Mechanism of alteration of sodium potassium pump of erythrocytes from patients with chronic renal failure

Pathophysiology and clinical management of hyperkalemia in chronic kidney disease

Adaptive increases in kidney and gastrointestinal excretion of K+ help to prevent hyperkalemia in patients with chronic kidney disease (CKD) as long as the glomerular filtration rate (GFR) remains >15-20 mL/min. K+ balance is maintained by increased secretion per functioning nephron, which is mediated by elevated plasma K+ concentration, aldosterone, increased flow rate, and enhanced Na+-K+-ATPase activity. Fecal losses of potassium also increase in CKD. These mechanisms are effective in preventing hyperkalemia if urine output is in excess of 600 mL/day and the GFR exceeds 15 mL/min. Development of hyperkalemia with only mild to moderate reductions in GFR should prompt a search for intrinsic disease of the collecting duct, disturbances in mineralocorticoid activity, and/or decreased delivery of sodium to the distal nephron. The initial approach to treatment is to review the patient's medication profile and whenever possible discontinue drugs that impair kidney K+ excretion. Patients should be educated on sources of K+ in the diet and should be strongly encouraged to avoid the use of K+ containing salt substitutes as well as herbal remedies since herbs may be a hidden source of dietary K+. Effective diuretic therapy and correction of metabolic acidosis are effective strategies to minimize the potential for hyperkalemia. Discontinuation or use of submaximal doses of renin-angiotensin blockers should be discouraged given the cardiovascular protective effect these drugs provide. Potassium binding drugs can be useful to enable use of these drugs and potentially allow liberalization of the diet in CKD patients.

Blockage of the Na-K-ATPase signaling-mediated oxidant amplification loop elongates red blood cell half-life and ameliorates uremic anemia induced by 5/6th PNx in C57BL/6 mice

We have previously demonstrated that the Na-K-ATPase signaling-mediated oxidant amplification loop contributes to experimental uremic cardiomyopathy and anemia induced by 5/6th partial nephrectomy (PNx). This process can be ameliorated by systemic administration of the peptide pNaKtide, which was designed to block this oxidant amplification loop. The present study demonstrated that the PNx-induced anemia is characterized by marked decreases in red blood cell (RBC) survival as assessed by biotinylated RBC clearance and eryptosis as assessed by annexin V binding. No significant change in iron homeostasis was observed. Examination of plasma samples demonstrated that PNx induced significant increases in systemic oxidant stress as assessed by protein carbonylation, plasma erythropoietin concentration, and blood urea nitrogen. Systemic administration of pNaKtide, but not NaKtide (pNaKtide without the TAT leader sequence) and a scramble "pNaKtide" (sc-pNaKtide), led to the normalization of hematocrit, RBC survival, and plasma protein carbonylation. Administration of the three peptides had no significant effect on PNx-induced increases in plasma erythropoietin and blood urea nitrogen without notable changes in iron metabolism. These data indicate that blockage of the Na-K-ATPase signaling-mediated oxidant amplification loop ameliorates the anemia of experimental renal failure by increasing RBC survival.NEW & NOTEWORTHY The anemia of CKD is multifactorial, and the current treatment based primarily on stimulating bone marrow production of RBCs with erythropoietin or erythropoietin analogs is unsatisfactory. In a murine model of CKD that is complicated by anemia, blockade of Na-K-ATPase signaling with a specific peptide (pNaKtide) ameliorated the anemia primarily by increasing RBC survival. Should these results be confirmed in patients, this strategy may allow for novel and potentially additive strategies to treat the anemia of CKD.

Inductively-Coupled Plasma Mass Spectrometry-Novel Insights From an Old Technology Into Stressed Red Blood Cell Physiology

BACKGROUND

Ion and metal homeostasis are critical to red blood cell physiology and Inductively Coupled Plasma (ICP) is a decades old approach to pursue elemental analysis. Recent evolution of ICP has resulted in its coupling to mass spectrometry (MS) instead of atomic absorption/emission.

METHODS

Here we performed Inductively-coupled plasma mass spectrometry (ICP-MS) measurements of intra- and extra-cellular Na, K, Ca, Mg, Fe, and Cu in red blood cells undergoing ionic, heat, or starvation stress. Results were correlated with Ca measurements from other common platforms (e.g., fluorescence-based approaches) and extensive measurements of red blood cell metabolism.

RESULTS

All stresses induced significant intra- and extracellular alterations of all measured elements. In particular, ionomycin treatment or hypertonic stress significantly impacted intracellular sodium and extracellular potassium and magnesium levels. Iron efflux was observed as a function of temperatures, with ionic and heat stress at 40°C causing the maximum decrease in intracellular iron pools and increases in the supernatants. Strong positive correlation was observed between calcium measurements via ICP-MS and fluorescence-based approaches. Correlation analyses with metabolomics data showed a strong positive association between extracellular calcium and intracellular sodium or magnesium levels and intracellular glycolysis. Extracellular potassium or iron were positively correlated with free fatty acids (especially mono-, poly-, and highly-unsaturated or odd-chain fatty acid products of lipid peroxidation). Intracellular iron was instead positively correlated with saturated fatty acids (palmitate, stearate) and negatively with methionine metabolism (methionine, S-adenosylmethionine), phosphatidylserine exposure and glycolysis.

CONCLUSION

In the era of omics approaches, ICP-MS affords a comprehensive characterization of intracellular elements that provide direct insights on red blood cell physiology and represent meaningful covariates for data generated via other omics platforms such as metabolomics.

Up-down biphasic volume response of human red blood cells to PIEZO1 activation during capillary transits

In this paper we apply a novel JAVA version of a model on the homeostasis of human red blood cells (RBCs) to investigate the changes RBCs experience during single capillary transits. In the companion paper we apply a model extension to investigate the changes in RBC homeostasis over the approximately 200000 capillary transits during the ~120 days lifespan of the cells. These are topics inaccessible to direct experimentation but rendered mature for a computational modelling approach by the large body of recent and early experimental results which robustly constrain the range of parameter values and model outcomes, offering a unique opportunity for an in depth study of the mechanisms involved. Capillary transit times vary between 0.5 and 1.5s during which the red blood cells squeeze and deform in the capillary stream transiently opening stress-gated PIEZO1 channels allowing ion gradient dissipation and creating minuscule quantal changes in RBC ion contents and volume. Widely accepted views, based on the effects of experimental shear stress on human RBCs, suggested that quantal changes generated during capillary transits add up over time to develop the documented changes in RBC density and composition during their long circulatory lifespan, the quantal hypothesis. Applying the new red cell model (RCM) we investigated here the changes in homeostatic variables that may be expected during single capillary transits resulting from transient PIEZO1 channel activation. The predicted quantal volume changes were infinitesimal in magnitude, biphasic in nature, and essentially irreversible within inter-transit periods. A sub-second transient PIEZO1 activation triggered a sharp swelling peak followed by a much slower recovery period towards lower-than-baseline volumes. The peak response was caused by net CaCl2 and fluid gain via PIEZO1 channels driven by the steep electrochemical inward Ca2+ gradient. The ensuing dehydration followed a complex time-course with sequential, but partially overlapping contributions by KCl loss via Ca2+-activated Gardos channels, restorative Ca2+ extrusion by the plasma membrane calcium pump, and chloride efflux by the Jacobs-Steward mechanism. The change in relative cell volume predicted for single capillary transits was around 10-5, an infinitesimal volume change incompatible with a functional role in capillary flow. The biphasic response predicted by the RCM appears to conform to the quantal hypothesis, but whether its cumulative effects could account for the documented changes in density during RBC senescence required an investigation of the effects of myriad transits over the full four months circulatory lifespan of the cells, the subject of the next paper.

The potential role of Na-K-ATPase and its signaling in the development of anemia in chronic kidney disease

Chronic kidney disease (CKD) is one of the most prominent diseases affecting our population today. According to the Factsheet published by Centers for Disease Control and Prevention (CDC), it effects approximately 15% of the total population in the United States in some way, shape, or form. Within the myriad of symptomatology associated with CKD, one of the most prevalent factors in terms of affecting quality of life is anemia. Anemia of CKD cannot be completely attributed to one mechanism or cause, but rather has a multifactorial origin in the pathophysiology of CKD. While briefly summarizing well-documented risk factors, this review, as a hypothesis, aims to explore the possible role of Na-K-ATPase and its signaling function [especially recent identified reactive oxygen species (ROS) amplification function] in the interwoven mechanisms of development of the anemia of CKD.

Change in hydration indices associated with an increase in total water intake of more than 0.5 L/day, sustained over 4 weeks, in healthy young men with initial total water intake below 2 L/day

This secondary data analysis addressed gaps in knowledge about effects of chronic water intake. Longitudinal data from the Adapt Study were used to describe effects of prescribing a sustained increase in water intake relative to baseline, for 4 weeks, on multiple indices of total body water (TBW) flux, regulation, distribution, and volume in five healthy, free-living, young men, with mean total water intake initially below 2 L/day. Indices were measured weekly. Within-person fixed effect models tested for significant changes in indices over time and associations between changes in indices. Agreement between indices was described. Mixed models tested if baseline between-person differences in hydration indices modified changes in indices over time. Body water flux: The half-life of water in the body decreased significantly. Body water regulation: Serum osmolality decreased significantly. Urine anti-diuretic hormone, sodium, potassium, and osmolality decreased significantly. Plasma aldosterone and serum sodium increased significantly. Body water distribution: No significant changes were observed. Body water volume: Saliva osmolality decreased significantly. Body weight increased significantly by a mean ± SEM of 1.8% ± 0.5% from baseline over 4 weeks. Changes in indices were significantly inter-correlated. Agreement between indices changed over 4 weeks. Baseline saliva osmolality significantly modified responses to chronic water intake. The results motivate hypotheses for future studies: Chronic TBW deficit occurs in healthy individuals under daily life conditions and increases chronic disease risk; Sustained higher water intake restores TBW through gradual isotonic retention of potassium and/or sodium; Saliva osmolality is a sensitive and specific index of chronic hydration status.

On the Mechanism of Human Red Blood Cell Longevity: Roles of Calcium, the Sodium Pump, PIEZO1, and Gardos Channels

In a healthy adult, the transport of O₂ and CO₂ between lungs and tissues is performed by about 2 · 10¹³ red blood cells, of which around 1.7 · 10¹¹ are renewed every day, a turnover resulting from an average circulatory lifespan of about 120 days. Cellular lifespan is the result of an evolutionary balance between the energy costs of maintaining cells in a fit functional state versus cell renewal. In this Review we examine how the set of passive and active membrane transporters of the mature red blood cells interact to maximize their circulatory longevity thus minimizing costs on expensive cell turnover. Red blood cell deformability is critical for optimal rheology and gas exchange functionality during capillary flow, best fulfilled when the volume of each human red blood cell is kept at a fraction of about 0.55-0.60 of the maximal spherical volume allowed by its membrane area, the optimal-volume-ratio range. The extent to which red blood cell volumes can be preserved within or near these narrow optimal-volume-ratio margins determines the potential for circulatory longevity. We show that the low cation permeability of red blood cells allows volume stability to be achieved with extraordinary cost-efficiency, favouring cell longevity over cell turnover. We suggest a mechanism by which the interplay of a declining sodium pump and two passive membrane transporters, the mechanosensitive PIEZO1 channel, a candidate mediator of P_sickle in sickle cells, and the Ca²⁺-sensitive, K⁺-selective Gardos channel, can implement red blood cell volume stability around the optimal-volume-ratio range, as required for extended circulatory longevity.

Human atrial cell models to analyse haemodialysis-related effects on cardiac electrophysiology: work in progress

During haemodialysis (HD) sessions, patients undergo alterations in the extracellular environment, mostly concerning plasma electrolyte concentrations, pH, and volume, together with a modification of sympathovagal balance. All these changes affect cardiac electrophysiology, possibly leading to an increased arrhythmic risk. Computational modeling may help to investigate the impact of HD-related changes on atrial electrophysiology. However, many different human atrial action potential (AP) models are currently available, all validated only with the standard electrolyte concentrations used in experiments. Therefore, they may respond in different ways to the same environmental changes. After an overview on how the computational approach has been used in the past to investigate the effect of HD therapy on cardiac electrophysiology, the aim of this work has been to assess the current state of the art in human atrial AP models, with respect to the HD context. All the published human atrial AP models have been considered and tested for electrolytes, volume changes, and different acetylcholine concentrations. Most of them proved to be reliable for single modifications, but all of them showed some drawbacks. Therefore, there is room for a new human atrial AP model, hopefully able to physiologically reproduce all the HD-related effects. At the moment, work is still in progress in this specific field.

Abnormalities of serum potassium concentration in dialysis-associated hyperglycemia and their correction with insulin: a unique clinical/physiologic exercise in internal potassium balance

The absence of significant losses of potassium in the urine makes dialysis-associated hyperglycemia (DH) a model for the study of the internal potassium balance. Studies of DH have revealed that hyperkalemia is frequent at presentation, insulin infusion is usually the only treatment required, and the magnitude of the decrease in serum potassium concentration (K(+)) during treatment of DH with insulin depends on the starting serum K(+) level, the decreases in serum glucose concentration and tonicity, and the increase in serum total carbon dioxide level. We present an analysis of these findings based on previously studied actions of insulin. Calculations of transcellular potassium shifts based on the combined effects of insulin-the increase in the electrical potential differences (hyperpolarization) of the cell membranes and the correction of the hyperglycemic intracellular dehydration through decrease in serum glucose concentration-produced quantitative predictions of the decrease in serum K(+) similar to the reported changes in serum K(+) during treatment of DH with insulin. The lessons from analyzing serum K(+) changes during treatment of DH with insulin are applicable to other conditions where internal potassium balance is called upon to protect serum K(+), such as the postprandial state. The main questions related to internal potassium balance in DH that await clarification include the structure and function of cell membrane potassium channels, the effect of insulin on these channels, and the mechanisms of feedforward potassium regulation.

L-carnitine supplementation and EPO requirement in children on chronic hemodialysis

L-carnitine supplementation has been the subject of heated discussion in the context of the treatment of pediatric hemodialysis patients. The aim of this study was to analyze the effect of intravenous L-carnitine supplementation on the erythropoetin (EPO) requirement in six pediatric hemodialysis patients. All patients were on intravenous L-carnitine (2.5 g per session for patients >30 kg and 1 g for those <30 kg) for 9 months. The EPO dose was adapted monthly to maintain a target hemoglobin (Hb) level of 11-13 g/dl. Prior to the initiation of L-carnitine supplementation, the EPO requirement was 1.15 +/- 0.22 (range 0.37-1.75) microg/kg darbepoetin alpha. Free carnitine (FC) levels were measured before (40.4 +/- 4.9 micromol/l), immediately after the 9-month L-carnitine supplementation period (378.5 +/- 77.3 micromol/l), and 4 months after withdrawal of L-carnitine (95.6 +/- 4.0 micromol/l). After 9 months, the EPO dose was 0.47 +/- 0.10 microg/kg (p < 0.002). The Hb levels increased from 12.2 +/- 0.97 to 14.0 +/- 0.54 g/dl (p < 0.05) within the first 2 months, and the EPO dose was then decreased in a stepwise manner. In conclusion, following intravenous carnitine supplementation, FC levels were higher and persisted longer than expected. This rise was associated with increased Hb levels and decreased EPO requirement. Since controls were missing for this study, prospective long-term multi-center studies on a large number of patients are required to provide solid answers to the controversial question of L-carnitine supplementation in hemodialyzed children.

Exercise-induced changes in plasma composition increase erythrocyte Na+,K+-ATPase, but not Na+-K+-2Cl- cotransporter, activity to stimulate net and unidirectional K+ transport in humans

We tested the hypothesis that exercise-induced changes in plasma composition result in peak stimulation of erythrocyte unidirectional K+ (JK,in) and net K+ (JK,net) transport within the first 120 s. In experimental series 1 (7 men; 2 women), plasma [K+] was continuously measured in vitro (37 degrees C) after the addition of red blood cells (RBCs) obtained from rested subjects (resting RBCs) into an exercise-simulated plasma (ESP; increased plasma osmolality, [K+], [H+], [lactate] and [adrenaline] (epinephrine)), and JK,net calculated. In experimental series 2 (7 men; 4 women), resting RBCs were incubated in true exercise plasma (TEP) obtained after two 30 s bouts of high intensity leg cycling exercise to determine JK,net and JK,in (via RBC 86Rb accumulation). JK,net of resting RBCs increased from 0.9 +/- 28.7 in resting plasma to 285 +/- 164 mmol (l RBCs)-1 h-1 in ESP and to 178 +/- 60 mmol (l RBCs)-1 h-1 after 10 s in TEP. Both JK,net and JK,in peaked within 10 s of incubation and decreased rapidly during the initial 120 s. The use of inhibitors for the Na+,K+-ATPase (ouabain) and the Na+-K+-2Cl- cotransporter (NKCC; bumetanide) indicated that rapid increases in JK,in and JK,net upon incubation of resting RBCs in TEP were due primarily to increased Na+,K+-ATPase activity; the NKCC appeared to be involved only when the Na+,K+-ATPase was blocked. It is concluded that RBCs rapidly increase JK,in and JK,net in response to exercise-induced changes in plasma composition.

The effect of hemodialysis on the transport of sodium in erythrocytes from chronic renal failure patients maintained on hemodialysis

Studies were undertaken to evaluate the modulatory effect of maintenance hemodialysis on ouabain sensitive (OS) and ouabain insensitive (OIS) 22Na(+) uptake in erythrocytes (E) of 8 chronic renal failure patients of both sexes. Following the receipt of informed consent, the blood samples were obtained just before and after Dialysis. The % 22Na(+) uptake of the total 22Na(+) present in the assay media was determined in the purified E just before and after Dialysis. The assay medium was composed of 100 mM NaCl, 5 mM KCl, 10 mM trisbase, 10 mM MOPS, 10 mM D-glucose and 60 mM sucrose, pH 7.4 with and without ouabain. Five different concentrations of E, ranging from 0.75 to 2.00 x 10(9)/mL were used for this study. We observed a linear relationship between the 22Na(+) uptake and E concentrations in both of the assay systems (OS and OIS). The mean total 22Na(+) uptake per 6.5 x 10(9) E/mL in OS and OIS before and after hemodialysis were 3.28 +/- 0.4 (OS) and 3.26 +/- 0.42 (OIS), and 3.42 +/- 0.54 (OS) and 3.42 +/- 0.68 (OIS) respectively. The relative % differences between pre- and post-Dialysis were 4 and 5%, which were statistically not significant. From this study, we conclude that hemodialysis does not affect E membrane properties influencing 22Na(+) transport.

Hyperkalemia: An adaptive response in chronic renal insufficiency

BACKGROUND

Hyperkalemia is a common feature of chronic renal insufficiency, usually ascribed to impaired K+ homeostasis. However, various experimental observations suggest that the increase in extracellular [K+] actually functions in a homeostatic fashion, directly stimulating renal K+ excretion through an effect that is independent of, and additive to, aldosterone.

METHODS

We have reviewed relevant studies in experimental animals and in human subjects that have examined the regulation of K+ excretion and its relation to plasma [K+].

RESULTS

Studies indicate that (1) extracellular [K+] in patients with renal insufficiency correlates directly with intracellular K+ content, and (2) hyperkalemia directly promotes K+ secretion in the principal cells of the collecting duct by increasing apical and basolateral membrane conductances. The effect of hyperkalemia differs from that of aldosterone in that K+ conductances are increased as the primary event. The changes in principal cell function and structure induced by hyperkalemia are indistinguishable from the effects seen in adaptation to a high K+ diet.

CONCLUSIONS

We propose that hyperkalemia plays a pivotal role in K+ homeostasis in renal insufficiency by stimulating K+ excretion. In patients with chronic renal insufficiency, a new steady state develops in which extracellular [K+] rises to the level needed to stimulate K+ excretion so that it again matches intake. When this new steady state is achieved, plasma [K+] remains stable unless dietary intake increases, glomerular filtration rate falls, or drugs are given that disrupt the new balance.

Cell age-related monovalent cations content and density changes in stored human erythrocytes

Conversion of erythrocyte membrane protein 4.1b to 4.1a occurs through a non-enzymatic deamidation reaction in most mammalian erythrocytes, with an in vivo half-life of approximately 41 days, making the 4.1a/4.1b ratio a useful index of red cell age [Inaba and Maede, Biochim. Biophys. Acta 944 (1988) 256-264]. Normal human erythrocytes distribute into subpopulations of increasing cell density and cell age when centrifuged in polyarabinogalactan density gradients. We have observed that, when erythrocytes were stored at 4 degrees C under standard blood bank conditions, the deamidation was virtually undetectable, as cells maintained the 4.1a/4.1b ratio they displayed at the onset of storage. By measuring the 4.1a/4.1b values in subpopulations of cells of different density at various time points during storage, a modification of the normal 'cell age/cell density' relationship was observed, as erythrocytes were affected by changes in cell volume in an age-dependent manner. This may stem from a different impact of storage on the imbalance of monovalent cations, Na(+) and K(+), in young and old erythrocytes, related to their different complement of cation transporters.

Electrolyte and pH dependence of heart rate during hemodialysis: a computer model analysis

The influence of hemodialysis-induced modifications in extracellular fluid characteristics on heart rate was investigated by using a detailed computer model of sinus-node electrical activity. Changes similar to those occurring in the course of hemodialysis in extracellular concentrations of sodium (from 138 to 140 mM), potassium (from 6 to 3.3 mM), and calcium (from 1.2 to 1.5 mM) ions as well as in pH (from 7.31 to 7.4) and intracellular volume were simulated. The model predicted that such changes may largely influence the rhythm of the sinoatrial node pacemaker, causing the heart rate to range from 69 to 86 bpm. Heart rate increases after removing potassium (up to 7 bpm) and also after calcium perfusion (up to 11 bpm) whereas restoring pH slows heart beat (up to 6 bpm). Extracellular sodium has no significant influence, but the heart rate strictly depends on intracellular sodium concentration (5 bpm/mM). A complex dependence of heart rate on electrolytes and pH was also recognized. Providing extracellular potassium concentration is maintained above 5 mM, heart rate exhibits low sensitivity to changes in calcium and potassium. When potassium concentration is reduced below 4.5 mM, heart rate sensitivity to calcium and potassium increases significantly to 10 and 30 bpm/mM, respectively. A sustained increase in heart rate always corresponds to an increase in intracellular sodium concentration.

A non ouabain-like inhibitor of the sodium pump in uremic plasma ultrafiltrates and urine from healthy subjects

A non ouabain-like inhibitor of the sodium pump was separated from uremic plasma ultrafiltrates and normal urine. Under the same chromatographic conditions (C18 column and a gradient of acetonitrile as eluant), ouabain was eluted in a fraction different from the inhibitor. Affinity chromatography based on the formation of a complex between Na,K-ATPase and the inhibitor achieved the differentiation ouabain. Without magnesium and sodium phosphate, ouabain could not bind to enzyme whereas the inhibitor did. A study of Na,K-ATPase enzyme kinetics showed the inhibitor was not competitive for K+, which further differentiates it from ouabain. It was uncompetitive for ATP and seemed competitive for Na+. These results indicate that the inhibitor acts inside the cell, unlike ouabain, and thus its action mechanism appears to be original.

Sodium pump and Na+/H+ activities in uremic erythrocytes. A microcalorimetric and pH-metric study

The sodium pump and Na+/H+ antiport activities in red blood cells from uremic hemodialyzed patients were measured concomitantly. The patients selected (n = 35) were normotensive and free of intercurrent illness known to affect Na transport. The Na pump activity of intact red blood cells in suspension in their own plasma was measured by flow microcalorimetry. The Na+/H+ antiport activity of the erythrocytes from the same patients was determined by a titrimetric technique. The mean global value of the sodium pump was lower in uremics than in controls (13.3 +/- 0.6 vs. 11.3 +/- 0.8 mW/l cells, P < 0.05). The Na+/H+ antiport maximal activity was decreased in uremics (2.9 +/- 0.3 vs. 4.6 +/- 0.5 mmol H+/l cells/h, P < 0.05). Our results thus confirm that uremia per se can affect sodium transport. Moreover it has been shown that a decrease in Na+/H+ antiport activity is able to counteract an impairment of sodium pump. The decrease found in this study could thus explain, at least in part, the absence of hypertension in the patients studied despite their decreased sodium pump activity.

Increased lymphocytic Na+/H+ exchange activity after hemodialysis: evidence for an endogenous inhibitor of Na+/H+ exchange in patients with end-stage renal failure

The Na+/H+ exchange antiport activity was measured in lymphocytes from 16 patients with end-stage renal failure pre- and postdialysis. In addition the effect of the patients' plasma on lymphocytes from healthy subjects was tested. Resting pH (pHi) was not significantly different in lymphocytes pre- and postdialysis. On the other hand, the Na+/H+ exchange activity was significantly lower in lymphocytes before hemodialysis (6.22 +/- 0.73 x 10(-3) pHi/s) than after hemodialysis (9.32 +/- 1.58 x 10(-3) pHi/s; n = 16; p < 0.05). The buffer capacity was not significantly different before and after hemodialysis. The incubation of lymphocytes from healthy control subjects with plasma from patients with end-stage chronic renal failure significantly reduced the lymphocytic Na+/H+ exchange activity. The addition of ultrafiltrate also significantly reduced the Na+/H+ exchange activity in lymphocytes from healthy control subjects. The study indicates the existence of an endogenous inhibitor of the Na+/H+ exchange that is accumulated in plasma from patients with end-stage chronic renal failure.

Low erythrocyte Na/K-pump activity and number in northeast Thailand adults: evidence suggesting an acquired disorder

Healthy northeastern Thais have a higher erythrocyte sodium concentration and a lower erythrocyte membrane Na,K-adenosine triphosphatase (ATPase) activity than central Thais. To elucidate whether the defect is hereditary or acquired, we studied plasma sodium and potassium and erythrocyte sodium, potassium, Na,K-ATPase activity, and ouabain-binding sites (OBS) in the following groups: healthy newborns of ethnic central Thais (group 1), healthy newborns of ethnic northeast Thais (group 2), healthy adults of central Thailand ethnicity who lived in the rural central region (group 3) or in Bangkok (group 4), healthy adults of northeast Thailand ethnicity who lived in the rural northeast region (group 5) or who migrated to work in Bangkok for at least 1 year (group 6). Erythrocyte Na was higher in group 2 than in group 1. Group 3 had lower erythrocyte Na,K-ATPase activity than group 4, and it was lower in group 5 than in group 6. Among all groups, group 5 had the highest erythrocyte Na (11.6 mmol/L,F < 0.0001) and the lowest Na,K-ATPase activity (63 mmol Pi/mg x h, F < 0.0001) and erythrocyte OBS (397 sites per cell, F < 0.05) than the other adult groups. There was a positive correlation between erythrocyte Na,K-ATPase and erythrocyte OBS (r = .416, P < .0001). Multiple regression analysis demonstrated a correlation between erythrocyte Na as a dependent variable and erythrocyte OBS, plasma potassium, erythrocyte potassium, and erythrocyte Na,K-ATPase (r = .517, P < .0001). The erythrocyte Na,K-ATPase/OBS ratio, an expression of Na,K-ATPase activity equalized for the number of Na,K-pump units, was lowest among rural adults of the central region (group 3) and the northeast region (group 5) (F < 0.0002). Our data suggest that rural dwellers in Thailand tend to have lower erythrocyte Na,K-ATPase activity than urban dwellers and that this is probably acquired after birth. It was more severe among those from the northeast versus the central region, and was less severe among those who migrated to an urban area. This defect in northeast rural dwellers was probably associated with low numbers of Na,K-pump units and a defect of the pump to express activity, whereas in central rural dwellers it was probably associated with the latter condition. We postulate that there might be circulating Na,K-pump inhibitors and metabolic disturbances that cause attenuation of Na,K-ATPase function and synthesis in the northeast Thailand rural population, and that such substances may have an environmental origin. There may be a relationship between these abnormalities and sudden unexpected deaths.

Urea inhibits NaK2Cl cotransport in human erythrocytes

We examined the effect of urea on NaK2Cl cotransport in human erythrocytes. In erythrocytes from nine normal subjects, the addition of 45 mM urea, a concentration commonly encountered in uremic subjects, inhibited NaK2Cl cotransport by 33 +/- 7%. Urea inhibited NaK2Cl cotransport reversibly, and in a concentration-dependent fashion with half-maximal inhibition at 63 +/- 10 mM. Acute cell shrinkage increased, and acute cell swelling decreased NaK2Cl cotransport in human erythrocytes. Okadaic acid (OA), a specific inhibitor of protein phosphatase 1 and 2A, increased NaK2Cl cotransport by nearly 80%, suggesting an important role for these phosphatases in the regulation of NaK2Cl cotransport. Urea inhibited bumetanide-sensitive K influx even when protein phosphatases were inhibited with OA, suggesting that urea acted by inhibiting a kinase. In cells subjected to shrinking and OA pretreatment, maneuvers expected to increase the net phosphorylation, urea inhibited cotransport only minimally, suggesting that urea acted by causing a net dephosphorylation of the cotransport protein, or some key regulatory protein. The finding that concentrations of urea found in uremic subjects inhibited NaK2Cl cotransport, a widespread transport pathway with important physiological functions, suggests that urea is not only a marker for accumulation of other uremic toxins, but may be a significant uremic toxin itself.

Red blood cell cation transports in uraemic anaemia: evidence for an increased K/Cl co-transport activity. Effects of dialysis and erythropoietin treatment

This study examines the role of uraemia and the effect of different dialysis treatments on red cell cation transport. We evaluated the main cation transport systems in erythrocytes of non-dialysed end-stage renal disease (ESRD) subjects, of patients undergoing haemodialysis (HD) and continuous ambulatory peritoneal dialysis (CAPD), as well as the changes induced by human recombinant erythropoietin (r-HuEPO) administration. In uraemic undialysed and dialysed patients, we observed an increase in K/Cl co-transport activity and in shrinkage-induced amiloride-sensitive (HMA-sensitive) Na efflux (Na/H exchange) and a decrease in Na/K pump and Na/K/Cl co-transport activity, while Na/Li exchange was increased only in dialysed patients. In uraemic erythrocytes, we showed for the first time an increased K/Cl co-transport activity, which was cell age independent. Generally, the different method of dialysis (CAPD or HD) did not modify the cation transport abnormalities observed. During the treatment with r-HuEPO, all the systems, with the exception of the Na/K pump and Na/K/Cl co-transport, increased their activities following the increase of circulating young red cells. The changes produced under r-HuEPO administration were transient and cation transports returned to the baseline values within 100 days of treatment, indicating a primary and prominent pathogenetic role of uraemia in modulating the red cell membrane cation transport activities.

A circulating inhibitor of the RBC membrane calcium pump in chronic renal failure

A humoral inhibitor of the membrane calcium pump was studied in plasma from 28 normal controls, 33 patients receiving long-term hemodialysis, and 26 with chronic renal failure (CRF; creatinine clearance range was 6 to 97 ml/min). Calcium pump activity was measured as the rate of Sr2+ efflux in normal erythrocytes (RBCs) loaded with Sr2+ (a substitute of Ca2+ in the calcium pump). Plasma, and plasma ultrafiltrates from hemodialysis patients strongly inhibited calcium pump activity compared with controls without plasma (36 +/- 18 vs. 25 +/- 12, %INHIBITION/CONTROL, P < 0.05). Inhibition markedly decreased with acute hemodialysis (16 +/- 12 vs. 5 +/- 14, %INHIBITION/NORMAL PLASMA, N = 15, P < 0.001). In CRF, degree of inhibition correlated with the serum creatinine concentration (r = 0.75, P < 0.001). A kinetic study showed that plasma decreased the maximal rate of the Ca2+ pumps (Vmax) without affecting the apparent affinity for internal cations (KSr). Moreover, the plasma inhibitory factor had a low molecular weight, and was dialyzable and heat stable. In conclusion, we found evidence for an RBC membrane calcium pump inhibitor in uremic plasma, which correlates with the degree of renal insufficiency. Possibly, it may increase calcium content in RBCs and other cells and could thus be related to uremic toxicity and/or hypertension.

Extrarenal potassium tolerance in chronic renal failure: implications for the treatment of acute hyperkalemia

The role of extrarenal potassium homeostasis is well recognized as a major mechanism for the acute defense against the development of hyperkalemia. The purpose of this report is to examine whether or not the various mechanisms of extrarenal potassium regulation are intact in patients with end-stage renal disease (ESRD). The available data suggest that with the development of ESRD and the uremic syndrome there is impaired extrarenal potassium metabolism that is related to a defect in the Na,K-adenosine triphosphatase (ATPase). The responsiveness of uremic patients to the various effector systems that regulate extrarenal potassium handling is discussed. Insulin is well positioned to play an important role in the regulation of plasma potassium concentration in patients with impaired renal function. The role of basal insulin may be even more important than previously appreciated, since somatostatin infusion causes a much greater increase in the fasting plasma potassium in rats with renal failure than in controls. Furthermore, stimulation of endogenous insulin by oral glucose results in a greater intracellular translocation of potassium in uremic rats than in controls. Under at least two common physiologic circumstances, feeding and vigorous exercise, endogenous catecholamines might also act to defend against acute increments in extracellular potassium concentration. However, it is important to appreciate that the response to beta 2-adrenoreceptor-mediated internal potassium disposal is heterogeneous as judged by the variable responses to epinephrine infusion. Based on the evidence presented in this report, a regimen for the treatment of life-threatening hyperkalemia is outlined. Interpretation of the available data demonstrate that bicarbonate should not be relied on as the sole initial treatment for severe hyperkalemia, since the magnitude of the effect of bicarbonate on potassium is variable and may be delayed. The initial treatment for life-threatening hyperkalemia should always include insulin plus glucose, as the hypokalemic response to insulin is both prompt and predictable. Combined treatment with beta 2-agonists and insulin is also effective and may help prevent insulin-induced hypoglycemia.

Improvement of leucocytic Na+ K+ pump activity in uremic patients on low protein diet

Leucocytic Na+ K+ pump activity was assessed in 20 patients with advanced renal failure. Na+ K(+)-ATPase activity was reduced when compared with the values obtained from normal subjects (101.8 +/- 48.6 versus 165.13 +/- 8.9 microM of Pi hr-1.g-1; P less than 0.001) and the mean 86Rb uptake by U 937 cells was depressed by 38% after the addition of patients' sera. Subsequently, patients were put on a diet providing 0.3 g protein/kg body weight daily and supplemented with ketoacids. After three months of dietary treatment Na+ K(+)-ATPase activity increased to 142 +/- 48.3 (P less than 0.01) and reached normal values at the sixth month (162.8 +/- 54.70 microM of Pi hr-1.g-1; P less than 0.001) whereas 86Rb uptake increased by 23 percent when compared to initial values. These data suggest that among the different mechanisms which have been advanced to explain the defects in the Na+ pump observed in uremic patients, circulating inhibitors deriving from alimentary protein intake may affect cation transport.

Membrane ATPase, erythrocyte sodium and potassium in haemodialysis patients

This study was undertaken to evaluate the effect of chronic renal failure as well as dialysate sodium concentration during haemodialysis on membrane ATPase activity and erythrocyte sodium and potassium concentration. Intracellular Na and K were not changed in patients when compared to normal subjects. There was, however, a significant decrease of Na-K-ATPase activity in patients versus controls. Erythrocyte sodium increased during haemodialysis with low and normal sodium dialysate. The present results suggest that sodium dialysate concentration has an influence on the intracellular cationic homeostasis.

Effects of acute and chronic uremia on active cation transport in rat myocardium

As abnormalities of active cation transport could contribute to the genesis of uremic cardiomyopathy, we investigated myocardial sodium pump function in rats with acute renal failure (ARF) and with a model of experimental chronic renal failure (CRF) that has metabolic similarities to advanced chronic uremia in humans. CRF rats were hypertensive and had left ventricular hypertrophy (33% higher heart:body weight ratio; P less than 0.01) at four weeks compared to pair-fed sham-operated rats. Importantly, both ouabain- and furosemide-sensitive 86Rb uptake rates were unchanged in left ventricular myocardial slices from CRF, and the intracellular sodium concentration was not different from that of control rats even though skeletal muscle sodium was increased, as we found previously (J Clin Invest 81:1197, 1988). Insulin-stimulated, ouabain-sensitive 86Rb influx was also preserved. There also were no abnormalities in myocardium cation transport in rats with ARF. However, [3H]ouabain binding was decreased 45% in CRF rats (P less than 0.01); it was unchanged in acute uremia. Decreased ouabain binding in chronic uremia was due entirely to fewer low affinity [3H]ouabain binding sites (the binding affinity for ouabain was unaffected). We conclude that in chronic, (but not acute) renal failure, sodium pump number is reduced in myocardium but intracellular sodium is unchanged and active cation flux rates are maintained. These results emphasize that in rats with chronic uremia, intracellular sodium homeostasis is preserved in myocardium, despite the presence of marked abnormalities of active cation transport in skeletal muscle that are characteristic of chronic uremia.

High-resolution NMR studies of transmembrane cation transport in uremic patients

Cation transport in erythrocytes of some uremic patients is impaired. Most studies have focused on the defect of the erythrocyte Na+/K+ pump in these diseased states. Herein, this cation transport defect was studied by using nuclear magnetic resonance spectroscopy (NMR) which is a non-invasive method permitting study on living erythrocytes. Firstly, we verified that the Na+ transport defect in uremic erythrocytes was not due to non-specific causes such as membrane alteration or a modification of the intracellular metabolism. The proton relaxation data, determined using a paramagnetic doping method, are consistent with a lack of erythrocytic membrane damage in uremic patients. Also, 31P-NMR results showed that in our experimental conditions, uremic and normal erythrocytes exhibit similar variations of ATP level over time. Lastly, the use of anionic paramagnetic shift reagent in 23Na-NMR revealed a defect in the Na+/K+ pump of erythrocytes from uremic patients with high Nain concentration. This defect seems to be due to a reduced number of pump units and to the presence of an endogenous inhibitor in uremic plasma.

Erythrocyte sodium transport in malignant hypertension

Twenty-three patients with treated malignant hypertension (MH), 23 patients with treated non-malignant hypertension (NMH) and 46 normotensive control subjects were investigated with regard to intraerythrocyte sodium (Na) and potassium (K) levels, as well as transmembrane fluxes of sodium (Na-influx and Na-efflux rate constant). Intraerythrocyte Na and K concentrations were determined by flame photometry. The Na-influx and Na-efflux rate constant were calculated from uptake values for 22Na in vitro. In NMH the Na-influx and Na-efflux rate constant were significantly higher while intraerythrocyte Na and K levels did not differ from the controls. Patients with MH tended to have an elevated intraerythrocyte Na concentration, but an unchanged Na-influx and Na-efflux rate constant relative to controls. The increased rate of erythrocyte membrane transport of Na in treated NMH could be due to a stimulatory effect of antihypertensive treatment on cellular Na transport. Patients with treated MH do not show this effect, and in addition, tend to have an elevated intraerythrocyte Na concentration, which is compatible with the existence of a more pronounced abnormality of cellular ion transport in MH.

Nifedipine inhibits calcium-activated K transport in human erythrocytes

The effect of nifedipine on K transport across human erythrocytes was investigated. Nifedipine had no effect on K influx mediated by the Na-K pump, Na-K-2Cl cotransport, or the passive residual K flux. However, it inhibited the K and water loss from ATP-depleted cells in the presence of external Ca (Cao). Similar inhibition of Ca-activated K [K(Ca)] efflux was observed in fresh cells exposed to Cao and A23187 or ionomycin. The inhibition was observed even when nifedipine was added after initiation of the K(Ca) efflux and was not readily reversed by washing cells with drug-free media. When K(Ca) efflux was plotted as a function of external free Ca, nifedipine reduced the maximum K(Ca) efflux but had no effect on the Ca concentration required for half-maximum K(Ca) efflux. The inhibition of K(Ca) efflux by nifedipine was not consequent to its effect on conductive Cl permeability, because valinomycin-induced K efflux in Cl media was enhanced rather than reduced by nifedipine and because the inhibition was also seen with SCN, a nonlimiting anion. Nifedipine inhibited the K(Ca) efflux with a dissociation constant (Kd) of 4 microM. The inhibitory capacity of nifedipine was reduced by increasing external K. Nifedipine reduced not only the basic conductance but also the zero-current K conductance with a Kd of 23 microM. Other Ca-channel blockers, such as verapamil and diltiazem, did not inhibit K(Ca) efflux, but other dihydropyridines, including BAY K 8644, a Ca-channel agonist, were effective in inhibiting K(Ca) efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of volume-sensitive K transport in human erythrocytes: evidence for asymmetry

The kinetic properties of volume-sensitive K fluxes in swollen human erythrocytes were investigated. Swelling-activated Cl-dependent K influx was a saturable function of external K concentration with a low affinity (apparent Km of 115-130 mM) and high capacity [maximal velocity (Vmax) = 20-30 mmol.l original cells-1.h-1 (mmol.loc-1.h-1)]. The Vmax and apparent Km for Cl-dependent K efflux were lower (Km = 47 mM; Vmax = 2.2 mmol.loc-1.h-1). The Hill coefficients for both K efflux and influx were close to unity, suggesting a single binding site for K. The increase of external K trans-stimulated K efflux, but the increase of intracellular K had no effect on Cl-dependent K influx in swollen cells. Under zero trans conditions, the Vmax (18 vs. 3 mmol.loc-1.h-1) and Km (138 vs. 32) were markedly different for influx and efflux, respectively. These results provide evidence for intrinsic functional asymmetry, such that the transporter is more prevalent and stable in the outward-facing conformation. The mean ratio of Km to Vmax for efflux (12.1) was 1.56 times larger than the same ratio for influx (7.8), but the difference between the means did not reach statistical significance. These kinetic observations are analyzed in terms of the simple carrier and the cotransport models.

Hemodialysis and red cell cation transport in uremia: role of membrane free fatty acids

Active and facilitated cation transport in erythrocytes of uremic patients may be improved acutely by hemodialysis, although the mechanisms remain unknown. As nonesterified fatty acids (NEFA) can affect Na+ pump activity in vitro, changes in plasma and red cell membrane NEFA content following a single hemodialysis procedure were examined and compared with acute changes in erythrocyte cation flux rates in 34 hemodialysis patients. In nonsodium-loaded cells, small changes in Na+ pump flux with dialysis did correlate with changes in intracellular Na+ content (r = 0.59; N = 17; P less than 0.01). On average, neither maximal Na+ pump activity nor Na+/Li+ counter-transport flux improved with dialysis, but Na+/K+/Cl- cotransport rates rose 25% post-dialysis (P less than 0.02). Plasma NEFA levels rose 87% following hemodialysis but erythrocyte membrane NEFA content declined by 23% (P less than 0.001). Importantly, 24 of the 34 subjects studied had a decrease in erythrocyte membrane NEFA content of greater than 10%, and in these patients, the fall in membrane NEFA correlated with an increase in ouabain-sensitive Na+ efflux (r = 0.564; P less than 0.01). The effects of hemodialysis on both erythrocyte NEFA content and Na+ pump flux could be reproduced by incubating pre-dialysis cells in fatty acid-free albumin. We conclude that acute changes in membrane NEFA may modulate active cation transport in uremic erythrocytes.

Erythrocyte sodium transport in dialyzed uremic patients

To investigate the status of the Na+ concentration and ionic fluxes in red cells of human subjects with dialyzed chronic uremia, the authors measured the Na(+)-K+ pump activity as well as Na(+)-K+ cotransport (CoT), Na(+)-Li+ countertransport (CTT) and Na+ passive permeability in erythrocytes from 37 normal subjects and 23 chronic uremic patients receiving maintenance hemodialysis. The mean intracellular Na+ concentration [Na+]i value in the pre-dialytic group was significantly lower than that in control subjects (p less than .0001), but tended to recover to the normal value of [Na+]i in the post-dialytic group. The mean intracellular K+ concentration value in the post-dialytic group was significantly higher than that of the control group (p less than .001), but not significantly different from that of the pre-dialytic group. It was found that the Na(+)-K+ pump activity of erythrocytes in the pre- and post-dialytic groups markedly decreased over that of the normal control group with statistical significance (p less than .0001, respectively). The Na(+)-K+ pump activity in the post-dialytic group, however, tended to recover, but not significantly. The rate constant for ouabain-sensitive Na+ efflux in the post-dialytic group was significantly decreased over that of the normal controls (p less than .05). The authors observed a significant decrease of the Na+ CoT value (p less than .001 respectively) and rate constant for Na+ CoT (p less than .05, respectively) in the patients with pre- and post-dialytic uremia vs. that of normal subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal cation transport in uremia. Mechanisms in adipocytes and skeletal muscle from uremic rats

The cause of the abnormal active cation transport in erythrocytes of some uremic patients is unknown. In isolated adipocytes and skeletal muscle from chronically uremic chronic renal failure rats, basal sodium pump activity was decreased by 36 and 30%, and intracellular sodium was increased by 90 and 50%, respectively, compared with pair-fed control rats; insulin-stimulated sodium pump activity was preserved in both tissues. Lower basal NaK-ATPase activity in adipocytes was due to a proportionate decline in [3H]ouabain binding, while in muscle, [3H]ouabain binding was not changed, indicating that the NaK-ATPase turnover rate was decreased. Normal muscle, but not normal adipocytes, acquired defective Na pump activity when incubated in uremic sera. Thus, the mechanism for defective active cation transport in CRF is multifactorial and tissue specific. Sodium-dependent amino acid transport in adipocytes closely paralleled diminished Na pump activity (r = 0.91), indicating the importance of this defect to abnormal cellular metabolism in uremia.

Endogenous cardiac glycosidelike compounds

The possibility that endogenous inhibitors of the sodium pump exist and bind to the cardiac glycoside binding site on Na+,K+-adenosine triphosphatase (ATPase) has been a source of much controversy. Although numerous hormones and inorganic ions that modulate Na+,K+-ATPase activity have been described, most of these affect the sodium pump indirectly by varying the intracellular sodium concentration or by increasing the number of enzyme units. None of these endogenous compounds has been shown conclusively to modulate sodium pump activity by binding to the cardiac glycoside binding site on Na+,K+-ATPase. However, the near-universal presence of three high-affinity binding sites on the alpha-subunit of the enzyme has engendered much speculation that endogenous ligands for these receptors must exist. In addition, none of the hormones known to indirectly affect sodium pump activity in vivo has been shown to modulate Na+,K+-ATPase activity in response to extracellular volume expansion or to play a role in the pathogenesis of hypertension or chronic renal failure, conditions in which a circulating inhibitor of Na+,K+-ATPase has been implicated. This report presents a condensed history of the search for endogenous inhibitors of Na+,K+-ATPase and describes recent advances in the field. Despite progress in identifying and characterizing compounds that could affect Na+,K+-ATPase activity in vivo, definitive proof for the existence of endogenous ligands for the cardiac glycoside binding site remains elusive.

Effects of L-carnitine on sodium transport in erythrocytes from dialyzed uremic patients

Red blood cell (RBC) sodium transport systems were studied by cation flux methodology, measuring both the ouabain-sensitive Na-K pump and the ouabain-insensitive Na-K cotransport (CoT), as well as the Na-lithium (Li) countertransport (CTT), in eight patients on chronic hemodialysis and a control group of eight normal individuals. Intracellular sodium content and passive Na permeability were also determined. The effect of L-carnitine on RBC sodium transport in the uremic group was evaluated by supplementation with oral (1 g/day) and i.v. (1 g post-hemodialysis) L-carnitine for 60 days. Mean Na efflux through the ouabain-sensitive Na-K pump was 30.7% lower in uremic patients than in controls (3.49 +/- 1.52 vs. 5.04 +/- 0.72 mmol/liter RBCxhr; P less than 0.025). Intracellular Na content was higher in uremic patients (11.57 +/- 3.38 vs. 8.86 +/- 0.88 mEq/liter RBC; P less than 0.05), but no differences were found in Na-K CoT, Na-Li CTT or passive Na permeability. L-carnitine treatment increased the ouabain-sensitive Na efflux in uremic patients (4.76 +/- 1.6 vs. 3.49 +/- 1.52 mmol/liter RBCxhr; P less than 0.05), with no change in CoT, CTT, intracellular Na content or passive Na permeability. We conclude that L-carnitine deficiency may play a major role in uremic Na-K pump disfunction.

A kinetic study of cation transport in erythrocytes from uremic patients

We previously described in red blood cells (RBCs) from uremic patients on dialysis a reduction in sodium (Na) efflux through the Na, potassium (K) cotransport system (Na,K CoT) while Na efflux through the Na,K pump was normal. We then examined Na efflux in fresh cells and in cells loaded to obtain one level of intracellular sodium (Nai) concentration at about 25 mmol/liter cell. In the present study we used similar cation flux methodology to examine the kinetics of cation efflux through the Na,K pump and Na,K CoT in uremic patients on dialysis. RBCs were Na-loaded to attain five different levels of Nai concentration over a range of 5 to 50 mmol/liter cells using the ionophore nystatin. At each level of Na-loading, the Nai achieved was similar in RBCs from controls and patients. Ouabain-sensitive Na efflux through the Na,K pump showed no difference in rate between normals and dialysis patients. When the kinetic parameters of this transport pathway were considered, the apparent affinity (K0.5) for sodium was not significantly different between controls and patients (18.4 +/- 2.3 vs. 20.0 +/- 2.6 mmol/liter cell) and the maximal velocity of efflux (Vmax) was also not different between controls and patients (9.6 +/- 0.7 vs. 8.5 +/- 1.2 mmol/liter cell/hr). Comparison of Nai-activated Na versus K efflux rates through the Na,K CoT in normal subjects demonstrated similar saturation kinetics, (K0.5 15.8 +/- 3.3 vs. 12.2 +/- 2.8 mmol/liter cell, Vmax 0.81 +/- 0.1 vs. 0.78 +/- 0.1 mmol/liter cell/hr) consistent with the known stoichiometric ratio of 1 Na:1 K:2 Cl described for this mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of N-ethylmaleimide on K transport in density-separated human red blood cells

N-ethylmaleimide (NEM) is a sulfhydryl-reacting agent known to stimulate chloride-dependent K transport in a variety of red cells. In high K sheep red cells, NEM-induced K movements are greater in magnitude in young cells compared with old cells. We hypothesized that human red cells might respond to NEM like high K sheep red cells. To test this idea, cells of various age were exposed to 0.5 mM NEM. We found that, after a 4-h incubation, young cells lost 50% of cell K, compared with 10% K loss in older cells. K loss in all fractions was inhibited by chloride replacement or furosemide.

The search for a hypothalamic Na+,K+-ATPase inhibitor

Accumulating experimental evidence suggests that natriuresis in response to intravascular volume expansion is promoted by an endogenous regulator of Na+,K+-adenosine triphosphatase (ATPase). Efforts to purify this substance by a number of laboratories have as yet been unsuccessful. The properties of partially purified inhibitors from plasma, urine, and tissue often fail to possess the characteristics thought to be consistent with those of a physiological regulator. These include potency (Ki of approximately 1 nM), reversibility of inhibition, specificity for Na+,K+-ATPase, and responsiveness to relevant physiological stimuli. Two rather different candidate substances, extracted from urine and hypothalamus, have been purified to a high degree. Neither is a peptide, and both are of low molecular weight and resistant to acid hydrolysis. The substance from urine is rather nonpolar and interacts with digoxin-specific antibodies, while that from hypothalamus is polar and does not appear to share epitopes with the cardiac glycosides. On the serosal surface of the toad urinary bladder, the hypothalamic substance causes a reversible inhibition of Na+ transport, inhibits rubidium uptake in red blood cells by acting on the membrane's exterior surface, inhibits binding of ouabain to purified Na+,K+-ATPase, and reversibly inhibits hydrolysis of adenosine 5'-triphosphate by the enzyme with a Ki of 1.4 nM. The hypothalamic inhibitor may be differentiated from ouabain by their respective ionic requirements for optimal inhibition of enzymatic activity, and although both ouabain and the hypothalamic inhibitor fix Na+,K+-ATPase in its E2 conformation, the hypothalamic inhibitor does not promote phosphorylation of the enzyme by inorganic phosphate in the presence of Mg2+. Ionic requirements for inhibition also differentiate the hypothalamic inhibitor from vanadate ion, as does the inhibitor's activity in the presence of norepinephrine. Further enzymological and physiological studies will be facilitated by structural characterizations of the inhibitory substances and by the availability of a method to measure their concentrations in physiological fluids.

Erythrocyte sodium and potassium in patients on peritoneal dialysis

Intracellular sodium and potassium concentrations were determined on erythrocytes obtained, before and after treatment, from patients with end-stage renal disease undergoing 48-hour intermittent peritoneal dialysis. Erythrocyte sodium increased from 7.5 +/- 0.3 to 8.6 +/- 0.4 mmol/L cells with a mean of 1.1 +/- 0.1 mmol/L cells (P less than .001), but erythrocyte potassium and cellular water content were virtually unchanged. Plasma potassium decreased during dialysis from 4.2 +/- 0.2 to 3.3 +/- 0.1 mEq/L (P less than .001). The increase in red-cell sodium correlated with this decrease in plasma potassium (r = .51, P less than .01). In contrast, erythrocyte sodium and potassium in undialyzed control patients with chronic renal failure did not change over a similar period, and plasma potassium was unchanged (4.3 +/- 0.1 mEq/L before and 4.3 +/- 0.2 mEq/L after 48 hours). Incubation of postdialysis erythrocytes from the dialysis patients in their own plasma at varying potassium concentrations showed that the rise in cell sodium was blunted as the plasma potassium was increased from 3.2 +/- 0.1 to 4.5 +/- 0.2 mEq/L. These results suggest that unlike hemodialysis, which is not associated with short-term changes in red-cell electrolytes, intermittent peritoneal dialysis results in a reversible increase in erythrocyte sodium. This change appears to be causally related to the decrease in extracellular potassium concentration.

Endogenous digitalis-like factors in hypertension and chronic renal insufficiency

Endogenous digitalis-like factors have been implicated in the adaptations that accompany renal insufficiency and in the pathogenesis of hypertension. We recently described several fractions of normal human plasma that inhibit NaK-ATPase and exhibit apparent digoxin-like immunoreactivity. To determine if hypertension and/or renal insufficiency affect plasma levels of these factors, we examined four patient groups: normotensive controls; hypertensive subjects with normal renal function; hypertensives with moderate renal insufficiency; and chronic dialysis patients. Plasma levels of digoxin-like immunoreactivity and NaK-ATPase inhibitory activity were significantly increased in hypertensive patients with mild renal failure (7.6 +/- 1.1 ouabain equivalents, mean +/- SEM, N = 21 vs 4.1 +/- 1.1 in normotensive controls, N = 20, P less than 0.05). NaK-ATPase inhibitory activity tended to be higher in patients with primary hypertension and normal renal function (5.5 +/- 0.7 ouabain equivalents, P less than 0.07); in dialysis patients, it was not different from controls. There was no correlation between NaK-ATPase inhibitory activity and blood pressure in any group. There was a significant rise in plasma NaK-ATPase inhibitory activity during dialysis (+ 1.8 +/- 0.7 ouabain equivalents, N = 22, P less than 0.03). As we have found that NaK-ATPase inhibitory activity in the plasma of normal humans can be separated into three distinct fractions, EI1, EI2, and EI3, we analyzed the plasma of 10 dialysis patients further. The increase in NaK-ATPase inhibitory activity could be attributed to fractions EI1 and EI3. These results suggest that plasma NaK-ATPase inhibitors increase with chronic renal insufficiency, but not hypertension alone. Although hemodialysis may acutely raise plasma levels, long-term dialysis returns them to the normal range.

Sodium transport in red blood cells from dialyzed uremic patients

Studies on red blood cell (RBC) sodium (Na) transport in chronic renal failure have described abnormalities in the ouabain-sensitive Na, K pump. We now report Na transport in RBC using cation flux methodology, measuring both the ouabain-sensitive Na, K pump and the ouabain-insensitive Na, K cotransport (CoT) and Na, lithium (Li) countertransport (CTT) in 28 subjects on hemodialysis, eight subjects on chronic ambulatory peritoneal dialysis (CAPD) and 29 control subjects. Intracellular cation content and passive permeability of Na were also examined. Mean Na efflux through the ouabain-sensitive Na, K pump was not reduced in dialysis patients when compared to normal subjects, whether measured in fresh cells (1.41 +/- 0.05 vs. 1.30 +/- 0.03 mmole/liter RBC/hr; P less than 0.05) or in Na-loaded cells (7.10 +/- 0.24 vs. 6.90 +/- 0.22; NS). There was, however, a marked and uniform suppression of the CoT pathway in Na-loaded cells from dialysis patients versus controls (0.14 +/- 0.02 vs. 0.41 +/- 0.05 mmole/liter RBC/hr; P less than 0.001). Mean CTT activity, as measured by Li efflux, was not different between dialysis and normal subjects. Uremic and normal RBC had similar intracellular Na or K content as well as passive permeability for either ion. This indicates that intracellular cationic homeostasis is maintained, perhaps secondary to balanced changes in cationic flux activity through these transport pathways.

Sodium pump parameters of red blood cells in men, women and women taking oral contraceptives

Three parameters of the sodium pump, the sodium efflux rate, the intracellular sodium content (Nai) and the number of sodium, potassium ATPase sites per red blood cell (sites/rbc) were measured simultaneously on erythrocytes from normal men and women and from women taking oral contraceptives. Significant (p less than 0.002) differences were obtained between normal men and women for sodium efflux (1.68 +/- 0.15 vs 1.52 +/- 0.12 mmoles/L rbc/hr). Among the three measured parameters, there was an inverse relationship (r = -0.74) between Nai and sites/rbc and a linear correlation (r = 0.89) between efflux per site and Nai. A positive correlation (r = 0.78) between the sites/rbc and a first-order rate constant for Na efflux/rbc suggests that one component of the rate constant is the number of sites/rbc. Values for a second-order rate constant, calculated from the assumption that efflux/rbc depends on both Nai and the number of sites/rbc, were similar for normal men and women but significantly (p less than 0.001) higher for women taking oral contraceptives.

Kinetics of Cl-dependent K influx in human erythrocytes with and without external Na: effect of NEM

The majority of the ouabain-insensitive K influx in human erythrocytes is dependent on the presence of Cl. Recent studies have shown that a portion of the Cl-dependent K influx persists in the absence of external Na (Nao). It has been suggested that this Nao-independent component represents (K + Cl) cotransport, whereas the remainder of the Cl-dependent K influx seen on addition of external Na represents (Na + K + 2Cl) cotransport. In the present studies, the kinetics of Cl-dependent K influx were examined in the presence and absence of external Na, by varying external K and external Cl. Our studies suggest that the Nao-independent Cl-dependent pathway has a relatively low affinity for external K (Km 17-30 mM) in contrast to the high affinity of the Nao-augmented component (Km 3-4 mM). N-ethylmaleimide (NEM) stimulates the maximal velocity of the Nao-independent Cl-dependent K influx achievable without alteration of intracellular solutes but does not alter its Km for external K. In contrast, NEM has no stimulatory effect on the Nao-augmented component. The Cl dependence of the Nao-independent K influx is best described by a relatively flat curve with a mild upward concavity. The kinetic properties of the Nao-independent component of Cl-dependent K transport are very similar to those of the putative (K + Cl) cotransport pathway seen in low-K sheep erythrocytes.(ABSTRACT TRUNCATED AT 250 WORDS)