Sharon L.H. Ong

How Do Glucocorticoids Cause Hypertension: Role of Nitric Oxide Deficiency, Oxidative Stress, and Eicosanoids

The exact mechanism by which glucocorticoid induces hypertension is unclear. Several mechanisms have been proposed, although there is evidence against the role of sodium and water retention as well as sympathetic nerve activation. This review highlights the role of nitric oxide-redox imbalance and their interactions with arachidonic acid metabolism in glucocorticoid-induced hypertension in humans and experimental animal models.

REACTIVE OXYGEN SPECIES AND GLUCOCORTICOID-INDUCED HYPERTENSION

1 There is increasing evidence for a role of oxidative stress and nitric oxide deficiency in experimental glucocorticoid‐induced hypertension, as evidenced by increased biomarkers of oxidative stress; the effectiveness of antioxidants or reduced NADPH oxidase antagonists in lowering blood pressure; and secondary upregulation of endogenous antioxidant enzymes in response to oxidative stress. 2 In the vasculature, the main sources of superoxide are NADPH oxidase, xanthine oxidase, uncoupled endothelial nitric oxide synthase (eNOS) and mitochondria. 3 NADPH oxidase plays a significant role in the pathogenesis of glucocorticoid‐induced hypertension in the rats, but xanthine oxidase and uncoupled eNOS pathways are not important sources of reactive oxygen species in these models. The role of mitochondrial reactive oxygen species in glucocorticoid‐induced hypertension remains to be clarified.

The role of 20-hydroxyeicosatetraenoic acid in adrenocorticotrophic hormone and dexamethasone-induced hypertension

Objective 20-hydroxyeicosatetraenoic acid (20-HETE) is a potent constrictor in small arteries and also has natriuretic properties. Urinary 20-HETE excretion is increased in adrenocorticotrophic hormone (ACTH)-induced hypertensive rats. In the present study, we investigated the effect of a specific enzyme inhibitor of 20-HETE production, N-hydroxy-N′-(4-butyl-2-methylphenyl) formamidine (HET0016), on glucocorticoid-induced hypertension in rats, a sodium-independent model. Methods Male Sprague–Dawley rats were treated with physiological saline (0.9% NaCl), ACTH (0.2 mg/kg per day) or dexamethasone (0.03 mg/rat per day) subcutaneously for 13 days. HET0016 (10 mg/kg per day) or its vehicle (10% lecithin in physiological saline) was coadministered (intraperitoneally) a day before (prevention study) or at day 8 of treatment (reversal studies). Systolic blood pressure was measured by the tail–cuff method. Results Relative to physiological saline, systolic blood pressure was increased by ACTH (P < 0.001) and dexamethasone (P < 0.01). HET0016 reversed ACTH-induced (P < 0.01) but not dexamethasone-induced hypertension. HET0016 also prevented the development of hypertension induced by ACTH (P < 0.01). ACTH, but not dexamethasone, increased renal microsome 20-HETE formation and plasma F2-isoprostane concentrations. HET0016 inhibited renal 20-HETE formation but had no effect on plasma F2-isoprostane concentrations or renal cytochrome P450 4A1 expression. Conclusion Inhibition of 20-HETE production by HET0016 prevents and reverses ACTH-induced but not dexamethasone-induced hypertension. These results suggest that 20-HETE may play a role in the genesis of ACTH-induced hypertension but not in dexamethasone-induced hypertension.

Hemodynamics of dexamethasone-induced hypertension in the rat

Although dexamethasone (DEX) is known to cause hypertension in humans and in animals, the hemodynamic characteristics of DEX-induced hypertension (DEX-HT) in the rat remain unclear. This study evaluated central and regional hemodynamics, and the role of total peripheral resistance (TPR) using a vasodilator minoxidil. Rats were divided into four groups, namely saline (n=20), DEX (n=21), minoxidil+saline (n=10) and minoxidil+DEX (n=10). Tail-cuff systolic blood pressure was recorded every second day. After 10–14 days of treatment, central (saline: n=9, DEX: n=10) and regional (saline: n=11, DEX: n=11) hemodynamic parameters were measured. Central hemodynamic data were also obtained from minoxidil-treated rats. DEX increased blood pressure (P<0.0005) in association with an increase in TPR (P<0.05). However, individual assessments of renal, mesenteric and hindquarter circulations did not detect any significant increase in resistance in these beds. Minoxidil increased cardiac output (P′<0.01) and cardiac index (P′<0.005) as well as decreased TPR (P′<0.05) without affecting DEX-HT. DEX prevented weight gain and decreased thymus weight. The increase in TPR in DEX-HT in rats was not simply explained by isolated alterations to resistance in the renal, mesenteric or hindquarter circulations. Minoxidil effectively prevented the increase in TPR but not the increase in blood pressure, suggesting that an increase in TPR is not essential for DEX-induced blood pressure increase.

Mechanisms of Dexamethasone-Induced Hypertension

Hypertension is a well-recognized complication of excess glucocorticoids, both naturally-occurring and synthetic. Dexamethasone is a potent synthetic glucocorticoid, which has widespread clinical applications. As dexamethasone has purely glucocorticoid activity with negligible mineralocorticoid effects, dexamethasone-induced hypertension (DEXHT) models have been used for studying the mechanisms of glucocorticoid-induced hypertension. This review examines the characteristics and mechanisms of DEX-HT, both in the human and experimental animal models. The roles of hemodynamics, volume, renin-angiotensin-aldosterone system, sympathetic nervous system, vasodilators including nitric oxide, vasoconstrictors and reactive oxygen species in the pathogenesis of DEX-HT are reviewed and differences from hypertension due to naturally occurring steroids discussed.

Effects of Sepiapterin Supplementation and NOS Inhibition on Glucocorticoid-Induced Hypertension

BACKGROUND Glucocorticoid-induced hypertension is associated with imbalance between nitric oxide (NO) and superoxide. One of the pathways that causes this imbalance is endothelial NO synthase (eNOS) uncoupling. In the present study, adrenocorticotrophic hormone (ACTH)- and dexamethasone-treated rats were further treated with sepiapterin, a precursor of tetrahydrobiopterin, or N-nitro-L-arginine (NOLA), an inhibitor of NOS, to investigate the role of eNOS uncoupling in glucocorticoid-induced hypertension. METHODS Male Sprague-Dawley (SD) rats (n = 7-13/group) were treated with either sepiapterin (5 mg/kg/day, IP) or saline (sham) 4 days before and during ACTH (0.2 mg/kg/day, SC), dexamethasone (0.03 mg/kg/day, SC), or saline treatment. NOLA (0.4 mg/ml in drinking water) was given to rats 4 days before and during dexamethasone treatment. Systolic blood pressure (SBP) was measured by the tail-cuff method. RESULTS Both ACTH (116 +/- 2 to 135 +/- 3 mm Hg (mean +/- s.e.m.), P < 0.001) and dexamethasone (114 +/- 4 to 133 +/- 3 mm Hg, P < 0.0005) increased SBP. Sepiapterin alone did not alter SBP. Sepiapterin did not prevent ACTH- (129 +/- 4 mm Hg, NS) or dexamethasone-induced hypertension (135 +/- 3 mm Hg, NS), although plasma total biopterin concentrations were increased. NOLA increased SBP in rats prior to dexamethasone or saline treatment. NOLA further increased SBP in both saline- (133 +/- 4 to 157 +/- 3 mm Hg, P < 0.05) and dexamethasone-treated rats (135 +/- 5 to 170 +/- 6 mm Hg, P < 0.05). ACTH and dexamethasone increased plasma F(2)-isoprostane concentrations. Neither sepiapterin nor NOLA significantly affected this marker of oxidative stress. CONCLUSION Sepiapterin did not prevent ACTH- or dexamethasone-induced hypertension. NOLA exacerbated dexamethasone-induced hypertension. These data suggest that eNOS uncoupling does not play a major role in the genesis of glucocorticoid-induced hypertension in the rat.

The Effect of Alpha-Lipoic Acid on Mitochondrial Superoxide and Glucocorticoid-Induced Hypertension

Aims. To examine the effect of alpha-lipoic acid, an antioxidant with mitochondrial superoxide inhibitory properties, on adrenocorticotrophic hormone- (ACTH-HT) and dexamethasone-induced hypertensions (DEX-HT) in rats and if any antihypertensive effect is mediated via mitochondrial superoxide inhibition. Methods. In a prevention study, rats received ground food or alpha-lipoic-acid-laced food (10 mg/rat/day) for 15 nights. Saline, adrenocorticotrophic hormone (ACTH, 0.2 mg/kg/day), or dexamethasone (DEX, 10 μg/rat/day) was injected subcutaneously from day 5 to day 11. In a reversal study, rats received alpha-lipoic-acid-laced food 4 days after commencement of saline or DEX. Tail-cuff systolic blood pressure (SBP) was measured second daily. Kidney mitochondrial superoxide was examined using (MitoSOX) Red (MitoSOX) via flow cytometry. Results. SBP was increased by ACTH (P < 0.0005) and DEX (P < 0.0005). Alpha-lipoic acid alone did not alter SBP. With alpha-lipoic acid pretreatment, SBP was increased by ACTH (P′ < 0.005) but not by DEX. Alpha-lipoic partially prevented ACTH-HT (P′ < 0.0005) and fully prevented DEX-HT (P′ < 0.0005) but failed to reverse DEX-HT. ACTH and DEX did not increase MitoSOX signal. In ACTH-hypertensive rats, high-dose alpha-lipoic acid (100 mg/rat/day) did not decrease SBP further but raised MitoSOX signal (P < 0.001), suggesting prooxidant activity. Conclusion. Glucocorticoid-induced hypertension in rats is prevented by alpha-lipoic acid via mechanisms other than mitochondrial superoxide reduction.

Blood pressure measurement in pregnancy: the effect of arm circumference and sphygmomanometer cuff size

This study aimed to assess the difference in blood pressure readings between the standard and large cuff and to determine if such a difference applies over a range of arm circumferences (ACs) in pregnancy. We measured blood pressure on 219 antenatal women. Six blood pressure readings were taken, three with a standard ‘adult’ and three with a ‘large’ cuff, in random order. A random zero sphygmomanometer was used by a trained observer. Women with an AC >33 cm were similar in age, gestational age and parity but were heavier and had more hypertension than those with AC ≤33 cm. There was a systematic difference between the standard and large cuff of 5–7 mmHg with little effect due to AC. We were unable to demonstrate an association between the standard and large cuff blood pressure difference and increasing blood pressure. Our study has shown that both systolic and diastolic blood pressure measurements are more dependent on the cuff size used than AC and for the individual it is difficult to predict the magnitude of effect the different cuff sizes will have on blood pressure measurements. This study has shown the presence of an average difference in blood pressure measurement between standard and large cuffs in pregnancy, and does not support the arbitrary 33 cm ‘cut-off’ recommended in guidelines for the use of a large cuff in pregnancy.

1100 ADEQUACY OF BLOOD PRESSURE CONTROL IN CHRONIC KIDNEY DISEASE

Background: In essential hypertension (EHT), clinic blood pressure measurement (CBP) misclassifies blood pressure (BP) in patients with white coat hypertension (WCHT) and masked hypertension (MHT). Accurate BP measurement in the chronic kidney disease (CKD) population is important in controlling cardiovascular complications and renal disease progression. The aim of this study was to characterise BP control in CKD population. Methods: 50 EHT and 387 CKD patients underwent CBP, ambulatory BP monitoring and central pressure measurement by pulse wave analysis (PWA). Patients were classified as having controlled hypertension (CHT), sustained hypertension (SHT), WCHT or MHT by comparing CBP to awake ambulatory BP average. BP control in the different CKD stages- stages 1 &2 (CKD-1,2, n = 153), stage 3 (CKD-3, n = 172) and stages 4 and 5 (CKD-4,5, n = 67) were compared with EHT (n = 50). Results: CKD-3 and CKD-4,5 were older than EH and CKD-1,2 (p′ < 0.01). There were no significant differences in the prevalence of CHT, SHT, WCHT or MHT; or type of antihypertensive used between EHT and CKD. Central pulse pressure (CPP) was lower in EHT than CKD-3 and CKD-4,5 (p′ < 0.01). Augmentation index was lower in EHT than all CKD groups (p′ < 0.01). Among the combined CKD groups, patients with CHT had significantly lower central systolic pressure and CPP than patients with SHT, WCHT and MHT (p′ < 0.05). However, central pressure measurements were not significantly different between MHT, WCHT and SHT. Conclusions: CBP misclassifies BP control in CKD and EHT. PWA may identify those patients with CHT but dose not distinguish between SHT, WCHT and MHT

883 BLOOD PRESSURE, FLUID VOLUME, VASCULAR AND VENTRICULAR FUNCTION IN CHRONIC KIDNEY DISEASE

Background: Hypertension (HT) is an early feature of renal disease, and is highly prevalent in patients with chronic kidney disease (CKD) of any stage. HT increases the risk of loss of residual renal function and is associated with increased cardiovascular morbidity and mortality in CKD patients. Fluid retention due to CKD is commonly attributed as a pathogenic cause of hypertension in this population. The aim of this study was to investigate relationships between fluid volume, vascular and ventricular function in different CKD stages. Methods: Patients with essential hypertension (EHT, n = 17), CKD stages 3b-5 (CKD, n = 53), renal transplantation (Tx, n = 31) and peritoneal dialysis (PD, n = 21) underwent ambulatory blood pressure monitoring, augmentation index (AI), cardiac echocardiogram and total body water (TBW) measurement. Results: There was no difference in medication use between groups. 24 hour average diastolic pressure was lower in CKD than EHT, PD and Tx (68 ± 1 vs 76 ± 3* vs 75 ± 3* vs 75 ± 2*mmHg, *p’ < 0.05). AI was lower in Tx than EHT and CKD (17 ± 2 vs 25 ± 2* vs 30 ± 3*, *p’ < 0.05) but not PD (19 ± 3). TBW was higher in CKD than Tx and PD (34 ± 2 vs 15 ± 3* vs 14 ± 4*L, *p’ < 0.05) but not EHT (27 ± 4L, NS). Left ventricular diastolic dysfunction (LVDD) was less prevalent in EHT than CKD, Tx and PD (33% vs 75%* vs 58%* vs 89%*, *p < 0.005) Conclusions: CKD is accompanied by an increase in TBW, AI and LVDD, even when blood pressure is well-controlled. TBW and AI may be reduced by Tx and PD, however, there is persisting LVDD.