The relative importance of genetic and environmental factors in hypertension in black subjects

Hypertension and the genetics of red cell membrane abnormalities

Hypertension represents the upper 15-25% of the blood pressure distribution in industrialized countries. The trait is practically absent in primitive societies and is made manifest by diet and lifestyles in industrialized countries. High blood pressure is an important risk factor for strokes, heart disease and renal disease. The frequency of hypertension is higher among blacks than among whites in the USA. Various twin, family and adoption studies indicate a strong genetic effect on blood pressure. The genetic mechanisms are unknown. Membrane transport variability has been studied in red cells as a surrogate for analogous alterations in smooth muscle or renal cells. Among the various transport systems, erythrocyte sodium-lithium countertransport (CT) has been consistently elevated in variable proportions of Caucasian hypertensives. Genetic studies of countertransport levels have shown familial aggregation and higher concordance for monozygotic than dizygotic twins. Complex segregation analysis suggests the action of a major gene superimposed on a polygenic background. The postulated gene (B) raises CT activity and has a population frequency of 0.25. CT levels of the common AA homozygotes and AB heterozygotes cannot be distinguished from each other, whereas CT activity of BB homozygotes (6% of the population) is significantly elevated. Although the CT gene contributes only 2.7% to 3.5% of the variability of blood pressure over its entire range, 14% to 20% of persons with systolic hypertension (greater than 140 mmHg) are BB homozygotes rather than the expected 6% to 7%. A much lower frequency of elevated countertransport activity among black hypertensives suggests genetic heterogeneity in the pathogenesis of high blood pressure. Further investigations on the mechanism and genetic linkage relationships of the putative CT gene may aid in elucidating an important mechanism of blood pressure elevation and will allow molecular approaches in the future.

Renal perfusion and function in healthy African Americans

BACKGROUND

Despite their increased risk of nephropathy, remarkably little is known about renal perfusion and function in healthy African Americans.

METHODS

We enrolled 32 healthy African Americans and compared renal perfusion and function in 82 age-matched healthy Caucasians. Studies were performed on a diet containing 200 mmol of sodium and 100 mmol of potassium per day. In a separate study of 28 subjects, 10 African American and 18 Caucasians, the contribution of the renin-angiotensin system was assessed by measuring renal hemodynamic responses to angiotension II (Ang II) and captopril.

RESULTS

Although glomerular filtration rate (GFR) was similar, renal plasma flow (RPF) was significantly less in age-matched African Americans (568 +/- 18) than Caucasians (620 +/- 13 mL/min/1.73 m(2), P = 0.0063). After captopril, African Americans had a sevenfold greater vasodilator response and a rise in RPF (35.3 +/- 4.9 vs. 4.9 +/- 12.4 mL/min/1.73 m(2) in African Americans and Caucasians, respectively, P < 0.028). Ang II administration caused a significantly smaller vasoconstrictor response in African Americans (Ang II-induced fall in RPF, -97 +/- 18 vs. -150 +/- 9 mL/min/1.73 m(2), P = 0.05), and angiotensin-converting enzyme (ACE) inhibition enhanced the response to Ang II in African Americans significantly.

CONCLUSIONS

A reduction in RPF, blunting of the renal vascular response to Ang II, and an accentuated renal vasodilator response to captopril, which in turn corrects the blunting of responsiveness to Ang II, all suggest activation of the renin system in apparently healthy African Americans. As PRA was identical in Caucasians and African Americans, the findings suggest that it is the intrarenal-renin system that is activated in African Americans. This difference in normal control mechanisms could predispose to nephropathy.

Racial/ethnic analysis of selected intermediate outcomes for hemodialysis patients: results from the 1997 ESRD Core Indicators Project

Principal goals of the End-Stage Renal Disease (ESRD) Core Indicators Project are to improve the care provided to ESRD patients and to identify categorical variability in intermediate outcomes of dialysis care. The purpose of the current analysis is to extend our observations about the variability of intermediate outcomes of ESRD care among different racial and gender groups to a previously unreported group, Hispanic Americans. This group is a significant and growing minority segment of the ESRD population. A random sample of Medicare-eligible adult, in-center, hemodialysis patients was selected and stratified from an end-of-year ESRD patient census for 1996. Of the 6,858 patients in the final sample, 45% were non-Hispanic whites, 36% were non-Hispanic blacks, and 11% were Hispanic. Whites were older than blacks or Hispanics (P < 0.001). Hispanics were more likely to have diabetes mellitus as a primary diagnosis than either blacks or whites (P < 0.001). Even though they received longer hemodialysis times and were treated with high-flux hemodialyzers, blacks had significantly lower hemodialysis doses than white or Hispanic patients (P < 0.001). The intradialytic weight losses were greater for blacks (P < 0.05). The delivered hemodialysis dose was lower for blacks than for whites or Hispanics whether measured as a urea reduction ratio (URR) or as the Kt/V calculated by the second generation formula of Daugirdas (median 1. 32, 1.36, and 1.37, respectively, P < 0.001). Hispanics and whites had modestly higher hematocrits than blacks (33.2, 33.2, and 33.0%, respectively, P < 0.01). There was no significant difference among groups in the weekly prescribed epoetin alfa dose ( approximately 172 units/kg/week). A significantly greater proportion of Hispanic patients had transferrin saturations >/=20% compared with the other two groups (P < 0.001). Logistic regression modeling revealed that whites were significantly more likely to have serum albumin <3. 5(BCG)/3.2(BCP) gm/dL (OR 1.4, p < 0.01); blacks were significantly more likely to have a delivered Kt/V < 1.2 (OR 1.4, P < 0.001) and hematocrit <30%, (OR 1.2; P < 0.05) and both blacks and Hispanics were significantly more likely to have a delivered URR < 65% (OR 1.5, P < 0.001 and 1.2, P < 0.05, respectively).

Essential hypertension in blacks: epidemiology, characteristics, and possible roles of racial differences in sodium, potassium, and calcium regulation

Racial differences in the regulation of Na+, K+, and Ca2+ have been shown both at the systemic and cellular levels. These include a higher incidence of "salt sensitivity," lower urinary K+ excretion, lower plasma renin activity, and higher circulating levels of immunoreactive parathyroid hormone and 1.25 dihydroxyvitamin D in blacks than in whites. Blacks exhibit a higher erythrocyte Na+ concentration, coupled with a lower maximal initial reaction velocity of erythrocyte Na,K-ATPase. Blacks also appear to differ from whites in erythrocyte Na+, K+ cotransport and Na-Li countertransport. Moreover, they show a higher activity of the Na(+)-H+ antiport in skin fibroblasts and a greater response of cellular Ca2+ signaling to agonists in serum. Mechanisms linking some of these racial differences in ionic metabolism to the increased propensity of blacks to develop essential hypertension are proposed, and the epidemiology and characteristics of this disease in blacks are reviewed.

The epidemiology of stroke in an urban black population

Hospital admissions for acute stroke were monitored over a twelve month period. Only patients age 20 and over were studied. Out of a population at risk of 114931 there were 116 cases, giving an overall incidence of 1.01 admissions per 1000 population per year over age 20. Cerebral hemorrhages were present in 32.8%, large artery thromboses in 31.9%, cerebral embolism in 13.8% and lacunar infarctions in 20.7% of cases. Atrial fibrillation was the presumed cause of cerebral embolism in 6.9% of patients. Hypertension was present in 69.8% of patients. Further studies are needed to establish the characteristics of stroke in the Third World.

Effects of age and race on clinical response to acebutolol in essential hypertension

Data from three multicenter controlled clinical trials of acebutolol carried out in the United States were reviewed to determine the efficacy and safety of acebutolol in patients greater than or equal to 65 years and in black patients. In 54 elderly patients, acebutolol decreased diastolic blood pressure by 12.8 mm Hg, and 86.5% were classified as responders. Acebutolol was superior to placebo, propranolol, and hydrochlorothiazide in terms of response rate. In 56 black patients, acebutolol decreased diastolic blood pressure by 8.5 mm Hg, and 53% were classified as responders. Acebutolol was superior to placebo and propranolol but not as effective as hydrochlorothiazide in terms of response rate. Acebutolol was well tolerated in both patient populations.

Red cell sodium countertransport and cotransport in normotensive and hypertensive blacks

We have previously described elevated Lii -Nao countertransport (CT) and Na-K cotransport (CO) in red cells of Caucasian patients from Boston. In this study, we report both transport systems in black patients from Philadelphia. The maximal rate (Vmax) of CT was assayed by measuring the Nao-stimulated Li efflux from cells containing +/- 6 mmol Li/liter. The Vmax of outward cotransport was assayed by measuring the furosemide-sensitive component of Na and K efflux into Mg medium from cells containing 50 mmol/liter of both ions. The mean value of CT for 18 normotensive (NT) subjects with no family history of hypertension, (-) FHH , was 0.18 +/- 0.05 (mmol/liter cells X hour); and in 14 hypertensive (HT) patients, 0.18 +/- 0.07. The mean values of Na and K cotransport were, respectively (mmol/liter cells X hour), in 18 NT subjects with (-) FHH , 0.38 +/- 0.24 and 0.50 +/- 0.28 in 18 HT subjects, 0.25 +/- 0.17 and 0.24 +/- 0.14. We conclude that there is no difference in the Vmax for CT between the two groups of black subjects, but that the Vmax for Na-K CO was significantly reduced in the HT group. Notably, the offspring of HT patients (age 14 years, n = 17) also had a marked reduction in the Vmax of Na (0.15 +/- 0.17) K cotransport (0.19 +/- 14) in comparison with the mean value of Na (0.40 +/- 0.2) and K (0.60 +/- 0.3) cotransport measured in offspring (n = 10) of NT subjects (age 14 years).(ABSTRACT TRUNCATED AT 250 WORDS)