Effect of dietary sodium reduction on red blood cell sodium concentration and sodium-lithium countertransport

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.

Erythrocyte sodium transport, intraplatelet pH, and calcium concentration in salt-sensitive hypertension

We evaluated changes in erythrocyte sodium transport systems, platelet pH, and calcium concentration induced by low and high salt intakes in a group of 50 essential hypertensive patients classified on the basis of their salt sensitivity. Patients received a standard diet with 20 mmol NaCl daily for 2 weeks supplemented in a single-blind fashion by placebo tablets the first 7 days and NaCl tablets the following 7 days. Salt sensitivity, defined as a significant rise (P <.05) in 24-hour mean blood pressure obtained by ambulatory blood pressure monitoring, was diagnosed in 22 (44%) patients. The remaining 28 (56%) were considered to have salt-resistant hypertension. In the entire group of hypertensive patients, high salt intake promoted a significant increase (P <.05) in the maximal rate of erythrocyte NA(+)-Li(+) countertransport (from 271 +/- 19 to 327 +/- 18 microM/(L cells/h) and of the Na(+)-dependent HCO3(-)-CL(-) exchanger (from 946 +/- 58 to 1237 +/- 92 microM/L cells/h) as well as in platelet pH (from 7.15+/-0 0.01 to 7.19+/-0.02 and calcium concentration (from 49+/-2 to 57 +/-2 nmol/L). Depending on salt sensitivity, high salt intake promoted opposing changes in some of the sodium transport systems studied. Salt-sensitive patients increased the maximal rate of the erythrocyte Na(+)-K(+) pump (fom 7.0 +/- 0.4 to 8.8 +/- 0.4 mmol/(L cells/h), Na(+)-K(+)-Cl(-) cotransport (from 416 +/- 37 to 612 +/- 41 micromol/(L cells/h), Na(+)-Li(+) countertransport (from 248 +/- 20 to 389 +/- 17 micromol/(L cells/h) at the end of the high salt period. Conversely, salt-resistant patients decreased the Na(+)-K(+) pump (from 8.0 +/- 0.4 to 6.9 +/- 0.3 mmol/(L cells/h) and Na(+)-K(+)-Cl(-) cotransport (from 578 +/- 53 to 481 +/- 43 micromol/(L cells/h). We conclude that modulation of erythrocyte sodium transport systems by high salt intake depends on salt sensitivity. The Na(+)-K(+) pump, Na(+)-K(+)-Cl(-) cotransport, and Na(+)-Li(+) countertransport increase in salt-sensitive patients, whereas the activity of these sodium transport systems tends to decrease in salt-resistant patients. Independent of salt sensitivity, high salt intake promotes a significant increase in the erythrocyte Na(+)-dependent HCO3(-)-Cl(-) exchanger, platelet pH, and calcium concentration in essential hypertensive patients.

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.

Sodium-lithium countertransport activity is decreased after weight loss in healthy obese men

Maximal red blood cell (RBC) sodium-lithium countertransport activity has been consistently related to essential hypertension and may be a marker for risk of developing hypertension. Although there is strong evidence for genetic control of sodium-lithium countertransport, increasing evidence suggests that obesity and insulin-glucose metabolism are related to countertransport activity. This study was performed to determine whether countertransport activity decreases with weight loss in healthy obese adults. Forty-five healthy, white, obese adults were studied at baseline and after 6 months of behavioral dietary intervention. Weight loss was 11.5 kg (25.4 lb) in 24 men and 8.1 kg (17.8 lb) in 21 women. Sodium-lithium countertransport activity decreased 55.0 mumol Li/L RBC/h in men (P < .001, paired t test) and 14.6 mumol Li/L RBC/h in women (NS). Change in countertransport activity was correlated with change in body mass index (BMI) in men (r = .52, P < .01) and women (r = .27, NS) and was also strongly correlated with change in fasting glucose levels in both men and women (r = .50 and r = .56, respectively; P < .01) and with change in fasting insulin levels in men (r = .42, P = .04). Change in countertransport activity was not significantly related to change in physical exercise or serum lipid levels. There was a large decrease in systolic blood pressure in men (10.0 mm Hg, P < .001) and a smaller decrease in women (4.1 mm Hg, P < .05). These changes were significantly correlated with change in weight, but not with change in countertransport or baseline countertransport activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlates of sodium-lithium countertransport. Findings from the Gubbio Epidemiological Study. The Gubbio Collaborative Study Group

BACKGROUND

Numerous reports have presented evidence for a positive association between the maximal velocity of the sodium-lithium countertransport (Na-Li CT) in erythrocytes and hypertension. The nature of this association remains to be clarified.

METHODS AND RESULTS

This report presents correlates of Na-Li CT in a population sample of 3,591 people aged 5-74 years. Males had higher mean age-specific levels of Na-Li CT than females except for the 5-14-year age stratum. In adults aged 25-74, for both men (n = 1,044) and women (n = 1,192), body mass index, plasma uric acid and glucose, alcohol consumption, and red blood cell mean corpuscular volume were positively related to countertransport in multivariate analyses; plasma high-density lipoprotein (HDL) cholesterol and plasma potassium were inversely related. Plasma non-HDL cholesterol was independently and directly related to Na-Li CT in women, and plasma sodium was inversely associated with Na-Li CT in men. These relations prevailed for men when persons with hypertension were excluded from the analyses and prevailed in part for women. When stepwise regression analyses were done for all men and women combined (n = 2,236), sex ceased to be significantly related to countertransport with plasma uric acid and alcohol intake in the model. In adults of either sex, no independent association was detected between Na-Li CT and age, heart rate, or the ratios of sodium to potassium or of sodium to creatinine in overnight untimed urine.

CONCLUSIONS

In both sexes, Na-Li CT is significantly and independently associated with a number of metabolic variables (plasma uric acid, plasma glucose, body mass index, plasma potassium, and life-style habits [e.g., alcohol intake]). Further research is needed to elucidate the meaning of the significant associations between Na-Li CT and the foregoing variables (all of them also related to blood pressure).

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.

Erythrocyte Na+, K+ pump inhibition after saline infusion in essentially hypertensive subjects: effects of canrenone administration

The effects of a 2-litre isotonic saline infusion, with and without prior oral canrenone (150 mg) administration, on erythrocyte Na+, K+ pump, urinary sodium excretion and arterial pressure were evaluated in nine patients with essential hypertension. Ouabain-sensitive Na+ efflux in fresh erythrocytes was used as an index of Na+, K+ pump activity, and the inhibitory effect on this ion efflux of preincubation of erythrocytes in plasma was used to test the presence of a circulating ouabain-like substance. Erythrocyte Na+, K+ pump activity decreased significantly (P less than 0.01) after saline infusion; canrenone administration was able to prevent this inhibition. Plasma from hypertensive patients obtained before saline infusion significantly (P less than 0.01) inhibited the Na+, K+ pump of erythrocytes from normal subjects, while plasma taken after the saline infusion plus canrenone was unable to produce any significant inhibition. Both systolic and diastolic arterial pressure fell significantly (P less than 0.05) only at the end of saline infusion with prior canrenone administration. This study supports the hypothesis that protection of Na+, K+ pump against endogenous inhibitors, other than exogenous, seems to be a pharmacological effect of canrenone, and may partly explain its antihypertensive activity.

Erythrocyte sodium content and transport in borderline and mild hypertension

Sodium content and transport of red blood cells were examined in 98 male blood donors. Regarding their blood pressure they were classified into the following groups: (a) 57 normotensives, (RR less than 140/90 mm Hg); (b) 24 borderline hypertensives (140/90 less than or equal to RR less than 160/95 mm Hg); and (c) 17 hypertensives (RR greater than 160/100 mm Hg). Compared with the normotensives the borderline hypertensives have significantly reduced red cell sodium content. The ouabain-resistant net Na+ uptake and the relative Na+ uptake, as a measure of the Na+/K+ pump, were significantly increased. With rising blood pressures the measured values turn to normal, so that no difference exists between the normotensive and hypertensive groups. It is supposed that in the initial or even prehypertensive state a considerable enhancement of the pump activity occurs, simultaneously accompanied by less marked increases in sodium influx, leading to a reduced intracellular sodium content. In the course of hypertension, possibly caused by the formation of a pump inhibitor, the sodium content of red cells turns to normal or supernormal values.

The inheritance of intraerythrocytic sodium level

Intraerythrocytic sodium level (RBC Na) was measured on 1,800 normotensive members of 64 Utah pedigrees ascertained through hypertensive or normotensive probands, sibs with early stroke death, or brothers with early coronary disease. Likelihood analysis provided evidence that RBC Na was determined by four alleles at a single locus. Each allele was recessive to all alleles associated with a lower mean level. The four resultant distributions occurred in the frequencies: 0.8%, 89.3%, 9.7%, and 0.2% with corresponding means (mmol/1 RBC) of 4.32, 6.67, 9.06, and 12.19, respectively. The major locus explained 29.0% of the variance in RBC Na; polygenic inheritance explained another 54.6%. A higher frequency of the genotypes for high RBC Na in pedigrees when the proband was hypertensive than normotensive provided evidence that this major locus increases susceptibility to hypertension.

Epidemiological studies of sodium transport and hypertension

Red blood cell membrane cation transport was measured in five population-based surveys and two randomized, controlled, dietary intervention studies to examine its associations with demographic, biological, and dietary variables in free-living individuals. A total of 508 individuals, 255 with high blood pressure, were studied. Both sexes, blacks and whites, and several age groups were represented. The intervention studies included short-term dietary sodium restriction in normotensive adolescents, and a 4-year multifactorial trial on weight, sodium, and alcohol in hypertensive adults. The findings from these surveys and intervention studies are summarized in this report. Sodium-stimulated lithium countertransport was significantly related to diastolic blood pressure in white adults (r = 0.28, p less than 0.001), and to systolic blood pressure in black children (r = 0.50, p less than 0.005) and white adolescents (r = 0.31, p less than 0.05). Lithium countertransport was related to sex and race, but not age. Body mass index had an independent relationship with lithium countertransport in some age groups. Lithium countertransport was lower in normotensive adults than in both younger and older hypertensive adults. Lithium countertransport did not differ significantly between subjects with hypertension treated with antihypertensive medications and those with untreated hypertension. Short-term dietary sodium restriction did not influence lithium countertransport in normotensive adolescents. Long-term dietary intervention was associated with low lithium countertransport in hypertensive adults able to maintain blood pressure control without medication. These findings indicate that lithium countertransport is related to blood pressure and hypertension among free-living individuals.

Sodium-lithium countertransport and blood pressure in healthy blood donors

Studies finding an increased maximal rate of Na-Li countertransport in red blood cells from persons with essential hypertension and their normotensive offspring have raised the possibility that Na-Li countertransport may serve as a marker for the genetic predisposition to hypertension. We studied Na-Li countertransport in 238 randomly selected blood donors representative of the population of Rochester, Minnesota. The mean value (+/- SD) for Na-Li countertransport in units of mmoles of lithium efflux per liter of red blood cells per hour was 0.29 +/- 0.12. The distribution of Na-Li countertransport values among the donors was continuous. An analysis for multimodality, however, detected significant evidence of bimodality with 72% of the population predicted to belong to the lower mode with a mean of 0.24 mmol/L red blood cells per hour and 28% of the population to belong to the upper mode with a mean of 0.42 mmol/L red blood cells per hour. There was a positive association between Na-Li countertransport and blood pressure; after adjustment for weight and age, Na-Li countertransport predicted approximately 3% of the variation in blood pressure. Persons belonging to the upper mode of the Na-Li countertransport distribution may be at increased risk of acquiring elevated blood pressure as they age.