Erythrocyte sodium-potassium-ATPase and sodium transport in obesity

Altered Brown Adipose Tissue and Na,K Pump Activities During Diet-Induced Obesity and Weight Loss in Rats

Brown adipose tissue (BAT) thermogenesis is an uncoupled ATPase-independent thermogenic mechanism. Ion transport by the Na,K pump is an ATPase-dependent thermogenic mechanism. Both have been proposed as mechanisms of altered energy expenditure during states of dietary energy surfeit and deficit. Our aim was to study these mechanisms during diet-induced obesity and weight loss. Over 36 weeks rats were fed lard- or tallow-based diets (63% energy as fat), or a control diet (12% energy as fat). During periods of restriction rats were fed 50% of the energy intake of controls in the form of a control diet. Several components of thermogenic response increased in rats eating high fat diets and decreased following dietary restriction. BAT activation occurred, particularly with a lard-based diet, as indicated by increased GDP binding and uncoupling protein (UCP) content. Na,K pump activity in thymocytes increased with the feeding of both high fat diets at some time points. Plasma T3 level increased in rats eating the lard-based diet and decreased with dietary restriction regardless of previous diet. Resting metabolic rate (RMR) of the animals was unchanged despite increases in these thermogenic components and was decreased in all groups following dietary restriction. Our results indicate a lack of any major role for activated BAT thermogenesis in mitigating the extent of the obesity induced by the high fat diets. The reasons for the differences in response to the two different sources of saturated fat, lard, and tallow, are not clear.

Cellular sodium turnover in obese hypertensive men

Seventeen moderately obese middle-aged male outpatients with untreated mild hypertension (OH) and 47 normotensive men with normal weight (C) were investigated with respect to intraerythrocyte electrolytes, transmembrane sodium fluxes, PRA and urinary excretion of aldosterone (U-ALDO). There was no difference between the groups in intraerythrocyte sodium but red cell potassium was significantly elevated in OH compared to C. Sodium influx and the rate constant for sodium efflux were also significantly higher in OH. There was a significant positive correlation between sodium influx and U-ALDO (r = 0.56, p less than 0.05).

Physicochemical and functional modifications induced by obesity on human erythrocyte membranes

BACKGROUND

Na+,K(+)-ATPase activity was evaluated in relation to membrane composition and molecular organization in erythrocyte membranes from obese patients by the amphyphylic molecule 6-dodecanoyl-2-dimethylamino-naphthalene (Laurdan). Its possible relationship with fat distribution and hyperinsulinaemia was also investigated.

DESIGN

Subjects were 10 obese men (OM), 12 women with subcutaneous obesity (FSO), 10 women with abdominal obesity (FAO) and 41 healthy lean subjects, 26 women (FC) and 15 men (MC). An oral glucose tolerance test was administered to all subjects to evaluate insulin secretion and glucose tolerance.

RESULTS

Na+,K(+)-ATPase activity was increased in all obese patients. Values were higher in FSO and FAO than in FC (with FAO greater than FSO) and in OM than in MC. The erythrocyte membrane cholesterol-to-phospholipid ratio was increased in obese patients and was significantly different in FSO patients compared with FC. The erythrocyte membrane protein-to-phospholipid ratio was also increased in all obese subjects, reaching statistical significance only in FSO vs. FC. The liquid crystalline phase, as tested by Laurdan generalized polarization (GP), was decreased in obese patients, indicating the presence of greater molecular environmental order; all patients groups showed lower GP values than control subjects, but only FAO reached statistical significance compared with FC. There was no evident correlation between membrane Na+,K(+)-ATPase activity and insulin levels, nor did membrane composition and properties show any evident relationship with insulin levels.

CONCLUSION

Both increased Na+,K(+)-ATPase activity and altered fluidity and lipid composition were observed in the erythrocyte membrane of all obese patients. These findings are in line with previous observations by our group and indicate that the changes in Na+,K(+)-ATPase activity observed in obese patients could be related to changes in plasma membrane organization and composition.

Erythrocyte Na+/K(+)-ATPase and membrane and serum lipid profiles: as related to alcohol, body mass index and blood pressure

Erythrocyte Na+/K(+)-pump activities have been measured in hypertensives, alcohol consumers and obese persons, but the results have been variously reported as decreased, increased or unchanged. We analyzed the relationships between erythrocyte Na+/K(+)-ATPase activities and the membrane and serum lipid profiles in 83 middle-aged men, to clarify the reasons for these inconsistencies. Increases in erythrocyte Na+/K(+)-ATPase activity related closely to decreases in cholesterol to phospholipid (C/P) ratio of the erythrocyte membrane. Decreases in the C/P ratio in turn related closely to elevations of serum triglycerides (TG) with increasing body mass index, and weakly to the volume of alcohol consumed. Thus, erythrocyte Na+/K(+)-ATPase activities depend largely on the membrane and serum lipid profiles as related to body weight and alcohol consumption, and which may be a cause of the previous conflicting findings. Erythrocyte Na+/K(+)-ATPase showed a positive association with blood pressure, independently of age, body mass index and serum gamma-glutamyl transpeptidase levels. Although the biological link of elevated erythrocyte Na+/K(+)-ATPase with the rise in blood pressure remains unclear, it may be a reflection of hyperinsulinemia in the subjects with a higher blood pressure due to overweight or excessive alcohol consumption.

Primary Na+/H+ exchanger dysfunction: a possible explanation for insulin resistance syndrome

Insulin resistance has been recently distinguished as a syndrome associated with a clustering of metabolic disorders, including non-insulin dependent diabetes mellitus (NIDDM), obesity, hypertension, dyslipidemia and atherosclerosis. To date, it is thought that all of these disorders are the resulting consequences of primary insulin resistance. We propose that insulin resistance and the metabolic diseases mentioned can be caused by primary overactivity of the Na+/H+ exchange. This hypothesis has practical connotations for understanding the pathogenesis of the insulin resistance syndrome.

Human genes containing polymorphic trinucleotide repeats

Expansions of trinucleotide repeats within gene transcripts are responsible for fragile X syndrome, myotonic dystrophy and spinal and bulbar muscular atrophy. To identify other human genes with similar features as candidates for triplet repeat expansion mutations, we screened human cDNA libraries with repeat probes and searched databases for transcribed genes with repeats. From both strategies, 40 genes were identified and 14 characterized. Five were found to contain repeats which are highly polymorphic including the N-cadherin, BCR, glutathione-S-transferase and Na+/K+ ATPase (beta-subunit) genes. These data demonstrate the occurrence of other human loci which may undergo this novel mechanism of mutagenesis giving rise to genetic disease.

Changes of ATPase activities in erythrocytes of rats with hypothalamic obesity

Na,K- and Ca,Mg-ATPase activities in the membrane of red blood cells (RBC) were determined in glutamate treated obese rats (GOR). Both activities are related oppositely. In the obese rats the Na,K-ATPase is higher but the efficiency in maintenance of Na/K-ion concentration gradients is diminished. Ca,Mg-ATPase is decreased in GOR. Under the hypermetabolic condition of cold adaption the Na,K-ATPase activity decreases and the Ca,Mg-ATPase activity rises in both animal groups. The Na,K-ATPase activity in RBC-membranes is positively related to fat accumulation and age.

[Arterial hypertension in patients with obesity. Role of hyperinsulinism and insulin resistance]

The association between arterial hypertension and obesity has been known for many years and demonstrated by epidemiological studies. The physiopathological mechanisms involved consist of increased extracellular volumes, hyperactivity of the sympathetic nervous system and the renin-angiotensin-aldosterone system, and abnormal ion exchanges between extra- and intracellular compartments. Recent studies have demonstrated an association between arterial hypertension and insulin resistance. Insulin resistance may well be the most important aetiological factor in this type of arterial hypertension as it stimulates both renal sodium reabsorption and sympathetic nervous system activity and reduces vascular Na-K-ATPase activity.

Effect of age, weight, race and sex on blood pressure and erythrocyte sodium pump characteristics

In order to examine whether erythrocyte membrane handling of sodium is influenced by factors other than hypertension, measurements of red cell sodium transport were studied in one hundred normotensive volunteers. Erythrocyte sodium content was found to increase with increasing age, body weight and mean arterial pressure (MAP). It is also significantly correlated with age, body weight and MAP. Total sodium efflux was found to be reduced and negatively correlated with age and body weight. A reduction in ouabain-sensitive sodium efflux was also observed with increasing age and body weight. In males, the rate of ouabain-sensitive sodium efflux is higher than in females. Race was found to have no effect on erythrocyte electrolyte content and cationic flux rates of subjects. These data suggest that when studies in hypertension are going to be carried out, control subjects carefully matched for age, body weight and sex should be used if confounding results are not to be obtained.

(Na+ -K+)ATPase activity in erythrocyte membranes of spontaneously, one kidney-one wrapped, and deoxycorticosterone acetate--NaCl hypertensive rats

(Na+ -K+)ATPase activity in erythrocyte membranes of spontaneously (SHR), one kidney-one wrapped, and deoxycorticosterone acetate (DOCA)-NaCl hypertensive rats was studied. (Na+ -K+) ATPase activity decreased in both prehypertensive (6 weeks old) and hypertensive (14 weeks old) stages of SHR, suggesting that the alteration of this enzymic activity may be due to a pre-existing defect in the membrane rather than being a consequence of hypertension. By contrast, (Na+ -K+)ATPase activity remained unchanged in the one kidney-one wrapped hypertensive rats, whereas that of one kidney-one wrapped normotensive rats as well as that of DOCA-NaCl hypertensive rats was increased significantly (P less than 0.05). These changes were specific for (Na+ -K+) ATPase, since Mg2+-ATPase activity was not altered. The susceptibility of (Na+ -K+)ATPase to the inhibitory action of ouabain was not changed significantly. These findings indicate that (Na+ -K+)ATPase activities of erythrocyte membranes isolated from the different types of hypertensive rats were subject to different changes. Whether this phenomenon applies to the clinical distinctions among the various types of hypertension remains a subject for further investigation.

Diminished [3H] ouabain binding and 86Rb influx by leukocytes in anorexia nervosa

Leukocyte Na-K ATPase was assessed in ten patients with anorexia nervosa. The indices of Na-K ATPase measured were 86Rb influx and [3H]-ouabain binding. Both 86Rb influx and [3H]-ouabain binding were significantly lower in patients with anorexia nervosa than in controls. Following weight gain (4.1 to 11.9 kg) in seven patients both 86Rb influx and [3H]-ouabain binding increased in each patient to levels similar to those in controls, although the patients still remained underweight. Acute oral glucose challenge (75 g) also resulted in an increase in 86Rb influx and [3H]-ouabain binding in each of the eight patients tested. There was a significant correlation between 86Rb influx and [3H]-ouabain binding. We conclude that the leukocytes of patients with anorexia nervosa have a significantly diminished number of Na-K ATPase units with a parallel decrease in 86Rb influx. Weight gain and acute glucose challenge result in an increase in both indices of Na-K ATPase. Nutrition appears to play an important role in the modulation of this enzyme.

Erythrocyte Na-K-ATPase membrane activity in obese patients fed over a long-term period with a very-low-calorie diet

In a search for the role of long-term hypocaloric feeding on the expression of the erythrocyte Na pump in obesity, we examined three groups of subjects. Group 1 consisted of 10 obese subjects who had been under treatment for a long period of time with a very-low-calorie diet (500 kcal/d) while group 2 consisted of 10 age-, sex-, and body mass index-matched obese subjects on their usual diet; in the third group, 12 normal-weight subjects on a free diet served as controls. There was no difference between the groups in the number of erythrocyte binding sites per cell. On the contrary, the Na-K-ATPase activity was significantly lower in the obese group 1 (0.35 +/- 0.09 mumol Pi x mg protein-1 x h-1) compared to that observed in the obese group 2 (0.42 +/- 0.07, P less than .05) and in control subjects (0.45 +/- 0.06, P less than .05). Sex, duration of hypocaloric feeding, and the amount of weight loss before the study in the obese group 1 seemed not to be related to the Na pump parameters. We conclude that long-term severe hypocaloric feeding may be a factor in altered erythrocyte Na-K-ATPase in obese individuals.

Leucocyte sodium pump activity after meals or insulin in normal and obese subjects: cause for increased energetic efficiency in obesity?

As cellular sodium pumping is an energy consuming process and differences in the obese may account for their energetic efficiency, leucocyte sodium-22 efflux was studied in obese and normal volunteers both in the fasting state and after a test meal or infusion of glucose and insulin intravenously. The 22Na ouabain sensitive efflux rate constant was significantly higher in obese subjects than normal (mean (1 SD) 2.69 (0.40)/h v 2.35 (0.49)/h). Two hours after a 4.2 MJ (1000 kcal) meal there was an increase in the efflux rate constant from its fasting value in normal weight subjects (2.39 (0.33)/h to 2.71 (0.40)/h) but not in obese subjects (2.65 (0.54)/h to 2.61 (0.58)/h). The rise in ouabain sensitive efflux rates was significantly higher in normal than obese subjects. Both groups showed a rise in intracellular sodium concentrations. The euglycaemic clamp produced similar results. Feeding or infusion of insulin increases sodium pump activity more in normal than obese subjects. This difference may contribute to any defective dietary thermogenesis in obesity, which may lead to energetic efficiency and a tendency to gain weight.

Increased leucocyte Na-K ATPase in obesity: reversal following weight loss

Ouabain-sensitive 86Rb influx and [3H] ouabain binding capacity were investigated in the leucocytes of 17 obese patients and 15 control subjects. Both were significantly increased in the obese when compared with controls. Following dietary restriction and a 4% to 5% weight reduction in the obese over 2 weeks, [3H] ouabain binding and ouabain-sensitive 86Rb influx (a model for K+ influx) decreased to levels similar to those in controls. This shows that the number of Na-K ATPase sites on leucocyte membranes of the obese are significantly increased and that this is associated with accelerated 86Rb transport. Since both of these indices decreased following 4% to 5% reduction in body weight while the patients were still obese, increased Na-K ATPase is neither a marker of nor cardinal to the pathogenesis of obesity. We conclude that (1) increase in Na-K ATPase units and 86Rb influx are not characteristic of obesity itself and (2) dietary restriction over the short-term with limited weight reduction restores Na-K ATPase units and 86Rb influx to normal.

The effect of oral glucose on the leucocyte sodium pump in normal and obese subjects

The effect of oral glucose (40 g/m2 body surface area) on the leucocyte 22Na efflux rate constants (ERC) was studied in 13 normal weight and 10 obese subjects. The ouabain-sensitive 22Na ERC was higher in leucocytes isolated from fasting obese subjects (median [range] for obese 2.77 [2.33-3.11] vs normals 1.91 [1.57-2.77] h-1, P less than 0.001). There was no difference in the ouabain-resistant 22Na ERC. Oral glucose raised the ouabain-sensitive 22Na ERC after 2 h in normal subjects (1.91 [1.57-2.77] to 2.41 [2.11-3.02] h-1, P less than 0.001). The ouabain-resistant 22Na ERC fell from 0.71 [0.32-1.10] to 0.46 [0.35-0.68] h-1, P less than 0.008. Conversely, in obese subjects, the ouabain-sensitive ERC fell (2.77 [2.33-3.11] to 2.59 [2.11-2.92] h-1, P less than 0.06). There was no significant change in ouabain-resistant 22Na ERC 2 h after oral glucose. The fasting leucocyte 22Na ouabain-sensitive ERC correlated with fasting plasma insulin levels and insulin resistance (rs = 0.48, P less than 0.01 for both). The change in this ERC with oral glucose correlated with the incremental insulin response over 2 h (rs = -0.53, P less than 0.006) and to the insulin resistance (rs = -0.56, P less than 0.003). The failure of oral glucose to stimulate the leucocyte sodium pump in obesity could partially account for the defect in dietary thermogenesis in obesity. This defect in stimulation of the sodium pump is related to insulin resistance.

Differences in erythrocyte cation (sodium) transport between Chinese and non Chinese males

Erythrocyte sodium content, sodium pump activity (ouabain sensitive efflux rate, efflux rate constant, ouabain binding sites and Na+, K+-ATPase activity), sodium-lithium countertransport and sodium-potassium cotransport activities were measured in 10 European males, 11 non Chinese Asian males and 12 Chinese males. There were no differences between the 3 groups in any of the measurements of active transport. The sodium-lithium countertransport was higher and sodium-potassium cotransport (measured as lithium-potassium cotransport) was lower in Chinese compared to either the Europeans or non Chinese Asians. There were no differences in counter and cotransport activities between the Europeans and non Chinese Asians. Multiple regression analysis showed that the co and countertransport pathways contribute little to the maintenance of erythrocyte sodium content in healthy subjects.

Effects of sodium and potassium adenosine-triphosphatase on circulating lymphocytes: an approach to human obesity

The methodological aspects of (Na+, K+)-ATPase-dependent uptake of 86Rb, a potassium analog, were examined on human lymphocytes isolated from peripheral blood. The study of the time-course, the kinetic parameters, i.e., maximum velocity (Vmax) and Michaelis constant (Km) and the ouabain inhibition curve of 86Rb+ uptake confirm that circulating lymphocytes represent a suitable model for the study of (Na+,K+)-ATPase in human diseases. An application to human obesity is reported: the results indicate that 86Rb+ uptake on circulating lymphocytes is similar in obese and non-obese subjects. Therefore, (Na+,K+)-ATPase does not seem to be involved in the pathogenesis of human obesity.

The leucocyte sodium pump in healthy and obese subjects: the association of insulin with its activity

Leucocyte ouabain-sensitive 22Na+ efflux was studied in 35 normal and 12 obese subjects. This efflux rate constant was raised in the obese (2.72 +/- SEM 0.13 vs 2.31 +/- 0.08 h-1, P less than 0.006), indicating a higher activity of the sodium pump in vivo, There was a significant correlation between this efflux rate constant and fasting insulin level in both the whole population and in the normals alone (rs = 0.36, P less than 0.007, and rs = 0.40, P less than 0.009 respectively). A hyperinsulinaemic-euglycaemic clamp was performed on seven normal volunteers. After 2 h, there was a significant stimulation of the leucocyte efflux rate constant (from 2.86 +/- 0.17 to 3.33 +/- 0.18 h-1, P less than 0.01). In-vitro incubation of leucocytes with insulin produced a maximal stimulation of the Na+-K+-ATPase activity of about 35% at 2 h with half-maximal stimulation achieved at 46 mU/l. Insulin (100 mU/l) also stimulated the leucocyte ouabain-sensitive 22Na+ efflux rate constant in vitro by about 11% with or without 1 h of preincubation with the insulin. These findings may explain the hypokalaemic and sodium retaining effects of insulin in man; they may also partially explain the raised Na+ efflux rate constants in obesity.

Sodium-potassium, magnesium, and calcium ATPase activities in erythrocyte membranes from manic-depressive patients responding to lithium

Erythrocyte membrane Na+-K+ adenosine triphosphatase (ATPase), Mg2+ ATPase, and Ca2+ ATPase activities were compared among 23 euthymic manic-depressive patients responding to lithium therapy and 24 healthy controls. The two groups were similar in age, sex composition, body mass index, and community background. No significant differences were noted in mean ATPase activities between the two groups. However, plasma lithium concentration correlated positively with Na+-K+ ATPase and Mg2+ ATPase activities within the patient group, and six patients with plasma lithium levels in the range of 0.85-1.2 mM had Na+-K+ ATPase activities 62% greater than the control group mean. Possible biochemical mechanisms for the effects of lithium therapy on erythrocyte membrane functions are discussed.

A reproducible procedure for measuring sodium transport in cultured human fibroblasts from normal and obese donors

Studies on erythrocyte sodium pump activity in obesity have yielded conflicting results probably because the erythrocyte is an atypical cell, and it may not reflect ATPase activity of other cells in the body. This study was undertaken to establish a reproducible procedure to measure sodium transport in human diploid fibroblasts, and apply this method to explore any differences in the cells from obese and nonobese humans. Cell cultures were established from 12 nonobese (body mass index (BMI) = wt in kg/height in m2 (Khosla and Lowe, Br J Prev Soc Med 1967; 21: 122-128); less than 27 kg/m2) and 10 obese (BMI) greater than 35 kg/m2) subjects. Triplicate measurements of sodium efflux rate constant were made with and without ouabain (1 mmol/l) to determine the total, active (ouabain-sensitive) and passive (ouabain-insensitive) components. Reproducible results were obtained as suggested by a coefficient of variation (CV) of less 10% on successive experiments on the same cell-line, and 11 and 15% of the active sodium efflux rate constant measured in fibroblasts from nonobese and obese subjects, respectively. The active sodium efflux rate constant in fibroblasts from nonobese (0.202 +/- 0.023 (SD)) was not significantly different from that obtained in the cells from obese subjects (0.21 +/- 0.030; p greater than 0.10). These results suggest that there is no intrinsic differences in basal sodium pump activity in fibroblasts related to obesity.

Heterogeneity of the erythrocyte Na-K pump status in human obesity

The number of Na-K pump units, the Na-K-ATPase activity, the K transport turnover rate per pump unit and the intracellular Na and K concentrations were measured in the erythrocytes of 56 obese patients and 20 normal subjects. No differences were found between the two groups. In obese patients, we failed to observe any influence of dietary habits, age of onset, or family history of obesity on the Na pump status. On the other hand, we found that the number of pump units was not a close reflection of the membrane cation transport and in some patients with an abnormally high number of pump units, an inappropriately low Na-K-ATPase activity was observed. We also identified two small groups of obese patients with, respectively, abnormally high or low K transport turnover rate per pump unit. Our study seems to support the hypothesis that abnormalities in the erythrocyte Na-K pump system are not usual in the obese population but are probably present only in a limited number of selected patients.

Evidence for coordinate genetic control of Na,K pump density in erythrocytes and lymphocytes

The erythrocyte is widely used as a model cell for studies of the Na,K pump in health and disease. However, little is known about the factors that control the number of Na,K pumps expressed on the erythrocytes of a given individual, nor about the extent to which erythrocytes can be used to validly assess the pump density on other cell types. In this report, we have compared the interindividual variance of Na,K pump density in erythrocytes of unrelated individuals to that seen with identical twins. Unlike unrelated individuals, in whom pump parameters, ie, ouabain binding sites. 86Rb uptake, cell Na concentration vary widely, identical twin pairs showed no significant intrapair variation for these values. Thus, a role for genetic factors is suggested. In addition, we established and validated a method for determining Na,K pump density and pump-mediated 86Rb uptake in human peripheral lymphocytes. Using this method, we show that whereas Na,K pump density differs markedly between erythrocytes (mean of 285 sites per cell) and lymphocytes (mean 40,600 sites per cell), there is a strong and highly significant correlation (r = 0.79, P less than 0.001) between the pump density in these cell types in any given individual. Taken together, these studies suggest that genetic factors are important determinants of Na,K pump expression, and that pump density appears to be coordinately regulated in two cell types in healthy individuals.

Insulin-resistant Na+ pump activity in adipocytes from obese humans

Basal and maximally insulin-stimulated Na+-pump activity was measured in adipocytes from subjects with normal glucose tolerance over a range of body mass indexes (BMI). In a comparison of 13 lean (BMI less than 25) vs. 15 extremely obese (BMI greater than 40) subjects basal activities per unit surface area were similar, but the maximally insulin-stimulated activity was significantly reduced in the extremely obese group [9.2 +/- 0.6 vs. 12.1 +/- 1.0 (min X dam2)-1, P less than 0.05]. The mean percent insulin stimulation of the Na+ pump above basal activity was 48 +/- 7% for the lean compared with 14 +/- 2% for the extremely obese group (P less than 0.001). A similar relationship was observed in these subjects for glucose transport where basal activities per unit surface area again were similar but the maximally insulin-stimulated transport was reduced in the extremely obese subjects (2.2 +/- 0.3 vs 5.1 +/- 0.6 attol/um2 X s, P less than 0.001). These results indicate that alterations in Na+-pump activity may be a manifestation of the insulin-resistant state that could contribute to the development of obesity via decreased cellular thermogenesis.

Regulation of mammalian aspartate-4-decarboxylase: its possible role in oxaloacetate and energy metabolism

A newly discovered enzyme in mammalian tissues, aspartate-4-decarboxylase (EC 4.1.1.12), catalyzes the exothermic conversion of aspartate to alanine and CO2. The occurrence of this enzyme poses at least two important questions. First, what is the purpose of such an enzyme in cell physiology? There are alternate ways to convert aspartate to alanine which are rapid and which conserve energy. Second, since the synthesis of aspartate is an energy-requiring process, how can the cell limit undue energy drain by this, seemingly pointless, beta-decarboxylation of aspartate? It is demonstrated that rat liver aspartate-4-decarboxylase is inhibited by acetyl-coenzyme A and stimulated by glutamate. These regulatory properties were predicted a priori. It was suggested that, in coordination with pyruvate carboxylase, aspartate-4-decarboxylase is important in regulating the metabolic fate of oxaloacetate and thus plays a role in determining the efficiency of carbohydrate metabolism. Furthermore, reciprocal regulation of rat liver pyruvate carboxylase and aspartate-4-decarboxylase would assure a limit on the extent of futile cycling that may occur between these enzymes.

Erythrocyte heat production in human obesity: microcalorimetric investigation of sodium-potassium pump and cell metabolism

Changes in overall cellular metabolism, induced by specific inhibition of the Na-K-pump, were determined in erythrocytes from 33 normal and 25 obese subjects. Cellular metabolism was determined by measurement of heat production rates in erythrocytes suspended in plasma with and without the cardioactive glycoside, ouabain. Specific inhibition with ouabain induced the same decrease of the heat production rate in the two groups (14 +/- 5 mW/L for normal subjects and 13 +/- 5 mW/L for obese subjects). In neither group was there a correlation between the ouabain-inhibitable rate of metabolism and body weight. The present study results do not give support to the suggestion that a defect in the Na-K-pump activity would exist in the erythrocyte of human obese subjects and could not therefore be of importance in the pathogenesis of the disease.

Erythrocyte ouabain binding and intracellular Na+ in normotensive obese women and obese women receiving medication for hypertension

People with "primary obesity" may be hypertensive because they have lost their ability to compensate for the effect of low Na+-K+-ATPase levels on blood pressure. In obese patients receiving hypertensive medication (n = 13), but not in normotensive nonmedicated patients (n = 42), diastolic blood pressure was inversely correlated with erythrocyte ouabain binding (P less than 0.02) and directly correlated with intracellular Na+ concentration (P less than 0.01). Moreover, there was a stronger inverse relationship between ouabain binding and intracellular Na+ in patients receiving medication for hypertension (P less than 0.01) than in normotensive patients (P less than 0.05). These data suggest that patients receiving hypertensive medication may be less able to compensate than normotensive patients, (a) for the potential effect of Na+-K+-ATPase levels on intracellular Na+ and (b) for the potential effect of intracellular Na+ concentration on diastolic blood pressure. We propose that obese people with low levels of ouabain binding (primary obesity) may have an increased risk of developing hypertension if their compensatory mechanisms fail.

Elevated skeletal muscle sodium-potassium-ATPase in human obesity

To determine whether muscle sodium-potassium-ATPase activity, an expression of energy turnover, is increased in obesity, we studied a group of nine non-obese and seven obese subjects undergoing elective cholecystectomy. The muscle ATPase activity was 87 per cent higher (p less than 0.001) in obese subjects compared with non-obese subjects. The increase in the ATPase activity positively correlated with both the body mass index (r = 0.81) and excess body weight (r = 0.75). No relationship was found between erythrocyte and muscle sodium-potassium-ATPase activities (r = 0.25). These findings demonstrate an increased ATPase activity in skeletal muscle obtained from obese subjects and that the erythrocyte is an inaccurate marker of this enzyme with respect to other body tissue.

Erythrocyte sodium pump activity in human obesity

In an attempt to resolve the prevailing confusion about erythrocyte sodium pump activity in obesity, we measured sodium-potassium-ATPase, ouabain-inhibitable (active) sodium efflux rate constant and intracellular sodium concentration in erythrocytes from 107 non-obese and obese subjects, with a body-mass index ranging from 17 to 54 kg X m-2. All the three independently measured variables were not significantly different between the two groups and no correlations were found between these three indices and body-mass index. The expression of ATPase activity in units of membrane protein allowed our previous data to be compared with this study and other reports. Our studies and most of the published reports suggest that there is no difference in erythrocyte sodium-potassium-ATPase and sodium transport between the vast majority of obese and non-obese subjects, but there is a subgroup of obese subjects (about 5%) with abnormally high erythrocyte sodium pump activity. The variable treatment of data from this subgroup and the small numbers of obese subjects studied by various investigators are largely responsible for the conflicting results about erythrocyte sodium pump activity.

Changes in cell membrane Na,K-ATPase activity associated with induction of dietary obesity in the rat

The effect of diet-induced obesity on tissue Na, K-ATPase activity ("sodium pump") has been determined in the intact rat exposed to a cafeteria diet. Mature female Charles River rats showed significant increases in carcass lipid on this regimen (P less than 0.01), whereas male rats exposed to cafeteria diet and control male and female animals on laboratory chow showed no increase in carcass lipid over the 54 to 103 days that the animals were studied. In the female cafeteria-diet group, red blood cell membrane Na, K-ATPase activity and carcass lipid were highly correlated (r = 0.847, P less than 0.001). Significant trends in Na, K-ATPase activity as a function of carcass lipid did not occur in either kidney or liver crude membrane preparations from cafeteria-diet females. No correlation was seen in red cell, liver, or kidney membrane Na, K-ATPase with carcass lipid in male cafeteria-diet animals or in the control males and females. In this animal model of nongenetic obesity, changes in tissue Na, K-ATPase activity can be induced by dietary manipulation and are sex-specific and organ-specific.

Sodium-potassium pump in cultured fibroblasts from obese donors; no evidence for an inherent decrease of basal or insulin-stimulated activity

Na, K-ATPase activity may have a significant role in cellular thermogenesis. Reduced thermogenesis and an increased accumulation of unused calories in the form of fat could result from reduced basal or insulin-stimulated Na,K-ATPase activity in obese insulin-resistant man. We have previously demonstrated reduced Na,K-ATPase activity in intact red cells and their isolated membranes from obese humans. To determine if the reduced enzyme activity in obese subjects is the result of inherent cellular defects in the regulation of Na,K-pump activity, basal and insulin-stimulated rates of ouabain-inhibitable Rb uptake were measured in diploid fibroblasts from subjects with a range of body mass indices (BMI). Cell cultures were established from five extremely obese subjects (BMI greater than 40 kg/m2) with fasting hyperinsulinemia (38 +/- 6 microU/mL) and in four control (BMI less than 30 kg/m2) normoinsulinemic (14 +/- 3 microU/mL) subjects. Basal (17 +/- 3 v 23 +/- 2 nmol/L/min/10(10) cells +/- SEM) and maximal insulin-stimulated Na,K-pump activities (26 +/- 3 v 32 +/- 3 nmol/L/min/10(10) cells) were similar in the obese and control subjects. Maximal insulin stimulation for both groups was observed in four to eight minutes, and one-half maximal response required 2.5 ng/ml insulin. Cell density was negatively correlated with basal (r = 0.75, p less than 0.001) and maximally stimulated Na,K-pump activity (r = -0.73, p less than 0.001). Adjustment for this relationship did not influence the conclusions. Comparison of the results from the obese and control groups indicates (a) no evidence for an intrinsic cellular difference in basal Na,K-pump activity related to obesity and (b) no difference in insulin regulation of Na,K-pump activity, in fibroblasts from obese subjects.

Sodium-potassium-ATPase activity is influenced by ethnic origin and not by obesity

Previous investigations have suggested that red-cell ouabain binding (an indirect measure of sodium-potassium-ATPase activity) is lower in severely obese patients than in normal controls. We now confirm that ouabain binding measures sodium-potassium-ATPase activity, and we demonstrate that the level of this activity is genetically determined. The activity of this enzyme differs in various ethnic and racial groups, relatively high levels being encountered in non-Jewish white subjects, particularly those with some Scandinavian ancestry. On the other hand, black, Asian, and Jewish white subjects have lower sodium-potassium-ATPase activity. In contrast, no difference was found in red-cell sodium-potassium-ATPase activity between severely obese and normal persons, nor could we confirm a putative effect of food intake on the level of the red-cell enzyme. We suggest that in most earlier studies in which differences were found between normal and severely obese persons, those differences could have been due to differences in the ethnic origins of the obese and control populations.

Inverse relationship between ouabain sites on human erythrocytes and body mass index in normal healthy subjects

Binding of [3H]ouabain by erythrocytes and levels of (Na+ + K+) ATPase, cell sodium and potassium, and plasma triiodothyronine (T3) were studied in 16 normal, apparently healthy men. The subjects were between 28 and 50 years of age. Based on their economic status, the subjects were divided into two groups of eight each. Group A consisted of subjects with a higher economic status; these subjects had a mean body mass index (weight/height2) of 24.1 +/- 1.2 (mean +/- SEM). Group B comprised subjects with a lower economic status; these subjects had a mean body mass index of 18.8 +/- 1.0. The number of ouabain binding sites per red cell in group B was higher (466 +/- 29) compared with that in group A (348 +/- 21; P less than 0.01). The dissociation constant (Kd) between the two groups was not different. The (Na+ + K+) ATPase activity per 10(10) cells in group B (0.50 +/- 0.07) was higher compared with that in group A (0.29 +/- 0.03). The increased number of pump sites and increased enzyme activity in group B were associated with a lower [Na]-to-[K] ratio in the cell (P less than 0.01). The plasma T3 level in group A (202 +/- 16.7 ng/dL) was not statistically different from that of group B (163 +/- 17.6 ng/dL). The number of sodium pump sites showed an inverse relationship with body mass index (r = -0.55; P less than 0.05) and with the cell [Na]-to-[K] ratio (r = -0.74; P less than 0.01). The number of pump sites in group A showed a positive correlation (r = 0.60) with plasma T3 levels. Such a relationship was, however, weak in group B (r = 0.36). The results lead to the conclusion that there was increased utilization of available cell energy by sodium pump activity in the subjects in group B. This may be a physiologic adaptation for efficient utilization of ingested nutrients via the sodium pump in response to marginal nutrient deficits in these subjects. Dissociation of increased pump sites from plasma T3 levels may mean that the adaptive phenomenon does not represent a wasteful loss of energy.

Erythrocyte sodium-potassium-stimulated adenosine triphosphatase activity is not related to obesity

Altered erythrocyte sodium potassium (Na,K)-stimulated adenosine triphosphatase (ATPase) activity has been cited as having pathophysiologic significance in morbidly obese man. Previous studies have failed to consider obese patients after weight loss and, therefore, did not clarify the role of ATPase deficiency as a cause or effect of the obese state. To define more completely the possible alteration of cellular thermogenesis in obesity, a study was made of three groups of people: (1) normal weight controls; (2) morbidly obese; and (3) formerly morbidly obese patients who had lost over 100 pounds after gastric bypass surgery. Erythrocyte ATPase activity was determined by use of an assay that coupled ATPase activity with NADH oxidation in the presence of excess pyruvate kinase, lactic dehydrogenase, and phosphoenolpyruvate. This coupled assay produced a continuous slope so that activity could be calculated from the initial, maximal, linear portion of the decay trace. Results did not demonstrate any statistically significant differences in Na,K-ATPase activity between groups by analysis of variance. A nonsignificant correlation of 0.086 was seen between obesity index and Na,K-ATPase activity. It is concluded that (1) erythrocyte Na,K-ATPase activity is similar in both normal and obese individuals, (2) erythrocyte Na,K-ATPase does not change with weight loss, and (3) therefore, disordered erythrocyte thermogenesis does not have a role in the development or maintenance of obesity.

Altered erythrocyte Na+ + K+ pump in adolescent obesity

The number of Na/K pump units and the cation transport activity of the pump were measured in erythrocytes from two etiologically different groups of obese adolescents and a group of normal controls. There was a significant reduction in the number of pump units, as measured by saturation ouabain binding, in erythrocytes from adolescents with idiopathic, early onset obesity. Individuals whose obesity developed subsequent to the appearance of a variety of hypothalamic lesions showed no reduction in the red cell complement of Na/K pump when compared to controls and the cation transport activity of their cells was higher than both the controls and the subjects with idiopathic obesity. These results support data obtained in adults that reduced red cell Na/K pump levels are seen in a group of individuals with idiopathic obesity. They further suggest that such reductions are not likely to be secondary to the obese state per se.