Hypertension caused by a truncated epithelial sodium channel gamma subunit: genetic heterogeneity of Liddle syndrome

Sensitivity of blood pressure to dietary salt is a common feature in subjects with hypertension. These features are exemplified by the mendelian disorder, Liddle's syndrome, previously shown to arise from constitutive activation of the renal epithelial sodium channel due to mutation in the beta subunit of this channel. We now demonstrate that this disease can also result from a mutation truncating the carboxy terminus of the gamma subunit of this channel; this truncated subunit also activates channel activity. These findings demonstrate genetic heterogeneity of Liddle's syndrome, indicate independent roles of beta and gamma subunits in the negative regulation of channel activity, and identify a new gene in which mutation causes a salt-sensitive form of human hypertension.

Renin-dependent cardiovascular functions and renin-independent blood-brain barrier functions revealed by renin-deficient mice

Renin plays a key role in controlling blood pressure through its specific cleavage of angiotensinogen to generate angiotensin I (AI). Although possible existence of the other angiotensin forming enzymes has been discussed to date, its in vivo function remains to be elucidated. To address the contribution of renin, we generated renin knockout mice. Homozygous mutant mice show neither detectable levels of plasma renin activity nor plasma AI, lowered blood pressure 20-30 mm Hg less than normal, increased urine and drinking volume, and altered renal morphology as those observed in angiotensinogen-deficient mice. We recently found the decreased density in granular layer cells of hippocampus and the impaired blood-brain barrier function in angiotensinogen-deficient mice. Surprisingly, however, such brain phenotypes were not observed in renin-deficient mice. Our results demonstrate an indispensable role for renin in the circulating angiotensin generation and in the maintenance of blood pressure, but suggest a dispensable role for renin in the blood-brain barrier function.

A genetic defect resulting in mild low-renin hypertension

Severe low-renin hypertension has few known causes. Apparent mineralocorticoid excess (AME) is a genetic disorder that results in severe juvenile low-renin hypertension, hyporeninemia, hypoaldosteronemia, hypokalemic alkalosis, low birth weight, failure to thrive, poor growth, and in many cases nephrocalcinosis. In 1995, it was shown that mutations in the gene (HSD11B2) encoding the 11beta-hydroxysteroid dehydrogenase type 2 enzyme (11beta-HSD2) cause AME. Typical patients with AME have defective 11beta-HSD2 activity, as evidenced by an abnormal ratio of cortisol to cortisone metabolites and by an exceedingly diminished ability to convert [11-3H]cortisol to cortisone. Recently, we have studied an unusual patient with mild low-renin hypertension and a homozygous mutation in the HSD11B2 gene. The patient came from an inbred Mennonite family, and though the mutation identified her as a patient with AME, she did not demonstrate the typical features of AME. Biochemical analysis in this patient revealed a moderately elevated cortisol to cortisone metabolite ratio. The conversion of cortisol to cortisone was 58% compared with 0-6% in typical patients with AME whereas the normal conversion is 90-95%. Molecular analysis of the HSD11B2 gene of this patient showed a homozygous C-->T transition in the second nucleotide of codon 227, resulting in a substitution of proline with leucine (P227L). The parents and sibs were heterozygous for this mutation. In vitro expression studies showed an increase in the Km (300 nM) over normal (54 nM). Because approximately 40% of patients with essential hypertension demonstrate low renin, we suggest that such patients should undergo genetic analysis of the HSD11B2 gene.

Increased energy expenditure, dietary fat wasting, and resistance to diet-induced obesity in mice lacking renin

An overactive renin-angiotensin system is associated with obesity and the metabolic syndrome. However, the mechanisms behind it are unclear. Cleaving angiotensinogen to angiotensin I by renin is a rate-limiting step of angiotensin II production, but renin is suggested to have angiotensin-independent effects. We generated mice lacking renin (Ren1c) using embryonic stem cells from C57BL/6 mice, a strain prone to diet-induced obesity. Ren1c(-/-) mice are lean, insulin sensitive, and resistant to diet-induced obesity without changes in food intake and physical activity. The lean phenotype is likely due to a higher metabolic rate and gastrointestinal loss of dietary fat. Most of the metabolic changes in Ren1c(-/-) mice were reversed by angiotensin II administration. These results support a role for angiotensin II in the pathogenesis of diet-induced obesity and insulin resistance.

Pathophysiology of protracted acute renal failure in man

Postischemic acute renal failure (ARF) induced by cardiac surgery is commonly prolonged and may be irreversible. To examine whether persistence of postischemic, tubular cell injury accounts for delayed recovery from ARF, we studied 10 patients developing protracted (36 +/- 4 d) ARF after cardiac surgery. The differential clearance and excretion dynamics of probe solutes of graded size were determined. Inulin clearance was depressed (5.0 +/- 1.7 ml/min), while the fractional urinary clearance of dextrans (radii 17-30 A) were elevated above unity. Employing a model of conservation of mass, we calculated that 44% of filtered inulin was lost via transtubular backleak. The clearance and fractional backleak of technetium-labeled DTPA ([99mTc]DTPA, radius = 4 A) were identical to those of inulin (radius 15 A). The time at which inulin or DTPA excretion reached a maximum after an intravenous bolus injection was markedly delayed when compared with control subjects with ARF of brief duration, 102 vs. 11 min. Applying a three-compartment model of inulin/DTPA kinetics (which takes backleak into account) revealed the residence time of intravenously administered inulin/DTPA in the compartment occupied by tubular fluid and urine to be markedly prolonged, 20 vs. 6 min in controls, suggesting reduced velocity of tubular fluid flow. We conclude that protracted human ARF is characterized by transtubular backleak of glomerular ultrafiltrate, such that inulin clearance underestimates true glomerular filtration rate by approximately 50%, and by sluggish tubular fluid flow, which strongly suggests the existence of severe and generalized intraluminal tubular obstruction. Because all patients also exhibited extreme hyperreninemia (16 +/- 2 ng/ml per h) that was inversely related to inulin clearance (r value = -0.83) and urine flow (r value = -0.70), we propose that persistent, angiotensin II-mediated renal vasoconstriction may have delayed healing of the injured tubular epithelium.

Brain-targeted (pro)renin receptor knockdown attenuates angiotensin II-dependent hypertension

The (pro)renin receptor is a newly discovered member of the brain renin-angiotensin system. To investigate the role of brain (pro)renin receptor in hypertension, adeno-associated virus-mediated (pro)renin receptor short hairpin RNA was used to knockdown (pro)renin receptor expression in the brain of nontransgenic normotensive and human renin-angiotensinogen double-transgenic hypertensive mice. Blood pressure was monitored using implanted telemetric probes in conscious animals. Real-time PCR and immunostaining were performed to determine (pro)renin receptor, angiotensin II type 1 receptor, and vasopressin mRNA levels. Plasma vasopressin levels were determined by ELISA. Double-transgenic mice exhibited higher blood pressure, elevated cardiac and vascular sympathetic tone, and impaired spontaneous baroreflex sensitivity. Intracerebroventricular delivery of (pro)renin receptor short-hairpin RNA significantly reduced blood pressure, cardiac and vasomotor sympathetic tone, and improved baroreflex sensitivity compared with the control virus treatment in double-transgenic mice. (Pro)renin receptor knockdown significantly reduced angiotensin II type 1 receptor and vasopressin levels in double-transgenic mice. These data indicate that (pro)renin receptor knockdown in the brain attenuates angiotensin II-dependent hypertension and is associated with a decrease in sympathetic tone and an improvement of the baroreflex sensitivity. In addition, brain-targeted (pro)renin receptor knockdown is associated with downregulation of angiotensin II type 1 receptor and vasopressin levels. We conclude that central (pro)renin receptor contributes to the pathogenesis of hypertension in human renin-angiotensinogen transgenic mice.

Renin-1 is essential for normal renal juxtaglomerular cell granulation and macula densa morphology

The secretion of renin from granules stored in renal juxtaglomerular cells plays a key role in blood pressure homeostasis. The synthesis and release of renin and the extent of granulation is regulated by several mechanisms including signaling from the macula densa, neuronal input, and blood pressure. Through the use of a gene-targeting vector containing homology arms generated using the polymerase chain reaction, we have inactivated the Ren-1(d) gene, one of two mouse genes encoding renin, and report that lack of renin-1(d) results in altered morphology of the macula densa of the kidney distal tubule and complete absence of juxtaglomerular cell granulation. Furthermore, Ren-1(d-/-) mice exhibit sexually dimorphic hypotension. The altered growth morphology of the macula densa in Ren-1(d)-null mice should provide a tool for the investigation of the JG cell-macula densa signaling. Furthermore, the current data indicate that expression of the Ren-1(d) gene is a prerequisite for the formation of storage granules, even though the related protein renin-2 is present in these mice, suggesting that renin-1(d) and renin-2 are secreted by distinct pathways in vivo.

Diagnosis and treatment of low-renin hypertension

Plasma renin levels can be used to classify hypertension. A significant proportion of hypertensive individuals display a low-renin profile and thus low-renin hypertension (LRH) requires appropriate diagnosis and treatment. LRH includes essential, secondary and genetic forms, the most common of which are low-renin essential hypertension and primary aldosteronism. Several studies have investigated the relationship between PRA status and clinical response to different antihypertensive therapies. The present review will discuss the differential diagnosis of LRH subtypes and the most appropriate treatment options based on the pathophysiological background of this condition.

Liddle syndrome: genetics and mechanisms of Na+ channel defects

Our current understanding of Na+ transport defects has been greatly expanded over the last several years and has provided new insights into unusual clinical syndromes resulting from mutations of specific ion transporters. These genetic disorders affect Na+ balance, with both Na+ retaining and Na+ wasting conditions being the consequence. A major focus of these studies has been the epithelial sodium channel (ENaC), which can be directly affected by mutations (eg, Liddle syndrome, autosomal recessive pseudohypoaldosteronism, type I) or by changes in the response to (autosomal recessive pseudohypoaldosteronism, type I), or production of mineralocorticoids (apparent mineralocorticoid excess syndrome, glucocorticoid-remediable aldosteronism). As a result, we now have clearly defined syndromes in which ENaC activity is "dysregulated" with subsequent development of disorders of systemic blood pressure that can be attributed to a primary renal mechanisms. The focus of the current review is on Liddle syndrome ("pseudoaldosteronism").

Evidence of prostacyclin deficiency in the syndrome of hyporeninemic hypoaldosteronism

Hyporeninemic hypoaldosteronism is an important cause of hyperkalemia and is characterized by low renin secretion. We found that prostacyclin, a potent vasodilator and renin secretagogue, was markedly reduced--as reflected by its stable urinary metabolite 6-keto-prostaglandin F1 alpha--in seven patients with hyporeninemic hypoaldosteronism as compared with seven matched controls with renal insufficiency and as compared with 12 normal volunteers (mean +/- SE, 42 +/- 7 vs. 185 +/- 37 and 164 +/- 20 ng per gram of creatinine, respectively; P less than 0.001). In contrast, renal prostaglandin E2 excretion was similar in all three groups. A low-dose infusion of calcium or norepinephrine (known stimulants of prostacyclin) increased renal prostacyclin release in normal subjects and controls with renal insufficiency. Neither agonist, however, increased the low basal prostacyclin excretion in the patients (49.6 +/- 11 [basal] vs. 62 +/- 20 [norepinephrine] and 47.5 +/- 16 [calcium]; P greater than 0.8). To evaluate the functional importance of the altered prostacyclin production, we studied the responses of renal blood flow and blood pressure to the calcium infusion. The calcium infusion did not alter blood pressure or renal blood flow in the normal subjects or the controls with renal insufficiency. In contrast, the same dose of calcium in the patients with hyporeninemic hypoaldosteronism produced a rise in mean blood pressure (from 91 +/- 6 to 104 +/- 8 mm Hg, P less than 0.05) and a fall in renal blood flow (from 673 +/- 58 to 560 +/- 42 ml per minute per 1.73 m2, P less than 0.05). These results indicate that a deficiency of prostacyclin could explain the low active-renin concentration and altered vasomotor tone seen in hyporeninemic hypoaldosteronism.

Primary adrenal insufficiency in childhood and adolescence: advances in diagnosis and management

OBJECTIVES

Primary adrenal insufficiency occurring in childhood and adolescence is due to abnormalities of gland development, gland responsiveness, and steroid biosynthesis or target organ response. Causes include autoimmune Addison's disease, tuberculosis, HIV, adrenoleukodystrophy, adrenal hypoplasia congenita and syndromes including triple A and IMAGe. We aimed to define the causes of adrenal insufficiency for a cohort of children in Melbourne.

METHODS

We reviewed the frequency and variety of presentation of primary adrenal insufficiency to the Royal Children's Hospital over the past 10 years through an audit of patient records, collating demographic information, presentation and investigations.

RESULTS

Sixteen cases (13 male, 3 female) of primary adrenal insufficiency were diagnosed at this hospital between January 1993 and July 2003. Median age at presentation was 7.7 years (range: birth to 14.8 years). Symptoms at presentation included weakness, increased pigmentation, abdominal pain, nausea, developmental delay or a reduction in school performance. Four patients presented with adrenal crisis. Median adrenocorticotrophic hormone (ACTH) at diagnosis was 246 pmol/L (range 30-969 pmol/L). Autoantibodies were positive in five patients. Five patients had elevation of very long chain fatty acids. Five patients were diagnosed with autoimmune adrenal insufficiency, five with adrenal hypoplasia congenita, five with adrenoleukodystrophy and one with IMAGe syndrome.

CONCLUSIONS

A high index of suspicion results in earlier detection and possible prevention of adrenal crisis with a reduction in associated morbidities. Definitive diagnosis is now possible for almost all cases of primary adrenal insufficiency using technologies for screening autoimmunity, adrenoleukodystrophy (ALD) and genetic screening.

AT1 receptor blockade is superior to conventional triple therapy in protecting against end-organ damage in Cyp1a1-Ren-2 transgenic rats with inducible hypertension

OBJECTIVE

In the present study we compared the effects of treatment with the AT1 receptor antagonist candesartan and of 'triple therapy' (hydralazine, hydrochlorothiazide, reserpine) on the course of blood pressure, cardiac hypertrophy and angiotensin II concentrations after induction of hypertension in transgenic rats with inducible expression of the mouse renin gene (Cyp1a1-Ren-2 rats).

METHODS

Hypertension was induced in Cyp1a1-Ren-2 rats through dietary administration of the natural xenobiotic indole-3-carbinol (I3C, 0.3%) for 4 days. Starting on the day before administration of I3C, rats were treated either with candesartan or received triple therapy for 9 days. Systolic blood pressure was measured in conscious animals. Rats were decapitated and angiotensin II levels in plasma and in whole kidney and left ventricular tissues were determined by radioimmunoassay.

RESULTS

Administration of I3C resulted in the development of severe hypertension and cardiac hypertrophy that was accompanied by marked elevations of plasma and tissue angiotensin II concentrations. Candesartan treatment prevented the development of hypertension and cardiac hypertrophy and was associated with a reduction of tissue angiotensin II concentrations. In contrast, triple therapy, despite maintaining systolic blood pressure in the normotensive range, did not prevent the development of cardiac hypertrophy and tissue angiotensin II augmentations.

CONCLUSIONS

Our findings indicate that hypertension in Cyp1a1-Ren-2 rats is a clearly angiotensin II-dependent model of hypertension with elevated circulating and tissue angiotensin II concentrations, and that antihypertensive treatment with AT1 receptor blockade is superior to conventional triple therapy in effective protection against hypertension-induced end-organ damage in this rat model.

Effects of angiotensin-converting enzyme inhibitor and angiotensin type 1 receptor antagonist in deoxycorticosterone acetate-salt hypertensive mice lacking Ren-2 gene

We previously reported that inhibition of angiotensin-converting enzyme (ACE) prevented the hypertension and left ventricular hypertrophy induced by deoxycorticosterone acetate-salt (DOCA-salt) in 129/SvEvTac mice, which have 2 renin genes (Ren-1 and Ren-2). In the present study, we induced hypertension by uninephrectomy and DOCA-salt in mice having only the Ren-1 gene (C57BL/6J) and investigated the effect of an ACE inhibitor (ramipril, 4 mg. kg(-)(1). d(-)(1)) and an angiotensin type 1 (AT(1)) receptor antagonist (L-158809, 4 mg. kg(-)(1). d(-)(1)) on the development of hypertension, cardiac hypertrophy, and renal injury. After 4 weeks of treatment, systolic blood pressure in DOCA-salt mice was significantly increased (128+/-2 mm Hg) compared with controls (109+/-2 mm Hg) (P:<0.001), while plasma renin concentration was decreased by 97% (P:<0.001). DOCA-salt also induced left ventricular and renal hypertrophy and renal damage as manifested by proteinuria. Collagen content in the left ventricle and kidney was significantly higher in DOCA-salt mice (P:<0.001). Urinary albumin (P:<0.05) and proliferating cell nucleic antigen-positive cells in the tubules and interstitium of the renal cortex (P:<0.001) were significantly increased in the DOCA-salt group. Neither the ACE inhibitor nor the AT(1) antagonist had any antihypertensive effect; however, they partially prevented cardiac hypertrophy and completely inhibited left ventricular collagen deposition. In the kidney, both the ACE inhibitor and AT(1) antagonist partially reduced the increase in collagen but had no effect on hypertrophy. They also significantly prevented the effect of DOCA-salt on urinary albumin and proliferating cell nucleic antigen expression in the kidney. Despite the lack of an antihypertensive effect, both ACE inhibitor and AT(1) antagonist prevented cardiac remodeling and renal damage. Our results indicate that ACE inhibitors and AT(1) antagonists exert beneficial effects on the heart and kidney in DOCA-salt hypertensive mice independently of their effects on blood pressure.

Possible role for impaired renal prostaglandin production in pathogenesis of hyporeninaemic hypoaldosteronism

A 57-year-old woman with hypertension and moderate renal insufficiency had chronic unexplained hyperkalaemia. Metabolic balance studies confirmed a diagnosis of hyporeninaemic hypoaldosteronism. Two observations suggested that impaired renal prostaglandin production contributed to the pathogenesis of the patient's disorder. Baseline renal-prostaglandin synthesis (as determined by urinary excretion of P.G.E and P.G.F) was was substantially depressed when compared with that in nine normal females. Infusion of low doses of P.G.A1 produced a significant increase in serum-aldosterone and urinary potassium excretion; it also led to a dramatic fall in blood-pressure and serum-potassium. It appears from these studies that a defect in renal prostaglandin synthesis has an important role in the pathogenesis of hyporeninaemic hypoaldosteronism.

Paradoxical glucose-induced hyperkalemia. Combined aldosterone-insulin deficiency

Severe hyperkalemia associated with spontaneous hyperglycemia as well as with the intravenous infusions of glucose occurred in an insulin-requiring diabetic patient in the absence of potassium administration, the use of diuretics which inhibit urinary potassium excretion or acidemia. Metabolic balance studies revealed, in addition to diabets, the presence of isolated aldosterone deficiency of the hyporeninemic type. Intravenous glucose infusions (0.5 g/kg body weight) produced significant hyperkalemia but desoxycortisone acetate (DOCA) therapy (10 mg/day) prevented the glucose-induced hyperkalemia. In this patient, the serum potassium concentration increases after the intravenous infusions of glucose because there is insufficient aldosterone and insulin to reverse the transfer of potassium to the extracellular fluid which normally occurs after hypertonic infusions of glucose. Although DOCA replacement modifies the distribution of potassium in the extracellular fluid and blunts the hyperkalemic effect of intravenous infusions of glucose, a rise in the insulin level is required for the usual hypokalemic response to intravenously administered glucose. These studies illustrate the risk of raising blood glucose levels in patients with combined aldosterone and insulin deficiency and the tendency towards hyperkalemia in diabetic patients under certain clinical conditions.

Disorders of chloriuretic hormone secretion

Experimental evidence supports the existence of a circulating substance, natriuretic hormone, which augments electrolyte excretion. Because such a hormone probably acts by inhibiting chloride reabsorption in the thick, ascending limb of the loop of Henle it would more accurately be called chloriuretic hormone. Chloriuretic hormone must have an action which resembles that of loop diuretics such as frusemide and ethacrynic acid. An excess of chloriuretic hormone could explain all the manifestations of Bartter's syndrome, whereas a deficiency could account for Gordon's syndrome. Hyporeninaemic hypoaldosteronism may develop in subjects who are unable to increase chloriuretic hormone concentrations appropriately in response to progressive impairment of renal function.

Dissociation of renal blood flow and filtration rate autoregulation by renin depletion

Renal blood flow (RBF) and glomerular filtration rate (GFR) autoregulation during changes in renal artery pressure (RAP) were examined in dogs fed a "normal" diet (group 1, n = 10) and in renin-depleted dogs (group 2, n = 11) which received a high-sodium diet and deoxycorticosterone acetate (DOCA) injections for a minimum of 21 days prior to the study. Renal venous plasma renin activity was undetectable in group 2 by radioimmunoassay of angiotensin I and did not increase even when RAP was reduced to less than 70 mmHg. Autoregulation of RBF was not impaired by renin depletion. However, GFR autoregulation, which was very effective in group 1 dogs, was markedly impaired in group 2. Average GFR in group 2 decreased progressively to 58 +/- 7% of the control value as RAP was reduced in steps from the control value of 137 +/- 3 to 69 +/- 1 mmHg. In normal dogs, the filtration fraction either increased slightly or did not change when RAP was reduced in steps, whereas in renin-depleted dogs the filtration fraction decreased progressively during reductions in RAP. Thus, chronic sodium loading and DOCA administration causes renin depletion and dissociates the autoregulation of RBF and GFR. These data are consistent with the hypothesis that the renin-angiotensin system participates in the control of GFR, possibly by an efferent arteriolar mechanism.

The angiotensin-converting enzyme inhibitor, perindopril, prevents cardiac hypertrophy in low-renin hypertensive rats

1. To examine whether an angiotensin-converting enzyme (ACE) inhibitor prevents left ventricular (LV) hypertrophy even in low-renin hypertension, we studied the effect of the administration of perindopril on cardiac hypertrophy induced by partial renal ablation in hypertensive rats. 2. Rats that had undergone partial nephrectomy were randomly divided into four groups that received the following as drinking water: Group A, tap water; Group B, 1% sodium chloride (NaCl); Group C, NaCl + perindopril 3 mg/kg per day; and Group D, NaCl + perindopril 1 mg/kg per day. Plasma renin activity (PRA), angiotensin-II (AII) concentration and cardiac tissue AII were measured. 3. Supplementation of NaCl following nephrectomy increased the blood pressure and cardiac weight compared with rats that had undergone nephrectomy alone (P < 0.05). Treatment with perindopril (3 mg/kg per day) did not affect the blood pressure and plasma AII but inhibited the increase of cardiac weight (P < 0.05). Left ventricular AII was decreased in cases of reduced renal mass hypertension, but was not changed by treatment with perindopril. 4. These results demonstrate that perindopril may be able to prevent LV hypertrophy even in low-renin hypertension, which was not mediated by a reduction of blood pressure or suppression of the circulating and cardiac renin-angiotensin systems. Other mechanisms of ACE inhibitors may contribute to the cardioprotective effects.