2272 had antihypertensive actions and attenuated remodeling in models of systemic arterial hypertension. It also unloaded the heart in experimental congestive heart failure. BAY 41-2272 reduced pulmonary vascular resistance in acute and chronic experimental pulmonary arterial hypertension. Furthermore, BAY 41-2272 inhibited platelet aggregation in vitro and leukocyte adhesion in vivo. These findings make direct sGC stimulation with BAY 41-2272 a promising new therapeutic strategy for cardiovascular diseases and warrant further studies. Finally, the significance of the novel NO- and heme-independent sGC activator BAY 58-2667, which activates two forms of NO-insensitive sGC, is briefly discussed.

Endothelial cell-mediated regulation of aldosterone release from human adrenocortical cells

Endothelial cells play an important role in the development and functioning of endocrine tissue and endothelial cell-derived factors have been shown to regulate mineralocorticoid release in bovine adrenal cells. In the present study, we analysed the role of human endothelial cells in the synthesis and release of aldosterone from adrenocortical cells (NCI-H295R). Endothelial cell-induced aldosterone release was rapid and lasted as a long-term effect over a period of 48 h. This stimulant effect was influenced by the duration of endothelial cell conditioning and decreased linearly with increasing dilutions of the conditioned medium. At the molecular level, an increase in the mRNA transcripts of aldosterone synthase and StAR could be observed. Cellular interaction with endothelial cell-factors enhanced the activation of CRE, and the promoter activity of both StAR and SF-1 reporter genes. In conclusion, human endothelial cells are important intra-adrenal regulators of human aldosterone synthesis and release.

Endothelium-derived steroidogenic factor enhances angiotensin II-stimulated aldosterone release by bovine zona glomerulosa cells

Endothelium-derived steroidogenic factor (EDSF) is an endothelial peptide that stimulates aldosterone release from bovine adrenal zona glomerulosa (ZG) cells. The regulation of aldosterone release by combinations of EDSF and angiotensin II (AII) or EDSF and ACTH was investigated. Endothelial cells (ECs) and EC-conditioned media (ECCM) increased aldosterone release from ZG cells, an activity attributed to EDSF. AII (10(-12) to 10(-8) M) and ACTH (10(-12) to 10(-9) M) also stimulated the release of aldosterone from ZG cells. The stimulation by AII, but not ACTH, was greatly enhanced when ZG cells were coincubated with ECs. AII was metabolized by ECs to peptides identified by mass spectrometry as angiotensin (1-7) and angiotensin IV. There was very little metabolism of AII by ZG cells. Neither of these two AII metabolites altered aldosterone release from ZG cells, so they could not account for the enhanced response with ECs. AII-induced aldosterone release from ZG cells was enhanced by ECCM but not cell-free conditioned medium. This enhanced response was not due to increased EDSF release from ECs by AII. The synergistic effect of EDSF and AII was apparent when AII was added during or after the generation of ECCM and not observed when the AII component of the enhancement was blocked by the AII antagonist, losartan. These studies indicate that EDSF enhances the steroidogenic effect of AII. In the adrenal gland, ECs are in close anatomical relationship with ZG cells and may sensitize ZG cells to the steroidogenic action of AII by releasing EDSF.

Teaching aldosterone regulation and basic scientific principles using a classic paper by Dr. James O. Davis and colleagues

Classroom discussion of scientific articles can be an effective means of teaching scientific principles and methodology to both undergraduate and graduate science students. The availability of classic papers from the American Physiological Society Legacy Project has made it possible to access articles dating back to the early portions of the 20th century. In this article, we discuss a classic paper from the laboratory of Dr. James O. Davis on the regulation of aldosterone synthesis from the adrenal zona glomerulosa cell. Dr. Davis has conducted much of the seminal research investigating the renin-angiotensin system and the regulation of aldosterone release by angiotensin II. In addition to a characterization of the effects of ACTH on aldosterone regulation, this study is useful for discussing the basic principles of negative feedback pathways of the hypothalamic-pituitary axis. This study also provides examples of early bioassay techniques for the detection of angiotensin II and of the importance of quantitative measurements when investigating physiological responses. Three figures and one table are reproduced from the original article along with a series of discussion questions designed to facilitate discovery learning.

Characterization of L-arginine transport in adrenal cells: effect of ACTH

Nitric oxide synthesis depends on the availability of its precursor L-arginine, which could be regulated by the presence of a specific uptake system. In the present report, the characterization of the L-arginine transport system in mouse adrenal Y1 cells was performed. L-arginine transport was mediated by the cationic/neutral amino acid transport system y+L and the cationic amino acid transporter (CAT) y+ in Y1 cells. These Na+-independent transporters were identified by their selectivity for neutral amino acids in both the presence and absence of Na+ and by the effect of N-ethylmaleimide. Transport data correlated to expression of genes encoding for CAT-1, CAT-2, CD-98, and y+LAT-2. A similar expression profile was detected in rat adrenal zona fasciculata. In addition, cationic amino acid uptake in Y1 cells was upregulated by ACTH and/or cAMP with a concomitant increase in nitric oxide (NO) production.

Mechanisms of histamine-induced relaxation in bovine small adrenal cortical arteries

Adrenal steroidogenesis is closely correlated with increases in adrenal blood flow. Many reports have studied the regulation of adrenal blood flow in vivo and in perfused glands, but until recently few studies have been conducted on isolated adrenal arteries. The present study examined vasomotor responses of isolated bovine small adrenal cortical arteries to histamine, an endogenous vasoactive compound, and its mechanism of action. In U-46619-precontracted arteries, histamine (10(-9)-5 x 10(-6) M) elicited concentration-dependent relaxations. The relaxations were blocked by the H(1) receptor antagonists diphenhydramine (10 microM) or mepyramine (1 microM) (maximal relaxations of 18 +/- 6 and 22 +/- 6%, respectively, vs. 55 +/- 5% of control) but only partially inhibited by the H(2) receptor antagonist cimetidine (10 microM) and the H(3) receptor antagonist thioperamide (1 microM). Histamine-induced relaxations were also blocked by the nitric oxide synthase inhibitor N-nitro-L-arginine (L-NA, 30 microM; maximal relaxation of 13 +/- 7%) and eliminated by endothelial removal or L-NA combined with the cyclooxgenase inhibitor indomethacin (10 microM). In the presence of adrenal zona glomerulosa (ZG) cells, histamine did not induce further relaxations compared with histamine alone. Histamine (10(-7)-10(-5) M) concentration-dependently increased aldosterone production by adrenal ZG cells. Compound 48/80 (10 microg/ml), a mast cell degranulator, induced significant relaxations (93 +/- 0.6%), which were blocked by L-NA plus indomethacin or endothelium removal, partially inhibited by the combination of the H(1), H(2), and H(3) receptor antagonists, but not affected by the mast cell stabilizer sodium cromoglycate (1 mM). These results demonstrate that histamine causes direct relaxation of small adrenal cortical arteries, which is largely mediated by endothelial NO and prostaglandins via H(1) receptors. The potential role of histamine in linking adrenal vascular events and steroid secretion requires further investigation.

Measurement of steroid synthesis in zona glomerulosa cells by liquid chromatography–electrospray ionization–mass spectrometry: inhibition by nitric oxide

A liquid chromatography-electrospray ionization-mass spectrometry method was developed to simultaneously determine the concentrations of aldosterone, corticosterone, cortisol, deoxycorticosterone, pregnenolone, and progesterone in bovine adrenal zona glomerulosa (ZG) cells. Steroids were extracted by liquid-liquid extraction, separated on a reverse-phase C18 column, ionized by electrospray, and detected by single-quadrupole mass spectrometry in a positive ion mode. All steroids formed sodium adducts at high abundance. Factors affecting the formation and signal of sodium adducts were investigated. The limits of detection (S/N=3) using selected ion monitoring are 2 pg for these steroids and 10 pg for pregnenolone. DETA NONOate, a nitric oxide donor, inhibited the basal, angiotensin-II-stimulated, and 25-hydroxycholesterol-stimulated syntheses of these steroids in ZG cells in a concentration-dependent manner. The technique demonstrates the ability to determine the individual steroid in each enzymatic step of aldosterone synthesis and the activity of steroidogenic enzymes in adrenal ZG cells.

NO Synthase Inhibition Increases Aldosterone in Humans

To test the hypothesis that NO influences aldosterone production in humans, we examined the effect of N G -nitro- l -arginine methyl ester ( l -NAME) on aldosterone concentrations in the presence and absence of the NO precursor l -arginine (3 g TID) and the angiotensin-converting enzyme inhibitor ramipril (10 mg QD). Ten normal subjects were given l -NAME (66 μg/kg per min for 30 minutes) or vehicle in random order on separate days during placebo and after randomized, double-blind treatment with l -arginine, ramipril, or l -arginine plus ramipril. Infusion of l -NAME significantly increased systolic blood pressure (all P <0.05) and decreased heart rate (all P ≤0.02) during all 4 treatment arms. After placebo pretreatment, serum aldosterone was significantly higher during l -NAME infusion than during vehicle (6.6±1.7 versus 3.3±0.5 ng/dL; P =0.045). Combined treatment with l -arginine plus ramipril abolished this effect. There was no effect of l -NAME on plasma renin activity (PRA; P =0.297) or angiotensin II concentrations ( P =0.537). However, there was a significant interactive effect of l -NAME and time on serum potassium ( P =0.039). There was a significant linear relationship between PRA and aldosterone concentration after vehicle infusion ([aldosterone]=3.9·PRA+1.9; r 2 =0.476; P =0.027) and l -NAME infusion ([aldosterone]=7.2·PRA+3.1; r 2 =0.457; P =0.032), and the intercepts of these lines were different ( P =0.029). There was a significant linear relationship between serum potassium and aldosterone during l -NAME ([aldosterone]=8.2 · [potassium]−28.9; r 2 =0.609; P =0.008) but not during vehicle ( P =0.313). These data suggest that endogenous NO modulates aldosterone synthesis in humans.

Chronic administration of nitric oxide reduces angiotensin II receptor type 1 expression and aldosterone synthesis in zona glomerulosa cells

Acute nitric oxide (NO) inhibits angiotensin II (ANG II)-stimulated aldosterone synthesis in zona glomerulosa (ZG) cells. In this study, we investigated the effects of chronic administration of NO on the ANG II receptor type 1 (AT1) expression and aldosterone synthesis. ZG cells were treated daily with DETA NONOate (10(-4) M), an NO donor, for 0, 12, 24, 48, 72, and 96 h. Chinese hamster ovary (CHO) cells, stably transfected with the AT1B receptor, were used as a positive control. Western blot analysis indicated that AT1 receptor expression was decreased as a function of time of NO administration in both CHO and ZG cells. ANG II binding to its receptors was determined by radioligand binding. NO treatment of ZG cells for 96 h resulted in a decrease in ANG II binding compared with control. The receptor density was decreased to 1,864 +/- 129 fmol/mg protein from 3,157 +/- 220 fmol/mg protein (P < 0.005), but the affinity was not changed (1.95 +/- 0.22 vs. 1.88 +/- 0.21 nM). Confocal Raman microspectroscopy and immunocytochemistry both confirmed that the expression of AT1 receptors in ZG cells decreased with chronic NO administration. In addition, chronic NO administration also decreased the expression of cholesterol side-chain cleavage enzyme in ZG cells and inhibited ANG II- and 25-hydroxycholesterol-stimulated aldosterone synthesis in ZG cells. This study demonstrates that chronic administration of NO inhibits aldosterone synthesis in ZG cells by downregulation of the expression of both AT1 receptors and cholesterol side-chain cleavage enzyme.

Substrate binding favors enhanced NO binding to P450cam

Ferric cytochrome P450cam from Pseudomonas putida (P450cam) in buffer solution at physiological pH 7.4 reversibly binds NO to yield the nitrosyl complex P450cam(NO). The presence of 1R-camphor affects the dynamics of NO binding to P450cam and enhances the association and dissociation rate constants significantly. In the case of the substrate-free form of P450cam, subconformers are evident and the NO binding kinetics are much slower than in the presence of the substrate. The association and dissociation processes were investigated by both laser flash photolysis and stopped-flow techniques at ambient and high pressure. Large and positive values of S and V observed for NO binding to and release from the substrate-free P450cam complex are consistent with the operation of a limiting dissociative ligand substitution mechanism, where the lability of coordinated water dominates the reactivity of the iron(III)-heme center with NO. In contrast, NO binding to P450cam in the presence of camphor displays negative activation entropy and activation volume values that support a mechanism dominated by a bond formation process. Volume profiles for the binding of NO appear to be a valuable approach to explain the differences observed for P450cam in the absence and presence of the substrate and enable the clarification of the underlying reaction mechanisms at a molecular level. Changes in spin state of the iron center during the binding/release of NO contribute significantly to the observed volume effects. The results are discussed in terms of relevance for the biological function of cytochrome P450 and in context to other investigations of the related reactions between NO and imidazole- and thiolate-ligated iron(III) hemoproteins.

Effects of nitric oxide on aldosterone synthesis and nitric oxide synthase activity in glomerulosa cells from bovine adrenal gland

This study investigated the effects of two NO-releasing agents, diethylenetriamine-NO (deta-NO) and sodium nitroprusside (SNP), on basal, ACTH-, and angiotensin II (AngII)-stimulated aldosterone production in glomerulosa cells from bovine adrenal gland. NO donors inhibited basal and ACTH- or AngII-stimulated aldosterone synthesis in a concentration-dependent manner. Deta-NO and SNP also provoked a concentration-dependent stimulation of cGMP production. However, cGMP was not responsible for the inhibition of aldosterone secretion, because a cGMP analog did not reproduce the inhibitory effect. Moreover, soluble guanylyl cyclase or protein kinase G inhibitors did not revert the inhibitory effect of NO on aldosterone production. NO donors did not modify ACTH-stimulated cAMP production or AngII-stimulated PLC activity stimulation, but inhibited 22[R] hydroxycholesterol- or pregnenolone-stimulated aldosteronogenesis. NO can be synthesized in bovine glomerulosa cells because nitrite production was determined and characterization of NOS activity was also performed. Nitrite accumulation was not modified in the presence of ACTH, AngII, or other factors used to induce iNOS. NOS activity that showed a Michaelis-Menten kinetic was NADPH- and calcium-dependent and was inhibited by two competitive inhibitors, L-NAME and L-NMMA. These results show that NO inhibits aldosterone production in glomerulosa cells acting on P450scc and other P450-dependent steroidogenic enzymes, and these cells display NOS activity suggesting that NO can be produced by constitutive NOS isozymes.

Characterization and application of Raman labels for confocal Raman microspectroscopic detection of cellular proteins in single cells

A method using confocal Raman microspectroscopy for the detection of cellular proteins in single intact cells was developed. Two approaches were used to improve the detection of these cellular components. First, compounds with high Raman scattering were investigated for potential use as Raman labels. Raman labels were conjugated to either biomolecules or biotin and used as markers in the detection of cellular enzymes and receptors. Second, silver colloids were used to increase the surface-enhanced Raman scatter (SERS) of these Raman labels. Cresyl violet and dimethylaminoazobenzene are Raman labels that provide very sensitive SERS detection by a confocal Raman microscope with a HeNe laser at wavelength of 632.8 nm. The detection of 12-lipoxygenase and cyclooxygenase-1 in single bovine coronary artery endothelial cells and the binding of angiotensin II to its receptors in zona glomerulosa cells was demonstrated.

cGMP-dependent protein kinase type II regulates basal level of aldosterone production by zona glomerulosa cells without increasing expression of the steroidogenic acute regulatory protein gene

The renin-angiotensin-aldosterone system plays a pivotal role in the regulation of salt and water homeostasis. Here, we demonstrate the expression and functional role of cGMP-dependent protein kinases (PKGs) in rat adrenal cortex. Expression of PKG II is restricted to adrenal zona glomerulosa (ZG) cells, whereas PKG I is localized to the adrenal capsule and blood vessels. Activation of the aldosterone system by a low sodium diet up-regulated the expression of PKG II, however, it did not change PKG I expression in adrenal cortex. Both, activation of PKG II in isolated ZG cell and adenoviral gene transfer of wild type PKG II into ZG cells enhanced aldosterone production. In contrast, inhibition of PKG II as well as infection with a PKG II catalytically inactive mutant had an inhibitory effect on aldosterone production. Steroidogenic acute regulatory (StAR) protein that regulates the rate-limiting step in steroidogenesis is a new substrate for PKG II and can be phosphorylated by PKG II in vitro at serine 55/56 and serine 99. Stimulation of aldosterone production by PKG II in contrast to stimulation by PKA did not activate StAR gene expression in ZG cells. The results presented indicate that PKG II activity in ZG cells is important for maintaining basal aldosterone production.

Caspase-independent cell death by low concentrations of nitric oxide in PC12 cells: involvement of cytochrome C oxidase inhibition and the production of reactive oxygen species in mitochondria

We reported previously that low levels of nitric oxide (NO) induced cell death with properties of apoptosis, including chromatin fragmentation and condensation in undifferentiated PC12 pheochromocytoma cells. The present study demonstrates that cytotoxicity of low concentrations of NO is mediated by inhibition of mitochondrial cytochrome c oxidase and generation of reactive oxygen species (ROS). An NO donor, (+/-)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR3) induced cell death even at low concentrations (10-100 microM), whereas peroxynitrite and a peroxynitrite generator, 3-(4-morpholinyl)-sydnonimine (SIN-1), did not have a significant effect on cell viability up to a concentration of 0.5 mM. The NOR3-induced cell death was unaffected by pretreatment with superoxide dismutase (SOD) or its mimetic peroxynitrite scavenger, manganese(III) tetrakis(benzoic acid)porphyrin chloride (Mn-TBAP), or with uric acid. These findings indicate that peroxynitrite does not contribute to this cell death. Furthermore, neither the release of cytochrome c from mitochondrial membranes, the cleavage of poly-ADP ribose polymerase (PARP), nor the activation of caspase-3-like activities was observed. Inhibitors of PARP, benzamide, and aminobenzamide, had no effect on the NOR3-induced cell death. In addition, pretreatment with general or selective caspase inhibitors, benzyloxy-carbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD-fmk), N-acetyl-Asp-Glu-Val-Asp-aldehyde (Ac-DEVD-CHO), and benzyloxycarbonyl-Asp-2,6-dichlorobenzoyloxymethylketone (Z-Asp-Ch(2)-DCB) did not prevent NOR3-induced cell death. Taken together, these findings suggest that cell death induced by NOR3 occurs by a caspase-independent mechanism. In contrast, we found an early increase in mitochondrial H(2)O(2) production during NOR3 exposure using the fluorescent dye 2',7'-dichlorofluorescin-diacetate (DCFH-DA) and dihydrorohdamine123 (DHR123), and these events were accompanied by strong inhibition of cytochrome c oxidase activity in the cells. Furthermore, we observed that several antioxidants, such as ascorbate, glutathione (GSH), cysteine, tetrahydrobiopterin, and dithiothreitol (DTT), all effectively prevented the NOR3-induced cell death. NOR3 treatment decreased the level of total intracellular GSH, but did not affect the activities of antioxidant enzymes SOD, GSH-peroxidase (GPX), and catalase. These results suggest that cell death induced at physiologically low concentrations of NO is mediated by ROS production in mitochondria, most likely resulting from the inhibition of cytochrome c oxidase, with ROS acting as an initiator of caspase-independent cell death.

Overexpression of V-1 prevents nitric oxide-induced cell death: involvement of enhanced tetrahydrobiopterin biosynthesis

Previously we reported that the synthesis of catecholamines, dopamine, and noradrenaline was enhanced by overexpression of V-1 protein, a neuronal protein active in the initial stage of development of the rat cerebellum, in the neuronal cell line PC12D, a model of dopamine cells (Yamakuni et al. [1998] J. Biol. Chem. 273:27051-27054). To investigate the physiological role of this protein, we examined the effect of V-1 overexpression on cell toxicity induced by nitric oxide (NO) used at low concentrations. Two clones of PC12D cells overexpressing V-1, transfectants termed V1-46 and V1-69, were significantly more resistant to NOR3 (an NO donor) but not to etoposide (an inhibitor of topoisomerase II)-induced apoptotic cell death than the control cells (termed C-7 and C-9) that had been transfected with the vector alone. The addition of L-DOPA, dopamine, or noradrenaline to the medium did not abolish NOR3-induced cell death in PC12D cells. Moreover, pretreatment of V1-46 and V1-69 cells with L-alpha-methyl-p-tyrosine (alpha-MPT), an inhibitor of tyrosine hydroxylase, to inhibit catecholamine biosynthesis did not affect the resistance to NO toxicity. These results indicate that the catecholamine levels increased by V-1 overexpression did not produce the protection against NOR3-induced toxicity. We further showed that overexpression of V-1 enhanced the synthesis of (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH(4)). In addition, pretreatment with BH(4) or with sepiapterin, which is converted to BH(4) intracellularly, significantly protected PC12D cells in a dose-dependent manner. The increased BH(4) synthesis by V-1 overexpression was dose dependently inhibited by pretreatment with diaminohydroxypyrimidine (DAHP), an inhibitor of GTP-cyclohydrolase I, which is the rate-limiting enzyme for the biosynthesis of BH(4), concomitantly with the loss of protective effect afforded by V-1 overexpression. Furthermore, the addition of BH(4) or sepiapterin to DAHP-pretreated V146 and V1-69 cells restored cell viability. Taken together, these results indicate that V1 protein plays an important role in protection against cell death induced by NO at low levels by promoting the synthesis of BH(4). Moreover, these findings suggest the up-regulation of V1 expression as a possible therapeutic target for protection against the insult of NO-induced oxidative stress.

Expression of nitric oxide synthases in rat adrenal zona fasciculata cells

Nitric oxide (NO) synthase (NOS) expression was analyzed in rat adrenal zona fasciculata. Both neuronal NOS and endothelial NOS mRNAs were detected by RT-PCR, immunohistochemistry, and immunoblot analysis. The biochemical characterization of adrenal zona fasciculata NOS enzymatic activity confirmed the presence of a constitutive isoform. In a cell line derived from mouse adrenal cortex, only endothelial NOS expression was detected by both RT-PCR and immunoblot analysis. Nitrate plus nitrite levels in Y1 cell incubation medium were increased in the presence of L-arginine and the calcium ionophore A23187, but not D-arginine, indicating enzymatic activity. Moreover, a low, but significant, conversion of Larginine to L-citrulline, abolished by the NOS inhibitor, N(G)-nitro-L-arginine, was detected in Y1 cells. The effect of L-arginine on pregnenolone production was examined. L-Arginine decreased both basal and ACTH-stimulated pregnenolone production in Y1 cells. The inhibitory effect of L-arginine could be attributed to endogenously generated NO, because it was blocked by N(G)-nitro-L-arginine, and it was mimicked by the addition of a NO donor, diethylenetriamine-NO. An inhibitory effect of NO on pregnenolone production from 22Rhydroxycholesterol and on steroidogenic acute regulatory protein expression was also determined. Taken together, these results suggest that at least part of the adrenal NO could derive from steroidogenic cells and modulate their function.

Nitric oxide-dependent damage to neuronal mitochondria involves the NMDA receptor

Cytokine-stimulated astrocytes produce nitric oxide, which can inhibit components of the mitochondrial respiratory chain. We have previously demonstrated that prolonged exposure (48 h) to rat astrocytic nitric oxide damages complexes II--III and IV of neighbouring rat neurons in coculture, resulting in neuronal death. Expanding on these observations, we have now shown that the NMDA receptor antagonist, MK-801, prevents this damage, suggesting involvement of glutamate. We postulate that astrocyte-derived nitric oxide stimulates release of neuronal glutamate. Indeed we demonstrate that neurons incubated with nitric oxide-generating astrocytes display enhanced glutamate release. Furthermore, direct exposure to the nitric oxide donor, DETA-NONOate resulted in a loss of activity of all the neuronal mitochondrial complexes, which was again prevented by MK-801. Thus, nitric oxide, generated by both cytokine-stimulated astrocytes and by a nitric oxide donor, causes activation of the NMDA receptor leading to damage to the neuronal mitochondrial respiratory chain. Glutamate exposure is known to damage the neuronal mitochondrial respiratory chain via neuronal nitric oxide synthase. Therefore, we propose that astrocyte-derived nitric oxide is capable of eliciting neuronal glutamate release, which in turn activates the neuronal NMDA receptor and stimulates further formation of reactive nitrogen species via neuronal nitric oxide synthases, leading to mitochondrial damage and neuronal death. Our findings support the hypothesis that glutamate, reactive nitrogen species and mitochondrial dysfunction may have a role in the neurodegenerative process.

Mineralocorticoids upregulate arterial contraction to epidermal growth factor

The present studies test the hypothesis that contraction to EGF is dependent on mineralocorticoids and/or an elevation in systolic blood pressure (SBP). Endothelium-denuded thoracic aortas from sham normotensive, N(omega)-nitro-L-arginine (L-NNA) hypertensive, Wistar-Kyoto (WKY), and spontaneously hypertensive rats (SHR) were used in isolated tissue-bath experiments. Maximal contraction to epidermal growth factor [EGF; percentage of phenylephrine (PE; 10 umol/l)-induced contraction] was greater in strips from L-NNA (32 +/- 5%) and SHR (53 +/- 8%) rats compared with sham and WKY rats (17 +/- 1 and 12 +/- 4%, respectively). Wistar-Furth rats became only mildly hypertensive when given DOCA salt (134 +/- 6 mmHg) compared with Wistar rats (176 +/- 9 mmHg), but aortas from both strains had a similarly enhanced contraction to EGF (approximately 9 times the maximal contraction of sham aorta). Furthermore, in vitro incubation of aortas from Wistar and Wistar-Furth rats with aldosterone (10 nmol/l) increased EGF-receptor mRNA expression by >50%. These data indicate that arterial contraction to EGF may occur independent of hypertension and be stimulated by mineralocorticoids.

Local inhibition of nitric oxide temporarily stimulates aldosterone secretion in conscious sheep in vivo

The effect of localized blockage of endogenous nitric oxide (NO) on basal aldosterone secretion was studied in conscious sheep with autotransplanted adrenal glands. We have shown that infusion of the NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 130 microg/l blood flow) significantly stimulated basal aldosterone secretion rate (ASR). This stimulatory effect was seen up to 4 h of infusion. Beyond this time point, however, the elevated ASR level was not sustained, and it was seen to drop markedly to lower than control values at 5 h. L-NAME had no effect on cortisol secretion rate (FSR) during the first 4 h of infusion, but a significant reduction in FSR was seen by the 8-h time point. Adrenal blood flow was consistently decreased in association with long L-NAME infusion. Additionally, L-NAME was shown to have no effect on aldosterone secretion when infused systemically. We conclude that the relationship between NO and aldosterone secretion is an inhibitory one, in which NO seems to have a negative effect on basal aldosterone secretion.

Decreased steroid hormone synthesis from inorganic nitrite and nitrate: studies in vitro and in vivo

Nitrites and nitrates are consumed nonchalantly in diet. Organic nitrates are also used as vasodilators in angina pectoris, but the therapy is associated with tolerance whose mechanism remains elusive. Previously, we found inorganic nitrate inhibited steroidogenesis in vitro. Because adrenocorticoids regulate water and electrolyte metabolism, tolerance may ensue from steroid deficiency. We have studied the effects of nitrite and nitrate on in vitro synthesis and in vivo blood levels of steroid hormones. In vitro, nitrite was more potent than nitrate in inhibiting human chorionic gonadotropin (hCG)-stimulated androgen synthesis by Mouse Leydig Tumor cells. At concentrations above 42 mM, nitrite completely inhibited androgen synthesis, and, unlike nitrate, the inhibition was irreversible by increasing hCG concentration. The cAMP production remained intact but reduced with both ions. The nitric oxide (NO) scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxy-3-oxide (c-PTIO) significantly increased hCG- or cAMP-stimulated androgen synthesis in all buffers, suggesting that NO is a chemical species directly involved in the nitrite/nitrate-induced inhibition. This is further supported by c-PTIO countering the inhibitory action of methylene blue on androgen synthesis. Rats given distilled water containing 50 mg/L NaNO(2) or NaNO(3) for 4 weeks drank significantly less daily. At the end, their blood corticosterone and testosterone levels were significantly decreased. The adrenocortical histology showed bigger lipid droplets, which are pathogonomic of impaired steroidogenesis. Nitrite and nitrate are metabolized to NO, which binds heme in cytochrome P450 enzymes, thereby inhibiting steroidogenesis. Therapeutic nitrates likewise may decrease adrenal (and gonadal) steroidogenesis. Cortisol deficiency would impair water excretion causing volume expansion, and aldosterone deficiency would cause sodium loss and raised renin. Paradoxically, volume expansion without sodium retention and raised renin has all been reported in tolerance.

Endothelial cell nitric oxide inhibits aldosterone synthesis in zona glomerulosa cells: modulation by oxygen

The regulation of aldosterone synthesis by endogenous nitric oxide (NO) was examined in cultured cells of the adrenal cortex. Endothelial NO synthase (eNOS) was detected by Western blot in cultured adrenal endothelial cells (ECs) but not in zona glomerulosa (ZG) cells or adrenal fibroblasts. Neither inducible (iNOS) nor neuronal NOS (nNOS) isoforms were detected in the cells. Only ECs had NOS activity and converted [(3)H]L-arginine to [(3)H]L-citrulline. Angiotensin II (ANG II, 100 nM) increased EC production of nitrate/nitrite by 2.4-fold. Coincubation with ECs or treatment with DETA nonoate increased the fluorescence of ZG cells loaded with an NO-sensitive dye, diaminofluorescein 2 diacetate (DAF-2 DA). DETA nonoate inhibited ANG II (1 nM) and potassium (10 mM) -stimulated aldosterone release in a concentration-related manner. This inhibitory effect of NO was enhanced >10-fold by decreasing the oxygen concentration from 21 to 8%. Coincubation of EC and ZG cells in 8% oxygen inhibited ANG II-induced aldosterone release, and inhibition was reversed by blockade of NOS. These findings indicate that adrenal EC-derived NO inhibits aldosterone release by cultured ZG cells and that the sensitivity to NO inhibition is increased at low oxygen concentrations.

Is steroid deficiency the cause of tolerance in nitrate therapy?

The award of the Nobel Prize in Physiology and Medicine for 1998 bears witness to the 'explosive' field of nitric oxide (NO), and who would have thought the explosive nitroglycerin owed its therapeutic effectiveness to this little molecule? NO is also involved in causing penile erection, which has brought sildenafil to the aid of patients with erectile dysfunction. However, emerging evidence in animals and in vitro studies indicates that NO also inhibits steroidogenesis, which may have repercussions in humans. The decrease in androgen secretion may impact on secondary sexual characteristics, including penile size. The tolerance to the nitrate therapy in angina, characterized by volume expansion and not due to sodium retention, may also be related to steroid hormone deficiency. Decreased cortisol secretion may impair water excretion, resulting in volume expansion. Impaired aldosterone secretion would cause hyponatraemia with resultant raised renin. I hypothesize that continuous therapy with nitrates and sildenafil will result in diminished levels of steroid hormones with predicted sequelae.