Kinetics and stoichiometry of the human red cell Na+/H+ exchanger

Is post-hypertonic lysis of human red blood cells caused by excessive cell volume regulation?

Human red blood cells (RBC) exposed to hypertonic media are subject to post-hypertonic lysis - an injury that only develops during resuspension to an isotonic medium. The nature of post-hypertonic lysis was previously hypothesized to be osmotic when cation leaks were observed, and salt loading was suggested as a cause of the cell swelling upon resuspension in an isotonic medium. However, it was problematic to account for the salt loading since the plasma membrane of human RBCs was considered impermeable to cations. In this study, the hypertonicity-related behavior of human RBCs is revisited within the framework of modern cell physiology, considering current knowledge on membrane ion transport mechanisms - an account still missing. It is recognized here that the hypertonic behavior of human RBCs is consistent with the acute regulatory volume increase (RVI) response - a healthy physiological reaction initiated by cells to regulate their volume by salt accumulation. It is shown by reviewing the published studies that human RBCs can increase cation conductance considerably by activating cell volume-regulated ion transport pathways inactive under normal isotonic conditions and thus facilitate salt loading. A simplified physiological model accounting for transmembrane ion fluxes and membrane voltage predicts the isotonic cell swelling associated with increased cation conductance, eventually reaching hemolytic volume. The proposed involvement of cell volume regulation mechanisms shows the potential to explain the complex nature of the osmotic response of human RBCs and other cells. Cryobiological implications, including mechanisms of cryoprotection, are discussed.

Promises and Pitfalls of Parasite Patch-clamp

Protozoan parasites acquire essential ions, nutrients, and other solutes from their insect and vertebrate hosts by transmembrane uptake. For intracellular stages, these solutes must cross additional membranous barriers. At each step, ion channels and transporters mediate not only this uptake but also the removal of waste products. These transport proteins are best isolated and studied with patch-clamp, but these methods remain accessible to only a few parasitologists due to specialized instrumentation and the required training in both theory and practice. Here, we provide an overview of patch-clamp, describing the advantages and limitations of the technology and highlighting issues that may lead to incorrect conclusions. We aim to help non-experts understand and critically assess patch-clamp data in basic research studies.

Erythrocyte plasma membrane potential: past and current methods for its measurement

The plasma membrane functions both as a natural insulator and a diffusion barrier to the movement of ions. A wide variety of proteins transport and pump ions to generate concentration gradients that result in voltage differences, while ion channels allow ions to move across the membrane down those gradients. Plasma membrane potential is the difference in voltage between the inside and the outside of a biological cell, and it ranges from ~- 3 to ~- 90 mV. Most of the most significant discoveries in this field have been made in excitable cells, such as nerve and muscle cells. Nevertheless, special attention has been paid to some events controlled by changes in membrane potential in non-excitable cells. The origins of several blood disorders, for instance, are related to disturbances at the level of plasma membrane in erythrocytes, the structurally simplest red blood cells. The high simplicity of erythrocytes, in particular, made them perfect candidates for the electrophysiological studies that laid the foundations for understanding the generation, maintenance, and roles of membrane potential. This article summarizes the methodologies that have been used during the past decades to determine Δψ in red blood cells, from seminal microelectrodes, through the use of nuclear magnetic resonance or lipophilic radioactive ions to quantify intra and extracellular ions, to continuously renewed fluorescent potentiometric dyes. We have attempted to highlight the advantages and disadvantages of each methodology, as well as to provide a description of the technical aspects involved.

Determinants of Cation Permeation and Drug Sensitivity in Predicted Transmembrane Helix 9 and Adjoining Exofacial Re-entrant Loop 5 of Na+/H+ Exchanger NHE1

Mammalian Na(+)/H(+) exchangers (NHEs) regulate numerous physiological processes and are involved in the pathogenesis of several diseases, including tissue ischemia and reperfusion injuries, cardiac hypertrophy and failure, and cancer progression. Hence, NHEs are being targeted for pharmaceutical-based clinical therapies, but pertinent information regarding the structural elements involved in cation translocation and drug binding remains incomplete. Molecular manipulations of the prototypical NHE1 isoform have implicated several predicted membrane-spanning (M) helices, most notably M4, M9, and M11, as important determinants of cation permeation and drug sensitivity. Here, we have used substituted-cysteine accessibility mutagenesis and thiol-modifying methanethiosulfonate (MTS) reagents to further probe the involvement of evolutionarily conserved sites within M9 (residues 342-363) and the adjacent exofacial re-entrant loop 5 between M9 and M10 (EL5; residues 364-415) of a cysteine-less variant of rat NHE1 on its kinetic and pharmacological properties. MTS treatment significantly reduced the activity of mutants containing substitutions within M9 (H353C, S355C, and G356C) and EL5 (G403C and S405C). In the absence of MTS, mutants S355C, G403C, and S405C showed modest to significant decreases in their apparent affinities for Na(+) o and/or H(+) i. In addition, mutations Y370C and E395C within EL5, whereas failing to confer sensitivity to MTS, nevertheless, reduced the affinity for Na(+) o, but not for H(+) i. The Y370C mutant also exhibited higher affinity for ethylisopropylamiloride, a competitive antagonist of Na(+) o transport. Collectively, these results further implicate helix M9 and EL5 of NHE1 as important elements involved in cation transport and inhibitor sensitivity, which may inform rational drug design.

Na+/H+ exchangers

Tightly coupled exchange of Na(+) for H(+) occurs across the surface membrane of virtually all living cells. For years, the underlying molecular entity was unknown and the full physiological significance of the exchange process was not appreciated, but much knowledge has been gained in the last two decades. We now realize that, unlike most of the other transporters that specialize in supporting one specific function, Na(+)/H(+) exchangers (NHE) participate in a remarkable assortment of physiological processes, ranging from pH homeostasis and epithelial salt transport, to systemic and cellular volume regulation. In parallel, we have learned a great deal about the biochemistry and molecular biology of Na(+)/H(+) exchange. Indeed, it has now become apparent that exchange is mediated not by one, but by a diverse family of related yet distinct carriers (antiporters) sometimes present in different cell types and located in various intracellular compartments. Each one of these has unique structural features that dictate its functional role and mode of regulation. The biological relevance of Na(+)/H(+) exchange is emphasized by its evolutionary conservation; analogous exchangers are present from bacteria to man. Because of its wide distribution and versatile function, Na(+)/H(+) exchange has attracted an enormous amount of interest and therefore generated a vast literature. The vastness and complexity of the field has been compounded by the multiplicity of NHE isoforms. For reasons of space and in the spirit of this series, this overview is restricted to the family of mammalian NHEs.

Oxygen-dependent ion transport in erythrocytes

The present contribution reviews current knowledge of apparently oxygen-dependent ion transport in erythrocytes and presents modern hypotheses on their regulatory mechanisms and physiological roles. In addition to molecular oxygen as such, reactive oxygen species, nitric oxide, carbon monoxide, regional variations of cellular ATP and hydrogen sulphide may play a role in the regulation of transport, provided that they are affected by oxygen tension. It appears that the transporter molecules themselves do not have direct oxygen sensors. Thus, the oxygen level must be sensed elsewhere, and the effect transduced to the transporter. The possible pathways involved in the regulation of transport, including haemoglobin as a sensor, and phosphorylation/dephosphorylation reactions both in the transporter and its upstream effectors, are discussed.

Steady-state function of the ubiquitous mammalian Na/H exchanger (NHE1) in relation to dimer coupling models with 2Na/2H stoichiometry

We describe the steady-state function of the ubiquitous mammalian Na/H exchanger (NHE)1 isoform in voltage-clamped Chinese hamster ovary cells, as well as other cells, using oscillating pH-sensitive microelectrodes to quantify proton fluxes via extracellular pH gradients. Giant excised patches could not be used as gigaseal formation disrupts NHE activity within the patch. We first analyzed forward transport at an extracellular pH of 8.2 with no cytoplasmic Na (i.e., nearly zero-trans). The extracellular Na concentration dependence is sigmoidal at a cytoplasmic pH of 6.8 with a Hill coefficient of 1.8. In contrast, at a cytoplasmic pH of 6.0, the Hill coefficient is <1, and Na dependence often appears biphasic. Results are similar for mouse skin fibroblasts and for an opossum kidney cell line that expresses the NHE3 isoform, whereas NHE1^−/− skin fibroblasts generate no proton fluxes in equivalent experiments. As proton flux is decreased by increasing cytoplasmic pH, the half-maximal concentration (K_1/2) of extracellular Na decreases less than expected for simple consecutive ion exchange models. The K_1/2 for cytoplasmic protons decreases with increasing extracellular Na, opposite to predictions of consecutive exchange models. For reverse transport, which is robust at a cytoplasmic pH of 7.6, the K_1/2 for extracellular protons decreases only a factor of 0.4 when maximal activity is decreased fivefold by reducing cytoplasmic Na. With 140 mM of extracellular Na and no cytoplasmic Na, the K_1/2 for cytoplasmic protons is 50 nM (pH 7.3; Hill coefficient, 1.5), and activity decreases only 25% with extracellular acidification from 8.5 to 7.2. Most data can be reconstructed with two very different coupled dimer models. In one model, monomers operate independently at low cytoplasmic pH but couple to translocate two ions in “parallel” at alkaline pH. In the second “serial” model, each monomer transports two ions, and translocation by one monomer allosterically promotes translocation by the paired monomer in opposite direction. We conclude that a large fraction of mammalian Na/H activity may occur with a 2Na/2H stoichiometry.

Physiology and pathophysiology of Na+/H+ exchange and Na+ -K+ -2Cl- cotransport in the heart, brain, and blood

Maintenance of a stable cell volume and intracellular pH is critical for normal cell function. Arguably, two of the most important ion transporters involved in these processes are the Na+/H+ exchanger isoform 1 (NHE1) and Na+ -K+ -2Cl- cotransporter isoform 1 (NKCC1). Both NHE1 and NKCC1 are stimulated by cell shrinkage and by numerous other stimuli, including a wide range of hormones and growth factors, and for NHE1, intracellular acidification. Both transporters can be important regulators of cell volume, yet their activity also, directly or indirectly, affects the intracellular concentrations of Na+, Ca2+, Cl-, K+, and H+. Conversely, when either transporter responds to a stimulus other than cell shrinkage and when the driving force is directed to promote Na+ entry, one consequence may be cell swelling. Thus stimulation of NHE1 and/or NKCC1 by a deviation from homeostasis of a given parameter may regulate that parameter at the expense of compromising others, a coupling that may contribute to irreversible cell damage in a number of pathophysiological conditions. This review addresses the roles of NHE1 and NKCC1 in the cellular responses to physiological and pathophysiological stress. The aim is to provide a comprehensive overview of the mechanisms and consequences of stress-induced stimulation of these transporters with focus on the heart, brain, and blood. The physiological stressors reviewed are metabolic/exercise stress, osmotic stress, and mechanical stress, conditions in which NHE1 and NKCC1 play important physiological roles. With respect to pathophysiology, the focus is on ischemia and severe hypoxia where the roles of NHE1 and NKCC1 have been widely studied yet remain controversial and incompletely elucidated.

MAPKinase and regulation of the sodium-proton exchanger in human red blood cell

The sodium-proton exchanger is activated by various agonists, including insulin, even in human red blood cell. MAPKinase, a family of ubiquitous serine/threonine kinases, plays an important role in the signal transduction pathways which lead to sodium-proton exchanger activation. The aim of our study was to establish the existence of MAPKinase in human red blood cell and to investigate the effects of its activation by insulin and okadaic acid on the sodium-proton exchanger. Immunoblot with antiMAPK antibody revealed the presence of two isoforms, p44(ERK1) and p42(ERK2). Insulin stimulated MAPKinase activity and increased the phosphorylation of MAPK tyrosine residues, with a peak time between 3 and 5 min. Okadaic acid, an inhibitor of serine/threonine phosphatases, stimulated MAPKinase activity. In the presence of PD98059, an inhibitor of MEK, the upstream activator of MAPKinase, insulin and okadaic acid failed to stimulate MAPKinase. Insulin and okadaic acid increased the activity of the sodium-proton exchanger and this effect was abolished by PD98059. In conclusion, we first describe the presence and activity of MAPKinase in human red blood cell. Furthermore, we demonstrate that in human red blood cell, insulin modulates the sodium-proton exchanger through MAPKinase activation.

Independence of dimethylamiloride-sensitive Li+ efflux pathways and Na+-Li+ countertransport in human erythrocytes

The in vivo function of the erythrocyte Na+-Li+ countertransport (SLC) is unknown. Whether SLC may reflect an operational mode of the widespread Na+-H+ exchanger (NHE) or may otherwise be expression of an independent membrane transport, remains presently unclear. We explored the presence of 5-(N,N-dimethyl)-amiloride (DMA)-sensitive Li+ pathways in human erythrocytes where the activity of the Na+ pump, Na+-K+ cotransport and anion exchange were suitably inhibited. A total of 0.02 mM DMA had no effect on SLC as expected, but gave a significant inhibition of Li+ efflux into both Na+ and Na+-free media. This DMA-sensitive Li+ pathway, but not SLC, was significantly enhanced by hyperosmolar cell shrinkage, which is a characteristic feature of NHE. In conclusion, DMA-sensitive Li+ pathways, possibly mediated by NHE, are present in erythrocytes and coexist with the DMA-insensitive, SLC. This finding supports the notion that SLC is independent of amiloride-sensitive NHE.

Activation by calcium of erythrocyte Na+/H+ exchange in men

OBJECTIVE

To determine whether protein kinase C is necessary for the calcium activation of the Na+/H+ exchange in human erythrocytes by studying activation by calcium of erythrocyte Na+/H+ exchange in control cells, in protein kinase C-depleted cells after downregulation of protein kinase C with phorbol-12-myristate-13-acetate and in cells that had been treated beforehand with phorbol-12-myristate-13-acetate with and without the calpain inhibitor E-64d.

METHODS

Erythrocyte Na+/H+ exchange was measured by determining the initial rates of the influx of Na+ into Na+-depleted, acid loaded cells. The effects of various concentrations (0-1 mmol/l) of CaCl2 and the effects of 1 mmol/l CaCl2 on activation of the intracellular pH and on the external Na+ activation of Na+/H+ exchange were studied. The effects of 1 mmol/l CaCl2 on Na+/H+ exchange in control cells and cells that had been incubated beforehand with and without 1 micromol/l phorbol-12-myristate-13-acetate and with E-64d and 1 micromol/l phorbol-12-myristate-13-acetate for 1, 2, 3 and 24 h were also investigated.

RESULTS

Addition of Ca2+ to a concentration in the range 0-1 mmol/l in the presence of calcimycin resulted in stimulation of Na+/H+ exchange: 1 mmol/l CaCl2 increased (P< 0.001) the erythrocyte Na+/H+ exchange by 74%. Calcium increased the maximum rate for activations by intracellular pH and by external Na+ of Na+/H+ exchange, whereas it did not affect the Michaelis-Menten constants for activation by intracellular H+ and external Na+. However, calcium did not activate the Na+/H+ exchange in protein kinase C downregulated erythrocytes and administration of the calpain inhibitor E-64d could not prevent this inactivation.

CONCLUSION

Our data indicate that protein kinase C is necessary for the activation by calcium of the erythrocyte Na+/H+ exchange.

Band 3 Campinas: A Novel Splicing Mutation in the Band 3 Gene (AE1 ) Associated With Hereditary Spherocytosis, Hyperactivity of Na+/Li+ Countertransport and an Abnormal Renal Bicarbonate Handling

We have studied the molecular defect underlying band 3 deficiency in one family with hereditary spherocytosis using nonradioactive single strand conformation polimorphism of polymerase chain reaction (PCR) amplified genomic DNA of the AE1 gene. By direct sequencing, a single base substitution in the splicing donor site of intron 8 (position + 1G → T) was identified. The study of the cDNA showed a skipping of exon 8. This exon skipping event is responsible for a frameshift leading to a premature stop codon 13 amino acids downstream. The distal urinary acidification test by furosemide was performed to verify the consequences of the band 3 deficiency in α intercalated cortical collecting duct cells (αICCDC). We found an increased basal urinary bicarbonate excretion, associated with an increased basal urinary pH and an efficient distal urinary acidification. We also tested the consequences of band 3 deficiency on the Na+/H+ exchanger, by the measurement of Na+/Li+ countertransport activity in red blood cells. The Na+/Li+ countertransport activity was increased threefold to sixfold in the patients compared with the controls. It is possible that band 3 deficiency in the kidney leads to a decrease in the reabsorption of HCO−3 in αICCDC and anion loss, which might be associated with an increased sodium-lithium countertransport activity.

Band 3 Campinas: A Novel Splicing Mutation in the Band 3 Gene (AE1 ) Associated With Hereditary Spherocytosis, Hyperactivity of Na+/Li+ Countertransport and an Abnormal Renal Bicarbonate Handling

We have studied the molecular defect underlying band 3 deficiency in one family with hereditary spherocytosis using nonradioactive single strand conformation polimorphism of polymerase chain reaction (PCR) amplified genomic DNA of the AE1 gene. By direct sequencing, a single base substitution in the splicing donor site of intron 8 (position + 1G → T) was identified. The study of the cDNA showed a skipping of exon 8. This exon skipping event is responsible for a frameshift leading to a premature stop codon 13 amino acids downstream. The distal urinary acidification test by furosemide was performed to verify the consequences of the band 3 deficiency in α intercalated cortical collecting duct cells (αICCDC). We found an increased basal urinary bicarbonate excretion, associated with an increased basal urinary pH and an efficient distal urinary acidification. We also tested the consequences of band 3 deficiency on the Na+/H+ exchanger, by the measurement of Na+/Li+ countertransport activity in red blood cells. The Na+/Li+ countertransport activity was increased threefold to sixfold in the patients compared with the controls. It is possible that band 3 deficiency in the kidney leads to a decrease in the reabsorption of HCO−3 in αICCDC and anion loss, which might be associated with an increased sodium-lithium countertransport activity.

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.

Protein kinase C and insulin regulation of red blood cell Na+/H+ exchange

Insulin activation of red blood cell (RBC) Na+/H+ (NHE) and Na+/Li+ (NLiE) exchanges is mimicked by okadaic acid, thus suggesting that it may change the state of phosphorylation of serine/threonine NHE residues. To investigate the role of the serine/threonine protein kinase C (PKC) in insulin regulation, we evaluated the effect of phorbol 12-myristate 13-acetate (PMA; 1 microM) and insulin on PKC activity, membrane protein phosphorylation, and the activation kinetics of both exchangers. Our studies revealed that PMA decreased cytosolic PKC activity (4.1 +/- 0.6 to 2.3 +/- 0.5 pmol x mg protein(-1) x min(-1), n = 9, P < 0.001), increased membrane PKC activity (42.3 +/- 5 to 132 +/- 12 pmol x mg protein(-1) x min(-1), n = 11, P < 0.001), and enhanced serine phosphorylation of bands 3, 4.1, and 4.9 membrane proteins. PMA markedly reduced the Michaelis constant (Km) for intracellular H+ (415 +/- 48 to 227 +/- 38 nM, n = 11, P < 0.01) but had no effect on the maximal transport rate (Vmax) of NHE and the Km for Na+ of NLiE. NHE activation and PKC activity were affected differently by insulin (100 microU/ml) and PMA. Insulin increased the Vmax of NHE and the Km for Na+ of NLiE but had no effect on the Km for intracellular H+ and membrane PKC activity. These findings lead us to conclude that in the human RBC, NHE is modulated by PKC and insulin through different biochemical mechanisms.

Membrane potential and human erythrocyte shape

Altered external pH transforms human erythrocytes from discocytes to stomatocytes (low pH) or echinocytes (high pH). The process is fast and reversible at room temperature, so it seems to involve shifts in weak inter- or intramolecular bonds. This shape change has been reported to depend on changes in membrane potential, but control experiments excluding roles for other simultaneously varying cell properties (cell pH, cell water, and cell chloride concentration) were not reported. The present study examined the effect of independent variation of membrane potential on red cell shape. Red cells were equilibrated in a set of solutions with graduated chloride concentrations, producing in them a wide range of membrane potentials at normal cell pH and cell water. By using assays that were rapid and accurate, cell pH, cell water, cell chloride, and membrane potential were measured in each sample. Cells remained discoid over the entire range of membrane potentials examined (-45 to +45 mV). It was concluded that membrane potential has no independent effect on red cell shape and does not mediate the membrane curvature changes known to occur in red cells equilibrated at altered pH.

Microalbuminuria and erythrocyte sodium-hydrogen exchange in essential hypertension

Microalbuminuria (urinary albumin excretion between 20 and 200 micrograms/min) and abnormalities of red blood cell sodium-hydrogen exchange coexist in essential hypertensive patients. To evaluate how the two phenomena relate, we recruited 10 untreated microalbuminuric male essential hypertensive patients without diabetes to be compared with an equal number of matched essential hypertensive patients excreting albumin in normal amounts as well as 10 healthy control subjects. Sodium-hydrogen exchange values were increased to a comparable extent in microalbuminuric and normoalbuminuric hypertensive patients. Systolic and mean blood pressures were higher in microalbuminuric patients. Fasting insulin was greater and high-density lipoprotein cholesterol lower in patients than control subjects. Urinary albumin excretion correlated positively with both mean blood pressure and left ventricular mass values in the absence of a relationship with circulating lipid and insulin levels. In contrast with microalbuminuria, sodium-hydrogen exchange covaried only with high-density lipoprotein cholesterol and insulin levels. Thus, microalbuminuria and an abnormal sodium-hydrogen exchange are unrelated phenomena in essential hypertensive patients. Microalbuminuria appears to be a hemodynamically driven biological variable, while an accelerated sodium-hydrogen exchange seems primarily conditioned by the metabolic abnormalities of hypertension, possibly in the context of an insulin-resistant syndrome.

Insulin activation of red blood cell Na+/H+ exchange decreases the affinity of sodium sites

We have previously reported increased activity of Na+/H+ and Na+/Li+ exchanges in red blood cells (RBC) of patients with hypertension and diabetic nephropathy. The presence in human red blood cells (RBC) of insulin receptors has led us to examine the effects of this hormone on the kinetic parameters of Na+/H+ exchange as a first approach to define its mechanism of action. The antiporter activity was measured as net Na+ influx driven by an outward H+ gradient in acid-loaded, Na-depleted RBCs preincubated with or without (w/wo) insulin (0 to 100 microU/ml) for different time periods. The effects of insulin on the H+ and Na+ activation kinetics of Na+/H+ exchange were examined in RBCs of normal subjects fasted for 12 hours. Insulin (50 microU/ml for 1 hr) increased the Vmax from 28 +/- 6 to 49 +/- 8 mmol/liter cell x hr (N = 10, P < 0.0005) and the Km for Na+ from 72 +/- 10 to 142 +/- 19 mM (N = 4, P < 0.05) but did not change the Km for intracellular H+. Insulin also increased the Vmax of Na+/Li+ exchange at pHi 7.4 (0.34 +/- 0.03 to 0.45 +/- 0.04 mmol/liter cell x hr, N = 9, P < 0.005) as well as the Km for Na+ (31 +/- 3 to 6 +/- 10 mM, P < 0.0003). Therefore, insulin can modulate Na+ sites of Na+/Li+ or Na+/H+ exchanges independent of the occupancy of H+ sites to favor the release of bound Na+ into the cytoplasm. Insulin stimulation of Na+/H+ exchange required endogenous cytosolic Ca2+ levels. The kinetic effects of insulin on Na+/H+ and Na+/Li+ exchanges were imitated by okadaic acid (300 microM), an inhibitor of protein phosphatases which dephosphorylate serine-threonine residues. Okadaic acid increased the Vmax of Na+/H+ and Na+/Li+ exchanges and the Km for Na+ as insulin did. In conclusion, insulin stimulation of the Na+/H+ antiporter occurs by a novel kinetic mechanism leading to a decreased affinity for external Na+ without changes in the affinity for Hi. On the basis that insulin effects were imitated by okadaic acid, we hypothesize that this hormone may increase the phosphorylated state of serine-threonine residues of this antiporter protein.

Erythrocyte anion exchanger activity and intracellular pH in essential hypertension

The present study was designed to examine the activity of the sodium-independent chloride-bicarbonate anion exchanger and the sodium-proton exchanger in erythrocytes of 30 normotensive and 35 hypertensive subjects and its relation to the previously reported decrease in erythrocyte pH. Erythrocyte cytosolic pH was measured by the pH-sensitive fluorescent probe 2'-7'-bis(2-carboxyethyl)- 5(6)-carboxyfluorescein. The activity of the anion exchanger was determined by acidifying cell pH and measuring the initial rate of the net sodium-independent, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid-sensitive, bicarbonate influx driven by an outward proton gradient. The activity of the sodium-proton exchanger was determined by acidifying cell pH and measuring the initial rate of the net sodium-dependent proton efflux driven by an outward proton gradient. The activity of the anion exchanger was higher in hypertensive than control individuals (18,863 +/- 1081 vs 15,629 +/- 897 mmol/L cells per hour, P < .05). The activity of the sodium-proton exchanger was higher in hypertensive than control individuals (301 +/- 45 vs 162 +/- 23 mmol/L cells per hour, P < .005). Basal erythrocyte pH was lower in hypertensive than control individuals (7.27 +/- 0.02 vs 7.33 +/- 0.01, mean +/- SEM, P < .05). With the 100% confidence (lower) limit of the normotensive population as a cutoff point, a subgroup of 11 hypertensive patients had an abnormally low erythrocyte pH (< 7.19).(ABSTRACT TRUNCATED AT 250 WORDS)

Increased Na/H antiport activity and abundance in uremic red blood cells

Alterations in red blood cell sodium (Na) transport have been described in chronic renal failure. This study examines the possible impact of uremia on two ouabain-insensitive pathways, the Na/H antiporter and the Cl-/NaCO3- anion exchanger. The Vmax of Na/H antiporter measured as Na influx driven by outward H gradient in acid loaded red blood cells was significantly higher in uremic red blood cells versus controls (60.5 +/- 16.5 vs. 24.5 +/- 5.4 mmol/liter cells/hr, P < 0.025). This increase in activity was associated with an increased abundance of the Na/H antiporter as determined by immunologic analysis using an affinity purified polyclonal antibody to the human NHE-1 isoform of the antiporter. By contrast, the activity of the anion exchanger measured as the DIDS-sensitive lithium (Li) influx was similar in uremic versus control red blood cells (2.10 +/- 0.18 vs. 2.14 +/- 0.20 mmol/liter cells/hr). These experiments, when considered in conjunction with prior studies showing normal Na/Li countertransport in uremia indicate that there is a selective increase in the number of functional Na/H antiporters in uremic red blood cells and that Na/Li countertransport measurements may not be a valid marker for Na/H antiporter activity in red blood cells in patients requiring dialysis for end-stage renal failure.

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

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

Red blood cell Li+/Na+ exchange in patients with diabetic nephropathy and essential hypertension: therapeutic implications

Patients who develop diabetic nephropathy, one of the leading causes of end-stage renal diseases in Western communities, have an increased red cell Li+/Na+ countertransport (CT). Li+/Na+ CT is a membrane function which exchanges intracellular Li for extracellular Na in vitro. High Li+/Na+ CT reflects abnormal kinetic properties of red cell membrane Na/H exchange. A widespread abnormality of Na/H exchange could play a major role in the pathogenesis of diabetic nephropathy as well as of cardiovascular diseases since Na/H exchange is involved in the regulation of cell pH and cell volume; in the cellular response to hormones, mitogens, and growth factors; and in the renal reabsorption of Na and bicarbonate. Li+/Na+ CT is under genetic control and raised in a subgroup of patients with essential hypertension. Among these patients, high Li+/Na+ CT is associated with increased glomerular filtration rate, filtration fraction, proximal fractional Na reabsorption, microalbuminuria, plasma renin activity, and kidney and cardiac volume. Increased Li+/Na+ CT is often associated with hyperlipidemia, hyperuricemia, reduced insulin sensitivity, and obesity. The whole of these observations may explain why patients with diabetes or essential hypertension and increased Li/Na CT are at risk of early renal and cardiac impairment.

Cold activation of Na influx through the Na-H exchange pathway in guinea pig red cells

Previous work showed that amiloride partially inhibits the net gain of Na in cold-stored red cells of guinea pig and that the proportion of unidirectional Na influx sensitive to amiloride increases dramatically with cooling. This study shows that at 37 degrees C amiloride-sensitive (AS) Na influx in guinea pig red blood cells is activated by cytoplasmic H+, hypertonic incubation, phorbol ester in the presence of extracellular Ca2+ and is correlated with cation-dependent H+ loss from acidified cells. Cytoplasmic acidification increases AS Na efflux into Na-free medium. These properties are consistent with the presence of a Na-H exchanger with a H+ regulatory site. Elevation of cytoplasmic free Mg2+ above 3 mM greatly increases AS Na influx: this correlates with a Na-dependent loss of Mg2+, indicating the presence of a Na-Mg exchanger. At 20 degrees C activators of Na-H exchange have little or no further stimulatory effect on the already elevated AS Na influx. AS Na influx is much larger than either Na-dependent H+ loss or AS Na efflux at 20 degrees C. The affinity of the AS Na influx for cytoplasmic H+ is greater at 20 degrees C than at 37 degrees C. Depletion of cytoplasmic Mg2+ does not abolish the high AS Na influx at 20 degrees C. Thus, elevation of AS Na influx with cooling appears to be due to increased activity of a Na-H exchanger (operating in a "slippage" mode) caused by greater sensitivity to H+ at a regulatory site.

Red cell sodium-proton exchange is increased in Dahl salt-sensitive hypertensive rats

To investigate the relationship between red blood cell Na+/H+ exchange (EXC) and genetic factors in hypertension, we studied the maximal rate of the antiporter (mmol/liter cell x hr; flux units = FU) in three strains of genetically hypertensive rats. Salt-resistant Dahl rats (DR) were normotensive under low (0.02%) and high (8%) NaCl diets, while salt-sensitive Dahl rats (DS) became markedly hypertensive after four weeks on the high-NaCl diet. Na+/H+ exchange did not differ between DR and DS rats when both were fed with the low-NaCl diet (mean +/- SE, 31 +/- 3, N = 15, vs. 29 +/- 3 FU, N = 14). On the high-NaCl diet, the DR strain did not exhibit significant changes in blood pressure and antiporter activity, but the DS rats significantly increased their blood pressure and Na+/H+ exchange (57 +/- 4 FU, N = 13) versus DR rats (38 +/- 3 FU, N = 15, P < 0.02). DS rats also significantly increased blood pressure and antiporter activity when fed with high-NaCl diet for one week. These data indicate that high NaCl intake per se does not increase Na+/H+ EXC because the control DR strain did not exhibit transport and blood pressure alterations as observed in the DS strain. Milan hypertensive and spontaneously hypertensive rats (Charles River substrain) had higher blood pressures than Milan and Wistar-Kyoto normotensive rats when they were maintained for four weeks on a 1.5% NaCl diet; however, no differences were seen among normotensive and hypertensive strains in Na+/H+ exchange activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Erythrocyte Li+/Na+ and Na+/H+ exchange, cardiac anatomy and function in insulin-dependent diabetics

It has been proposed that an increased activity of cell membrane Na+/H+ exchange, mirrored by increased erythrocyte Li+/Na+ exchange, may facilitate cell hypertrophy and hyperplasia. Patients with insulin-dependent diabetes mellitus may develop a specific cardiomyopathy with systolic and diastolic abnormalities and increased thickness of the left ventricle. Therefore, we have investigated the relationships between erythrocyte Li+/Na+ and Na+/H+ exchange and echocardiographic parameters in 31 male insulin-dependent diabetics (aged 17-68), in good metabolic control. Three had untreated mild hypertension. In all patients the urinary albumin excretion rate was less than 200 micrograms min-1. Ten patients had a Li+/Na+ countertransport higher than 0.37 mmol l-1 cell h-1, the upper normal limit for our laboratory (0.49 +/- 0.10, mean +/- SD). In comparison with the patients with normal countertransport, they had increased interventricular septum thickness and relative wall thickness (h/r). End diastolic volume and cardiac index were reduced while blood pressure and urinary albumin excretion rate were similar. In the whole study group, interventricular septum thickness was significantly correlated to Li+/Na+ exchange (r = 0.61, P less than 0.001) and Na+/H+ exchange (r = 0.35, P less than 0.05), independently of the effect of age and blood pressure. Posterior wall thickness was correlated to Li+/Na+ exchange (r = 0.38, P less than 0.05) and h/r to Li+/Na+ exchange (r = 0.41, P less than 0.05) and to Na+/H+ exchange (r = 0.44, P less than 0.05). Li+/Na+ exchange was negatively correlated to cardiac index (r = -0.37, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Is increased erythrocyte sodium-lithium countertransport a useful marker for diabetic nephropathy?

Genetic predisposition to essential hypertension has been proposed as a risk factor for the development of diabetic nephropathy in type 1 (insulin-dependent) diabetes mellitus. An increased sodium-lithium countertransport activity (NaLiCT) has been suggested as a genetic marker for essential hypertension. We therefore evaluated NaLiCT in diabetic patients with (N = 39) or without (N = 23) diabetic nephropathy (DNP), patients with non-diabetic renal diseases (N = 42) and in healthy controls (N = 24). The NaLiCT was elevated in both diabetic patient groups compared to healthy controls (median 244; range 134 to 390 mumol.liter cells-1.hr-1), but was not different in patients with DNP (median 314; range 162 to 676), without DNP (median 325; range 189 to 627) and patients with non-diabetic renal disease (median 300; range 142 to 655). The genetic predisposition to DNP is illustrated by the fact that diabetic sibs of probands with DNP showed a higher occurrence of DNP than diabetic sibs of patients without DNP. We analyzed whether familial DNP clustered with an increased NaLiCT. The NaLiCT in sibs concordant for the presence of DNP (N = 10; median 307; range 217 to 428 mumol.liter cells-1.hr-1) was not significantly different from that in sibs concordant for absence of DNP (N = 15; median 279; range 189 to 442). We conclude that erythrocyte sodium-lithium countertransport activity cannot be used as a marker to identify patients at risk for the development of diabetic nephropathy.

Calcium-dependent zinc efflux in human red blood cells

Zinc efflux from human red blood cells is largely brought about by a saturable mechanism that depends upon extracellular Ca2+ ions. It has a Vmax of about 35 mumol/10(13) cells hr, a Km for external Ca2+ of 1 x 10(-4) M, and a Km for internal Zn2+ of 1 x 10(-9) M. External Zn2+ inhibits with a K0.5 of 3 x 10(-6) M. Sr2+ is a substitute for external Ca2+, but changes in monovalent anions or cations have little effect on the Zn2+ efflux mechanism. It is unaffected by most inhibitors of red cell transport systems, although amiloride and D-600 (methoxyverapamil, a Ca2+ channel blocker) are weakly inhibitory. The transport is capable of bringing about the net efflux of Zn2+, against an electrochemical gradient, provided Ca2+ is present externally. This suggests it may be a Zn2+:Ca2+ exchange, which would be able to catalyze the uphill movement of Zn2+ at the expense of an inward Ca2+ gradient, which is itself maintained by the Ca2+ pump.

Kinetic abnormalities of the red blood cell sodium-proton exchange in hypertensive patients

The present study was designed to examine the kinetics of Na(+)-H+ exchange in red blood cells of normotensive and hypertensive subjects and its relation to the previously reported abnormalities in Na(+)-Li+ exchange. The Na(+)-H+ antiporter activation kinetics were studied by varying cell pH and measuring net Na+ influx (mmol/l cell x hr = units) driven by an outward H+ gradient. The Na(+)-Li+ exchange was determined at pH 7.4 as sodium-stimulated Li+ efflux. Untreated hypertensive patients (n = 30) had a higher maximal rate of Na(+)-Li+ exchange (0.43 +/- 0.05 versus 0.26 +/- 0.02 units, p less than 0.0003), a higher maximal rate of Na(+)-H+ exchange (62.3 +/- 6.2 versus 47 +/- 4 units; p less than 0.02), but a similar affinity for cell pH compared with normotensive subjects (n = 46). The cell pH activation of the Na(+)-H+ antiporter exhibited a lower Hill coefficient than that of normotensive subjects (1.61 +/- 0.12 versus 2.56 +/- 0.14; p less than 0.0001). This index of occupancy of internal H+ regulatory sites was found reduced in most of the hypertensive patients (73%) whether their hypertension was untreated or treated. Hypertensive patients with Na(+)-Li+ exchange above 0.35 units (0.68 +/- 0.057 units, n = 16) did not exhibit elevated maximal rates of Na(+)-H+ exchange (57.3 +/- 10 units, NS) in comparison with those with Na(+)-Li+ exchange below 0.35 units (66.4 +/- 7.6 units, n = 26), but both groups exhibited reduced Hill coefficients. Hypertensive patients with enhanced Na(+)-H+ exchange activity (more than 90 units) had normal maximal rates of Na(+)-Li+ exchange.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of external and internal H+ and Na+ with Na+/Na+ and Na+/H+ exchange of rabbit red cells: evidence for a common pathway

We have studied the kinetic properties of rabbit red cell (RRBC) Na+/Na+ and Na+/H+ exchanges (EXC) in order to define whether or not both transport functions are conducted by the same molecule. The strategy has been to determine the interactions of Na+ and H+ at the internal (i) and external (o) sites for both exchanges modes. RRBC containing varying Nai and Hi were prepared by nystatin and DIDS treatment of acid-loaded cells. Na+/Na+ EXC was measured as Nao-stimulated Na+ efflux and Na+/H+ EXC as Nao-stimulated H+ efflux and delta pHo-stimulated Na+ influx into acid-loaded cells. The activation of Na+/Na+ EXC by Nao at pHi 7.4 did not follow simple hyperbolic kinetics. Testing of different kinetic models to obtain the best fit for the experimental data indicated the presence of high (Km 2.2 mM) and low affinity (Km 108 mM) sites for a single- or two-carrier system. The activation of Na+/H+ EXC by Nao (pHi 6.6, Nai less than 1 mM) also showed high (Km 11 mM) and low (Km 248 mM) affinity sites. External H+ competitively inhibited Na+/Na+ EXC at the low affinity Nao site (KH 52 nM) while internally H+ were competitive inhibitors (pK 6.7) at low Nai and allosteric activators (pK 7.0) at high Nai. Na+/H+ EXC was also inhibited by acid pHo and allosterically activated by Hi (pK 6.4). We also established the presence of a Nai regulatory site which activates Na+/H+ and Na+/Na+ EXC modifying the affinity for Nao of both pathways. At low Nai, Na+/Na+ EXC was inhibited by acid pHi and Na+/H+ stimulated but at high Nai, Na+/Na+ EXC was stimulated and Na+/H+ inhibited being the sum of both pathways kept constant. Both exchange modes were activated by two classes of Nao sites, cis-inhibited by external Ho, allosterically modified by the binding of H+ to a Hi regulatory site and regulated by Nai. These findings are consistent with Na+/Na+ EXC being a mode of operation of the Na+/H+ exchanger. Na+/H+ EXC was partially inhibited (80-100%) by dimethyl-amiloride (DMA) but basal or pHi-stimulated Na+/Na+ EXC (pHi 6.5, Nai 80 mM) was completely insensitive indicating that Na+/Na+ EXC is an amiloride-insensitive component of Na+/H+ EXC. However, Na+ and H+ efflux into Na-free media were stimulated by cell acidification and also partially (10 to 40%) inhibited by DMA; this also indicates that the Na+/H+ EXC might operate in reverse or uncoupled modes in the absence of Na+/Na+ EXC.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium-lithium countertransport activity in red cells of patients with insulin dependent diabetes and nephropathy and their parents

OBJECTIVE

To determine whether there are familial and genetic aspects of sodium-lithium countertransport activity in red cells in diabetic nephropathy.

DESIGN

Case-control study.

SETTING

Teaching hospital diabetic clinic.

SUBJECTS

40 Patients with insulin dependent diabetes, both of whose parents were alive: 20 with persistent proteinuria and 20 with normal albumin excretion matched for age, duration of diabetes, and body mass index. All 80 parents.

MAIN OUTCOME MEASURES

Sodium-lithium countertransport activity in red cells and arterial blood pressure.

RESULTS

Sodium-lithium countertransport activity in red cells was higher in the patients with proteinuria than in the patients with normoalbuminuria (mean (95% confidence interval) 0.47 (0.39 to 0.54) v 0.33 (0.28 to 0.38) mmol/l red cells/h respectively, p = 0.0036; mean difference 0.14 (0.04 to 0.22)). The mean countertransport activity for the two parents of each patient was calculated, and from this the mean value for each group of parents was calculated; the value was higher in the parents of the patients with proteinuria than in the parents of the patients with normoalbuminuria (0.40 (0.32 to 0.48) v 0.30 (0.26 to 0.33) mmol/l red cells/h respectively, p = 0.016; 0.10 (0.02 to 0.19)). Twenty-eight of the parents of the patients with proteinuria compared with 12 of the parents of the patients with normoalbuminuria had a countertransport activity that was above the median value in all 80 parents (p less than 0.001). Mean arterial blood pressure in the parents of the patients with proteinuria was related to that of their offspring (r = 0.46; p less than 0.01). There was a positive correlation between the sodium-lithium countertransport activity in red cells in the parents and their offspring when all parents and patients were considered (r = 0.37; p less than 0.001).

CONCLUSIONS

Increased sodium-lithium countertransport activity in red cells in the parents of diabetic patients with nephropathy provides further evidence that familial, and possibly genetic, factors related to a predisposition to arterial hypertension have a role in the susceptibility of diabetic renal disease.

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

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

Na+/H+ exchange is increased in sickle cell anemia and young normal red cells

Red cell volume regulation is important in sickle cell anemia because the rate and extent of HbS polymerization are strongly dependent on initial hemoglobin concentration. We have demonstrated that volume-sensitive K:Cl cotransport is highly active in SS whole blood and is capable of increasing MCHC. We now report that Na+/H+ exchange (Na/H EXC), which is capable of decreasing the MCHC of erythrocytes with pHi less than 7.2, is also very active in the blood of patients homozygous for HbS. The activity of Na/H EXC (maximum rate) was determined by measuring net Na+ influx (mmol/liter cell.hr = FU) driven by an outward H+ gradient in oxygenated, acid-loaded (pHi6.0), DIDS-treated SS cells. The Na/H EXC activity was 33 +/- 3 FU (mean +/- SE) (n = 19) in AA whites, 37 +/- 8 FU (n = 8) in AA blacks, and 85 +/- 15 FU (n = 14) in SS patients (P less than 0.005). Separation of SS cells into four density-defined fractions by density gradient revealed mean values of Na/H EXC four to five times higher in reticulocytes (SS1), discocytes (SS2) and dense discocytes (SS3), than in the fraction containing irreversibly sickled cells and dense discocytes (SS4). In contrast to K:Cl cotransport, which dramatically decreases after reticulocyte maturation, Na/H EXC persists well after reticulocyte maturation. In density-defined, normal AA red cells, Na/H EXC decreased monotonically as cell density increased. In SS and AA red cells, the magnitude of stimulation of Na/H EXC by cell shrinkage varied from individual to individual.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium-lithium countertransport in microalbuminuric insulin-dependent diabetic patients

A familial predisposition to arterial hypertension has recently been suggested as one important component of the susceptibility to diabetic kidney disease. Sodium-lithium countertransport activity, a marker of risk for essential hypertension, has been found to be increased in diabetic patients with overt nephropathy. We have measured red blood cell sodium-lithium counter-transport activity in 36 microalbuminuric insulin-dependent diabetic patients, a group at high risk of progression to clinical nephropathy and cardiovascular disease, and compared it with that of a matched group of 36 normoalbuminuric diabetic patients. Sodium-lithium countertransport was higher in the microalbuminuric (0.43 [95% confidence interval (CI) 0.38-0.47] mmol/l red blood cells [RBC]/hr) than in the normoalbuminuric diabetic patients (0.29 [0.25-0.33] mmol/l RBC/hr, mean difference 0.14 [0.08-0.20]; p less than 0.0001). Microalbuminuric patients had a higher frequency of parental hypertension than normoalbuminuric diabetic patients (56% vs. 28%, p less than 0.05). Sodium-lithium countertransport was related to mean arterial pressure in the microalbuminuric patients (r = 0.54, p less than 0.001) and to daily insulin requirements in both groups (microalbuminuric patients r = 0.39, p less than 0.05; normoalbuminuric patients r = 0.42, p less than 0.01). In a subset of patients in whom lipoproteins were measured, sodium-lithium countertransport activity was related to total and very low density lipoprotein triglycerides (r = 0.41, p less than 0.05 and r = 0.48, p less than 0.05) and to apolipoprotein B (r = 0.56, p less than 0.05), independently of body mass index, albumin excretion rate, glycemic control, and insulin dose.(ABSTRACT TRUNCATED AT 250 WORDS)