Mary Anne Wagner

Improvement in Blood Pressure With Inhibition of the Epithelial Sodium Channel in Blacks With Hypertension

Hypertension in blacks is more prevalent and less often controlled than the hypertension of other ethnic groups. We sought to explore the benefit of adding inhibitors of the epithelial sodium channel (ENaC), an aldosterone-regulated site of sodium reabsorption in the distal nephron, to the antihypertensive regimen of black hypertensive patients. In a prospective, randomized, placebo-controlled, double-blind clinical trial, we used a 2-by-2 factorial design with 4 treatment groups: amiloride (a direct inhibitor of ENaC), spironolactone (an aldosterone receptor antagonist), the combination of both drugs, and placebo. The subjects (n=98) had an elevated blood pressure despite treatment that included a diuretic and a calcium channel blocker; the level of plasma renin activity was ≤0.56 ng/L per second. The primary end points were changes from baseline in systolic and diastolic blood pressure over a 9-week period of treatment. The reductions in systolic and diastolic blood pressures (mm Hg) were, respectively, 9.8±1.6 (SE) and 3.4±1.0 for amiloride (P<0.001) and 4.6±1.6 (P=0.006) and 1.8±1.0 for spironolactone (P=0.07). Treatment with either amiloride or spironolactone or the combination was well tolerated; no patient experienced hyperkalemia. In a substudy, plasma endothelin-1 levels were observed to decrease after 3 weeks of treatment with spironolactone (P<0.001), consistent with a non–ENaC-related potential benefit of spironolactone. In conclusion, treatment with either amiloride or spironolactone can provide an additional reduction in blood pressure in blacks already receiving conventional antihypertensive therapy.

Racial Differences in Aldosterone Excretion and Plasma Aldosterone Concentrations in Children

Blacks are more likely to have hypertension, have lower levels of plasma renin activity, and typically consume less potassium than whites. Whether blacks and whites secrete different amounts of aldosterone is less clear. We estimated aldosterone secretion indirectly in 715 children, 249 of whom were black, by measuring their nocturnal rates of urinary excretion of aldosterone. Dietary sodium and potassium intakes were estimated from their excretion rates. The mean (+/- SE) aldosterone-excretion rate was lower in the black children than in the white children (0.045 +/- 0.003 vs. 0.078 +/- 0.004 nmol per micromole of creatinine per kilogram of body weight; P less than 0.001). The potassium-excretion rate was also lower in the black children than in the white children (0.13 +/- 0.01 vs. 0.18 +/- 0.01 mmol per micromole of creatinine per kilogram; P less than 0.001). Aldosterone excretion was highly correlated with potassium excretion (P less than 0.001), but the lower aldosterone-excretion rate in blacks was explained only in part by their lower dietary intake of potassium. Systolic blood pressure was higher in black children (P less than 0.001), as was diastolic pressure (P = 0.037). In a second study of 99 children, the plasma aldosterone level was found to be significantly lower in black children than in white children (230 +/- 30 vs. 400 +/- 30 pmol per liter; P less than 0.001). Plasma renin activity and plasma cortisol levels were the same in both groups. In summary, we found that black children secrete about 40 percent less aldosterone than white children. The role of the lower aldosterone-secretion rate in the genesis of the higher blood pressures observed in black children is not known.

Genetic Variants in the Epithelial Sodium Channel in Relation to Aldosterone and Potassium Excretion and Risk for Hypertension

Renin and aldosterone secretion is often lower in blacks than in whites, characteristics that resemble a milder form of Liddle syndrome in which a mutation in the amiloride-sensitive epithelial sodium channel (ENaC) of the kidney results in enhanced resorption of sodium. In the present study, we looked for evidence that the intrinsic level of ENaC activity is indeed higher in blacks than in whites. In overnight urine samples collected from young people (249 white and 181 black subjects, mean age 13.4 years), the urinary aldosterone/potassium ratio, which is typically very low in Liddle syndrome, was lower in blacks than in whites: 0.421+/-0.024 (mean+/-SE) versus 0.582+/-0.016 nmol/mmol (P<0.0001). In addition, all but 1 of 5 molecular variants in ENaC were much more common in blacks than in whites. G442V in the beta-subunit, present in 16% of the blacks and in only 1 white, was associated with parameters reflective of a greater Na retention and potentially a higher ENaC activity: a lower plasma aldosterone concentration (P=0.070), a lower urinary aldosterone excretion rate (P=0.052), a higher potassium excretion rate (P=0.048), and a lower urinary aldosterone/potassium ratio (P=0.027). In a second cohort consisting of 126 black and 161 white normotensive subjects and 232 black and 188 white hypertensive subjects, betaG442V did not show a significant association with hypertension (P=0.089). On the other hand, a variant that was twice as common in whites, alphaT663A, was associated with being normotensive both in blacks (P=0.018) and in whites (P=0.034). Expression of either betaG442V or alphaT663A in Xenopus oocytes did not result in a change in basal Na current, consistent with the variants being in linkage disequilibrium with alleles at active loci. In conclusion, several lines of evidence are presented to suggest that ENaC activity is higher in blacks than in whites, which could contribute to racial differences in Na retention and the risk for hypertension.

Ethnic Differences in Renal Responses to Furosemide

Blacks have a greater tendency to retain Na than whites. The present study sought evidence for ethnic differences in parameters reflective of Na uptake by the Na,K,2Cl cotransporter in the thick ascending limb, namely, the urine concentration and urinary excretion of certain cations before and after furosemide administration (40 mg IV). Subjects were healthy (ages 18 to 36 years). During the preceding overnight period, urine volume was lower, and osmolality was higher in blacks than in whites, an ethnic difference that disappeared when water intake was restricted to infused normal saline (60 mL/h). Plasma vasopressin levels were higher in black males than in other sex/ethnic groups. Baseline urinary excretion rates of K, Ca, and Mg were significantly lower in blacks than in whites. After furosemide (0 to 1 hour), K and Ca excretion rates increased, but the proportionate ethnic difference decreased from 44% to 22% and from 22% to 10%, respectively, consistent with blacks having more basal Na,K,2Cl cotransporter activity to inhibit. During a later postfurosemide period (1 to 5 hours), urinary concentrations of Ca and Mg recovered more slowly in blacks, consistent with greater reuptake in the thick ascending limb. In summary, there were distinct ethnic differences in renal handling of Ca and Mg basally and in response to furosemide that were consistent with a more active Na,K,2Cl cotransporter in the thick ascending limb in blacks. An increase in vasopressin levels appeared to explain greater urine concentrations in black males but not black females.

Racial Differences in Sensitivity of Blood Pressure to Aldosterone

Blacks in comparison with whites are at risk for a more serious form of hypertension with high rates of complications. Greater sodium retention is thought to underlie the blood pressure (BP)-determining physiology of blacks, but specific mechanisms have not been identified. In a prospective observational study of BP, 226 black children and 314 white children (mean age, 10.6 years) were enrolled initially. Assessments were repeated in 85 blacks and 136 whites after reaching adulthood (mean age, 31 years). The relationship of BP to plasma aldosterone concentration in the context of the prevailing level of plasma renin activity was studied in blacks and whites. In a secondary interventional study, 9-&agr; fludrocortisone was administered for 2 weeks to healthy adult blacks and whites to simulate hyperaldosteronism. BP responses in the 2 race groups were then compared. Although black children had lower levels of plasma renin activity and plasma aldosterone, their BP was positively associated with the plasma aldosterone concentration, an effect that increased as plasma renin activity decreased (P=0.004). Data from black adults yielded similar results. No similar relationship was observed in whites. In the interventional study, 9-&agr; fludrocortisone increased BP in blacks but not in whites. In conclusion, aldosterone sensitivity is a significant determinant of BP in young blacks. Although its role in establishing the risk of hypertension is not known, it could be as relevant as the actual level of aldosterone.

Adrenal androgen excretion during adrenarche. Relation to race and blood pressure.

We have previously shown that black children have higher blood pressures than white children. In the present study, we examined whether a possible racial difference in adrenal androgen production during adrenarche might contribute to the racial disparity in blood pressure. Adrenal androgen production was estimated from urinary excretion of adrenal androgen metabolites that showed cross-reactivity with antisera to dehydroepiandrosterone sulfate (DHEA-S). Urine samples were collected overnight in 798 children, one third of whom were black. Analyses were performed for two different age groups, less than 10 years and 10 years or more of age. In children less than 10 years of age, adrenal androgen excretion rates were 17% higher in blacks than in whites (p = 0.0099); adrenal androgen excretion rates tended to be higher in older black children as well, but differences here were not statistically significant. Adrenal androgen excretion rates were positively correlated with diastolic blood pressure in the older age group only (p = 0.014). However, when the relation of race to blood pressure was examined along with adrenal androgen excretion adjusted for age, sex, and weight, race remained an independent contributor to the level of blood pressure, suggesting that a difference in adrenal androgens could not explain the racial differences in blood pressure. In summary, black children produced more adrenal androgen, but this did not explain their higher blood pressures. In older children, where adrenal androgen excretion rates were higher, diastolic blood pressure and adrenal androgen excretion were positively related, suggesting that adrenal androgens participate in establishing the level of blood pressure in young people.

Molecular variations in the calcium-sensing receptor in relation to sodium balance and presence of hypertension in blacks and whites.

Sodium (Na) excretion is to an extent tied to calcium (Ca) excretion; increases in Ca result in increased Na excretion. We hypothesized that molecular variation in the calcium-sensing receptor (CaSR), which imparts certain of the influences of extracellular Ca, might be related to differences in Na balance and blood pressure. We further hypothesized that such an influence by CaSR is more pronounced in blacks than in whites, as the hypertension in blacks appears to be more dependent on Na retention. Three common molecular variants in CaSR were studied. Two were more frequent in the whites (A986S, P < .0001, and G990R, P = .093), whereas Q1011E was more frequent in the blacks (P < .0001). Two distinctly separate groups were studied: (1) healthy schoolchildren in whom levels of the renin-aldosterone axis and blood pressure were measured, and (2) normotensive and hypertensive adults. Studies of association were made separately in the whites and the blacks. No association of any of the variants with Na balance (as estimated from renin and aldosterone levels) was observed. In the black schoolchildren, Q1011E showed a marginal association with a higher blood pressure (P = .093 for systolic and P = .025 for diastolic), a relationship that was considered to be nonsignificant after adjusting for multiple comparisons. Nor was there a significant association of the variants with presence or absence of hypertension. In summary, studies of two cohorts that included whites and blacks did not suggest that molecular variations in the CaSR influence either Na balance or blood pressure.

Race, sex, and the regulation of urine osmolality: observations made during water deprivation.

A more concentrated urine is excreted by blacks than whites and by men than women. The purpose of this study was to explore the physiological bases for the race and sex effects during water deprivation when osmoregulation is challenged and differences are amplified. Drinking water was withheld from 17 blacks (10 men) and 19 whites (9 men) for 24 h. Vasopressin (VP) levels and osmolality in plasma (P(osmol)) and urine (U(osmol)) were measured basally and then every 4 h. U(osmol) was higher in blacks at baseline (P = 0.01) and during water deprivation (P = 0.046). Before and during water deprivation, no differences were seen in levels of VP, P(osmol), or the VP-U(osmol) relationship between blacks and whites. Although VP levels were initially higher in men (P < 0.02 for samples collected over the first 12 h), over the last 12 h of water deprivation, U(osmol) was higher (P = 0.027) and more responsive to the level of VP (in terms of slopes, P = 0.0001) in women than men. Our results suggest that, after a period of water deprivation, there develops a sensitivity of the collecting duct to VP that is greater in women. Although U(osmol) is higher in blacks, the race difference in water conservation did not appear to result from differences in the level of VP or the sensitivity of the collecting duct to VP. Upstream effects such as Na(+) uptake in the thick ascending limb, with its ensuing effects on water reabsorption, need to be considered in future studies of the relationship of race to water conservation.

Heart rate as a predictor of future blood pressure in schoolchildren

Heart rate (HR) has been shown to predict future blood pressures (BP) in studies in adults. We explored the relation of HR to future BP levels in a cohort of 344 black and 456 white schoolchildren ages 5 to 19 years, to examine the hypothesis that HR predicts subsequent BP even very early in life. After making baseline measurements, BP was assessed longitudinally 1 to 24 additional times (mean = 8.25) after the baseline period, at intervals of approximately 6 months. We found that HR was significantly related to future diastolic BP in the black boys (P = .016) after adjusting for baseline diastolic BP, age, and body mass index, but not in the black girls or in the white children. Because HR is reflective of sympathetic nervous system (SNS) activity that in turn can be related to the renin-angiotensin system (RAS), we also explored the relation of HR to the RAS by studying relationships to variants in the angiotensinogen gene and the angiotensin I-converting enzyme (ACE) gene. We found a significantly positive relationship of HR to the presence of the deletion allele of the ACE gene (P = .0015), but, again, only in the black boys. Because blacks in general appear to retain additional sodium when compared with whites, the SNS, as reflected in the HR, may influence BP more when individuals have increased sodium retention. In summary, baseline HR predicted future diastolic BP in the black boys but not in the black girls or in the white children.