Red cell sodium countertransport and cotransport in normotensive and hypertensive blacks

Decreased NKCC1 activity in erythrocytes from African Americans with hypertension and dyslipidemia

BACKGROUND

Recent studies demonstrated a key role of ubiquitous isoform of Na+,K+,2Cl- co-transport (NKCC1) in regulation of myogenic tone and peripheral resistance. We examined the impact of race, gender, and plasma lipid on NKCC1 activity in French Canadians and African Americans with hypertension and dyslipidemia.

METHODS

NKCC and passive erythrocyte membrane permeability to K+, measured as ouabain-resistant, bumetanide-sensitive, and (ouabain+bumetanide)-resistant 86Rb influx, respectively, were compared in 111 French-Canadian men, 107 French-Canadian women, 26 African-American men, and 45 African-American women with essential hypertension and dyslipidemia.

RESULTS

The African-American men and women were 7 years younger and presented twofold decreased plasma triglycerides compared to their French-Canadian counterparts (P < 0.01) whereas body mass index (BMI), total cholesterol, low-density lipoprotein, and high-density lipoprotein (HDL) were not different. NKCC was respectively 50 and 38% lower in the African-American men and women than in the French Canadians (P < 0.005) without any differences in passive erythrocyte membrane permeability for K+. We did not observe any impact of age on NKCC in all groups under investigation, whereas plasma triglycerides correlated positively with the activity of this carrier in the French-Canadian men only.

CONCLUSIONS

NKCC1 activity is lower in erythrocytes of African Americans with essential hypertension and dyslipidemia than in Caucasian counterparts. We suggest that decreased NKCC1 may contribute to the feature of the pathogenesis of salt-sensitive hypertension seen in African Americans.

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.

Modulation of the Na(+)-H(+) antiport activity by adrenaline on erythrocytes from normal and obese individuals

The effect of adrenaline on normal and obese human Na(+)-H(+) antiport (NHE 1) erythrocyte activity has been studied. Adrenaline increased both intracellular pH (pHi) and Na(+) influx in erythrocyte suspensions. This effect of adrenaline was inhibited by amiloride or EIPA, indicating that adrenaline stimulated NHE 1. Phorbol myristicate ester (PMA), a protein kinase C (PKC) stimulator, increased the activity of NHE 1 whereas calphostin C, a PKC inhibitor, partially inhibited NHE 1 activation induced by adrenaline. The effect of adrenaline to NHE 1 was counteracted by prazocin and by propranolol as well indicating the involvement of both alpha and beta 2 adrenergic receptors. The effect of adrenaline on erythrocyte NHE 1 activity was significantly more profound in obese compared to normal subjects. These data indicate that adrenaline induces an increase of pHi and Na(+) uptake of human erythrocytes through stimulation of NHE 1 activity. The significantly more profound stimulation of NHE 1 activity by adrenaline in obese as compared to normal subjects is discussed.

Abnormalities in insulin sensitivity, vascular resistance and erythrocyte cation transport are independent genetic traits in familial hypertension

Several isolated abnormalities have been noted in normotensive members of hypertensive families, including exaggerated forearm vascular resistance (FVR), decreased insulin sensitivity and elevated sodium-lithium (Na/Li) countertransport. No family study has investigated the aforementioned abnormalities concurrently within the same hypertensive families. It is therefore unknown whether these disturbances reflect single or different genetic traits. Thus, we studied cardiovascular reactivity of the forearm vasculature, cellular sodium transport mechanisms and insulin sensitivity concomitantly in normotensive (n = 24) and borderline hypertensive (n = 16) members of hypertensive families, compared with normotensive members (n = 24) of normotensive families. At least one abnormality was noted in 27 (67%) out the 40 subjects with a positive family history of hypertension. Na/Li-countertransport was increased in 15 (37%), FVR was increased in 14 (35%) and insulin sensitivity was decreased in 9 (22%) subjects with familial hypertension. The concomitant occurrence of at least 2 out of the 3 abnormalities mentioned was noted in 9 (22%) out of the 40 subjects with family history of hypertension. Decreased insulin sensitivity, increased basal FVR and increased Na/Li countertransport were concurrently observed in two (5%) subjects. This investigation documents the frequent occurrence of abnormalities such as decreased insulin sensitivity, increased basal FVR and Na/Li-countertransport in subjects with family history of hypertension. The concomitant occurrence of at least two of the mentioned abnormalities being observed in less than one-quarter of the subjects with family history of essential hypertension, it is assumed that insulin sensitivity, basal FVR and Na/Li-countertransport reflect more than one genetic trait predisposing to hypertension.

Modifications induced by plasma of gestational hypertensive women on the Na+/K+-ATPase obtained from human placenta

In order to investigate the molecular mechanisms of the inhibition of Na+/K+-ATPase in Gestational Hypertension (GH), we incubated Na+/K+-ATPase purified from human placenta of 6 healthy normotensive women with plasma from 6 GH women and 6 healthy controls. We determined the enzyme activity by the method of Esman, and the anthroyl-ouabain-binding capacity, dissociation constant (Kd) and average lifetime values (tau) by the static and dynamic fluorescence of anthroyl-ouabain. The lipid annulus of the enzyme was studied by static and dynamic fluorescence of 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5- hexatriene (TMA-DPH). The addition of total and protein-free GH plasma to normal Na+/K+-ATPase significantly inhibited the enzymatic activity even at the lowest concentration studied (1:100), as well as the ouabain-binding capacity, Kd and tau. GH plasma significantly decreased the fluorescence polarization and lifetime values of TMA-DPH. These observations indicate that the inhibition caused by GH plasma on Na+/K+-ATPase might be due to a reduction of the number of active molecules or a modification of the ouabain-binding site suggesting the existence of digitalis-like factor. A link between the modification of the lipid moiety of the enzyme and the Na+/K+-ATPase inhibition might be hypothesized.

Pathophysiology and treatment implications of hypertension in the African-American population

Regardless of the etiology, hypertension remains a major public health problem in African-Americans and is associated with significant morbidity and mortality. Additional data on the pathophysiology of this disease in this population are needed, as are data on the best therapies to decrease the high complication rate. Because many of the large studies on hypertension have included few African-Americans, recruitment of this ethnic group into clinical trials should be promoted. Further studies into the genetic factors in the pathophysiology of racial differences in hypertension may shed more light on this complex issue.

Sodium-lithium countertransport is associated with insulin resistance and urinary albumin excretion in young African-Americans

Increased activity of the sodium transporter, sodium-lithium countertransport (SLC), is reported in hypertensive white patients with evidence of cardiac and renal injury. The purpose of this study was to determine whether increased SLC activity detects risk for nephropathy or vascular disease in nondiabetic, young adult African-Americans. We examined 85 African-Americans aged 25 to 33 years with measurement of blood pressure, an oral glucose tolerance test to measure insulin response to glucose challenge, and an insulin clamp for insulin sensitivity (M). Fasting plasma lipids were measured, and the Vmax and Km for Na+ were assayed on red blood cells. Urinary albumin excretion (UAE) was measured on timed collections. There was a statistically significant correlation of the Vmax for SLC with M (r = -0.26, P = 0.02) and with UAE (r = 0.25, P = 0.02). The Km for Na+ to activate SLC was also elevated in the subgroup of subjects with elevated Vmax of SLC. There was no significant correlation of SLC with blood pressure in bivariate analysis. Step-wise multiple linear regression analysis of all variables on the Vmax SLC demonstrated that plasma triglyceride, UAE, body mass index, systolic blood pressure, M, and fasting insulin were step-wise selected into the linear regression model (F-ratio = 3.2, df = 77, R = 0.46, P < 0.009). In this young adult African-American population, elevated SLC activity is detected in association with metabolic and lipid alterations typical of insulin resistance. Elevated SLC activity is also associated with higher rates of UAE, suggesting possible evidence of early renal injury.

Insulin resistance in systemic hypertension: pharmacotherapeutic implications

Systemic hypertension, a vascular disease with multiple origins, now is being linked to subtle abnormalities in glucose metabolism, which include insulin resistance and hyperinsulinemia. These conditions often occur together in patients with obesity, noninsulin-dependent diabetes, or both. Hyperinsulinemia and insulin resistance may cause systemic hypertension through multiple mechanisms. Insulin has a salt-retaining effect on the kidney. Also, insulin can augment catecholamine release, increase vascular sensitivity to vasoconstrictor substances, and decrease vascular sensitivity to vasodilator substances. In addition, insulin can increase production of tissue growth factors and help retain sodium and calcium in cells. Insulin resistance in patients can be treated with regular aerobic exercise, weight reduction, and a high-fiber diet. Pharmacologic approaches include hypoglycemic drugs, weight-reducing agents, and certain antihypertensive drugs that may have a favorable impact on both blood pressure and insulin resistance.

Albuminuria in association with insulin and sodium-lithium countertransport in young African Americans with borderline hypertension

The purpose of this study was to determine whether early nephropathy, evidenced by urinary albumin excretion, can be detected in young African American subjects with only borderline hypertension, and whether there is a relationship of albuminuria with insulin resistance and with sodium-lithium countertransport activity. Clinically well young African American men and women including normotensive (blood pressure < 135/85 mm Hg, n = 41) and borderline hypertensive (blood pressure > or = 135/85 mm Hg, n = 26) individuals were studied. Each subject underwent an oral glucose tolerance test and euglycemic hyperinsulinemic clamp study. Albuminuria was measured on timed urine collections. Sodium-lithium countertransport activity was assayed in fresh red blood cells at 280 mmol/L Na+ for full saturation of external Na+ sites. The sum of insulin levels during glucose tolerance was significantly greater in the borderline hypertensive compared with the normotensive subjects (P = .014), and insulin-stimulated glucose utilization during the clamp was significantly lower in borderline hypertensive compared with normotensive subjects (P = .016). Albuminuria was greater in borderline hypertensive compared with normotensive subjects (P = .002). Albuminuria was significantly correlated with fasting plasma insulin concentration (r = .44, P < .002) and the sum of insulins during the glucose tolerance test (r = .45, P < .002). Sodium-lithium countertransport correlated with albuminuria (r = .31, P < .05) as well as significantly with insulin-stimulated glucose utilization during the clamp (r = .44, P < .001).(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in sodium metabolism as an etiological model for hypertension

An adequate matching for race, sex, stage of the menstrual cycle, family history of hypertension, and the amount of sodium and other electrolytes in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium in essential hypertensive patients in comparison with normal subjects. Alterations in intracellular sodium concentration and transmembrane sodium transport systems as causes of essential hypertension are postulated. This review article describes how this abnormal sodium and calcium metabolism translates into increased systemic vascular resistance through altered vasoactive responses and/or vasculature structural changes.

Comparison of lithium ratio between African-American and Caucasian bipolar patients

Lithium RBC/plasma ratio (LR) was studied in 34 bipolar subjects on therapeutic doses of lithium carbonate. The sample was divided into 22 Caucasians and 12 African-Americans to observe possible ethnic differences in LR as previously reported. The latter group demonstrated a higher LR as well as increased reports of side effects (p < .05), even after controlling previous confounding factors. Our findings suggest that African-Americans may be more susceptible to the side effects associated with lithium treatment, and consequently, lower dosages may be necessary for this group.

The link between hypertension and nephrosclerosis

Nephrosclerosis is literally defined as hardening of the kidneys (Greek derivation: nephros, kidney; sklerosis, hardening). It is the result of scarring or replacement of the normal renal parenchyma by dense collagenous tissue. In practice, nephrosclerosis refers to diseases with predominant pathologic changes occurring in the preglomerular microvasculature and secondarily involving the glomeruli and interstitium. The relationship between mild to moderate hypertension and either nephrosclerosis or end-stage renal disease (ESRD) remains circumstantial, although these syndromes have long been associated in the medical literature. Nephrologists credit hypertension as the etiology of nephrosclerosis in 25% of patients initiating Medicare-supported renal replacement therapy, even though other processes may cause similar renal pathologic findings. Strikingly, serum creatinine values infrequently increase in patients with long-standing mild to moderate hypertension. Patients classified as having hypertensive ESRD typically present with advanced disease, making the processes that initiated the renal disease difficult to detect. Nephrologists are twice as likely to label an African-American patient as having hypertensive nephrosclerosis, compared with a white patient, when presented with identical clinical histories. This review proposes that many patients classified as having hypertensive nephrosclerosis actually have intrinsic renal parenchymal diseases, renal artery stenosis, unrecognized episodes of accelerated hypertension, or a primary renal microvascular disease. The familial clustering of ESRD attributed to hypertension in African-Americans and the identification of genes associated with renal injury in animals support the concept that inherited factors may predispose to renal failure. African-American families often have members with ESRD from disparate etiologies, including hypertensive ESRD. This suggests that common mechanisms, be they inherited or environmental, underlie the development of progressive renal failure in diverse forms of nephropathy. Identification of the mechanisms producing susceptibility to progressive renal disease would support the concept that mild to moderate elevations in blood pressure per se are uncommon causes of nephrosclerosis.

Erythrocyte ion fluxes in essential hypertensive patients with left ventricular hypertrophy

BACKGROUND

Left ventricular hypertrophy (LVH) is an independent risk factor for cardiovascular morbidity and mortality in essential hypertension (EH). Several hemodynamic and nonhemodynamic factors have been involved in the development of LVH in hypertension, including abnormalities in cellular ion mobilization.

METHODS AND RESULTS

We measured different ion transport systems in erythrocytes from 50 patients with EH classified as having or not having LVH measured by M-mode echocardiography. Thirty-two EH patients (64%) exhibited criteria of LVH, and 18 (36%) did not. When the two groups were compared, patients with LVH were older (44.7 +/- 7.4 versus 37.6 +/- 9.2 years; P < .01) and exhibited higher rates of erythrocyte Na(+)-H+ exchange (9.8 +/- 4.1 versus 7.1 +/- 2.6 mmol.[L.cells.h]-1; P < .05) and higher intraerythrocyte Na+ content (8.5 +/- 1.3 versus 7.5 +/- 0.8 mmol/L per cell; P < .01). Systolic and diastolic blood pressure values, as well as biochemical, hormonal, and other erythrocyte ion transport systems studied did not differ between EH with or without LVH. The results of a multiple linear regression analysis using left ventricular mass index (LVMI) as the dependent variable showed that Na(+)-H+ exchange and the maximal rate of the Na(+)-K(+)-Cl- cotransport were the only two independently significant parameters associated with an increased LVMI.

CONCLUSIONS

The increased rate of the erythrocyte Na(+)-H+ exchange and the decreased maximal rate of the Na(+)-K(+)-Cl- cotransport system are both associated with the presence of LVH in EH patients. These abnormalities of ion transport pathways tend to increase the intracellular Na+ content and may be involved in the pathogenesis of LVH in EH.

Red blood cell sodium-proton exchange in hypertensive blacks with insulin-resistant glucose disposal

To define the potential pathogenic role of hyperinsulinemia as a mediator of alterations in sodium transport, we have examined red blood cell Na(+)-H+ and Na(+)-Li+ exchanges in a young adult black population characterized for blood pressure and insulin-mediated glucose disposal. Normotensive and mildly hypertensive blacks (blood pressure, 120 +/- 2/76 +/- 2 and 139 +/- 3/94 +/- 2 mm Hg, respectively) with a mean age of 26.1 years were studied for insulin sensitivity with the euglycemic hyperinsulinemic clamp (molar index of insulin sensitivity, M/I = moles glucose metabolized/insulin in milliliters of plasma). Na(+)-H+ exchange (U = mmol/L cell.h) was measured before and after the insulin clamp as a function of cell pH to determine the maximum transport rate. In the normotensive subjects, 18 were insulin sensitive (M/I = 9.37 +/- 0.6 x 10(4)) and 4 were insulin resistant (M/I = 3.64 +/- 0.6 x 10(4)). In the hypertensive subjects, 4 were insulin sensitive (M/I = 9.15 +/- 1.1 x 10(4)) and 16 were insulin resistant (M/I = 3.02 +/- 0.3 x 10(4)). The maximum rate of Na(+)-H+ exchange was significantly higher in all hypertensive vs normotensive individuals (35 +/- 3 vs 23 +/- 3 U, P < .005). Na(+)-H+ exchange activity was higher in insulin-resistant vs insulin-sensitive hypertensive subjects (40 +/- 3 vs 20 +/- 2 U, P < .001) but not in insulin-resistant normotensive subjects. Na(+)-Li+ exchange was not different in hypertensive and normotensive individuals but was higher in all insulin-resistant compared with all insulin-sensitive subjects (0.26 +/- 0.03 vs 0.16 +/- 0.02 U, P < .01). Na(+)-Li+ exchange also was higher in insulin-resistant vs insulin-sensitive normotensive subjects (0.35 +/- 0.03 vs 0.15 +/- 0.02 U, P < .001) and in insulin-resistant hypertensive subjects vs insulin-sensitive normotensive subjects (0.24 +/- 0.03 vs 0.15 +/- 0.02 U, P < .001). A stepwise multiple regression analysis for all variables revealed that with Na(+)-H+ exchange as a dependent variable the main determinant was blood pressure, which in turn had insulin sensitivity as the main determinant. In conclusion, these results indicate that in hypertensive blacks, insulin-resistant glucose disposal is strongly associated with elevated red blood cell Na(+)-H+ exchange activity. Thus, despite impaired insulin-mediated glucose disposal, cellular Na+ gain via enhanced activity of Na(+)-H+ exchange is not blunted in hypertensive blacks.

The familial risk of end-stage renal disease in African Americans

African Americans have higher overall incidence rates of end-stage renal disease (ESRD) compared with American whites. Hypertensive nephrosclerosis (HN), nephropathy secondary to diabetes mellitus types I and II, and chronic glomerulonephritis (CGN) all occur more frequently in African Americans. To explore the possibility that hereditary factors may play a role in the increased risk of ESRD in African Americans, the family history of 131 African American hemodialysis patients (cases) was compared with 115 age-, sex-, and race-matched non-ESRD controls. Odds ratios (ORs) were calculated to define the prevalence of a relative with ESRD among cases versus controls. Chi-square values were estimated from a log-linear model, while controlling for gender, to test for significance of ORs. Forty percent (12/30) of HN cases, 35% (18/51) of type II diabetes mellitus-induced renal failure cases, and 13% (5/38) of CGN cases had a first-, second-, or third-degree relative with ESRD. The presence of a first-degree relative with ESRD increased an African American's risk for developing ESRD ninefold (OR, 9.13; 95% confidence interval [CI], 2.6 to 31.8; P < 0.001). The presence of a first- or second-degree relative increased the risk fivefold (OR, 5.23; 95% CI, 2.2 to 12.3; P < 0.0002). First-, second-, or third-degree relatives with ESRD were more prevalent among cases with ESRD due to hypertension and type II diabetes mellitus compared with CGN (P < or = 0.05). Gender differences among the ORs were nonsignificant (P > 0.2) and socioeconomic class (level of education and income) did not differ markedly between cases and controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Racial aspects of blood pressure in children and adolescents

The incidence, prevalence, and severity of essential hypertension are higher in minority adult populations, especially black Americans. Studies have not uniformly shown that black children and adolescents have higher blood pressure values than whites of the same age. The goal of this article is to review data available comparing minority and white populations. A section discussing studies addressing racial differences in related topics is included to demonstrate the existing fields of research that may not be familiar to the general pediatrician.

Plasma lipids affect maximum velocity not sodium affinity of human sodium-lithium countertransport: distinction from essential hypertension

Inheritance is a major determinant of increased sodium-lithium countertransport (SLC) activity in hypertension. However, hyperlipidaemia can also cause increased SLC activity in some individuals and it is difficult to distinguish this effect from the effect of hypertension. Erythrocyte SLC activity and its kinetic determinants sodium affinity (km) and maximum velocity (Vmax) were measured in 25 hyperlipidaemic patients and 15 normal controls (NC). Increased SLC activity (0.31 +/- SEM 0.03 mmol Li/(h x 1 cells) vs. NC 0.20 +/- 0.01, P < 0.01) in the hyperlipidaemic patients was associated with increased Vmax (0.59 +/- 0.07 vs. NC 0.41 +/- 0.03, P < 0.01) but normal km (median 120 range [40-324] mmol l-1 vs. 140 [108-260]. Lipid-lowering therapy resulted in decreased SLC activity secondary to a fall in Vmax. Km remained constant despite the changes in lipids and Vmax. The mechanism of increased SLC activity in hyperlipidaemia is different from that in essential hypertension where increased sodium affinity is found. Measurement of the kinetic characteristics of SLC may discriminate between the independent influences of hypertension and hyperlipidaemia on the sodium-lithium countertransporter.

Insulin resistance, hyperinsulinemia, dyslipidemia, hypertension, and accelerated atherosclerosis

Hypertension is only one component of a multifaceted metabolic-hemodynamic complex that also includes obesity, subtle and overt glucose intolerance, dyslipidemia, enhanced vascular resistance and accelerated atherosclerosis. Results of a number of studies in the past 5 years have shown that even nonobese, nondiabetic individuals with hypertension display insulin resistance, which is located in peripheral tissues (primarily skeletal muscle), is limited to nonoxidative pathways of glucose disposal, and appears to be directly correlated with the severity of hypertension. Insulin resistance and associated hyperinsulinemia in hypertensive individuals are also associated with increased plasma triglyceride levels and decreased high-density lipoprotein concentrations, which likely contributes to enhanced atherosclerosis. Hyperinsulinemia may directly promote atherosclerosis by enhancing LDL-cholesterol accumulation in vessel walls, vascular smooth muscle migration, and proliferation, augmenting connective tissue synthesis in the vascular wall, and decreasing the regression of lipid plaques. The enhanced peripheral vascular resistance that characterizes insulin resistance/hyperinsulinemic states may be related to decreased vascular smooth muscle responses to insulin, which normally modulates (attenuates) vascular contractile responses to vasoactive agents.

Acute effects of angiotensin-converting enzyme inhibitor on erythrocyte sodium ion transport in essential hypertension

The acute effects of angiotensin-converting enzyme inhibitor, captopril, on sodium ion transport systems were investigated in essential hypertensive and normotensive subjects. The passive sodium efflux through the erythrocyte membrane was significantly higher and erythrocyte sodium-potassium cotransport was lower in patients with essential hypertension when compared with normal subjects. However, sodium-potassium pump activity and sodium-lithium countertransport did not differ significantly between the hypertensive patients and the normal subjects. Immediately after captopril administration, erythrocyte passive sodium efflux and sodium-potassium cotransport returned to normal levels in the hypertensive subjects. Although the plasma renin activity and plasma aldosterone concentration were altered by captopril, they did not correlate with changes in any sodium transport system. These results suggest that the changes in sodium transport systems which occur immediately after captopril administration may contribute, at least in part, to its antihypertensive action.

Erythrocyte sodium-lithium countertransport in Chinese: its relationship to family history of hypertension

Rates of sodium (Na+)-stimulated lithium (Li+) efflux (Na(+)-Li+ countertransport) and ouabain-sensitive Na+ efflux (Na+ pump) were determined in erythrocytes of Chinese normotensive and hypertensive subjects. Near-maximal rate of Na(+)-Li+ countertransport was found to be significantly higher in hypertensive than normotensive subjects. No significant difference was observed for the rate of Na+ pump between them. A second series of study involved normotensive subjects without and with hypertensive parent(s) (group A and B, respectively) and hypertensive subjects (group C). We found that the rate of Na(+)-Li+ countertransport in group A was significantly lower than that of group B and C, while no difference existed between group B and C. No significant difference was observed for the rate of Na+ pump among the three groups. Our results suggested that Na(+)-Li+ countertransport activity could be a genetic marker for essential hypertension in Chinese, similar to that as proposed in Caucasians.

Exclusion of the Na(+)-H+ antiporter as a candidate gene in human essential hypertension

The primary abnormalities that contribute to the pathogenesis of human essential hypertension are unknown. The known genetic contribution to this disorder suggests the possible use of genetic linkage analysis to test whether specific candidate genes contribute to the pathogenesis of either essential hypertension or intermediate phenotypes. Among such phenotypes, elevated erythrocyte Na(+)-Li+ countertransport (SLC) is the best known, supporting major gene inheritance by pedigree analysis. Striking similarities between SLC and Na(+)-H+ exchange suggest that mutations at the Na(+)-H+ antiporter gene locus (APNH) might result in elevated SLC and contribute to the subsequent pathogenesis of hypertension. We have tested these hypotheses by genetic linkage analysis, with APNH as a candidate gene. By determining genotypes at APNH and flanking loci in pedigrees that support major gene segregation of elevated SLC, we have excluded linkage of APNH and the major SLC locus with a LOD score of -5.91, an odds ratio of almost 1,000,000:1 against linkage. In the analysis of 93 hypertensive sibling pairs, we have further demonstrated that APNH explains none of the variance in SLC in hypertensive individuals (r2 = 6 x 10(-7), p greater than 0.99). Finally, we have directly tested for linkage of APNH to genes predisposing toward hypertension by linkage in hypertensive sibling pairs. Mean allele sharing at APNH is not greater than expected from random assortment in hypertensive siblings (0.92 versus 1.0, p greater than 0.80), and the upper 95% confidence limit of this value (1.04) indicates that mutations at APNH rarely if ever contribute to the pathogenesis of hypertension in this population.(ABSTRACT TRUNCATED AT 250 WORDS)

Red blood cell Na+ transport as a predictor of blood pressure response to Na+ load in young blacks and whites

The present study was designed to investigate the role of abnormalities in red blood cell sodium-potassium-chloride (Na-K-Cl) cotransport and Na+ pump as predictors of the pressor response to chronic oral Na+ loading in young whites and blacks. Subjects were healthy adults from 18 to 23 years of age and included normotensive whites (n = 24) and normotensive blacks (n = 35). Red blood cell transport studies were performed before Na+ loading. The Na+ load consisted of 10 g NaCl daily added to the usual diet. A sodium-sensitive response was defined as an increase of 5 mm Hg or more in mean arterial pressure after the Na+ load; a sodium-insensitive response was a less than 5 mm Hg increase in mean arterial pressure. A sodium-sensitive response occurred in 16% of whites and 57% of blacks. Black subjects have a significantly lower (p less than 0.005) maximal rate of furosemide-sensitive Na+ efflux and a higher Km for cellular Na+ (p less than 0.05) to activate Na-K-Cl cotransport than white subjects. Normotensive blacks with sodium-sensitive blood pressure response had a higher Km (14.4 +/- 6 mmol/l cell, n = 17, mean +/- SD) to activate the cotransport than sodium-insensitive blacks (9.9 +/- 3.7 mmol/l cell, n = 13, p less than 0.001). Normotensive whites had a significantly lower red blood cell Na+ content (p less than 0.05) and a higher maximal rate of cotransport (p less than 0.005) than young normotensive blacks.(ABSTRACT TRUNCATED AT 250 WORDS)

Variations in the apparent pH set point for activation of platelet Na-H antiport

To explore the role of the Na-H antiport in essential hypertension, we studied the kinetics of cytosolic pH and external sodium activation of this transport system in platelets from 65 normotensive and essential hypertensive subjects on and off antihypertensive medications. Subjects included both blacks and whites, as well as men and women. The fluorescent dye 2'7-bis(carboxyethyl)-5,6-carboxyfluorescein was used to monitor the cytosolic pH in these cells. Platelets from black (hypertensive and normotensive) men and hypertensive white men demonstrated a highly significant alkaline shift in the apparent cytosolic pH set point for activation of the Na-H antiport. For the hypertensive subgroups, the cytosolic pH set point values (mean +/- SEM) were: white men, 7.45 +/- 0.052; white women, 7.04 +/- 0.089; black men, 7.66 +/- 0.148; and black women, 7.20 +/- 0.082. For the normotensive subgroups, the cytosolic pH set point values were: white men, 7.13 +/- 0.034; white women, 7.05 +/- 0.036; black men, 7.50 +/- 0.110; and black women, 7.20 +/- 0.176 (p = 0.0016 for race and p = 0.0001 for gender, using a three-way analysis of variance by race, gender, and hypertension). There were no race-, gender-, or blood pressure-related differences among the various cohorts in the kinetics of sodium activation of the Na-H antiport, the cellular buffering power, and basal pH. These results suggest that at basal pH the Na-H antiport is quiescent in platelets from both black and white women and normotensive white men.(ABSTRACT TRUNCATED AT 250 WORDS)

Erythrocyte concentrations and transmembrane fluxes of sodium and potassium in essential hypertension: role of intrinsic and environmental factors

The intraerythrocyte sodium concentration is increased in the erythrocytes of Zaïrean Bantu with untreated hypertension, while the red blood cell potassium is not different from that of normotensive subjects. Compared with whites, normotensive healthy blacks have a higher intracellular concentration of sodium due to a depressed activity of the sodium-potassium pump. Normotensive healthy males with a positive familial background of hypertension display higher erythrocyte sodium and lower cotransport activity. None of the two measurements offer a clear-cut genetic marker of essential hypertension. In healthy women, the erythrocyte sodium concentration is lowered during the luteal as compared with the follicular phase of the menstrual cycle. This variability explains the difference observed between men and women. A low-sodium diet stimulates the activity of the sodium-potassium ATPase pump, which leads to a decrease in the erythrocyte sodium concentration. Both alterations reverse only slowly during sodium repletion. It is therefore suggested that an adequate matching for race, sex, stage of the menstrual cycle (in women), family history of hypertension, and the amount of sodium in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium.

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.

Changes in sodium-lithium countertransport correlate with changes in triglyceride levels and body mass index over 2 1/2 years of follow-up in Utah

We have previously reported from a cross-sectional study that plasma total cholesterol, triglycerides, and HDL-C were significantly and independently correlated with Na(+)-Li+ countertransport. These original participants were rescreened 2 1/2 years later (range of 20-58 months), with lipid, blood pressure, and Na(+)-Li+ countertransport measurements from both visits on 906 normotensive adults. The correlation found between age- and sex-adjusted triglyceride levels and Na(+)-Li+ countertransport at visit 1 (r = 0.34, p less than 0.0001) was reconfirmed at visit 2 (r = 0.32, p less than 0.0001). The Na(+)-Li+ countertransport correlations with HDL-C (r = -0.11, p less than 0.01) and body mass index (r = 0.24, p less than 0.0001) also remained at visit 2. After 30 months, there were significant increases of triglyceride, body mass index, blood pressure, and Na(+)-Li+ countertransport levels, and significant decreases of HDL-C and total cholesterol levels, after adjusting the changes in these variables between visit 2 and visit 1 for age, sex, time between the two visits, and the visit 1 level of the variable. Increases in triglycerides, cholesterol, and body mass index were significantly correlated with increases in Na(+)-Li+ countertransport (r = 0.23, r = 0.19, and r = 0.21, respectively). The correlations of the lipid and lipoprotein changes with Na(+)-Li+ countertransport changes were independent of body mass index and blood pressure changes. We conclude that increasing plasma triglyceride levels and body mass index are associated with increasing Na(+)-Li+ countertransport levels in both cross-sectional and longitudinal data.

Typology of Na+ transport abnormalities in erythrocytes from essential hypertensive patients. A first step towards the diagnosis and specific treatment of different forms of primary hypertension

Over the last 5 years, several authors have measured apparent affinities and maximal translocation rates of the different erythrocyte Na+ transport systems in essential hypertensive patients. These kinetic studies have clearly shown that no unique red cell Na+ transport defect characterizes the whole population of essential hypertensive patients. Conversely, several complex patterns of erythrocyte Na+ transport abnormalities may be present in different subsets of essential hypertensive patients. These kinetic studies are now providing a more profound biochemical insight into the molecular heterogeneity of primary hypertension. In particular, they may permit the diagnosis and specific treatment of different forms of primary hypertension in the next decade.

Race, sex, and family history of hypertension and erythrocyte sodium pump [3H]ouabain binding

We studied the binding properties of [3H]ouabain to erythrocytes from normotensive children (n = 83) between the ages of 10 and 18 years (mean resting arterial pressure: 102/57 mm Hg) from normotensive and essential hypertensive parents. Arterial blood pressures of 101/57 and 104/57 mm Hg (subjects with normotensive and hypertensive parents, respectively) were not significantly different between the two groups. Forty-four children had normotensive parents and 39 had hypertensive parents, 51 were white and 32 were black, and 41 were girls and 42 were boys. By using the [3H]ouabain-binding technique, we determined the density of sodium pump sites and the equilibrium dissociation constants in erythrocytes from these children. Possible effects of race, sex, or parental hypertension status on pump sites and dissociation constants were tested with a three-way analysis of variance (ANOVA). Race had a major effect on the dissociation constant: blacks had a significantly higher value than did whites (p = 0.002). We also found a race by sex by parental hypertension status interaction (p = 0.04) with black girls with hypertensive parents having the highest value. There was no effect of race, sex, or status on sodium pump site density. Age, height, weight, resting arterial blood pressure, and plasma Na+ and K+ concentrations did not correlate with the dissociation constants. These data suggest that, among the groups we studied, black girls with hypertensive parents had erythrocytes with the lowest binding affinity for ouabain.(ABSTRACT TRUNCATED AT 250 WORDS)

Red blood cell lithium-sodium countertransport in non-modulating essential hypertension

Abnormalities in erythrocyte Li-Na countertransport have been reported in hypertensive subjects, and the available evidence favors familial aggregation and striking heritability of this marker. It is uncertain, however, whether the abnormalities are associated with hypertension per se or whether they may be concentrated in a particular subset of hypertensive subjects. In the present study, maximal rates of Li-Na countertransport were measured in red blood cells of 82 white subjects, including 37 normotensive subjects and 45 normal- or high-renin hypertensive subjects previously classified as non-modulators (n = 21) or modulators (n = 24). Mean countertransport activity was significantly higher in non-modulators compared with normally modulating hypertensive or normotensive subjects (0.475 +/- 0.044 vs. 0.309 +/- 0.028 or 0.249 +/- 0.012 mmol/l cell x hr, respectively, p less than 0.001). Modulators did not differ significantly from normotensive subjects with regard to mean countertransport activity. Red blood cell sodium pump and Na-K-Cl cotransport were not significantly different in modulating and non-modulating hypertensive subjects. These relations remained unchanged after adjusting for age, body weight, and plasma cholesterol levels by analysis of covariance. A countertransport value exceeding 0.50 mmol/l cell x hr occurred in 40% of the non-modulators but in only one of the other subjects. In contrast , while one half of the modulators and normotensive subjects had a countertransport value less than 0.235 mmol/l cell x hr, none of the non-modulators did. Thus, elevated countertransport appears to aggregate in the non-modulating subset of essential hypertensive subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Erythrocyte sodium transport and blood pressure in white subjects

The mechanisms that define the relation between blood pressure and sodium handling are not yet well understood. Although several abnormalities in sodium transport have been associated with hypertension, a link between the blood pressure of normotensive subjects and the erythrocyte sodium-potassium adenosine triphosphatase pump, the principal sodium transporter of sodium, has not been previously demonstrated. Data from independent measurements of erythrocyte intracellular sodium, ouabain-sensitive sodium efflux, and the number of sodium pump sites per red blood cell were used to calculate a second-order rate constant for ouabain-sensitive sodium efflux. Among 20 normotensive white subjects, this rate constant correlated significantly (p less than 0.005) with mean arterial blood pressure. A significant correlation was not observed between the rate constant and the blood pressure of 22 hypertensive subjects. A hypothesis is proposed, which suggests that the sodium efflux rate constant of erythrocytes is related to the control of sodium reabsorption via the sodium pump of the renal tubules and that an elevated erythrocyte rate constant may be associated with chronic increased sodium reabsorption, which leads to volume expansion and the development of hypertension.

Sodium-lithium countertransport and blood pressure: the Gubbio Population Study

The relation of red blood cell sodium-stimulated lithium countertransport to blood pressure (BP) and prevalence of hypertension was assessed in univariate and multivariate analyses for 2,748 men and women aged 25-74 years who participated in the baseline examination of the Gubbio Population Study in north central Italy. Since age-specific countertransport values were consistently higher for men than women, all analyses were done for men and women separately. In simple correlation analyses, countertransport was significantly related to systolic and diastolic BP in both sexes (r values 0.107-0.163). In age-adjusted analyses, countertransport was significantly related to BP level of both men and women not receiving antihypertensive treatment; mean levels were high for hypertensive persons receiving antihypertensive therapy compared with normotensive persons. Age-adjusted prevalence rates of hypertension were progressively higher for both sexes in successively higher quintiles of countertransport, almost twice as high for those in the highest quintile compared with those in the lowest quintile. Correspondingly, age-adjusted logistic regression analyses showed countertransport to be related significantly to prevalence of hypertension for both men and women (p less than 0.001). Since age, body mass index, plasma total cholesterol, uric acid, glucose, urinary sodium/potassium excretion, pulse, and (for men) daily alcohol intake also were significantly correlated with BP, and in some instances with countertransport, relation of countertransport to BP was also assessed in multivariate analyses with control for these variables.(ABSTRACT TRUNCATED AT 250 WORDS)

Red-cell sodium-lithium countertransport and fractional excretion of lithium in normal and hypertensive humans

To examine the relations between erythrocyte sodium-lithium countertransport and renal proximal tubular sodium handling, we measured countertransport, and then subjected 30 normal and 32 hypertensive subjects, both white and black, to provocative maneuvers of volume expansion and contraction. The fractional excretions of sodium and lithium were measured simultaneously. In agreement with previous studies, we found that countertransport in erythrocytes was elevated in hypertensive patients compared with normal subjects. We also observed that whites have a higher level of countertransport than blacks. In the basal state, we found that fractional sodium excretion of hypertensive patients was no different than in normal subjects, whereas the fractional lithium excretion of hypertensive persons was increased compared with normotensive values. Volume expansion with 2 1 0.9% saline administered intravenously during a 4-hour period provoked an exaggerated natriuresis and a greater increase in fractional lithium clearance in hypertensive patients compared with the control group. With volume expansion and contraction, fractional lithium clearance and countertransport were directly correlated. Our data suggest that hypertensive persons do not have increased proximal tubular sodium reabsorption compared with normal subjects. Further, the exaggerated natriuresis of hypertension is, in part, the result of increased distal solute delivery. The fact that our hypertensive patients were older may partially explain the discrepancies between this report and previous observations.

The Na-K-Cl cotransport in essential hypertension: cellular functions and genetic environment interactions

The present paper examines factors involved in the expression of the Na-K-Cl cotransport system present in erythrocytes and in vascular cells. This transport system is modulated by vasoactive peptides such as atrial natriuretic factor and bradykinin in vascular smooth muscle and endothelial cells. The Vmax of the Na-K-Cl in human red cells displays large interindividual differences which can be mainly accounted for by genetic factors. Elevation of the Km for Na of the outward Na cotransport is found in red cells of some Caucasian hypertensives and in Black normotensives born of hypertensive parents whose blood pressure increases with salt loading. Reduction of Na intake from 200 to 10 mEq/day does not influence the activity of the cotransport in normotensive individuals but decreases the Km for Na of hypertensive subjects to values similar to those of the normotensives. These findings indicate that the Na-K-Cl cotransport is an important probe of genetic and environmental factors in the hypertensive process.

Influence of race, sex, and blood pressure on erythrocyte sodium transport in humans

Sodium transport of erythrocytes from normotensive and essential hypertensive subjects was evaluated by determining ouabain-sensitive and ouabain-insensitive sodium efflux rates, Na+-Li+ countertransport rates, Li+-K+ cotransport rate constants (lithium replacing sodium), intracellular sodium concentrations, and the number of Na+,K+-adenosine triphosphatase (ATPase) sites per erythrocyte. Subjects included men and women, blacks and whites. Hypertensive subjects had significantly higher sodium transport than did normotensive subjects for ouabain-sensitive sodium efflux (p less than 0.025) and Na+-Li+ countertransport (p less than 0.001). Sexual differences were noted for ouabain-sensitive (p less than 0.001) and ouabain-insensitive (p less than 0.001) sodium efflux, for intracellular sodium concentration (p less than 0.025), and for the Li+-K+ cotransport rate constant (p less than 0.005), all with higher values for men than for women. Racial differences were noted for ouabain-insensitive sodium efflux (p less than 0.005), Na+-Li+ countertransport (p less than 0.001), and the Li+-K+ cotransport rate constant (p less than 0.001); values were higher in whites than blacks for all three measurements. The number of [3H]ouabain binding sites was lower for blacks (p less than 0.001) and the intracellular sodium concentration was higher for blacks (p less than 0.001). Among all subjects, significant (p less than 0.001) correlations were found between intracellular sodium concentration and the number of Na+,K+-ATPase sites per erythrocyte (r = -0.78) and between the ouabain-sensitive sodium efflux per site and intracellular sodium concentration (r = 0.85, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Profile of systemic hypertension in black patients

Hypertension extracts a huge toll from the black community in terms of excess morbidity and mortality. The black hypertensive is more likely to die from the disease and to have stroke, end-stage renal disease or heart failure. Furthermore, contrary to previous beliefs, blacks are at least as likely to have coronary artery disease as whites. Although substantial overlap occurs, the black hypertensive is more likely to be volume-expanded, to have a lower plasma renin level, and to be classified, as salt-sensitive than is the white hypertensive. Decreased dietary potassium and calcium intake, altered intra-cellular handling of sodium and calcium, and psychosocial factors have also been implicated in the pathophysiology of hypertension in blacks.

Hypertension, non-insulin-dependent diabetes, and intracellular sodium metabolism

The present study was designed to investigate whether non-insulin-dependent diabetic hypertensive patients exhibit abnormalities in intracellular sodium metabolism similar to those described for essential hypertensive patients. Both normotensive and hypertensive non-insulin-dependent diabetic patients had similar average values of both Na+-Li+ countertransport and Na+-K+ cotransport compared with nondiabetic controls. Within the group of diabetic patients, hypertensive patients did not exhibit any abnormalities in either of the sodium transport pathways studied. The possible implications of these findings are addressed.

Sodium transport kinetics in erythrocytes from spontaneously hypertensive rats

Rat erythrocytes with five different amounts of Na+ content have been prepared by using a new, nondetrimental Na+-loading method (net NaHPO4-influx through the anion carrier). This method allowed the determination of 1) maximal translocation rates and apparent dissociation constants for internal Na+ of the Na+-K+ pump, outward Na+-K+ cotransport, and Na+-Li+ countertransport and 2) rate constants of Na+ leak in erythrocytes from spontaneously hypertensive rats of the Okamoto strain and Wistar-Kyoto normotensive controls aged 2 to 26 weeks. Two major abnormalities were found in erythrocytes from spontaneously hypertensive rats: 1) a decreased cotransport affinity for internal Na+, which was constantly observed from 2 to 26 weeks of age (mean intracellular Na+ content for half-maximal stimulation of outward Na+-K+ cotransport = 33.1 +/- 7.0 [SD] mmol/L cells in spontaneously hypertensive rats vs 16.7 +/- 4.7 mmol/L cells in Wistar-Kyoto rats; p less than 0.001), and 2) a decreased maximal pump rate in adult (15- to 26-week-old) spontaneously hypertensive as compared with that for age-matched Wistar-Kyoto rats (9-37 vs 34-70 mmol/L cells/hr). Therefore, the low cotransport affinity for internal Na+ appears to be a stable, possibly genetic defect of spontaneously hypertensive rats. Conversely, the decreased maximal pump rate may be a secondary event, possibly reflecting the appearance of endogenous pump inhibitors in the plasma of adult spontaneously hypertensive rats.

The Li+-Na+ exchange and Na+-K+-Cl- cotransport systems in essential hypertension

This review examines the physiological functions of the Li+-Na+ exchanger and Na+-K+-Cl- cotransport system in human red blood cells. Both transporters are family aggregated and determined mainly by genetic factors; they are present in kidney and vascular cells, where they are regulated by vasoactive substances. To assess the physiological function of these two transporters, we investigated their kinetic and equilibrium properties, and their modulation by vasoactive substances. Recent studies in red blood cells indicate that the Li+-Na+ exchanger may be a mode of operation of the Na+-H+ exchanger, which plays an important role in the regulation of cell pH, cell volume, and transtubular sodium transport. In vascular cells, Na+-H+ exchanger is modulated by vasoconstrictors such as growth factors and angiotensin, while Na+-K+-Cl- cotransport is modulated by vasodilators such as atrial natriuretic factor and bradykinin. Kinetic studies in red blood cells of hypertensive patients and their offspring indicate the presence of subsets with elevated Vmax of Li+-Na+ exchange or high Km for cell sodium for outward Na+-K+-Cl- cotransport. The latter alteration is found most frequently in young blacks born of hypertensive parents, and it appears to be dependent on their level of sodium intake. The relationship between the alterations of the red blood cell sodium exchanger and Na+-K+-Cl- cotransport and risk factors for hypertension indicates that they can provide a tool to examine the interaction of genetic, hormonal, and environmental factors in human hypertension.

Erythrocyte ghost Na+,K+-ATPase and blood pressure

To examine the relationship between body mass index, blood pressure, and the Na+,K+-adenosine triphosphatase (ATPase) system, we measured the erythrocyte ghost Na+,K+-ATPase and the erythrocyte Na+ concentration in 120 blacks and 127 whites (136 males and 111 females). Blacks showed a 13.9% higher erythrocyte Na+ (7.63 +/- 0.19 vs 6.70 +/- 0.11 [SEM] mEq/L; p = 0.0001) and a 16.1% lower erythrocyte ghost Na+,K+-ATPase activity (140.3 +/- 4.2 vs 167.3 +/- 4.7 nmol inorganic phosphate/mg protein/hr; p = 0.0002) than whites. Male subjects demonstrated a 6.4% higher erythrocyte Na+ (7.35 +/- 0.17 vs 6.91 +/- 0.14 mEq/L; p = 0.043) and an 11.5% lower Na+,K+-ATPase activity (145.7 +/- 3.7 vs 164.7 +/- 5.5 nmol inorganic phosphate/mg protein/hr; p = 0.0015) than female subjects. Significant (p less than 0.001) negative correlations were identified for the systolic, diastolic, and mean blood pressure levels and the erythrocyte ghost Na+,K+-ATPase. These findings were complemented by positive correlations for the blood pressure levels and erythrocyte Na+ concentrations. The body mass index was negatively correlated with erythrocyte ghost Na+,K+-ATPase and it accounted for 6.7%, 5.6%, and 6.1% of the variabilities in the systolic, diastolic, and mean blood pressure levels, respectively. Variabilities of 1.4% systolic, 12.3% diastolic, and 11.1% in mean arterial pressure were attributable to the erythrocyte ghost Na+,K+-ATPase activity. Provided that findings in erythrocytes also reflect the relative status of the vascular smooth muscle cell Na+,K+-ATPase, the predisposition of black, male, and obese persons to hypertension may relate, among other factors, to a lower activity of this enzyme system, which results in an increased vascular tone.

Natural variation in passive sodium permeability in human erythrocytes

The rate of influx of 22Na+ into human erythrocytes (RBC) varies greatly depending upon the donor. A high rate of influx may be related to a congenital predisposition to essential hypertension. In Northern Europeans, we find a threefold difference in the rate of 22Na+ influx between those with the least (LP) and most highly permeable (HP) RBC (from less than 0.15 to greater than 0.60 mmol Na+/liter RBC/hr). In order to further define determinants of these apparently hereditary differences in passive membrane Na+ transport, we identified two groups of normal laboratory and hospital personnel differing markedly (greater than twofold) in RBC 22Na+ influx rate. We find that the loop diuretics furosemide and bumetanide decrease by about 50% the influx of 22Na+ into HP RBC, but have a lesser influence on LP RBC. Impermeant polyanions such as citrate and pyrophosphate also specifically diminish 22Na+ influx into HP, but not LP, RBC. Therefore, the exaggerated 22Na+ influx into HP RBC probably occurs through a discrete pathway (perhaps via "Na/K/Cl cotransport"), which appears to be almost absent in LP RBC. The differences between HP and LP RBC most likely do not involve polymorphisms of RBC anion transport per se. The rate of RBC anion (35SO4(2-)) transport is the same in HP and LP RBC and is equally inhibited by furosemide and (to a lesser extent) bumetanide. Furthermore, the potent inhibitor of RBC anion transport, DIDS (diisothiocyanostilbene disulfonate) does not affect RBC Na+ permeability in either group. Nonetheless, the preferential reduction of Na+ permeation of HP RBC by loop diuretics may be of help in experimentally distinguishing HP from LP phenotypes. This information may be crucial in unraveling the structural basis of intrinsic differences in cell membrane Na+ permeability and their possible relationship to essential hypertension.