Decreased intracellular Na+ concentration is an early event in murine erythroleukemic cell differentiation

A decrease in Na+/K+-pump activity is an early event of Friend murine erythroleukemic (MEL) cell differentiation along the erythroid pathway. This decreased Na+/K+-pump activity has been proposed to be an essential step in differentiation which would cause a rise in intracellular Na+ concentration and then, by means of Na+/Ca2+ exchange, an increase in intracellular Ca2+. An increase in intracellular Ca2+ has been proposed to be essential for induction of differentiation. A critical prediction of this Na+-Ca2+ hypothesis is the rise in intracellular Na+. To test this prediction we have measured intracellular Na+ using a novel triple isotope method involving 3H2O, [14C]sucrose, and 22Na to measure total water, extracellular fluid, and Na+, respectively. 22Na equilibration occurred in less than 10 min. In uninduced cells, intracellular Na+ was 15.2 +/- 2.2 mM (S.D., n = 22); after induction for 14-16 h with dimethyl sulfoxide, intracellular Na+ decreased significantly (p less than 0.0001) to 8.4 +/- 1.4 mM (n = 21). The time course of the decline in intracellular Na+ paralleled that of the decrease in the Na+/K+-pump activity. These results are in direct contradiction to the Na+-Ca2+ hypothesis and suggest that observed changes in Na+/K+-pump activity can be explained solely on the basis of changes in intracellular Na+. The drop in intracellular Na+ is due to a decrease in Na+ influx. We suggest, however, that the decrease in the Na+ influx is not itself an essential event of differentiation, but may be induced by a change in the flux of another ion coupled to Na+.     

Muscle K+, Na+, and Cl disturbances and Na+-K+ pump inactivation: implications for fatigue.

Membrane excitability is a critical regulatory step in skeletal muscle contraction and is modulated by local ionic concentrations, conductances, ion transporter activities, temperature, and humoral factors. Intense fatiguing contractions induce cellular K(+) efflux and Na(+) and Cl(-) influx, causing pronounced perturbations in extracellular (interstitial) and intracellular K(+) and Na(+) concentrations. Muscle interstitial K(+) concentration may increase 1- to 2-fold to 11-13 mM and intracellular K(+) concentration fall by 1.3- to 1.7-fold; interstitial Na(+) concentration may decline by 10 mM and intracellular Na(+) concentration rise by 1.5- to 2.0-fold. Muscle Cl(-) concentration changes reported with muscle contractions are less consistent, with reports of both unchanged and increased intracellular Cl(-) concentrations, depending on contraction type and the muscles studied. When considered together, these ionic changes depolarize sarcolemmal and t-tubular membranes to depress tetanic force and are thus likely to contribute to fatigue. Interestingly, less severe local ionic changes can also augment subtetanic force, suggesting that they may potentiate muscle contractility early in exercise. Increased Na(+)-K(+)-ATPase activity during exercise stabilizes Na(+) and K(+) concentration gradients and membrane excitability and thus protects against fatigue. However, during intense contraction some Na(+)-K(+) pumps are inactivated and together with further ionic disturbances, likely precipitate muscle fatigue.     

Retinoic-acid-induced differentiation of HL-60 cells is associated with biphasic activation of the Na+− K+ pump

The human promyelocytic leukemia cell line, HL-60, can be induced to differentiate into granulocyte-like cells when cultured in the presence of 10(-6) M retinoic acid (RA) for several days. Following the addition of RA two kinds of changes occur. First, there are early changes that comprise an increase in the intracellular concentration of sodium ions [Na]i, which reaches its maximum after 6 h, and an increase in the activity of the Na+-pump, which is reflected by an ouabain-sensitive K+ influx that peaks at 8 h (170% of the control value) and that occurs without any change in the number of pump molecules, as measured by the binding of 3H-ouabain. Second, beginning after 12 h of culture with RA, a decrease in the number of ouabain-binding sites occurs, this being accompanied by an increase in the number of K+ ions actively transported by each site. The effect of modulation of Na+-pump activity on the RA-induced differentiation of HL-60 cells was studied using low, noncytotoxic concentrations of ouabain which, although alone having no differentiating effect, accelerated and potentiated the effect of RA on differentiation. When added in combination, these drugs induced rapid stimulation of the Na+-pump, which reached its peak after 2 h. These results indicate that a concomitant increase in the level of [Na+]i and in the activity of the Na+-pump constitute primary events in the interaction between RA and HL-60 cells, and that cation fluxes may play a role in the initiation of the process of differentiation.     

Inhibition of the Na+,K(+)-ATPase pump during induction of experimental colon cancer.

During the development of large bowel cancer alterations in colonic epithelial ion transport have been observed some of which result in altered intracellular ionic composition. In many tumors intracellular sodium and potassium become elevated and depressed, respectively. This observation suggests that mechanisms governing intracellular homeostasis for sodium and potassium are no longer tightly regulated. Changes in cell membrane permeability, sodium, potassium-ATPase K(+)-ATPase) pump activity, or both may be responsible for these alterations. It is not known when during initiation and development of cancer such changes may occur. To assess whether there are changes in the Na+, K(+)-ATPase pump early during the induction of large bowel cancer and prior to any notable histological changes, we measured the kinetics of the Na+, K(+)-pump in distal colonic mucosa of CF1 mice one week following only four weekly injections of the carcinogen 1,2-dimethyhydrazine (DMH). The kinetics of the pump were found to be best described by a model of highly cooperative binding. The VMAX of the pump in premalignant mucosa was lower for both sodium and potassium substrate activation (55-65% of control) with little change in other kinetic parameters. Depression of VMAX could not be attributed to an increased barium blockable potassium conductance of the basolateral membrane. Na+,K(+)-ATPase activity was also decreased by 50% in the distal colon of DMH treated mice, but was not affected in the less cancer susceptible proximal colon. These data demonstrate that alterations occur in the Na+,K(+)-pump in premalignant mucosa months before gross tumors develop, and these changes may partially explain the altered levels of Na+ and K+ in the cytoplasm of pre-malignant and malignant colonocytes.     

Mechanisms and consequences of Na+,K+-pump regulation by insulin and leptin.

Hormonal control of the Na+,K+-pump modulates membrane potential in mammalian cells, which in turn drives ion coupled transport processes and maintains cell volume and osmotic balance. Na+,K+-pump regulation is particularly important in the musculoskeletal, cardiovascular and renal systems. Decreased Na+,K+-pump activity can result in a rise in intracellular Na+ concentrations which in turn increase Na+/Ca2+ exchange, thereby raising intracellular Ca2+ levels. In cardiac and skeletal muscle, this could interfere with normal contractile activity. Similarly, in vascular smooth muscle the result would be resistance to vasodilation. Inhibition of the Na+,K+-pump can also reduce the driving force for renal tubular Na+ reabsorption, elevating Na+ excretion. By virtue of decreasing the membrane potential, thus allowing more efficient depolarization of nerve endings and by increasing intracellular Ca2+, inhibition of the Na+,K+-pump can increase nervous tone. The ability of insulin to stimulate the Na+,K+-pump in various cells and tissues, and the physiological significance thereof, have been well documented. Much less is known about the effect of leptin on the Na+,K+-pump. We have shown that leptin inhibits Na+,K+-pump function in 3T3-L1 fibroblasts. Defects in insulin and leptin action are associated with diabetes and obesity, respectively, both of which are commonly associated with cardiovascular complications. In this review we discuss the mechanisms of Na+,K+-pump regulation by insulin and leptin and highlight how, when they fail, they may contribute to the pathophysiology of hypertension associated with diabetes and obesity.     

Ionic behaviors and nerve growth factor dependence in developing chick ganglia. II. Studies with neurons of dorsal root ganglia

Using intact dorsal root ganglia (DRG) from embryonic (E) chick and measuring 22Na+ accumulation, the authors have recently shown that (i) ionic control by the ganglia has a complete requirement for exogenous NGF between E6 and E10, and (ii) control of ion pump mechanisms independent of exogenous NGF is progressively acquired by these ganglia from E10 to E16. Similar experiments have now been carried out using enriched suspensions of ganglionic neurons to test whether the acquisition of endogenous control by older ganglia was (1) due to the close association between neurons and nonneurons, and (2) correlated with a decreasing need by these neurons for exogenous NGF for survival in culture. In this enriched neuronal population, Na+ accumulation in the absence of NGF increases from E7 to E10, paralleling the increase in Na+ accessible space under ouabain, but then decreases conspicuously between E10 and E16, despite little change in the ouabain-sensitive Na+ space. NGF prevents Na+ accumulation during the early period, and becomes increasingly irrelevant for this behavior in later (after E10) development. K+ movements (traced with 86Rb+) behaved similarly. Active K+ influx (Na+, K+-pump mediated) also increases severalfold between E7 and E10. This K+ influx is sensitive to NGF at E7 and E10 but not at E14, paralleling the observed Na+ and K+ behaviors. These data suggest that the control of Na+, K+-pump performances acquired by these neurons between E10 and E16 represents the development of a neuronal self-sufficiency. This increase in ionic control is not due to an increase in pump molecules or pumping efficiency. No increases in the binding of [3H]ouabain or active K+ influx occur between E10 and E16, when ionic control is developing. The ionic dependence on NGF by the DRG neurons changes with their developmental age along the same temporal pattern displayed by their survival response to NGF in culture.     

Regulation of the sodium permeability of the luminal border of toad bladder by intracellular sodium and calcium: role of sodium-calcium exchange in the basolateral membrane

Sodium movement across the luminal membrane of the toad bladder is the rate-limiting step for active transepithelial transport. Recent studies suggest that changes in intracellular sodium regulate the Na permeability of the luminal border, either directly or indirectly via increases in cell calcium induced by the high intracellular sodium. To test these proposals, we measured Na movement across the luminal membrane (th Na influx) and found that it is reduced when intracellular Na is increased by ouabain or by removal of external potassium. Removal of serosal sodium also reduced the influx, suggesting that the Na gradient across the serosal border rather than the cell Na concentration is the critical factor. Because in tissues such as muscle and nerve a steep transmembrane sodium gradient is necessary to maintain low cytosolic calcium, it is possible that a reduction in the sodium gradient in the toad bladder reduces luminal permeability by increasing the cell calcium activity. We found that the inhibition of the influx by ouabain or low serosal Na was prevented, in part, by removal of serosal calcium. To test for the existence of a sodium- calcium exchanger, we studied calcium transport in isolated basolateral membrane vesicles and found that calcium uptake was proportional to the outward directed sodium gradient. Uptake was not the result of a sodium diffusion potential. Calcium efflux from preloaded vesicles was accelerated by an inward directed sodium gradient. Preliminary kinetic analysis showed that the sodium gradient changes the Vmax but not the Km of calcium transport. These results suggest that the effect of intracellular sodium on the luminal sodium permeability is due to changes in intracellular calcium.     

Regulation of Na+,K+ pump activity by nerve growth factor in chick embryo dorsal root ganglion cells

Nerve growth factor (NGF) is required for the growth and development of sensory and sympathetic neurons. Incubation of chick dorsal root ganglionic cells without NGF resulted in a decrease of active (Na+,K+-pump-mediated) K+ influx over a period of several hours. Addition of NGF to NGF-deprived cells caused 1) a return of the active K+ influx to the values occurring in cells continuously exposed to NGF, preceded by 2) a very rapid, but transient overstimulation of the Na+,K+-pump-mediated K+ influx. Restoration of normal Na+,K+-pump activity occurred at NGF concentrations of 1 biological unit/ml or greater, whereas the NGF concentration in the 1-100 biological unit/ml range affected the rapidity with which the pump restoration took place. The transient pump behavior was only observed in NGF-deprived cells and could not be elicited in NGF-supported steady-state cells or in cells having already received delayed NGF once. This transient Na+,K+-pump behavior was exclusively displayed in conjunction with a high intracellular Na+ concentration. Decreasing the external Na+ concentration below 70 mM reduced the hyperstimulation response to NGF, until at 10 mM Na+ the delayed presentation of NGF caused no overshoot at all. The effect of NGF on the Na+,K+-pump was specific for the NGF molecule and could not be mimicked by other proteins.     

Intracellular ionic changes in normal and transformed human fibroblasts after extracellular Ca2+ deprivation.

The lowering of extracellular Ca2+ concentration in the growth medium reversibly blocks normal, but not SV40-transformed WI38 diploid fibroblasts in the early G1/G0 phase of the cell cycle. This growth response is characterized by specific changes in ionic content and transport. Ca2+ deprivation (0.03 mM) has little effect on the K+ content of either normal or transformed cells. Na+ content, however, is increased nearly 2-fold in the normal cells. This increase is presumably due to a 3-fold increase in unidirectional Na+ influx in Ca2+-deprived cells. The increased intracellular Na+ also gives rise to a nearly 3-fold enhancement of the active (ouabain-sensitive) Na+ efflux. Ca2+ deprivation causes only slight increases in Na+ influx, ouabain-sensitive Na+ efflux and intracellular Na+ in the transformed cell. In contrast, the transformed cells lose nearly 60% of their intracellular Ca2+ on deprivation, whereas normal WI38 cells lose only 10%. The data suggest that the growth arrest exhibited by the normal cell but not the transformed cell may be related to different membrane-transport and permeability changes in response to Ca2+ deprivation.     

Stimulation of glucose transport in skeletal muscle by the sodium ionophore monensin

The tissue/medium distribution of the nonmetabolized glucose analog 3-O-methyl-D-glucose was measured in mouse diaphragm muscle and related to changes in 45Ca influx, Na+ content and Na+-pump activity. In the presence of external Ca2+ the sodium ionophore monensin greatly increased cellular Na+ content (and decreased K+ content) although 86Rb uptake, reflecting Na+-pump activity was increased. Concomitantly, 45Ca influx was stimulated, presumably through activation of Na+-Ca2+ exchange. In parallel to the rise in Ca2+ influx sugar transport was also increased. Sugar transport was also increased by monensin in the nominal absence of external Ca2+, when Ca2+ influx was minimal. To test if monensin releases Ca2+ from intracellular storage sites in the absence of external Ca2+, the ionophore was added to medium perfusing rat hind limb preparations and the total Ca content of muscle mitochondria was determined. When Ca2+ was present in the perfusate, monensin increased the mitochondrial Ca content. In the absence of Ca2+, the mitochondrial Ca content was lower and was further depressed by monensin, suggesting that elevation of internal Na+ by monensin may increase mitochondrial Ca2+ loss via activation of Na+-Ca2+ exchange across the mitochondrial membrane. The above results are consistent with the effect of monensin on sugar transport being due to alterations in Ca2+ distribution. They support the earlier conclusion that regulation of sugar transport in muscle is Ca2+ dependent.     

The mechanism of insulin stimulation of (Na+,K+)-ATPase transport activity in muscle

Since the mechanism underlying the insulin stimulation of (Na+,K+)-ATPase transport activity observed in multiple tissues has remained undetermined, we have examined (Na+,K+)-ATPase transport activity (ouabain-sensitive 86Rb+ uptake) and Na+/H+ exchange transport (amiloride-sensitive 22Na+ influx) in differentiated BC3H-1 cultured myocytes as a model of insulin action in muscle. The active uptake of 86Rb+ was sensitive to physiological insulin concentrations (1 nM), yielding a maximum increase of 60% without any change in 86Rb+ permeability. In order to determine the mechanism of insulin stimulation of (Na+,K+)-ATPase activity, we demonstrated that insulin also stimulates passive 22Na+ influx by Na+/H+ exchange transport (maximal 200% increase) and an 80% increase in intracellular Na+ concentration with an identical time course and dose-response curve as insulin-stimulated (Na+,K+)-ATPase transport activity. Incubation of the cells with high [Na+] (195 mM) significantly potentiated insulin stimulation of ouabain-inhibitable 86Rb+ uptake. The ionophore monensin, which also promotes passive Na+ entry into BC3H-1 cells, mimics the insulin stimulation of ouabain-inhibitable 86Rb+ uptake. In contrast, incubation with amiloride or low [Na+] (10 mM), both of which inhibit Na+/H+ exchange transport, abolished the insulin stimulation of (Na+,K+)-ATPase transport activity. Furthermore, each of these insulin-stimulated transport activities displayed a similar sensitivity to amiloride. These results indicate that insulin stimulates a large increase in Na+/H+ exchange transport and that the resulting Na+ influx increases the intracellular Na+ concentration, thus activating the internal Na+ transport sites of the (Na+,K+)-ATPase. This Na+ influx is, therefore, the mediator of the insulin-induced stimulation of membrane (Na+,K+)-ATPase transport activity classically observed in muscle.     

Changes in sodium pump transport activity and Na+, K+-ATPase expression level following lymphocytes activation in humans

Ouabain-inhibitable rubidium influxes, intracellular sodium content (Nai), and alpha 1-subunit abundance have been studied in human blood lymphocytes, stimulated by phytohemagglutinin (PHA) or by the phorbol 12,13-dibutyrate (PDBu), and calcium ionophore--ionomycin. It is shown that at early stages of PHA-induced activation, the Na/K pump expression (as determined by Wesrn blots of alpha 1 protein in membrane fractions of total cell lysates) does not change, and the increase in Rb influx is due to the increase in Nai and results from the enhanced transport activity of Na/K pumps present in plasma membrane. During the late stages of G0-->G1-->S transit (16-48 h), the increase in Rb influx occurs without changes in Nai, and monensin increases both Nai, and the Rb influx via the Na/K pump. To the end of the first day of mitogen activation, the alpha 1 protein content was found to increase by 5-7 times. A correlation was revealed between changes in ouabain-inhibitable Rb influxes, alpha 1 protein abundance, and the proliferation rate. It is concluded that blasttransformathion of normal human lymphocytes is accompanied by the increase in membrane-associated pool of alpha 1-subunit of Na+,K(+)-ATPase, and the enhanced activity of sodium pump during the G0-->G1-->S progression is provided by increased number of Na+,K(+)-ATPase pumps in plasma membrane.     

Mechanisms controlling choline transport and acetylcholine synthesis in motor nerve terminals during electrical stimulation

Electrical stimulation of the chick ciliary nerve leads to a frequency-dependent increase in the Na+-dependent high affinity uptake of [3H]choline (SDHACU) and its conversion to acetylcholine (ACh) in the nerve terminals innervating the iris muscle. The forces that drive this choline (Ch) uptake across the presynaptic membrane were evaluated. Depolarization with increased [K+] out or veratridine decreases Ch accumulation. In addition to the electrical driving force, energy is provided by the Na+ gradient. Inhibition of the Na,K-ATPase decreased the Ch taken up. Thus, changes in the rate of Ch transport are dependent on the electrochemical gradients for both Ch and Na+. Ch uptake and ACh synthesis were increased after a conditioning preincubation with high [K+] out or veratridine. As is the case for electrical stimulation, this acceleration of Ch uptake and ACh synthesis was strongly dependent on the presence of Ca++ in the incubation medium. Na+ influx through a TTX-sensitive channel also contributed to this acceleration. Inasmuch as membrane depolarization reduces the initial velocity of Ch uptake and ACh synthesis, their increases during electrical stimulation therefore cannot be the direct effect of the depolarization phase of the action potential. Instead they are the result of the ionic fluxes accompanying the presynaptic spike. It is concluded that stimulation of Ch uptake and ACh synthesis by nerve activity depends first, on the ACh release elicited by Ca++ influx after depolarization and second, on the activation of the Na,K-ATPase due to Na+ entry. Furthermore, it is suggested that the release of ACh after stimulation drives translocation of cytoplasmic ACh into a protected compartment (probably vesicular). This recompartmentation of intraterminal ACh stimulates ACh synthesis by mass action, allowing further accumulation of Ch.     

Changes in intracellular cations during the cell cycle in HeLa cells

Intracellular Na+, K+, and Mg2+ concentrations have been measured during the HeLa cell cycle and compared with changes in oxygen utilization and macromolecular synthesis. Cell water content remains relatively constant at 79 +/- 1% during the cell cycle. A biphasic change in intracellular Na+ occurs with low values as cells reach peak S phase and again in early G1. The decrease in S coincides with an increase in cell volume during increased macromolecular synthesis. The fall in intracellular Na+ during mitosis/early G1 coincides with decreased energy utilization as macromolecular synthesis decreases with a continued decrease in [Na+]i in G1 corresponding to a period of increasing cell volume and an increase in protein synthesis. Intracellular Na+ is relatively high during late S/G2 when phosphate incorporation into protein and phospholipid is maximal. Intracellular K+ concentrations largely parallel intracellular Na+ levels although the intracellular K+:Na+ ratio is significantly lower as the cell volume increases during late G2/mitosis. Additions of a Na+-pump inhibitor (strophanthidin) not only caused a rise in [Na+]i and fall in [K+]i but also inhibited protein synthesis. Conversely, addition of a protein synthesis inhibitor (cycloheximide) blocked amino acid incorporation and produces a fall in intracellular Na+ levels. These findings indicate that intracellular Na+ and K+ play an important role in regulating cell hydration during the cell cycle and that changes in Na+, K+-ATPase activity, synthesis and/or utilization of high energy phosphate compounds, fluid phase turnover (endocytosis), Na+:H+ exchange (pHi), Donnan forces, and ionic adsorption may all be involved.     

Ion channels and regulation of intracellular calcium in vascular endothelial cells

Endothelial cells in vivo form an interface between flowing blood and vascular tissue, responding to humoral and physical stimuli to secrete relaxing and contracting factors that contribute to vascular homeostasis and tone. The activation of endothelial cell-surface receptors by vasoactive agents is coupled to an elevation in cytosolic Ca2+, which is caused by Ca2+ entry via ion channels in the plasma membrane and by Ca2+ release from intracellular stores. Ca2+ entry may occur via four different mechanisms: 1) a receptor-mediated channel coupled to second messengers; 2) a Ca2+ leak channel dependent on the electrochemical gradient for Ca2+; 3) a stretch-activated nonselective cation channel; and 4) internal Na+-dependent Ca2+ entry (Na+-Ca2+ exchange). The rate of Ca2+ entry through these ion pathways can be modulated by the resting membrane potential. Membrane potential may be regulated by at least two types of K channels: inwardly rectifying K channels activated upon hyperpolarization or shear stress; and a Ca2+-activated K channel activated upon depolarization, which may function to repolarize the agonist-stimulated endothelial cell. After agonist stimulation, cytosolic Ca2+ increases in a biphasic manner, with an initial peak due to inositol 1,4,5-trisphosphate-mediated Ca2+ release from intracellular stores, followed by a sustained plateau that is dependent on the presence of [Ca2+]o and on membrane potential. The delay in agonist-activated Ca2+ influx is consistent with the coupling of receptor activation to Ca2+ entry via a second messenger. Oscillations in [Ca2+]i, which may involve both Ca2+ entry and release, have been observed in isolated and confluent endothelial cell monolayers stimulated by histamine and bradykinin. Receptor-mediated Ca2+ entry, release, and refilling of intracellular stores follows a cycle that involves the plasma membrane.     

Ionic responses and growth stimulation in rat astroglial cells: differential mechanisms of gangliosides and serum

Rat astroglial cells respond to fetal calf serum (FCS) and gangliosides, including GM1, by undergoing proliferation. Here, we show that addition of FCS but not GM1 causes an increase in Na+, K+-pump activity, as measured by ouabain-sensitive 86Rb+ influx. The increase of Na+, K+-pump activity by FCS was due to increased Na+ influx (measured with 22Na+). This increased Na+ influx was sensitive to amiloride, an inhibitor of Na+/H+ exchange. Amiloride also blocked the FCS-stimulated incorporation of [3H]thymidine into DNA. Two defined polypeptide growth factors, epidermal growth factor and fibroblast growth factor were also able to elicit an amiloride-sensitive Na+ influx and an ouabain-sensitive K+ uptake in these astroglial cells, in the presence of FCS or insulin. Thus, GM1 differs from serum and growth factors in the mechanisms by which these agents stimulate the proliferation of the astroglial cells used here.     

Ouabain-Insensitive Sodium Movements in the Human Red Blood Cell

Red blood cells exposed to ouabain are capable of net Na outflux against an electrochemical gradient; the net outflux is inhibited by the diuretic, furosemide. In ouabain-treated cells, both the unidirectional Na outflux and the unidirectional Na influx are inhibited by furosemide. Furosemide also inhibits the ouabain-sensitive Na-Na exchange accomplished by the Na-K pump in K-free solutions. From the interaction of extracellular K, furosemide, and ouabain with the transport system, it seems possible that the ouabain-insensitive Na outflux is accomplished by the same mechanism that is responsible for the ouabain-sensitive Na-K exchange. The ouabain-insensitive Na outflux is increased by extracellular Na, and the influx increases as the intracellular Na increases. In fresh cells, high extracellular K concentrations decrease the ouabain-insensitive Na outflux and increase the ouabain-insensitive Na influx. When the rate constant for sodium outflux and the rate constant for sodium influx in ouabain-treated cells are plotted against the extracellular K concentration, the curves obtained are mirror images of each other. In starved cells, extracellular K increases the ouabain-insensitive Na outflux as does extracellular Na, and it has little effect on the Na influx.     

Erythrocyte ion transport and membrane lipid composition in young and adult rats with NO-deficient hypertension

The aim of our study was to search for abnormalities of sodium and potassium transport in erythrocytes of male Wistar rats subjected to chronic L-NAME treatment (40 mg/kg/day) for 4 weeks either from weaning (4-week-old) or in adulthood (12-week-old). Sodium content, Na(+),K(+)-pump and Na(+),K(+)-cotransport activity, cation leaks as well as membrane cholesterol and phospholipid contents were determined in fresh erythrocytes. Chronic inhibition of NO synthase elicited similar blood pressure rise in both age groups which did not differ in the degree of NO synthase inhibition. No significant ion transport abnormalities were disclosed in erythrocytes of young NO-deficient rats, whereas erythrocyte Na(+) content, outward Na(+),K(+)-cotransport and inward Na(+) leak were significantly reduced in adult hypertensive animals compared to age-matched controls. It should be noted that the erythrocytes of adult control rats were characterized by higher activity of Na(+),K(+)-pump and Na(+),K(+)-cotransport, increased Na(+) and Rb(+) leaks and elevated membrane cholesterol content compared to those of young normotensive controls. Increased Na(+) leak and elevated membrane cholesterol content but reduced membrane phospholipid content were revealed in erythrocytes of adult hypertensive rats when compared to young hypertensive rats. It can be concluded that young and adult Wistar rats did not differ in the extent of NO synthase inhibition and blood pressure rise elicited by chronic L-NAME treatment. Our results exclude the important participation of classical sodium transport abnormalities in the pathogenesis of this NO-deficient form of experimental hypertension.     

Changes of intracellular Na~ concentration in erythrocytes caused by pulsed electrical field

Changes of sodium ionic concentration of human erythrocytes applied to pulsed electrical field (PEF) were studied by using shift reagent and NMR spectroscopy. The results show that the concentration of intracellular Na increases with the increasing intensity of PEF when the erythrocytes are applied to PEF with higher intensities. The relationship between intracellular Na concentrations and the intensities of PEF does not follow linear or exponen-tial behavior. As the intensities increase, the intracellular Na concentrations increase even faster by an exponential curve. However under effects of PEF at lower intensities, intracellular Na concentration decreases. Ouabain can in-hibit the decrease of intracellular Na concentration, and the inhibition increases with the increasing concentration of ouabain, suggesting that Na , K -ATPase on cell membrane can be activated by PEF at lower intensities. Direct measurement of activities of the enzyme by using Malachite green method has confirmed this observatio     

Characterisation of the potassium influx in rat erythrocytes.

In the rat erythrocyte membrane five different transport pathways for K+ are present. In addition to the well characterised K+ transport via the Na+ pump, the Na,K,Cl cotransport and the Ca(2+)-activated K+ channel, there are a K,Cl cotransport and a residual (leak) K+ transport. The K,Cl cotransport is already present under physiological conditions, and can be stimulated by N-ethylmaleimide treatment but not by a cell volume increase. A low ionic strength stimulated increase of the residual K+ influx can be demonstrated in rat erythrocytes after suppressing the K,Cl cotransport pathway. Between 11 and 19 weeks of age, rats show significant differences in all transport pathways of the erythrocyte potassium influx. Using influx data from individual rats a significant correlation between the total K+ influx and the ouabain-sensitive K+ influx has been found. Maintaining the rats on a diet poor in essential fatty acids leads to a significant change of the linoleic acid content of the erythrocyte membrane phospholipids. However, no significant effect on the various K+ transport pathways has been found. An analysis of the fatty acid composition of the erythrocyte membrane phospholipids showed significant correlations between the content of oleic acid, and arachidonic acid, and the ouabain-sensitive K+ influx (as well as the total K+ influx).