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(Hypertension. 2002;40:13.)
© 2002 American Heart Association, Inc.
Scientific Contributions |
From the Department of Physiological Medicine, St Georges Hospital Medical School, Cranmer Terrace, London, United Kingdom.
Correspondence to Dr Emma Baker, Department of Physiological Medicine, St Georges Hospital and Medical School, Cranmer Terrace, London, SW17 0RE. E-mail ebaker{at}sghms.ac.uk
| Abstract |
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5% of people of African origin and is significantly associated with high blood pressure. Although the T594M polymorphism could increase renal sodium absorption through affected channels, it is not known whether this polymorphism causes hypertension. Amiloride specifically inhibits overactive sodium channels and effectively controls blood pressure in Liddles syndrome, in which hypertension is caused by separate epithelial sodium channel mutations. The aim of this study was to determine whether amiloride was effective in lowering blood pressure in individuals with the T594M polymorphism. In an open, controlled study, 14 black hypertensive individuals with the T594M polymorphism were withdrawn from their usual medication and treated with amiloride. On entry to the study, individuals taking a mean of 2 drugs had blood pressure of 142/89±3/3 mm Hg. Amiloride alone (10 mg BID) controlled blood pressure effectively to the same level (140/91±4/2 mm Hg). When amiloride was withdrawn for 2 weeks, there was a large increase in blood pressure of 17/8±4/2 mm Hg (systolic, P<0.05; diastolic, P<0.01). On restarting amiloride, blood pressure was again controlled to 140/88±6/2 mm Hg. These results demonstrate that 10 mg BID amiloride is effective in controlling blood pressure in hypertensive individuals of African origin who have the T594M polymorphism. Our study supports the concept that the T594M polymorphism contributes to the elevation of blood pressure and suggests that consideration should be given to the use of amiloride in affected individuals.
Key Words: hypertension, essential blacks sodium channel polymorphism epithelium
| Introduction |
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Abnormal activity of the epithelial sodium channel may be an important cause of high blood pressure in black people. The epithelial sodium channel is present in the apical membrane of cells in distal renal tubule and collecting duct. It permits reabsorption of sodium from distal tubular fluid. Increased activity of the epithelial sodium channel caused by genetic mutations as seen in Liddles syndrome results in inappropriate reabsorption of sodium and the development of high blood pressure.5 Mutations of the sodium channel separate from those causing Liddles syndrome have also been identified in black people.6 We have previously shown that one polymorphism, the T594M polymorphism, is found in
5% of the black population7 and is significantly associated with high blood pressure.8 The T594M polymorphism affects the regulatory C-terminal region of the sodium channel ß-subunit and alters a putative binding site for protein kinase C.9 This binding site is thought to mediate inhibition of sodium channel activity. The T594M polymorphism could therefore contribute to the development of high blood pressure by disrupting negative regulation and increasing sodium channel activity in a similar but less florid way to that seen in Liddles syndrome.
In Liddles syndrome, the high blood pressure is controlled by amiloride, which blocks the overactive epithelial sodium channels.10 The aim of our study was to determine whether amiloride would also control blood pressure in hypertensive black people with the T594M polymorphism.
| Methods |
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140/90 mm Hg on no treatment after becoming accustomed to the measurement or were taking drug treatment for high blood pressure. Ethnicity was defined by skin color, place of birth or parents birth, and cultural identity. Subjects were excluded from entry into the study if they had renal impairment, plasma creatinine >150 µmol/L, serum potassium >5.0 mmol/L, or they had evidence of target organ damage, for example, previous stroke, heart failure, myocardial infarction, heart failure, or other evidence of ischemic heart disease. Women of childbearing potential were only admitted to the study if they were taking oral contraceptives. Subjects were to be withdrawn from the study if they had creatinine >200 µmol/L, serum K+ >5.5 mmol/L, or blood pressure consistently >180/110 mm Hg.
Informed consent was obtained from all individuals before entry into the study, which was approved by the Local Research Ethics Committee of Merton, Sutton, and Wandsworth. Procedures followed were in accordance with institutional guidelines.
Study Design
This was a longitudinal, open-label study. Subjects were first observed on their usual treatment, then underwent 4 treatment phases: change from usual treatment to amiloride, treatment with amiloride alone, withdrawal of amiloride, and reinstitution of amiloride. At entry to the study, all individuals were advised, if not already doing so, to reduce salt intake modestly; this modest reduction in salt intake was continued throughout the study with dietary advice being reinforced at each visit. All individuals were then observed for 1 month on their usual blood pressure therapy (Table 1). After this 1-month observation period, 5 mg BID amiloride was added to their usual medication, and over the ensuing month, patients underwent gradual reduction of their usual medication with an increase in dose of amiloride to a maximum of 10 mg BID under careful supervision. Amiloride alone was then continued for 1 month. Next, the amiloride was stopped and subjects remained on no treatment under close supervision for a period of 2 weeks. Amiloride was then reintroduced at an initial dose of 5 mg BID for 2 weeks and increased to 10 mg BID for 4 weeks. Appropriate measurements of blood pressure, weight, urea, electrolytes, creatinine, plasma renin activity, plasma aldosterone, and atrial natriuretic peptide, as well as 24-hour collections of urine for sodium, potassium, and creatinine excretion, were made throughout the study.
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Clinical Measurements
Blood pressure was measured with a semiautomatic Omron HEM-705CP oscillometric blood pressure recorder.11 Individuals rested for 5 minutes, after which blood pressure recordings were done in triplicate with the use of the appropriate cuff size, based on the upper mid-arm circumference. Blood pressure was measured with the patient sitting, and the values given are the mean of 3 recordings. Readings were therefore free of observer bias. Blood was taken without stasis and analyzed for sodium, potassium, creatinine, and urea and for plasma renin activity, plasma aldosterone, and atrial natriuretic peptide concentrations by radioimmunoassay.1214 Twenty-fourhour urine collections were started 1 day before attendance, and measurements were made of volume, urinary sodium, potassium, and creatinine excretion. Careful verbal and written advice was given to ensure that 24-hour urine collections were accurate. Weight was measured at each visit on the same set of digital electronic scales (Secca, Marsden).
Screening for the T594M Polymorphism
DNA was extracted from whole blood from each patient, and a 245-bp fragment of the sodium channel ß-subunit containing the region affected by the T594M polymorphism was amplified from DNA by polymerase chain reaction as previously described.7,8 Subjects were genotyped to look for the T594M polymorphism by restriction digestion of amplified DNA.
Polymerase chain reaction products were verified by electrophoresis and digested at 37°C overnight with 0.25 U of Nla III. The restriction digestion was predicted to produce fragments of the following sizes: T594 homozygotes; one fragment of 245 bp, T594M heterozygotes; three fragments of 53, 192, and 245 bp; M594 homozygotes; two fragments of 53 and 192 bp. Digest products were resolved on a 2% high-pure DNA agarose gel and visualized by ethidium bromide staining. The 192- and 245-bp fragments could be seen by this method, but the 53-bp fragment was too small to be resolved on the gel. Genotype was confirmed in a proportion of samples by direct sequence analysis with the use of a dye terminator kit on an ABI 377 automated sequencer.
Statistical Analysis
Group values are given as mean±SEM for normally distributed data and as median and interquartile range for plasma renin activity, aldosterone, and atrial natriuretic peptide concentrations, which are not normally distributed. Paired Student t tests were used to test for differences in variables that were normally distributed. Wilcoxon signed ranks tests were used to test for differences in plasma renin activity and plasma aldosterone and atrial natriuretic peptide concentrations. Two-tailed probability values of <0.05 were considered significant.
| Results |
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Blood Pressure Was Adequately Controlled on Amiloride Alone
In the remaining 14 subjects at entry into the study taking usual medication, blood pressure was 142/89±3/3 mm Hg (Table 1). After 4 weeks of a moderate reduction in salt intake in addition to usual medication, blood pressure was unchanged at 142/90±3/3 mm Hg. After 1 month during which amiloride was substituted for usual medication and a further month of treatment with 10 mg BID amiloride alone, blood pressure was controlled to the same level as that achieved by the 1.93±0.3 drugs taken at entry, to 140/91±4/2 mm Hg (Figure 1). Cessation of amiloride treatment for 2 weeks caused a significant increase in blood pressure of 17/8±4/2 mm Hg (systolic, P=0.048; diastolic, P=0.005). Twenty-fourhour sodium excretion was 102±15 mmol/24 hours on low salt diet and usual medication and did not change after 4 weeks of treatment with amiloride alone (103±12 mmol/24 hours) or after 2 weeks off all treatment (114±12 mmol/24 hours), indicating that sodium intake remained constant during the study.
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Changes in body weight and serum sodium and potassium occurred when commencing and stopping amiloride treatment. Body weight fell by 1.1±0.5 kg with amiloride treatment and increased significantly by 0.8±0.3 kg on cessation of amiloride (P=0.033). Serum sodium concentration fell by 1.3±0.6 mmol/L with amiloride treatment and increased significantly by 1.4±0.5 mmol/L on cessation of amiloride (P=0.024). Serum potassium concentration rose by 0.74±0.14 mmol/L with amiloride treatment and fell significantly by 0.77±0.12 mmol/L on cessation of amiloride (P<0.0001).
Plasma Hormone Changes Indicate Retention of Sodium After Stopping Amiloride
Plasma hormone activity was measured after 4 weeks of treatment with amiloride alone and repeated after 2 weeks on no treatment (Table 2). On cessation of amiloride, plasma renin activity fell by 0.73±0.27 ng/mL per hour (P=0.008), plasma aldosterone concentration fell by 720±222 pmol/L (P=0.001), and plasma atrial natriuretic peptide rose by 12.4±2.0 pmol/L (P=0.001).
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Confirmation of Antihypertensive Effect of Amiloride on Restarting Treatment
Paired measurements were available in 9 individuals who restarted amiloride after 2 weeks on no medication (Figure 2). Amiloride alone (10 mg BID) again controlled blood pressure to 140/88±6/2 mm Hg. Decrease in body weight and serum sodium and an increase in serum potassium again indicated that participants were taking amiloride tablets (Figure 2).
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Effect of Amiloride in Patients Previously Taking Thiazide Diuretics
Seven individuals were taking thiazide diuretics as part of combination antihypertensive therapy at entry into the study. Of these, 2 were taking a thiazide in combination with 1 other drug and 5 were taking a thiazide in combination with 2 other drugs (Table 2). Blood pressure in these 7 patients at entry was 144/89±5/4 mm Hg. Blood pressure was slightly but not significantly lower in these 7 patients when they had been changed to amiloride alone (10 mg BID) (blood pressure on amiloride) 137/89±6/3 mm Hg.
| Discussion |
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Amiloride, in conjunction with modest reduction in salt intake, is known to control blood pressure in patients with Liddles syndrome through specific blockade of upregulated sodium channels. Amiloride is not recognized currently as a powerful blood pressurelowering drug, although, apart from its use in Liddles syndrome, it has been used in primary aldosteronism. Its use until now has been as a mild, distally acting diuretic that will correct some of the decline in potassium when large amounts of loop or thiazide diuretics are given. In this study, amiloride alone controlled blood pressure in people with the T594M polymorphism at least as well as the more potent thiazide diuretics given with 1 or 2 other drugs. This observation suggests that the effect of amiloride in people with the T594M polymorphism is more than that expected from a general diuretic.
Our study appears strongly to support the concept that the T594M polymorphism does affect the activity of the sodium channel and that amiloride appears to help to reverse this abnormality. However, before reaching this conclusion, it is important to realize that there are some drawbacks to our study. First, we used a longitudinal, open-label design to investigate the effects of amiloride, which we chose because of ethical considerations. Initially we had no idea what the effect of amiloride would be in these individuals with the T594M polymorphism with severe hypertension who were taking two or more drugs to lower blood pressure. We thought that at this stage of our knowledge it was inappropriate to conduct a double-blind study with withdrawal of therapy before entry to the study. We therefore designed the study to ensure careful supervision of withdrawal of therapy and substitution of amiloride. At the same time, we were also concerned that there might be large rises in plasma potassium, particularly as the dose of amiloride was increased to 10 mg BID. In the light of these concerns, the study was not blinded, and, indeed, one individual had to be withdrawn because of an increase in plasma potassium when amiloride was commenced. Observer bias is always a risk in an open study, although in our study we used the Omron semiautomatic sphygmomanometer to ensure that measurement of blood pressure was observer-blind. As an additional safeguard to ensure accuracy of our observations, we checked the efficacy of amiloride at three different points in the study. First, amiloride alone controlled blood pressure as well as previous combination therapy. A short cessation of amiloride therapy for only 2 weeks resulted in a large increase in blood pressure, and a second period of amiloride treatment was as effective as the first. The study, therefore, in our view does show that amiloride is effective in black hypertensive patients with the T594M polymorphism. A further important consideration is that we have no control group to be certain that amiloride would not be equally effective in black hypertensive patients without the T594M polymorphism.
In view of the above considerations, we are now conducting a double-blind study comparing the effect of amiloride in black hypertensive people both with and without the T594M polymorphism. In the meantime, we feel that our study provides strong evidence that amiloride at a dose of 10 mg BID is very effective as a single agent combined with modest salt restriction in black hypertensive individuals with the T594M polymorphism. These results provide further support for the concept that the T594M polymorphism may, in part, be responsible for the rise in blood pressure in these individuals that would indicate that this is the most common cause of secondary hypertension in the black population. Currently, we feel that all black patients with high blood pressure should be genotyped for the T594M polymorphism, and, if positive, consideration should be given to the use of amiloride to control their blood pressure.
Perspectives
Genetic investigations have successfully identified rare disorders in which single gene mutations cause hypertension. These discoveries not only have allowed insight into the structure and function of genes that regulate blood pressure but also have provided clinical tools for diagnosis and targeted therapy of these conditions. The challenge for genetic research in hypertension is now to see if these studies can be extended to identify diagnostic and therapeutic targets for essential (primary) hypertension. In pursuit of this, many investigators have found allelic variants in genes that regulate blood pressure, and some of these are associated with hypertension or blood pressure variation in the general population. Our study is the first to suggest that possession of one of these genetic variants is associated with response to specific therapy targeted to the affected gene product. Our study is an early example of the use of pharmacogenomics, the association between genetics and drug response, in essential hypertension. New technologies including the rapid identification of single nucleotide polymorphisms and simultaneous analysis of multiple genes could to lead to rapid expansion of this field and further advances in the treatment of essential hypertension.
| Acknowledgments |
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Received August 31, 2001; accepted May 7, 2002.
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