Insulin induces translocation of the alpha 2 and beta 1 subunits of the Na+/K(+)-ATPase from intracellular compartments to the plasma membrane in mammalian skeletal muscle

Hyperkalemia in dialysis patients

Serious hyperkalemia is common in patients with end-stage renal disease (ESRD) and accounts for considerable morbidity and death. Mechanisms of extrarenal disposal of potassium (gastrointestinal excretion and cellular uptake) play a crucial role in the defense against hyperkalemia in this population. In this article we review extrarenal potassium homeostasis and its alteration in patients with ESRD. We pay particular attention to the factors that influence the movement of potassium across cell membranes. With that background we discuss the emergency treatment of hyperkalemia in patients with ESRD. We conclude with a review of strategies to reduce the risk of hyperkalemia in this population of patients.

Na(+)/K(+) pump expression in the L8 rat myogenic cell line: effects of heterologous alpha subunit transfection

We have characterized the physiological and biochemical properties of the Na(+)/K(+) pump and its molecular expression in L8 rat muscle cells. Pump properties were measured by [(3)H]ouabain binding and (86)Rb uptake. Scatchard plot analysis of specific ouabain binding indicated the presence of a single family of binding sites with a B(max) of approximately 135 fmol/ mg P and a K(D) of 3.3 x 10(-8). (86)Rb uptake due to specific pump activity was found to be 20% of the total in L8 cells. The results indicated lower affinity of L8 cells for ouabain and lower activity of the pump than that reported for chick or rat skeletal muscle in primary culture. Both the alpha(1) and beta(1) protein and mRNA isoforms were expressed in myoblasts and in myotubes, while the alpha(2), alpha(3), and beta(2) isoforms were not detectable. We attempted to overcome low physiological expression of the Na(+)/K(+) pump by employing a vector expressing an avian high affinity alpha subunit. This allowed identification of the transfected subunit separate from that endogenously expressed in L8 cells. Successful transfection into L8 myoblasts and myotubes was recognized by anti-avian alpha subunit monoclonal antibodies. Fusion index, Na(+)/K(+) pump activity, and the level of the transmembrane resting potential were all significantly greater in transfected L8 (tL8) cells than in non-tL8. The total amount of alpha subunit (avian and rat) in tL8 cells was greater than that (only rat) in non-tL8 cells. This relatively high abundance of the Na(+)/K(+) pump in transfected cells may indicate that avian and rat alpha subunits hybridize to form functional pump complexes.

Insulin increases the turnover rate of Na+-K+-ATPase in human fibroblasts

Insulin stimulates K+ transport by the Na+-K+-ATPase in human fibroblasts. In other cell systems, this action represents an automatic response to increased intracellular [Na+] or results from translocation of transporters from an intracellular site to the plasma membrane. Here we evaluate whether these mechanisms are operative in human fibroblasts. Human fibroblasts expressed the alpha(1) but not the alpha(2) and alpha(3) isoforms of Na+-K+-ATPase . Insulin increased the influx of Rb+, used to trace K+ entry, but did not modify the total intracellular content of K+, Rb+, and Na+ over a 3-h incubation period. Ouabain increased intracellular Na+ more rapidly in cells incubated with insulin, but this increase followed insulin stimulation of Rb+ transport. Bumetanide did not prevent the increased Na+ influx or stimulation of Na+-K+-ATPase. Stimulation of the Na+-K+-ATPase by insulin did not produce any measurable change in membrane potential. Insulin did not affect the affinity of the pump toward internal Na+ or the number of membrane-bound Na+-K+-ATPases, as assessed by ouabain binding. By contrast, insulin slightly increased the affinity of Na+-K+-ATPase toward ouabain. Phorbol esters did not mimic insulin action on Na+-K+-ATPase and inhibited, rather than stimulated, Rb+ transport. These results indicate that insulin increases the turnover rate of Na+-K+-ATPases of human fibroblasts without affecting their number on the plasma membrane or modifying their dependence on intracellular [Na+].

Glucocorticoids increase sodium pump alpha(2)- and beta(1)-subunit abundance and mRNA in rat skeletal muscle

Fourteen-day adrenal steroid treatment increases [(3)H]ouabain binding sites 22-48% in muscle biopsies from patients treated with adrenal steroids for chronic obstructive lung disease and in rats treated with dexamethasone (Dex). Ouabain binding measures plasma membrane sodium pumps (Na(+)-K(+)-ATPase) with isoform-dependent affinity. In this study we have established the specific pattern of Dex regulation of sodium pump isoform protein and mRNA levels in muscle. Rats were infused with Dex (0.1 mg/kg per day) or vehicle for 14 days. Abundance of sodium pump catalytic alpha(1)- and alpha(2)-subunits and glycoprotein beta(1)- and beta(2)-subunits was determined by immunoblot in soleus, extensor digitorum longus, whole gastrocnemius, and diaphragm and was normalized to the mean vehicle control value. Dex increased alpha(2) and beta(1) protein in all muscle types by 53-78% and ~50%, respectively. Dex increased alpha(1) protein only in diaphragm (65 +/- 7%). At the mRNA level in whole hindlimb muscle, Dex increased alpha(2) (6.4 +/- 0.5-fold) and beta(1) (1.54 +/- 0.15-fold) and decreased beta(2) (to 0.36 +/- 0.6 of control). In summary, alpha(2)beta(1) is the Dex-responsive pump in all skeletal muscles, and changes in alpha(2) and beta(1) mRNA levels can drive the 50% change in alpha(2)beta(1)-subunits, which can account for the reported increase in [(3)H]ouabain binding.

Cyclic AMP increases cell surface expression of functional Na,K-ATPase units in mammalian cortical collecting duct principal cells

Cyclic AMP (cAMP) stimulates the transport of Na(+) and Na,K-ATPase activity in the renal cortical collecting duct (CCD). The aim of this study was to investigate the mechanism whereby cAMP stimulates the Na,K-ATPase activity in microdissected rat CCDs and cultured mouse mpkCCD(c14) collecting duct cells. db-cAMP (10(-3) M) stimulated by 2-fold the activity of Na,K-ATPase from rat CCDs as well as the ouabain-sensitive component of (86)Rb(+) uptake by rat CCDs (1.7-fold) and cultured mouse CCD cells (1.5-fold). Pretreatment of rat CCDs with saponin increased the total Na,K-ATPase activity without further stimulation by db-cAMP. Western blotting performed after a biotinylation procedure revealed that db-cAMP increased the amount of Na,K-ATPase at the cell surface in both intact rat CCDs (1.7-fold) and cultured cells (1.3-fold), and that this increase was not related to changes in Na,K-ATPase internalization. Brefeldin A and low temperature (20 degrees C) prevented both the db-cAMP-dependent increase in cell surface expression and activity of Na,K-ATPase in both intact rat CCDs and cultured cells. Pretreatment with the intracellular Ca(2+) chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid also blunted the increment in cell surface expression and activity of Na,K-ATPase caused by db-cAMP. In conclusion, these results strongly suggest that the cAMP-dependent stimulation of Na,K-ATPase activity in CCD results from the translocation of active pump units from an intracellular compartment to the plasma membrane.

Immunochemical demonstration of a novel beta-subunit isoform of X, K-ATPase in human skeletal muscle

Recently we have identified mRNA encoding a hitherto unknown mammalian X,K-ATPase beta-subunit expressed predominantly in muscle tissue (Pestov, N. B. et al. (1999) FEBS Lett. 456, 243-248). Here we demonstrate the existence of the predicted protein, designated as beta(m) (beta(muscle)), in human adult skeletal muscle membranes using immunoblotting with beta(m)-specific antibodies generated against recombinant polypeptide formed by extramembrane beta(m) domains. The electrophoretic mobility of beta(m) was shown to be abnormally low due to the presence of Glu-rich sequences. In contrast to mature forms of other known X,K-ATPase beta-subunits, carbohydrate moiety of beta(m) is sensitive to endoglycosidase H and appears to be composed of short high-mannose or hybrid N-glycans. This finding argues in favor of an intracellular location of beta(m) in human skeletal muscle.

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Simultaneous phosphorylation of Ser11 and Ser18 in the alpha-subunit promotes the recruitment of Na(+),K(+)-ATPase molecules to the plasma membrane

Renal sodium homeostasis is a major determinant of blood pressure and is regulated by several natriuretic and antinatriuretic hormones. These hormones, acting through intracellular second messengers, either activate or inhibit proximal tubule Na(+),K(+)-ATPase. We have shown previously that phorbol ester (PMA) stimulation of endogenous PKC leads to activation of Na(+),K(+)-ATPase in cultured proximal tubule cells (OK cells) expressing the rodent Na(+), K(+)-ATPase alpha-subunit. We have now demonstrated that the treatment with PMA leads to an increased amount of Na(+),K(+)-ATPase molecules in the plasmalemma, which is proportional to the increased enzyme activity. Colchicine, dinitrophenol, and potassium cyanide prevented the PMA-dependent stimulation of activity without affecting the increased level of phosphorylation of the Na(+), K(+)-ATPase alpha-subunit. This suggests that phosphorylation does not directly stimulate Na(+),K(+)-ATPase activity; instead, phosphorylation may be the triggering mechanism for recruitment of Na(+),K(+)-ATPase molecules to the plasma membrane. Transfected cells expressing either an S11A or S18A mutant had the same basal Na(+),K(+)-ATPase activity as cells expressing the wild-type rodent alpha-subunit, but PMA stimulation of Na(+),K(+)-ATPase activity was completely abolished in either mutant. PMA treatment led to phosphorylation of the alpha-subunit by stimulation of PKC-beta, and the extent of this phosphorylation was greatly reduced in the S11A and S18A mutants. These results indicate that both Ser11 and Ser18 of the alpha-subunit are essential for PMA stimulation of Na(+), K(+)-ATPase activity, and that these amino acids are phosphorylated during this process. The results presented here support the hypothesis that PMA regulation of Na(+),K(+)-ATPase is the result of an increased number of Na(+),K(+)-ATPase molecules in the plasma membrane.

Exercise-induced translocation of Na(+)-K(+) pump subunits to the plasma membrane in human skeletal muscle

Six human subjects performed one-legged knee extensor exercise (90 +/- 4 W) until fatigue (exercise time 4.6 +/- 0.8 min). Needle biopsies were obtained from vastus lateralis muscle before and immediately after exercise. Production of giant sarcolemmal vesicles from the biopsy material was used as a membrane purification procedure, and Na(+)-K(+) pump alpha- and beta-subunits were quantified by Western blotting. Exercise significantly increased (P < 0.05) the vesicular membrane content of the alpha(2)-, total alpha-, and beta(1)-subunits by 70 +/- 29, 35 +/- 10, and 26 +/- 5%, respectively. The membrane content of alpha(1) was not changed by exercise, and the densities of subunits in muscle homogenates were unchanged. The ratio of vesicular to crude muscle homogenate content of the alpha(2)-, total alpha-, and beta(1)-subunits was elevated during exercise by 67 +/- 33 (P < 0.05), 23 +/- 6 (P < 0.05), and 40 +/- 14% (P = 0.06), respectively. It is concluded that translocation of subunits is an important mechanism involved in the short time upregulation of the Na(+)-K(+) pumps in association with human muscle activity.

Insulin stimulates transepithelial sodium transport by activation of a protein phosphatase that increases Na-K ATPase activity in endometrial epithelial cells

The objective of this study was to investigate the effects of insulin and insulin-like growth factor I on transepithelial Na(+) transport across porcine glandular endometrial epithelial cells grown in primary culture. Insulin and insulin-like growth factor I acutely stimulated Na(+) transport two- to threefold by increasing Na(+)-K(+) ATPase transport activity and basolateral membrane K(+) conductance without increasing the apical membrane amiloride-sensitive Na(+) conductance. Long-term exposure to insulin for 4 d resulted in enhanced Na(+) absorption with a further increase in Na(+)-K(+) ATPase transport activity and an increase in apical membrane amiloride-sensitive Na(+) conductance. The effect of insulin on the Na(+)-K(+) ATPase was the result of an increase in V(max) for extracellular K(+) and intracellular Na(+), and an increase in affinity of the pump for Na(+). Immunohistochemical localization along with Western blot analysis of cultured porcine endometrial epithelial cells revealed the presence of alpha-1 and alpha-2 isoforms, but not the alpha-3 isoform of Na(+)-K(+) ATPase, which did not change in the presence of insulin. Insulin-stimulated Na(+) transport was inhibited by hydroxy-2-naphthalenylmethylphosphonic acid tris-acetoxymethyl ester [HNMPA-(AM)(3)], a specific inhibitor of insulin receptor tyrosine kinase activity, suggesting that the regulation of Na(+) transport by insulin involves receptor autophosphorylation. Pretreatment with wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase as well as okadaic acid and calyculin A, inhibitors of protein phosphatase activity, also blocked the insulin-stimulated increase in short circuit and pump currents, suggesting that activation of phosphatidylinositol 3-kinase and subsequent stimulation of a protein phosphatase mediates the action of insulin on Na(+)-K(+) ATPase activation.

Endothelial membrane potential regulates production of both nitric oxide and superoxide--a fundamental determinant of vascular health

There is recent evidence that the membrane potential of vascular endothelium regulates not only nitric oxide (NO) synthesis, but also superoxide generation, such that hyperpolarization stimulates NO production while suppressing that of superoxide. Given that NO works in a variety of ways to inhibit atherothrombotic disease and hypertension, whereas superoxide not only vetoes the benefits of NO but also disrupts endothelial metabolism and promotes LDL oxidation through its oxidant activity, it is thus evident that endothelium membrane potential is a crucial determinant of cardiovascular risk. Membrane polarization can be enhanced by measures which increase the synthesis or availability of the Na+-K+-ATPase, moderately enhance serum K+ and increase the conductance of membrane K+ channels. Such measures may include high-K+/low-Na+ natural diets, insulin sensitizing modalities, 'euthyroid replacement therapy' and ACE inhibitors. Epidemiological correlations of insulin resistance with hypertension and cardiovascular risk may reflect the low membrane potential of insulin-resistant vascular endothelium. Adjunctive measures for suppressing the generation or half-life of endothelial superoxide are suggested.

Age-associated differential expression of Na(+)-K(+)-ATPase subunit isoforms in skeletal muscles of F-344/BN rats

Skeletal muscle expresses multiple isoforms of the Na(+)-K(+)-ATPase. Their expression has been shown to be differentially regulated under pathophysiological conditions. In addition, previous studies suggest possible age-dependent alterations in Na(+)-K(+) pump function. The present study tests the hypothesis that advancing age is associated with altered Na(+)-K(+)-ATPase enzyme activity and isoform-specific changes in expression of the enzyme subunits. Red and white gastrocnemius (Gast) as well as soleus muscles of male Fischer 344/Brown Norway (F-344/BN) rats at 6, 18, and 30 mo of age were examined. Na(+)-K(+)-ATPase activity, measured by K(+)-stimulated 3-O-methylfluorescein phosphatase activity, increased by approximately 50% in a mixed Gast homogenate from 30-mo-old compared with 6- and 18-mo-old rats. Advancing age was associated with markedly increased alpha(1)- and beta(1)-subunit, and decreased alpha(2)- and beta(2)-subunit in red and white Gast. In soleus, there were similar changes in expression of alpha(1)- and alpha(2)-subunits, but levels of beta(1)-subunit were unchanged. Functional Na(+)-K(+)-ATPase units, measured by [(3)H]ouabain binding, undergo muscle-type specific changes. In red Gast, high-affinity ouabain-binding sites, which are a measure of alpha(2)-isozyme, increased in 30-mo-old rats despite decreased levels of alpha(2)-subunit. In white Gast, by contrast, decreased levels of alpha(2)-subunit were accompanied by decreased high-affinity ouabain-binding sites. Finally, patterns of expression of the four myosin heavy chain (MHC) isoforms (type I, IIA, IIX, and IIB) in these muscles were similar in the three age groups examined. We conclude that, in the skeletal muscles of F-344/BN rats, advancing age is associated with muscle type-specific alterations in Na(+)-K(+)-ATPase activity and patterns of expression of alpha- and beta-subunit isoforms. These changes apparently occurred without obvious shift in muscle fiber types, since expression of MHC isoforms remained unchanged. Some of the alterations occurred between middle-age (18 mo) and senescence (30 mo), and, therefore, may be attributed to aging of skeletal muscle.

Insulin-induced stimulation of Na+,K(+)-ATPase activity in kidney proximal tubule cells depends on phosphorylation of the alpha-subunit at Tyr-10

Phosphorylation of the alpha-subunit of Na+,K(+)-ATPase plays an important role in the regulation of this pump. Recent studies suggest that insulin, known to increase solute and fluid reabsorption in mammalian proximal convoluted tubule (PCT), is stimulating Na+,K(+)-ATPase activity through the tyrosine phosphorylation process. This study was therefore undertaken to evaluate the role of tyrosine phosphorylation of the Na+,K(+)-ATPase alpha-subunit in the action of insulin. In rat PCT, insulin and orthovanadate (a tyrosine phosphatase inhibitor) increased tyrosine phosphorylation level of the alpha-subunit more than twofold. Their effects were not additive, suggesting a common mechanism of action. Insulin-induced tyrosine phosphorylation was prevented by genistein, a tyrosine kinase inhibitor. The site of tyrosine phosphorylation was identified on Tyr-10 by controlled trypsinolysis in rat PCTs and by site-directed mutagenesis in opossum kidney cells transfected with rat alpha-subunit. The functional relevance of Tyr-10 phosphorylation was assessed by 1) the abolition of insulin-induced stimulation of the ouabain-sensitive (86)Rb uptake in opossum kidney cells expressing mutant rat alpha1-subunits wherein tyrosine was replaced by alanine or glutamine; and 2) the similarity of the time course and dose dependency of the insulin-induced increase in ouabain-sensitive (86)Rb uptake and tyrosine phosphorylation. These findings indicate that phosphorylation of the Na+,K(+)-ATPase alpha-subunit at Tyr-10 likely participates in the physiological control of sodium reabsorption in PCT.

Rat skeletal muscle in culture expresses the alpha1 but not the alpha2 protein subunit isoform of the Na+/K+ pump

Studies from this laboratory have shown that the physiological expression of the Na+/K+ pump in primary cultures of rat skeletal muscle increases with development. The molecular mechanisms underlying these changes are not known. Therefore, we have examined the expression of alpha and beta subunits of the Na+/K+ pump at both the protein and mRNA levels during myogenesis of primary skeletal muscle cell cultures obtained from newborn rats. Protein isoforms were identified by Western blotting techniques with specific monoclonal and polyclonal antibodies and subunit mRNA was studied with specific cDNA probes. Freshly isolated skeletal muscle from newborn rats expressed both alpha1 and alpha2 protein subunits. From day 1 after plating, primary cultures expressed only the alpha1 protein isoform. In contrast, both beta1 and beta2 isoforms were expressed in freshly isolated muscle and in primary cultures, with beta1 expression being stronger in both preparations. Studies on RNA expression showed that mRNA for alpha1, alpha2, beta1, and beta2 isoforms was identified both in freshly isolated muscle and after plating of cells in culture. These findings indicate that the lack of alpha2 protein expression in primary muscle cell cultures reflects a form of posttranscriptional regulation. There did not appear to be a quantitative difference in isoform expression as a function of age or of fusion in spite of developmental increases in Na+/K+ pump activity and its dependence on cell fusion. The lack of expression of the alpha2 subunit isoform suggests that the developmental changes in physiological expression of the Na+/K+ pump in primary cultures of skeletal muscle may be attributable either to the changes in activity of the alpha1 subunit or to differential activities of alphabeta complexes involving either of the beta subunits.

Role of submaximal exercise in promoting creatine and glycogen accumulation in human skeletal muscle

We examined the effect of glycogen-depleting exercise on subsequent muscle total creatine (TCr) accumulation and glycogen resynthesis during postexercise periods when the diet was supplemented with carbohydrate (CHO) or creatine (Cr) + CHO. Fourteen subjects performed one-legged cycling exercise to exhaustion. Muscle biopsies were taken from the exhausted (Ex) and nonexhausted (Nex) limbs after exercise and after 6 h and 5 days of recovery, during which CHO (CHO group, n = 7) or Cr + CHO (Cr+CHO group, n = 7) supplements were ingested. Muscle TCr concentration ([TCr]) was unchanged in both groups 6 h after supplementation commenced but had increased in the Ex (P < 0.001) and Nex limbs (P < 0.05) of the Cr+CHO group after 5 days. Greater TCr accumulation was achieved in the Ex limbs (P < 0.01) of this group. Glycogen was increased above nonexercised concentrations in the Ex limbs of both groups after 5 days, with the concentration being greater in the Cr+CHO group (P = 0.06). Thus a single bout of exercise enhanced muscle Cr accumulation, and this effect was restricted to the exercised muscle. However, exercise also diminished CHO-mediated insulin release, which may have attenuated insulin-mediated muscle Cr accumulation. Ingesting Cr with CHO also augmented glycogen supercompensation in the exercised muscle.

Mechanical stretching of alveolar epithelial cells increases Na(+)-K(+)-ATPase activity

Alveolar epithelial cells effect edema clearance by transporting Na(+) and liquid out of the air spaces. Active Na(+) transport by the basolaterally located Na(+)-K(+)-ATPase is an important contributor to lung edema clearance. Because alveoli undergo cyclic stretch in vivo, we investigated the role of cyclic stretch in the regulation of Na(+)-K(+)-ATPase activity in alveolar epithelial cells. Using the Flexercell Strain Unit, we exposed a cell line of murine lung epithelial cells (MLE-12) to cyclic stretch (30 cycles/min). After 15 min of stretch (10% mean strain), there was no change in Na(+)-K(+)-ATPase activity, as assessed by (86)Rb(+) uptake. By 30 min and after 60 min, Na(+)-K(+)-ATPase activity was significantly increased. When cells were treated with amiloride to block amiloride-sensitive Na(+) entry into cells or when cells were treated with gadolinium to block stretch-activated, nonselective cation channels, there was no stimulation of Na(+)-K(+)-ATPase activity by cyclic stretch. Conversely, cells exposed to Nystatin, which increases Na(+) entry into cells, demonstrated increased Na(+)-K(+)-ATPase activity. The changes in Na(+)-K(+)-ATPase activity were paralleled by increased Na(+)-K(+)-ATPase protein in the basolateral membrane of MLE-12 cells. Thus, in MLE-12 cells, short-term cyclic stretch stimulates Na(+)-K(+)-ATPase activity, most likely by increasing intracellular Na(+) and by recruitment of Na(+)-K(+)-ATPase subunits from intracellular pools to the basolateral membrane.

Temporal responses of oxidative vs. glycolytic skeletal muscles to K+ deprivation: Na+ pumps and cell cations

When K+ output exceeds input, skeletal muscle releases intracellular fluid K+ to buffer the fall in extracellular fluid (ECF) K+. To investigate the mechanisms and muscle specificity of the K+ shift, rats were fed K+-deficient chow for 2-10 days, and two muscles at phenotypic extremes were studied: slow-twitch oxidative soleus and fast-twitch glycolytic white gastrocnemius (WG). After 2 days of low-K+ chow, plasma K+ concentration ([K+]) fell from 4.6 to 3.7 mM, and Na+-K+-ATPase alpha2 (not alpha1) protein levels in both muscles, measured by immunoblotting, decreased 36%. Cell [K+] decreased from 116 to 106 mM in soleus and insignificantly in WG, indicating that alpha2 can decrease before cell [K+]. After 5 days, there were further decreases in alpha2 (70%) and beta2 (22%) in WG, not in soleus, whereas cell [K+] decreased and cell [Na+] increased by 10 mM in both muscles. By 10 days, plasma [K+] fell to 2.9 mM, with further decreases in WG alpha2 (94%) and beta2 (70%); cell [K+] fell 19 mM in soleus and 24 mM in WG compared with the control, and cell [Na+] increased 9 mM in soleus and 15 mM in WG; total homogenate Na+-K+-ATPase activity decreased 19% in WG and insignificantly in soleus. Levels of alpha2, beta1, and beta2 mRNA were unchanged over 10 days. The ratios of alpha2 to alpha1 protein levels in both control muscles were found to be nearly 1 by using the relative changes in alpha-isoforms vs. beta1- (soleus) or beta2-isoforms (WG). We conclude that the patterns of regulation of Na+ pump isoforms in oxidative and glycolytic muscles during K+ deprivation mediated by posttranscriptional regulation of alpha2beta1 and alpha2beta2 are distinct and that decreases in alpha2-isoform pools can occur early enough in both muscles to account for the shift of K+ to the ECF.

Insulin, cation metabolism and insulin resistance

There is considerable evidence that insulin and insulin-like growth factors regulate a number of important physiological functions in a variety of tissues, some not considered to be classically insulin sensitive. Impaired biological responses to insulin and related insulin-like growth factors are referred to as insulin resistance. Persons with insulin resistance often display clinical abnormalities other than impaired glucose tolerance, including central obesity, hypertension, dyslipidemia, microalbuminuria, and abnormal coagulation and fibrinolytic systems. The mechanisms leading to development of insulin resistance are not fully understood. However, in addition to abnormalities of phosphorylation processes, it appears that alterations in cellular cation metabolism contribute to diminished cellular actions of insulin (i.e., glucose transport and hemodynamic actions). This review focuses on known cellular cation abnormalities and associated insulin resistance and cardiovascular disease.

Linkage of the Na,K-ATPase alpha 2 and beta 1 genes with resting and exercise heart rate and blood pressure: cross-sectional and longitudinal observations from the Quebec Family Study

OBJECTIVE

To investigate whether genetic variations in the genes encoding the alpha and beta subunits of the Na,K-ATPase are linked with hemodynamic phenotypes.

DESIGN AND PARTICIPANTS

Cross-sectional data based on 533 subjects (no antihypertensive medication) were obtained from 150 families of phase 2 of the Quebec Family Study, together with longitudinal data from 338 subjects (105 families) who had been measured 12 years earlier in phase 1 of the Quebec Family Study.

MAIN OUTCOME MEASURES

Restriction fragment length polymorphisms were examined at the alpha 2 (exon 1 and exon 21-22 with BglII) and beta 1 (Msp I and Pvu II) loci of Na,K-ATPase. Hemodynamic phenotypes measured included systolic and diastolic blood pressure, heart rate and rate-pressure product at rest and during low-intensity exercise.

RESULTS

Sib-pair analysis revealed relatively strong linkages (P = 0.0003-0.002) between the resting heart rate and rate-pressure product and the alpha 2 exon 21-22 marker and alpha 2 haplotype. Moreover, the alpha 2 exon 21-22 marker showed suggestive linkages (P = 0.01 to 0.043) with resting systolic blood pressure and exercise diastolic blood pressure, heart rate and rate-pressure product, and the alpha 2 haplotype with exercise diastolic blood pressure and rate-pressure product and the 12-year change in resting systolic blood pressure (P = 0.03 to 0.05). Both the beta 1 Msp I marker and the beta 1 haplotype were linked with the resting rate-pressure product (P = 0.007 and 0.003, respectively), and all beta 1 markers showed linkage with the change in resting systolic blood pressure (P = 0.00005 to 0.024). In men, there was a significant (P = 0.01) interaction between the alpha 2 exon 21-22 genotype and the postglucose plasma insulin level with regard to resting systolic blood pressure.

CONCLUSIONS

These data suggest that the alpha 2 and beta 1 genes of Na,K-ATPase contribute to the regulation of hemodynamic phenotypes in healthy subjects.

Isoform-specific effects of charged residues at borders of the M1-M2 loop of the Na,K-ATPase alpha subunit

The Na,K-ATPase is specifically inhibited by the cardiac glycoside, ouabain. Via a largely undefined mechanism, the ouabain affinity of the Na,K-ATPase can be manipulated by mutating the residues at the borders of the first extracellular (M1-M2) loop of the alpha subunit [Price, E. M., Rice, D. A., and Lingrel, J. B. (1990) J. Biol. Chem. 265, 6638-6641]. To address this issue, we compared the effects of two combinations of charged residues at the M1-M2 loop border, R113, D124 and D113,R124 (numbered according to the rat alpha1 subunit), on the ouabain sensitivity of the alpha1 and alpha2 isoforms. We report that ouabain sensitivity is dependent not only upon the identity of the residues at the M1-M2 loop border but also upon the context into which they are introduced. Furthermore, at low concentrations of ATP, the identity of the residues at the M1-M2 loop border affects the regulation of ATP hydrolysis by potassium in an isoform-specific manner. Analysis of chimeric alpha subunits reveals that the effects of potassium are determined primarily by the interaction of the N-terminus and M1-M2 loop with the C-terminal third of the alpha subunit. M1-M2 loop border residues may, therefore, influence ouabain sensitivity indirectly by altering the stability or structure of the intermediate of the Na,K-ATPase catalytic cycle which is competent to bind ouabain.

Mean arterial blood pressure and serum levels of the molar ratio of insulin-like growth factor-1 to its binding protein-3 in healthy centenarians

OBJECTIVE

Healthy centenarians have a greater molar ratio of plasma insulin-like growth factor-1 to insulin-like growth factor binding protein-3 than that of aged subjects. We investigated the question of whether differences in mean arterial pressure and in this plasma ratio were related in healthy centenarians.

SUBJECTS AND METHODS

We studied 52 subjects in total, 30 aged subjects (70-99 years) and 22 healthy centenarians (> 100 years) to determine differences in mean arterial pressure, endothelial function and intracellular cation levels.

RESULTS

In the healthy centenarians, the molar ratio of fasting plasma insulin-like growth factor-1 to its binding protein-3 was significantly correlated with mean arterial pressure (r = -0.66, P < 0.001). Baseline (19.3 +/- 1.5 versus 27.6 +/- 2.2 mumol/l, P < 0.05) and L-arginine-stimulated percentage increases in the plasma total nitrate: nitrite ratio (67 +/- 3.4 versus 48 +/- 4.5%, P < 0.03) were greater in the healthy centenarians than in the aged subjects. An L-arginine bolus elicited an increase in forearm blood flow which was correlated with the percentage increase in the plasma total nitrate: nitrite ratio (r = 0.79, P < 0.001) and with the fasting erythrocyte magnesium concentration (r = 0.80, P < 0.001) in healthy centenarians. Both correlations remained significant (P < 0.01) after adjustment for sex, body mass index and the waist: hip ratio. Moreover, the fasting plasma molar ratio of insulin-like growth factor-1 to its binding protein-3 was correlated with the percentage increase in forearm blood flow (r = 0.59, P < 0.005) and with the percentage increase in the plasma total nitrate: nitrite ratio (r = 0.54, P < 0.009) in healthy centenarians. The centenarians had higher baseline total erythrocyte magnesium and lower calcium concentrations than the aged subjects. The addition of insulin growth factor-1 to the incubation medium increased the total intracellular erythrocyte magnesium content and decreased the calcium content in both groups of subjects. Nevertheless, the percentage increase in total erythrocyte magnesium (33 +/- 3.8 versus 12 +/- 3.4%, P < 0.03) and decline in intracellular calcium (17 +/- 2.8 versus 8 +/- 3.1%, P < 0.02) concentrations were greater in the healthy centenarians than the aged subjects.

CONCLUSION

In healthy centenarians, insulin-like growth factor-1 may preserve endothelial function and modulate the intracellular cation content, thus contributing to a lower mean arterial pressure than that in aged subjects.

Stimulatory effect of insulin on creatine accumulation in human skeletal muscle

This study investigated the effect of insulin on plasma and muscle creatine accumulation and limb blood flow in humans after creatine administration. Seven men underwent a 300-min euglycemic insulin clamp combined with creatine administration on four separate occasions. Insulin was infused at rates of 5, 30, 55, or 105 mU. m-2. min-1, and on each occasion 12.4 g creatine was administered. During infusion of insulin at rates of 55 and 105 mU. m-2. min-1, muscle total creatine concentration increased by 4.5 +/- 1.4 (P < 0. 05) and 8.3 +/- 1.0 mmol/kg dry mass (P < 0.05), and plasma creatine concentrations were lower at specific time points compared with the 5 mU. m-2. min-1 infusion rate. The magnitude of increase in calf blood flow (plethysmography) was the same irrespective of the rate of insulin infusion, and forearm blood flow increased to the same extent as the three highest infusion rates. These findings demonstrate that insulin can enhance muscle creatine accumulation in humans but only when present at physiologically high or supraphysiological concentrations. This response is likely to be the result of an insulin-mediated increase in muscle creatine transport rather than creatine delivery.

Expression of alpha- and beta-subunits and activity of Na+K+ ATPase in pig thyroid cells in primary culture: modulation by thyrotropin and thyroid hormones

Na+ K+ ATPase located at the basolateral pole of thyroid epithelial cells, contributes to thyroid hormone synthesis by generating the driving force for the uptake of the substrate, iodide. We have investigated whether the expression of the alpha- and beta-subunits and activity of Na+ K+ ATPase were subjected to variations in response, (a) to TSH, that controls the expression of differentiation in thyroid cells and (b) to thyroid hormones as potential autocrine factors. Studies were carried out on pig thyroid cells cultured (a) without TSH to obtain thyroid cell monolayers (TCM) in basal state or (b) with TSH in the form of cell monolayers (TCM-T) or as reconstituted thyroid follicles (RTF). Iodide uptake activity, thyroperoxidase protein and thyroglobulin mRNA taken as parameters of thyroid cell differentiation were 6 to 25-fold higher in RTF and TCM-T than in TCM. Western blot analyses of Na+ K+ ATPase subunits revealed that the alpha-subunit (105 kDa) content of TCM-T and RTF was similar but 8-fold higher than that of TCM. In contrast, the beta-subunit (50 kDa) content of TCM-T and RTF was only about twice that of TCM. Similar relative variations were observed at the mRNA level for both alpha- and beta-subunits. Na+ K+ ATPase activity was only 40% higher in RTF and TCM-T than in TCM. A 48 h treatment of RTF by either T4 or T3 (1-100 nM) induced a 3-fold increase of the alpha-subunit but did neither alter the beta-subunit nor the Na+ K+ ATPase activity. In conclusion, Na+ K+ ATPase activity and the level of expression of its beta-subunit, known to control the assembly and targetting of alpha-beta oligomers and thus the amount of functional sodium pump at the plasma membrane, are only moderately altered when thyroid cells undergo major changes in their differentiation status. Our data show that the expression of the alpha-subunit of Na+ K+ ATPase by thyroid cells is up-regulated by TSH and thyroid hormones.

Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function

The Na-K-ATPase is characterized by a complex molecular heterogeneity that results from the expression and differential association of multiple isoforms of both its alpha- and beta-subunits. At present, as many as four different alpha-polypeptides (alpha1, alpha2, alpha3, and alpha4) and three distinct beta-isoforms (beta1, beta2, and beta3) have been identified in mammalian cells. The stringent constraints on the structure of the Na pump isozymes during evolution and their tissue-specific and developmental pattern of expression suggests that the different Na-K-ATPases have evolved distinct properties to respond to cellular requirements. This review focuses on the functional properties, regulation, and possible physiological relevance of the Na pump isozymes. The coexistence of multiple alpha- and beta-isoforms in most cells has hindered the understanding of the roles of the individual polypeptides. The use of heterologous expression systems has helped circumvent this problem. The kinetic characteristics of different Na-K-ATPase isozymes to the activating cations (Na+ and K+), the substrate ATP, and the inhibitors Ca2+ and ouabain demonstrate that each isoform has distinct properties. In addition, intracellular messengers differentially regulate the activity of the individual Na-K-ATPase isozymes. Thus the regulation of specific Na pump isozymes gives cells the ability to precisely coordinate Na-K-ATPase activity to their physiological requirements.

Protein kinase A induces recruitment of active Na+,K+-ATPase units to the plasma membrane of rat proximal convoluted tubule cells

1. The aim of this study was to investigate the mechanism of control of Na+,K+-ATPase activity by the cAMP-protein kinase A (PKA) pathway in rat proximal convoluted tubules. For this purpose, we studied the in vitro action of exogenous cAMP (10-3 M dibutyryl-cAMP (db-cAMP) or 8-bromo-cAMP) and endogenous cAMP (direct activation of adenylyl cyclases by 10-5 M forskolin) on Na+,K+-ATPase activity and membrane trafficking. 2. PKA activation stimulated both the cation transport and hydrolytic activity of Na+,K+-ATPase by about 40%. Transport activity stimulation was specific to the PKA signalling pathway since (1) db-cAMP stimulated the ouabain-sensitive 86Rb+ uptake in a time- and dose-dependent fashion; (2) this effect was abolished by addition of H-89 or Rp-cAMPS, two structurally different PKA inhibitors; and (3) this stimulation was not affected by inhibition of protein kinase C (PKC) by GF109203X. The stimulatory effect of db-cAMP on the hydrolytic activity of Na+,K+-ATPase was accounted for by an increased maximal ATPase rate (Vmax) without alteration of the efficiency of the pump, suggesting that cAMP-PKA pathway was implicated in membrane redistribution control. 3. To test this hypothesis, we used two different approaches: (1) cell surface protein biotinylation and (2) subcellular fractionation. Both approaches confirmed that the cAMP-PKA pathway was implicated in membrane trafficking regulation. The stimulation of Na+,K+-ATPase activity by db-cAMP was associated with an increase (+40%) in Na+, K+-ATPase units expressed at the cell surface which was assessed by Western blotting after streptavidin precipitation of biotinylated cell surface proteins. Subcellular fractionation confirmed the increased expression in pump units at the cell surface which was accompanied by a decrease (-30%) in pump units located in the subcellular fraction corresponding to early endosomes. 4. In conclusion, PKA stimulates Na+,K+-ATPase activity, at least in part, by increasing the number of Na+-K+ pumps in the plasma membrane in proximal convoluted tubule cells.

Receptor-mediated inhibition of renal Na(+)-K(+)-ATPase is associated with endocytosis of its alpha- and beta-subunits

The mechanisms involved in receptor-mediated inhibition of Na(+)-K(+)-ATPase remain poorly understood. In this study, we evaluate whether inhibition of proximal tubule Na(+)-K(+)-ATPase activity by dopamine is linked to its removal from the plasma membrane and internalization into defined intracellular compartments. Clathrin-coated vesicles were isolated by sucrose gradient centrifugation and negative lectin selection, and early and late endosomes were separated on a flotation gradient. Inhibition of Na(+)-K(+)-ATPase activity by dopamine, in contrast to its inhibition by ouabain, was accompanied by a sequential increase in the abundance of the alpha-subunit in clathrin-coated vesicles (1 min), early endosomes (2.5 min), and late endosomes (5 min), suggesting its stepwise translocation between these organelles. A similar pattern was found for the beta-subunit. The increased incorporation of both subunits in all compartments was blocked by calphostin C. The results demonstrate that the dopamine-induced decrease in Na(+)-K(+)-ATPase activity in proximal tubules is associated with internalization of its alpha- and beta-subunits into early and late endosomes via a clathrin-dependent pathway and that this process is protein kinase C dependent. The presence of Na(+)-K(+)-ATPase subunits in endosomes suggests that these compartments may constitute normal traffic reservoirs during pump degradation and/or synthesis.

Biochemical characterization of the subunits of the Na+/K+ ATPase expressed in insect cells

The Na+/K+ ATPase is composed of two subunits called alpha and beta chains. In insect cells, independently expressed alpha and beta chains are localized to intracellular membranes. Sucrose density gradient sedimentation, crosslinking analysis, and immunoprecipitation of radio-labeled proteins show that the alpha chains expressed alone are in large aggregates of different molecular weights with less than 4% being monomeric. Analysis by non-reducing SDS-PAGE and immunoblotting show that the beta chains expressed alone are in Triton X-100 insoluble, disulfide-linked aggregates. Co-expression of both subunits in insect cells results in only a small fraction (less than 15%) of the alpha chains being assembled as the active recombinant enzyme, with at least 22% of the active recombinant enzyme localized to the plasma membrane as determined by a biochemical assay. The small amount of beta chain at the plasma membrane in cells that express both subunits is beyond the limit of detection by the biochemical assay. Immunoprecipitation of Triton X-100 soluble alpha chains from radio-labeled cells expressing both subunits shows that the alpha chains are mostly in large aggregates containing beta chains. These results suggest that, in insect cells, the availability of correctly folded beta chains is the rate limiting step in the assembly of active Na+/K+ ATPase.

Dexamethasone upregulates the Na-K-ATPase in rat alveolar epithelial cells

Previous studies in kidney, heart, and liver cells have demonstrated that dexamethasone regulates the expression of Na-K-ATPase. In the lungs, Na-K-ATPase has been reported in alveolar epithelial type II (ATII) cells and is thought to participate in active Na+ transport and lung edema clearance. The aim of this study was to determine whether Na-K-ATPase would be regulated by dexamethasone in cultured rat ATII cells. Regulation of the Na-K-ATPase by dexamethasone could lead to a greater understanding of its role in active Na+ transport and lung edema clearance. Rat ATII cells were isolated, plated for 24 h, and exposed to 10(-7) and 10(-8) M dexamethasone. These cells were harvested at 0, 3, 6, 12, and 24 h after dexamethasone exposure for determination of steady-state Na-K-ATPase mRNA transcript levels, protein expression, and function. The steady-state Na-K-ATPase beta1-mRNA transcript levels increased in ATII cells 6, 12, and 24 h after dexamethasone exposure (P < 0.05). However, the steady-state alpha1-mRNA transcript levels were unchanged. The protein expression for the alpha1- and beta1-subunits increased in ATII cells exposed to dexamethasone compared with controls in association with a temporal increase in Na-K-ATPase function after dexamethasone exposure. These results suggest that dexamethasone regulates Na-K-ATPase in ATII cells possibly by transcriptional, translational, and posttranslational mechanisms.

Role of serine/threonine protein phosphatases in insulin regulation of Na+/K+-ATPase activity in cultured rat skeletal muscle cells

In this study, we examined the potential role of serine/threonine protein phosphatase-1 (PP-1) and PP-2A in the mechanism of Na+/K+-ATPase activation by insulin in the rat skeletal muscle cell line L6. Incubation of L6 cells with insulin caused a time- and dose-dependent stimulation of ouabain-sensitive plasma membrane Na+/K+-ATPase activity. Pretreatment with okadaic acid (OA; 0.1-1 microM) or calyculin A (1 microM) blocked insulin's effect on Na+/K+-ATPase activation. Low concentrations of OA that specifically inhibit PP-2A were ineffective. Immunoprecipitation of the enzyme from 32P-labeled cells with an antibody directed against the alpha-1 subunit of the enzyme revealed a 60% decrease in 110-kDa protein phosphorylation in insulin-treated cells. The presence of calyculin A blocked insulin-mediated dephosphorylation of Na+/K+-ATPase, whereas low concentrations of OA were ineffective. To further confirm the role of PP-1, we used L6 cell lines that overexpress the glycogen/SR-associated regulatory subunit of PP-1, PP-1G. Overexpression of PP-1G resulted in a 3-fold increase in insulin-stimulated PP-1 catalytic activity. This was accompanied by a 30% increase in basal Na+/K+-ATPase activity and a >2-fold increase in insulin's effect on pump activity. Inhibition of phosphatidylinositol-3 kinase with wortmannin blocked insulin-stimulated PP-1 activation as well as the dephosphorylation and activation of Na+/K+-ATPase. We conclude that insulin regulates the activity of Na+/K+-ATPase by promoting dephosphorylation of the alpha subunit via an insulin-stimulated PP-1 and that phosphatidylinositol-3 kinase-generated signals may mediate insulin activation of PP-1 and Na+/K+-ATPase.

Dopamine stimulates sodium transport and liquid clearance in rat lung epithelium

Pulmonary edema clearance is driven primarily by active sodium transport out of the alveoli, mediated predominantly by apical sodium channels and the basolateral NA,K-ATPase. We postulated that dopamine, analogous to its effects in other transporting epithelia, could regulate these sodium transport mechanisms and affect lung liquid clearance. We therefore studied the effects of dopamine on sodium transport and liquid clearance in isolated perfused rat lungs. Instillation of dopamine into the airways caused a dose-dependent increase in liquid clearance from isolated rat lungs of up to 33% above control values at 10(-8) to 10(-4) M concentrations. 10(-6) M amiloride, which selectively inhibits apical sodium channels, decreased basal liquid clearance by 34% but did not inhibit the dopamine-mediated stimulation of lung liquid clearance. Instillation of 10(-4) M amiloride into rat airways, which inhibits other sodium transport mechanisms non-selectively, decreased basal lung liquid clearance by 49% and inhibited the dopamine-mediated stimulation of lung liquid clearance. Perfusion of rat lungs with 5 x 10(-4) M ouabain to specifically inhibit Na,K-ATPase reduced both basal clearance (by 55%) and the dopamine-stimulated increase in lung fluid clearance. Conceivably, the stimulation of lung liquid clearance by dopamine is due to a modulation of Na,K-ATPase in the pulmonary epithelium.

Studies on the mechanism of short-term regulation of pulmonary artery endothelial cell Na/K pump activity

The Na/K pump is critically important in maintenance of cell homeostasis in the face of injury. Little is known about the regulation of endothelial cell Na/K-pump activity. We previously reported that short-term (30-minute) oxidant-induced endothelial cell perturbation increased Na/K-pump activity in intact monolayers of bovine pulmonary artery endothelial cells (BPAECs). In this study we investigated the mechanism of oxidant-induced increases in endothelial Na/K-pump activity, focusing on short-term modulation of alpha1-pump subunit. By using immunofluorescence microscopy and confocal scanning laser microscopy, we found alpha1 subunit on both apical and basal aspects of BPAECs without polarized distribution. Short-term (30-minute) incubation of PAEC monolayers with H2O2 (1 mmol/L) did not change the relative amounts of alpha1 subunit in membrane fractions, as assessed by immunoblotting. Phosphorylation of the alpha1 subunit also was not affected by H2O2 treatment. Because protein kinases have been reported to alter Na/K-pump activity in several tissues and because H2O2 has been reported to increase PKC activity of endothelial cells, we determined the effects of inhibition and activation of protein kinase C (PKC) on Na/K-pump activity quantitated as ouabain-inhibitable uptake of 86Rb. We also determined the effects of PKC activation and inhibition on H2O2-induced increases in Na/K-pump activity. Inhibitors of PKC increased Na/K-pump activity over a 30-minute period in intact monolayers. Inhibition or depletion of PKC did not prevent H2O2-induced increases in pump activity. These results indicate that PKC is an endogenous regulator of pulmonary artery endothelial cell Na/K-pump activity but that the effects of H2O2 are not mediated by activation of PKC or by changes in the expression or phosphorylation of alpha1 subunit.

Parallel detection of Na,K-ATPase alpha subunit isoforms by pan-specific monoclonal mAb 9A7

While emphasis has been placed upon those proteins which either mediate or respond to the rapid influx of calcium following depolarization, there has been little emphasis upon those proteins which aid in the reequilibration of the membrane potential. In an effort to identify presynaptic membrane proteins implicated in neurosecretion, monoclonal antibodies were screened against proteins which cosegregated with neuronal voltage-dependent calcium channels (VDCC) following immunoprecipitation. One monoclonal antibody (mAb 9A7) identified a 110-kDa protein. Micropeptide sequencing of (i) the mAb 9A7 immunoaffinity purified antigen and (ii) the 110-kDa protein present in the neuronal (N-type) VDCC preparation (McEnery et al., 1991, Proc. Natl. Acad. Sci. 88, 11095-11099) indicated identity with the alpha subunit(s) of the Na,K-ATPase. Further characterization by Western blotting, immunochemical localization, and immunoaffinity purification indicated that mAb 9A7 not only recognized the alpha3 isoform which is predominant in neuronal tissues but also identified the alpha1 and alpha2 isoforms. mAb 9A7 exhibited a wide cross-species reactivity and recognized human, rat, and mouse alpha subunit isoforms at an internal epitope. The pan-specificity of mAb 9A7 and the differential mobility of the alpha1 isoform relative to the alpha2 and alpha3 permitted parallel detection of multiple alpha isoforms. Western blot analysis of undifferentiated rat pheochromocytoma cell line (PC12) and human neuroblastoma (IMR32) cells indicated coexpression of the alpha1 and alpha3 isozymes. Upon differentiation of IMR32 cells by dibutrylyl-cAMP, a substantial increase in the alpha3 relative to the alpha1 isoform was observed. While the enrichment of total Na,K-ATPase may reflect the increased demand for ATP-dependent ion transport as IMR32 cells become more excitable, the specific increase in the alpha3 isoform suggests a unique role of this isoform during IMR32 cell differentiation.

IGF-I regulation of Na(+)-K(+)-ATPase in rat arterial smooth muscle

Insulin-like growth factor I (IGF-I) is vasodilatory and mitogenic for vascular smooth muscle cells (VSMC). Alteration in VSMC Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) activity is hypothesized to underlie abnormal vascular tone and growth in hypertension and diabetes. Therefore, we investigated effects of IGF-I on Na(+)-K(+)-ATPase activity in rat aortic VSMC. IGF-I increases pump activity in a dose- and time-dependent manner: the minimal dose required was 10(-10) M, and the minimal time required was 20 min (at 10(-8) M) to increase activity. Similar effects persisted through 12 h. In Na(+)-loaded cells, IGF-I does not further stimulate activity. Blockade of Na+/H+ exchange attenuates IGF-I-induced increases in activity after 30 min but has no effect after 12 h. Northern blot analyses reveal that expression of the alpha 1- and the alpha 2-subunits of the pump were unaffected by IGF-I. Plasma membrane alpha 1- and alpha 2-protein were also unaffected, suggesting translocation of preformed pools was not responsible for the increases. Inhibitors revealed that neither tyrosine kinase activity, RNA transcription, protein synthesis, nitric oxide synthase activity, or protein kinase C activity mediated this IGF-I effect. Therefore, IGF-I regulates Na pump activity in the short term by an Na+/H+ exchange-dependent but transcription/translocation-independent mechanism. These data suggest that IGF-I, known to be produced by VSMC, may regulate tone and growth responses abnormal in disease states such as hypertension and diabetes.

The molar ratios of alpha and beta subunits of the Na+-K+-ATPase differ in distinct subcellular membranes from rat skeletal muscle

The Na+-K+-ATPase consists of alpha and beta subunits proposed to function as an alpha-beta heterodimer. Skeletal muscle is characterized by expression of alpha1, alpha2, beta1, and beta2 subunit isoforms. The relative molar proportions of each subunit or each protein isoform are not known, yet their subcellular distribution and expression in muscles of different fiber types are markedly different. In this study, the molar ratio of each pump subunit isoform was measured in purified membranes from skeletal muscle and compared with those in kidney and brain microsomes. Recombinant proteins were used as standards to quantitate each isoform by immunoblotting in combination with measurements of [3H]ouabain binding. The results indicate that in kidney microsomes, which express predominantly alpha1 and beta1 isoforms, the alpha:beta subunit molar ratio is approximately 1:1. In brain microsomes, the sum of all alpha (alpha1, alpha2, and alpha3) and all beta (beta1 and beta2) subunits also yielded a molar ratio of approximately 1:1. In contrast, in red (oxidative) skeletal muscles, the all alpha:beta subunit ratio was 0.2 in plasma membranes and 0.4 in intracellular membranes. The ratio of alpha2 subunits to alpha1 subunits ranged from 1.6 in surface membranes to up to 7 in internal membranes, while the beta1 subunits exceeded the beta2 subunits by approximately 4-fold in all membrane fractions. Thus, intracellular membranes of red skeletal muscles contain primarily alpha2 and beta1 subunits. When these intracellular membranes were further subfractionated by velocity gradient centrifugation, the alpha2:beta1 subunit ratio was 0.5 in the faster migrating (larger) membranes and 1.0 in the slower migrating (smaller) ones. This was due to a progressive decrease in abundance of the beta1 subunits without a change in the concentration of alpha2 subunits per unit protein. The Na+-K+-ATPase hydrolytic activity was higher in the larger vesicles than in the smaller ones along the sucrose gradient. These results suggest that the ratio of beta to alpha subunits may serve to regulate the catalytic activity of the Na+-K+-ATPase in skeletal muscle.

Desipramine modulates 3H-ouabain binding in rat hypothalamus

We have previously shown that Na+, K(+)-ATPase activity in hypothalamus is increased after administration of an acute dose of desipramine, a noradrenaline uptake inhibitor (Viola et al., Cell Molec Neurobiol 9:263-271, 1989). In this report the same treatment (10 mg per kg) was applied to evaluate 3H-ouabain binding in rat brain sections by quantitative autoradiography. Results disclosed an increase in the number of ouabain binding sites in hypothalamus but not in cerebral cortex. Concomitantly, such acute DMI treatment enhanced K(+)-stimulated-p-nitrophenylphosphatase activity in hypothalamus membranes whereas it failed to modify cerebral cortex membranes. A direct interaction of DMI with the enzyme was ruled out since in vitro DMI is known to inhibit the enzyme. It may be speculated that DMI indirectly stimulates Na+, K(+)-ATPase through the increase in noradrenaline which acts in turn on the external phosphorylated site of the enzyme.

Skeletal muscle Na,K-ATPase alpha and beta subunit protein levels respond to hypokalemic challenge with isoform and muscle type specificity

During potassium deprivation, skeletal muscle loses K+ to buffer the fall in extracellular K+. Decreased active K+ uptake via the sodium pump, Na,K-ATPase, contributes to the adjustment. Skeletal muscle expresses alpha1, alpha2, beta1, and beta2 isoforms of the Na, K-ATPase alphabeta heterodimer. This study was directed at testing the hypothesis that K+ loss from muscle during K+ deprivation is a function of decreased expression of specific isoforms expressed in a muscle type-specific pattern. Isoform abundance was measured in soleus, red and white gastrocnemius, extensor digitorum longus, and diaphragm by immunoblot. alpha2 expression was uniform across control muscles, whereas alpha1 and beta1 were twice as high in oxidative (soleus and diaphragm) as in fast glycolytic (white gastrocnemius) muscles, and beta2 expression was reciprocal: highest in white gastrocnemius and barely detectable in soleus and diaphragm. Following 10 days of potassium deprivation plasma K+ fell from 4.0 to 2.3 mM, and there were distinct responses in glycolytic versus oxidative muscles. In glycolytic white gastrocnemius alpha2 and beta2 fell 94 and 70%, respectively; in mixed red gastrocnemius and extensor digitorum longus both fell 60%, and beta1 fell 25%. In oxidative soleus and diaphragm alpha2 fell 55 and 30%, respectively, with only minor changes in beta1. Although decreases in alpha2 and beta2 expression are much greater in glycolytic than oxidative muscles during K+ deprivation, both types of muscle lose tissue K+ to the same extent, a 20% decrease, suggesting that multiple mechanisms are in place to regulate the release of skeletal muscle cell K+.

Expression of a Na,K-ATPase beta 3 subunit during development of the zebrafish central nervous system

Zebrafish beta 3, a full length cDNA clone encoding a zebrafish Na,K-ATPase beta subunit, was isolated. The protein shares highest homology with the beta 3 subunits of amphibians and mammals, slightly less homology with the beta 2 subunits, and is distinct from the beta 1 subunits. The fish beta subunit co-assembled with alpha subunits to form Na,K-ATPase enzymes when expressed in Xenopus oocytes. Embryonic expression was first detected by whole-mount in situ hybridization between 8-12 hr post-fertilization (hpf) in the head mesoderm. Subsequently, and up to 24 hpf, the mRNA was confined to four dorsal domains in the anterior neural tube. After a transient downregulation during the second day, expression was again conspicuous in the nervous system of 3-day-old larvae. Based on its distribution pattern, the fish beta subunit could be involved in setting up regional identities in the developing fish CNS and in the differentiation of distinct cell types.

Na-K-ATPase in lacrimal gland acinar cell endosomal system: correcting a case of mistaken identity

Na-K-ATPase is associated with a variety of membrane populations in lacrimal acinar cells. Acinus-like structures formed by rabbit acinar cells in primary culture were incubated with horseradish peroxidase (HRP) to label basolateral and endosomal membranes and then analyzed by electron microscopy cytochemistry with the 3-3'-diaminobenzidine reaction or by fractionation and measurement of marker catalytic activities or immunoreactivities. HRP adsorbed to basolateral membranes at 4 degrees C. Fractionation showed it associated with low-density membranes enriched in acid phosphatase and TGN38 but containing only minor amounts of Na-K-ATPase. Cells internalized HRP to cytoplasmic vesicles, Golgi structures, and lysosomes at 37 degrees C. The major endosomal compartment revealed by fractionation coincided with major peaks of Na-K-ATPase and Rab6 and secondary peaks of galactosyltransferase and gamma-adaptin. Carbachol (10 microM) increased lysosomal and Golgi labeling. Thus most of the Na-K-ATPase is located in the basolateral membrane-oriented endosomal system, concentrated in a compartment possibly related to the trans-Golgi network. Constitutive and stimulation-accelerated traffic to and from this compartment may serve several exocrine cell functions.

Identification of the mammalian Na,K-ATPase 3 subunit

We have isolated and characterized cDNA clones encoding the human and rat Na,K-ATPase beta3 subunit isoform. The human cDNA encodes a polypeptide of 279 amino acids that exhibits primary sequence and secondary structure similarities to Na,K-ATPase beta subunit isoforms. Sequence comparisons showed that the human beta3 subunit closely resembles the beta3 subunit of Xenopus laevis (59% amino acid identity) and is less similar to the human Na,K-ATPase beta1 and beta2 subunits (38% and 48% amino acid identity, respectively). By analyzing the segregation of restriction fragment length polymorphisms among recombinant inbred strains of mice, we localized the beta3 subunit gene to murine chromosome 7. Northern blot analysis revealed that the beta3 subunit gene encodes two transcripts that are expressed in a variety of rat tissues including testis, brain, kidney, lung, stomach, small intestine, colon, spleen, and liver. Identification of the mammalian beta3 subunit suggests an even greater potential for Na,K-ATPase isoenzyme diversity than previously realized.

Insulin stimulates the Na+,K(+)-ATPase and the Na+/K+/Cl- cotransporter of human fibroblasts

Insulin regulation of K+ (Rb+) transport was investigated in cultured human fibroblasts using a non-radioactive method which allows the simultaneous determination of the intracellular concentration of other monovalent cations. Insulin stimulated Rb+ influx through the Na+,K(+)-ATPase and the Na+/K(+)/Cl- cotransporter in human fibroblasts. Insulin stimulation was very rapid and maximal effect was observed within 10 min. Insulin stimulation of Rb+ uptake via the Na+,K(+)-ATPase and the Na+/K(+)/Cl- cotransporter was dose-dependent, with half-maximal stimulation at 2-3 nM of hormone. Insulin increased the V(max) of both transporters involved, affecting only minimally their Km. In other cells, insulin stimulates the Na+,K(+)-pump by increasing Na+ availability through the Na+/H+ exchanger. In human fibroblasts, insulin stimulation of Na+,K(+)-ATPase occurred in the presence of ethyl-isopropyl amiloride, an inhibitor of the Na+/H+ exchanger, and without sustained changes in intracellular[Na+]. By contrast, insulin action on Na+,K(+)-ATPase was impaired by the protein kinase inhibitors staurosporine and genistein. These results indicate that, in human fibroblasts, insulin stimulates both the Na+,K(+)-ATPase and the Na+/K+/Cl- cotransporter, that stimulation of the Na+,K(+)-ATPase occurs in the absence of changes in intracellular [Na+], and that protein kinase activity is essential for this insulin action.

Insulin-induced translocation of Na+-K+-ATPase subunits to the plasma membrane is muscle fiber type specific

We have previously shown that an acute insulin treatment induces redistribution of the alpha 2- and beta 1- isoforms of the Na+-K+-ATPase from intracellular membranes to plasma membranes detected on subcellular fractionation of mixed muscles and immunoblotting with isoform-specific antibodies (H. S. Hundal et al. J. Biol. Chem. 267: 5040-5043, 1992). In the present study we give both biochemical and morphological evidence that this insulin effect is operative in muscles composed mostly of oxidative (red) fibers but not in muscles composed mostly of glycolytic (white) fibers. The redistribution of the Na+-K+-ATPase alpha 2- and beta 1-isoforms after insulin injection was detected in membranes isolated from and muscles (soleus, red gastrocnemius, red rectus femoris, and red vastus lateralis) but not in membranes from white muscles (white gastrocnemius, tensor fasciae latae, white rectus femoris, and white vastus lateralis). After insulin injection, the potassium-dependent 3-O-methylfluorescein phosphatase activity of the enzyme was higher by 22% in the plasma membrane-enriched fraction and lower by 15% in the internal membrane fraction isolated from red but not from white muscles. Quantitative immunoelectron microscopy of ultrathin muscle cryosections showed that in vivo insulin stimulation augmented the density of Na+-K+-ATPase alpha 2- and beta 1- isoforms at the plasma membrane of soleus muscle by 80 and 124%, respectively, with no change in white gastrocnemius muscle. The effect of insulin to increase the content of Na+-K+-ATPase alpha 2- and beta 1-subunits in isolated plasma membranes was still observed when glycemia was prevented from dropping by using hyperinsulinemic-euglycemic clamps. We conclude that the insulin-induced redistribution of the alpha 2- and beta 1-isoforms of the Na+-K+-ATPase from an intracellular pool to the plasma membrane in restricted to oxidative fiber-type skeletal muscles. This may be related to the selective expression of beta 1-subunits in these fibers and implies that the beta 2-subunit, typical of glycolytic muscles, does not sustain translocation of alpha 2 beta 2-complexes.

Sedimentation and immunological analyses of GLUT4 and alpha 2-Na,K-ATPase subunit-containing vesicles from rat skeletal muscle: evidence for segregation

In skeletal muscle insulin induces the translocation of both the GLUT4 glucose transporter and the alpha 2 subunit of the Na,K-ATPase from an intracellular membrane (IM) compartment to the plasma membrane (PM). Fractionation studies of rat skeletal muscle using a discontinuous sucrose gradient have indicated that the insulin-induced loss of both proteins occurs from a fraction containing intracellular membranes (IM) of common density. This raises the possibility that both proteins may be colocalized in a single intracellular compartment or are present in separate membrane vesicles that are of similar buoyant density. In this study we report the membrane vesicles from the insulin-responsive IM fraction can in fact be separated on the basis of differences in their sedimentation velocities; immunoblot analyses of fractions collected from a sucrose velocity gradient revealed the presence of two separate peaks for GLUT4 and the alpha 2 subunit of the Na,K-ATPase. One of these peaks representing a fast sedimenting population of vesicles (with a sedimentation coefficient of 2697 +/- 57 S) reacted against antibodies to the alpha 2 subunit of the Na,K-ATPase, whereas, the second peak contained a population of much slower sedimenting vesicles (with a sedimentation coefficient of 209 +/- 4 S) were practically devoid of the alpha 2-subunit. By contrast, the slow sedimenting vesicles were enriched by approximately 32-fold in GLUT4 relative to the starting IM fraction when the fractional protein content was taken into account. Immunoprecipitation of GLUT4-containing vesicles from the insulin-sensitive IM fraction revealed that no immunoreactivity towards either the alpha 2 or the beta 1 subunits of the Na,K-ATPase could be observed, signifying that the insulin-responsive subunits of the Na,K-ATPase and GLUT4 were present in different membrane vesicles and that it was unlikely, therefore, that the insulin-induced redistribution of these proteins to the PM occurs from a common intracellular pool.

Alterations in the properties and isoform ratios of brain Na+/K(+)-ATPase in streptozotocin diabetic rats

In this study we analysed the changes in the properties of rat cerebral cortex Na+K(+)-ATPase in streptozotocin induced diabetes (STZ-diabetes). Special attempt was made to determine whether insulin treatment of diabetic animals could restore the altered parameters of this enzyme. Na+/K(+)-ATPase activity was found to be decreased by 15% after 2 weeks, and by 37% after 4 weeks in diabetic rat brains with a parallel decrease in maximal capacity of low affinity ouabain binding sites. There was no significant change in the high affinity ouabain binding sites. The Kd values did not change significantly. Western blot analysis of brain Na+/K(+)-ATPase isoforms indicated a 61 +/- 5.8% and 20 +/- 2.8% decrease of the alpha 1 and alpha 3 isoforms, respectively in 4 weeks diabetic animals. Change in the amount of the alpha 2 isoform proved to be less characteristic. Both types of beta subunit isoform showed a significant decrease in four weeks diabetic rats. Our data indicate a good correlation in diabetic rats between changes in Na-/K(+)-ATPase activity, low affinity ouabain binding capacity and the level of alpha 1 isoform. While insulin treatment of diabetic animals restored the blood glucose level to normal, a complete reversal of diabetes induced changes in Na+/K(+)-ATPase activity, ouabain binding capacity and Na+/K(+)-ATPase isoform composition could not be achieved.

Action of insulin on Na(+)-K(+)-ATPase and the Na(+)-K(+)-2Cl- cotransporter in 3T3-L1 adipocytes

The Na(+)-K(+)-ATPase presents several different isoforms of its alpha- and beta-subunits. We detected alpha 1- and beta 1-mRNA transcripts and polypeptides in 3T3-L1 fibroblasts; during differentiation into adipocytes, alpha 1-mRNA decreased, alpha 2-mRNA was induced, beta 1-mRNA dropped to undetectable levels, and beta 2-mRNA was never expressed, suggesting that 3T3-L1 adipocytes may express an unidentified Na(+)-K(+)-ATPase beta-subunit isoform. Insulin rapidly increased ion pump activity [ouabain-sensitive 86Rb+(K+) uptake] in 3T3-L1 fibroblasts and adipocytes without changing the plasma membrane concentration of alpha 1- or alpha 2-subunits as determined by subcellular membrane fractionation and immunoblotting or by [3H]ouabain binding to intact cells. Monensin, which raises the concentration of intracellular Na+, increased Na(+)-K+ pump activity, and no further stimulation was achieved with insulin. The stimulation of the pump by insulin was reduced by bumetanide, an inhibitor of the Na(+)-K(+)-2Cl- cotransporter, and was prevented by omission of extracellular Cl-. Insulin increased both ouabain-sensitive and bumetanide-sensitive 86Rb+(K+) uptake. These results suggest that insulin activation of the Na(+)-K(+)-ATPase in 3T3-L1 adipocytes is mediated by an elevation in intracellular Na+ that is likely the consequence of Na(+)-K(+)-2Cl- cotransporter activation.

Hepatic Na(+)-K(+)-ATPase enzyme activity correlates with polarized beta-subunit expression

We have examined underlying causes for observations made in hepatocytes in which catalytic subunits of Na(+)-K(+)-ATPase are found both in bile canalicular (apical) and sinusoidal (basolateral) membrane domains, whereas functional activity is associated preferentially with sinusoidal membrane sites. In a series of parallel studies, we determined by both light and electron microscopy that Na(+)-K(+)-ATPase alpha-subunits were localized to both membrane domains of hepatocytes. With the use of purified liver plasma membrane subfractions, ouabain inhibition curves demonstrated similar inhibition constants (inhibition constant 10(-5) M), and immunoblots using alpha 1-, alpha 2-, and alpha 3-polyclonal and monoclonal antibodies demonstrated antigenic sites predominantly for alpha 1 in both membrane fractions. Also, Northern blot hybridization analysis revealed only the alpha 1-isoform in hepatocytes. In contrast to the bipolar distribution of the alpha 1-subunit, the beta-subunit was identified only at the sinusoidal surface using fluorescence labeling with a monoclonal antibody. The beta 1-isoform was demonstrated by Northern blot analysis and was present predominantly at the sinusoidal domain by immunoblotting with polyclonal antibodies. In addition to the bipolar distribution of alpha 1, immunoblotting of liver plasma membrane subfractions demonstrated a symmetrical distribution of fodrin, ankyrin, actin, and E-cadherin at both domains. These results suggest that functionally competent alpha/beta-complexes form at the sinusoidal domain, whereas only alpha 1-subunits are present at the apical pole.

The GLUT4 glucose transporter and the alpha 2 subunit of the Na+,K(+)-ATPase do not localize to the same intracellular vesicles in rat skeletal muscle

The GLUT4 glucose transporter and the alpha 2 subunit of the Na+,K(+)-ATPase of rat skeletal muscle are two proteins which redistribute from intracellular membranes to plasma membranes following in vivo insulin stimulation. Here we show that although both proteins co-segregate after subcellular fractionation of unstimulated rat hindlimb muscles, they do not share the same intracellular residence inside the muscle fibre. By immunogold single- and double-labeling on ultrathin muscle cryosections with specific antibodies, the GLUT4 glucose transporter and the Na+,K(+)-ATPase alpha 2 subunit were observed on different vesicular structures within the cell. GLUT4 was detected on subsarcolemmal and perinuclear membranes, and at the junction between myofibrillar A and I bands where triads are localized. The alpha 2 subunit of the Na+,K(+)-ATPase was observed at the plasma membrane and in distinct subsarcolemmal vesicles and intermyofibrillar membranes. Quantitative analysis of double-labeling of GLUT4 and Na+,K(+)-ATPase alpha 2 subunit revealed that less than 6% of the two proteins co-localize in the same continuous vesicular structures. The differential intracellular localization of the two proteins was further confirmed by immunopurification of GLUT4-containing membranes from muscle homogenates, in which the alpha 2 subunit of the Na+,K(+)-ATPase was found only at the same extent as the alpha 1 subunit of the enzyme, a protein exclusively present at the plasma membrane.

Presence of two isoforms of Na, K-ATPase with different pharmacological and immunological properties in the rat kidney

Previous studies have demonstrated the presence of two populations of Na,K-ATPase with distinct kinetic, pharmacological and immunological characteristics along the rabbit nephron, indicating that the proximal segments of the nephron express exclusively the alpha 1 isoform of the catalytic subunit, whereas the collecting duct expresses an alpha 3-like isoform. Because pharmacological studies have shown the existence of two populations of Na,K-ATPase with different sensitivities to ouabain in the rat cortical collecting duct, which may result from the presence in the same nephron segment of the two isoforms demonstrated in the different segments of the rabbit nephron, the present study was undertaken to characterize the properties of Na,K-ATPase along the rat nephron. Results indicate that each segment of the rat nephron contains two subpopulations of Na,K-ATPase: a component highly sensitive to ouabain (IC50 approximately 5.10(-6) M) which is recognized by an anti-alpha 3 antibody and another moiety of lower affinity for ouabain (IC50 approximately 5.10(-4) M) which is recognized by an anti-alpha 1 antibody. Whether these two subpopulations correspond to different isoforms of the alpha subunit of Na,K-ATPase (alpha 1 and alpha 3-like) remains to be determined.