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Abstract: Sodium is the main determinant of body fluid distribution. Sodium accumulation causes water retention and, often, high blood pressure. At the cellular level, the concentration and active transport of sodium is handled by the enzyme Na+,K+-ATPase, whose appearance enabled evolving primitive cells to cope with osmotic stress and contributed to the complexity of mammalian organisms. Na+,K+-ATPase is a platform at the hub of many cellular signaling pathways related to sensing intracellular sodium and dealing with its detrimental excess. One of these pathways relies on an intracellular sodium-sensor network with the salt-inducible kinase 1 (SIK1) at its core. When intracellular sodium levels rise, and after the activation of calcium-related signals, this network activates the Na+,K+-ATPase and expel the excess of sodium from the cytosol. The SIK1 network also mediates sodium-independent signals that modulate the activity of the Na+,K+-ATPase, like dopamine and angiotensin, which are relevant per se in the development of high blood pressure. Animal models of high blood pressure, with identified mutations in components of multiple pathways, also have alterations in the SIK1 network. The introduction of some of these mutants into normal cells causes changes in SIK1 activity as well. Some cellular processes related to the metabolic syndrome, such as insulin effects on the kidney and other tissues, also appear to involve the SIK1. Therefore, it is likely that this protein, by modulating active sodium transport and numerous hormonal responses, represents a “crossroad” in the development and adaptation to high blood pressure and associated diseases. [ABSTRACT FROM AUTHOR]

Abstract: Chronic hypertension is characterized by a persistent increase in vascular tone. Sodium-rich diets promote hypertension; however, the underlying molecular mechanisms are not fully understood. Variations in the sodium content of the diet, through hormonal mediators such as dopamine and angiotensin II, modulate renal tubule Na+,K+-ATPase activity. Stimulation of Na+,K+-ATPase activity increases sodium transport across the renal proximal tubule epithelia, promoting Na+ retention, whereas inhibited Na+,K+-ATPase activity decreases sodium transport, and thereby natriuresis. Diets rich in sodium also enhance the release of adrenal endogenous ouabain-like compounds (OLC), which inhibit Na+,K+-ATPase activity, resulting in increased intracellular Na+ and Ca2+ concentrations in vascular smooth muscle cells, thus increasing the vascular tone, with a corresponding increase in blood pressure. The mechanisms by which these homeostatic processes are integrated in response to salt intake are complex and not completely elucidated. However, recent scientific findings provide new insights that may offer additional avenues to further explore molecular mechanisms related to normal physiology and pathophysiology of various forms of hypertension (i.e. salt-induced). Consequently, new strategies for the development of improved therapeutics and medical management of hypertension are anticipated. [Copyright &y& Elsevier]

Parathyroid hormone (PTH) inhibits Na+,K+-ATPase activity through protein kinase C-(PKC) and extracellular signal-regulated kinase-(ERK) dependent pathways and increases serine phosphorylation of the α1-subunit. To determine whether specific serine phosphorylation sites within the Na+,K+-ATPase αl-subunit are involved in the Na+,K+-ATPase responses to PTH, we examined the effect of PTH in opossum kidney cells stably transfected with wild type rat Na+,K+-ATPase α1-subunit (WT), serine 11 to alanine mutant αl-subunit (S11A), or serine 18 to alanine mutant αl-subunit (S18A). PTH increased phosphorylation and endocytosis of the Na+,K+-ATPase α1-subunit into clathrin-coated vesicles in cells transfected with WT and S18A rat Na+,K+-ATPase α1-subunits. PTH did not increase the level of phosphorylation or stimulate translocation of Na+,K+-ATPase α1-subunits into clathrin-coated vesicles in cells transfected with the S11A mutant. PTH inhibited ouabain-sensitive 86Rb uptake and Na+,K+-ATPase activity (ouabain-sensitive ATP hydrolysis) in WT-and S18A-transfected opossum kidney cells but not in S11A-transfected cells. Pretreatment of the cells with the PKC inhibitors and ERK inhibitor blocked PTH inhibition of 86Rb uptake, Na+,K+-ATPase activity, α1-subunit phosphorylation, and endocytosis in WT and S18A cells. Consistent with the notion that ERK phosphorylates Na+,K+-ATPase α1-subunit, ERK was shown to be capable of causing phosphorylation of Na+,K+-ATPase α1-subunit immunoprecipitated from WT and S18A but not from S11A-transfected cells. These results suggest that PTH regulates Na+,K+-ATPase by PKC and ERK-dependent α1-subunit phosphorylation and that the phosphorylation requires the expression of a serine at the 11 position of the Na+,K+-ATPase α1-subunit. [ABSTRACT FROM AUTHOR]

Demonstrates that treatment with phorbol ester leads to an increased amount of sodium/potassium-ATPase in the plasmalemma, which is proportional to the increased enzyme activity. Phosporylation; Triggering mechanism for recruitment of sodium/potassium ATPase molecules.

Presents information on a study which investigated whether sodium and potassium adenosine triphosphatase undergoes acute changes in its catalytic activity in response to beta-adrenergic-receptor stimulation. Pathophysiology of pulmonary edema; Experimental procedure; Results and discussion.

Presents information on a study which investigated whether Na...-K...-ATPase undergoes acute changes in its catalytic activity in response to beta-adrenergic-receptor stimulation. Background on pulmonary edema; Experimental procedures; Results and discussion.

Evaluates the physiological significance of the short-term regulation of Na:K pump activity by certain hormones and autacoids and the cellular mechanisms involved in the process. Physiological effects of short-term regulation of renal Na-K-ATPase activity; Intracellular messengers; Linking mechanisms between intracellular messengers and Na-K-ATPase system.

Investigates the relationship between protein kinase A (PKA) activation of 9-pS apical channel and actin filament organization. Use of patch clamp techniques; Actin phosphorylation; Effect of deoxyribonuclease on PKA-activated Na+ channel activity; Effect of cytochalasin D treatment on Na+ channel activity.

Motion of integral membrane proteins to the plasma membrane in response to G-protein-coupled receptor signals requires selective cargo recognition motifs that bind adaptor protein 1 and clathrin. Angiotensin II, through the activation of AT1 receptors, promotes the recruitment to the plasma membrane of Na,K-ATPase molecules from intracellular compartments. We present evidence to demonstrate that a tyrosine-based sequence (IVVY-255) present within the Na,K-ATPase α1-subunit is involved in the binding of adaptor protein 1. Mutation of Tyr-255 to a phenylalanine residue in the Na,K-ATPase α1-subunit greatly reduces the angiotensin II-dependent activation of Na,K-ATPase, recruitment of Na,K-ATPase molecules to the plasma membrane, and association of adaptor protein 1 with Na,K-ATPase α1-subunit molecules. To determine protein-protein interaction, we used fluorescence resonance energy transfer between fluorophores attached to the Na,K-ATPase α1-subunit and adaptor protein 1. Although angiotensin II activation of AT1 receptors induces a significant increase in the level of fluorescence resonance energy transfer between the two molecules, this effect was blunted in cells expressing the Tyr-255 mutant. Thus, results from different methods and techniques suggest that the Tyr-255-based sequence within the NKA α1-subunit is the site of adaptor protein 1 binding in response to the G-protein-coupled receptor signals produced by angiotensin II binding to AT1 receptors. [ABSTRACT FROM AUTHOR]

Examines the role of inositol (Ins) hexakisphosphate in improving dynamin-medicated endocytosis in the pancreatic cell. Molecular mechanism regulating the membrane homeostasis; Metabolic control of the Ins; Formation of the phosphatidylinositol biphosphate following activation of protein kinase C.

Cardiac left ventricle hypertrophy (LVH) constitutes a major risk factor for heart failure. Although LVH is most commonly caused by chronic elevation in arterial blood pressure, reduction of blood pressure to normal levels does not always result in regression of LVH, suggesting that additional factors contribute to the development of this pathology. We tested whether genetic preconditions associated with the imbalance in sodium homeostasis could trigger the development of LVH without concomitant increases in blood pressure. The results showed that the presence of a hypertensive variant of α-adducin gene in Milan rats (before they become hypertensive) resulted in elevated expression of genes associated with LVH, and of salt-inducible kinase 2 (SIK2) in the left ventricle (LV). Moreover, the mRNA expression levels of SIK2, α-adducin, and several markers of cardiac hypertrophy were positively correlated in tissue biopsies obtained from human hearts. In addition, we found in cardiac myocytes that α-adducin regulates the expression of SIK2, which in turn mediates the effects of adducin on hypertrophy markers gene activation. Furthermore, evidence that SIK2 is critical for the development of LVH in response to chronic high salt diet (HS) was obtained in mice with ablation of the sik2 gene. Increases in the expression of genes associated with LVH, as well as increases in LV wall thickness upon HS, occurred only in sik2+/+ but not in sik2−/− mice. Thus LVH triggered by HS or the presence of a genetic variant of α-adducin requires SIK2 and is independent of elevated blood pressure. Inhibitors of SIK2 may constitute part of a novel therapeutic regimen aimed at prevention/regression of LVH. [ABSTRACT FROM AUTHOR]

To uncover the potential cardiovascular effects of human polymorphisms influencing transforming growth factor β1 (TGFβ1) expression, we generated mice with Tgfb1 mRNA expression graded in five steps from 10% to 300% normal. Adrenal expression of the genes for mineral-ocorticoid-producing enzymes ranged from 50% normal in the hypermorphs at age 12 wk to 400% normal in the hypomorphs accompanied with proportionate changes in plasma aldosterone levels, whereas plasma volumes ranged from 50% to 150% normal accompanied by marked compensatory changes in plasma angiotensin II and renin levels. The aldosterone/renin ratio ranged from 0.3 times normal in the 300% hypermorphs to six times in the 10% hypomorphs. which have elevated blood pressure. Urinary output of water and electrolytes are markedly decreased in the 10% hypomorphs without significant change in the glomerular filtration rate. Renal activities for the Na+, K+-ATPase, and epithelial sodium channel are markedly increased in the 10% hypomorphs. The hypertension in the 10% hypomorphs is corrected by spironolactone or amiloride at doses that do not change blood pressure in wild-type mice. Thus, changes in Tgfb1 expression cause marked progressive changes in multiple systems that regulate blood pressure and fluid homeostasis, with the major effects being mediated by changes in adrenocortical function. [ABSTRACT FROM AUTHOR]

The protein kinase liver kinase B1 (LKB1) regulates cell polarity and intercellular junction stability. Also, LKB1 controls the activity of salt-inducible kinase 1 (SIK1). The role and relevance of SIK1 and its downstream effectors in linking the LKB1 signals within these processes are partially understood. We hypothesize that SIK1 may link LKB1 signals to the maintenance of epithelial junction stability by regulating E-cadherin expression. Results from our studies using a mouse lung alveolar epithelial (MLE-12) cell line or human renal proximal tubule (HK2) cell line transiently or stably lacking the expression of SIK1 (using SIK1 siRNAs or shRNAs), or with its expression abrogated (sik1+/+ vs. sik1+/+ mice), indicate that suppression of SIK1 (~40%) increases the expression of the transcriptional repressors Snail2 (~12-fold), Zebl (~100%), Zeb2 (~50%), and TWIST (~20-fold) by activating cAMP-response element binding protein. The lack of SIK1 and activation of transcriptional repressors decreases the availability of E-cadherin (mRNA and protein expression by ~100 and 80%, respectively) and the stability of intercellular junctions in epithelia (decreases in transepithelial resistance). Furthermore, LKB1-mediated increases in E-cadherin expression are impaired in cells where SIK1 has been disabled. We conclude that SIK1 is a key regulator of E-cadherin expression, and thereby contributes to the stability of intercellular junctions. [ABSTRACT FROM AUTHOR]

Cardiac hypertrophy (CH) generally occurs as the result of the sustained mechanical stress caused by elevated systemic arterial blood pressure (BP). However, in animal models, elevated salt intake is associated with CH even in the absence of significant increases in BP. We hypothesize that CH is not exclusively the consequence of mechanical stress but also of other factors associated with elevated BP such as abnormal cell sodium homeostasis. We examined the effect of small increases in intracellular sodium concentration ([Na+]i) on transcription factors and genes associated with CH in a cardiac cell line. Increases in [Na+]i led to a timedependent increase in the expression levels of mRNA for natriuretic peptide and myosin heavy chain genes and also increased myocyte enhancer factor (MEF)2/nuclear factor of activated T cell (NFAT) transcriptional activity. Increases in [Na+]i are associated with activation of salt-inducible kinase 1 (snflk-1, SIK1), a kinase known to be critical for cardiac development. Moreover, increases in [Na+]i resulted in increased SIK1 expression. Sodium did not increase MEF2/ NFAT activity or gene expression in cells expressing a SIK1 that lacked kinase activity. The mechanism by which SIK1 activated MEF2 involved phosphorylation of HDAC5. Increases in [Na+]i activate SIK1 and MEF2 via a parallel increase in intracellular calcium through the reverse mode of Na+/Ca2+-exchanger and activation of CaMK1. These data obtained in a cardiac cell line suggest that increases in intracellular sodium could influence myocardial growth by controlling transcriptional activation and gene expression throughout the activation of the SIK1 network. [ABSTRACT FROM AUTHOR]

Abstract: Salt-inducible kinase 1 (SIK1) in epithelial cells mediates the increases in active sodium transport (Na+, K+-ATPase-mediated) in response to elevations in the intracellular concentration of sodium. In lung alveolar epithelial cells increases in active sodium transport in response to β-adrenergic stimulation increases pulmonary edema clearance. Therefore, we sought to determine whether SIK1 is present in lung epithelial cells and to examine whether isoproterenol-dependent stimulation of Na+, K+-ATPase is mediated via SIK1 activity. All three SIK isoforms were present in airway epithelial cells, and in alveolar epithelial cells type 1 and type 2 from rat and mouse lungs, as well as from human and mouse cell lines representative of lung alveolar epithelium. In mouse lung epithelial cells, SIK1 associated with the Na+, K+-ATPase α-subunit, and isoproterenol increased SIK1 activity. Isoproterenol increased Na+, K+-ATPase activity and the incorporation of Na+, K+-ATPase molecules at the plasma membrane. Furthermore, those effects were abolished in cells depleted of SIK1 using shRNA, or in cells overexpressing a SIK1 kinase-deficient mutant. These results provide evidence that SIK1 is present in lung epithelial cells and that its function is relevant for the action of isoproterenol during regulation of active sodium transport. As such, SIK1 may constitute an important target for drug discovery aimed at improving the clearance of pulmonary edema. [Copyright &y& Elsevier]

In mammalian cells, active sodium transport and its derived functions (e.g., plasma membrane potential) are dictated by the activity of the Na+,K+-ATPase (NK), whose regulation is essential for maintaining cell volume and composition, as well as other vital cell functions. Here we report the existence of a salt-inducible kinase-1 (SIK1) that associates constitutively with the NK regulatory complex and is responsible for increases in its catalytic activity following small elevations in intracellular sodium concentrations. In- creases in intracellular sodium are paralleled by elevations in intracellular calcium through the reversible Na+/Ca2+ exchanger, leading to the activation of SIK1 (Thr-322 phosphorylation) by a calcium calmodulin-dependent kinase. Activation of SIK1 results in the dephosphorylation of the NK tv-subunit and an increase in its catalytic activity. A protein phosphatase 2A/phosphatase methylesterase-1 (PME-1) complex, which constitutively associates with the NK α-subunit, is activated by SIKI through phosphorylation of PME-1 and its dissociation from the complex. These observations illustrate the existence of a distinct intracellular signaling network, with SIK1 at its core, which is triggered by a monovalent cation (Na+) and links sodium permeability to its active transport. [ABSTRACT FROM AUTHOR]

Abstract: Dopamine is a major regulator of sodium reabsorption in proximal tubule epithelia. It induces the endocytosis of plasma membrane Na,K-ATPase molecules, and this results in a reduced capacity of the cells to transport sodium. Dopamine induces the phosphorylation of Ser-18 in the α1-subunit of Na,K-ATPase. Fluorescence resonance energy transfer analysis of cells expressing YFP-α1 and β1-CFP reveals that treatment of the cells with dopamine increases energy transfer between CFP and YFP. This is consistent with a protein conformational change that results in the N-terminal end of α1 moving closer to the internal face of the plasma membrane. [Copyright &y& Elsevier]

Clathrin-dependent endocytosis of Na+,K+-ATPase molecules in response to G protein-coupled receptor signals is triggered by phosphorylation of the α-subunit and the binding of phosphoinositide 3-kinase. In this study, we describe a molecular mechanism linking phosphorylation of Na+,K+-ATPase α-subunit to binding and activation of phosphoinositide 3-kinase. Co-immunoprecipitation studies, as well as experiments using confocal microscopy, revealed that dopamine favored the association of 14-3-3 protein with the basolateral plasma membrane and its co-localization with the Na+,K+-ATPase α-subunit. The functional relevance of this interaction was established in opossum kidney cells expressing a 14-3-3 dominant negative mutant, where dopamine failed to decrease Na+,K+-ATPase activity and to promote its endocytosis. The phosphorylated Ser-18 residue within the α-subunit N terminus is critical for 14-3-3 binding. Activation of phosphoinositide 3-kinase by dopamine during Na+,K+-ATPase endocytosis requires the binding of the kinase to a proline-rich domain within the α-subunit, and this effect was blocked by the presence of a 14-3-3 dominant negative mutant. Thus, the 14-3-3 protein represents a critical linking mechanism for recruiting phosphoinositide 3-kinase to the site of Na+,K+-ATPase endocytosis. [ABSTRACT FROM AUTHOR]

Glucose-dependent exocytosis of insulin requires activation of protein kinase C (PKC). However, because of the great variety of isoforms and their ubiquitous distribution within the β-cell, it is difficult to predict the importance of a particular isoform and its mode of action. Previous data revealed that two PKC isoforms (α and ∈) translocate to membranes in response to glucose (Zaitzev, S. V., Efendic, S., Arkhammar, P., Bertorello, A. M., and Berggren, P. O. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 9712-9716). Using confocal microscopy, we have now established that in response to glucose, PKC-∈ but not PKC-α associates with insulin granules and that green fluorescent protein-tagged PKC-∈ changes its distribution within the cell periphery upon stimulation of β-cells with glucose. Definite evidence of PKC-∈ requirement during insulin granule exocytosis was obtained by using a dominant negative mutant of this isoform. The presence of this mutant abolished glucose-induced insulin secretion, whereas transient expression of the wildtype PKC-∈ led to a significant increase in insulin exocytosis. These results suggest that association of PKC-∈ with insulin granule membranes represents an important component of the secretory network because it is essential for insulin exocytosis in response to glucose. [ABSTRACT FROM AUTHOR]

The balance and cross-talk between natriuretic and antinatriuretic hormone receptors plays a critical role in the regulation of renal Na[sup +] homeostasis, which is a majot determinant of blood pressure. Dopamine and angiotensin II have antagonistic effects on renal Na[sup +] and water excretion, which involves regulation of the Na[sup +],K[sup +]-ATPase activity. Herein we demonstrate that angiotensin II (Ang II) stimulation of AT1 receptors in proximal tubule cells induces the recruitment of Na[sup +],K[sup +]-ATPase molecules to the plasmalemma, in a process mediated by protein kinase Cβ and interaction of the Na[sup +],K[sup +]-ATPase with adaptor protein 1. Ang II stimulation led to phosphorylation of the α subunit Ser-11 and Ser-18 residues, and substitution of these amino acids with alanine residues completely abolished the Ang II-induced stimulation of Na[sup +],K[sup +]-ATPase-mediated Rb[sup +] transport. Thus, for Ang II-dependent stimulation of Na[sup +],K[sup +]-ATPase activity, phosphorylation of these serine residues is essential and may constitute a triggering signal for recruitment of Na[sup +],K[sup +]-ATPase molecules to the plasma membrane. When cells were treated simultaneously with saturating concentrations of dopamine and Ang II, either activation or inhibition of the Na[sup +],K[sup +]-ATPase activity was produced dependent on the intracellular Na[sup +] concentration, which was varied in a very narrow physiological range (9-19 mM). A small increase in intracellular Na[sup +] concentrations induces the recruitment of D1 receptors to the plasma membrane and a reduction in plasma membrane AT1 receptors. Thus, one or more proteins may act as an intracellular Na[sup +] concentration sensor and play a major regulatory role on the effect of hormones that regulate proximal tubule Na[sup +] reabsorption. [ABSTRACT FROM AUTHOR]

Insulin secretion is controlled by the β cell's metabolic state, and the ability of the secretory granules to undergo exocytosis increases during glucose stimulation in a membrane potential-independent fashion. Here, we demonstrate that exocytosis of insulincontaining secretory granules depends on phosphatidylinositol 4-kinase (PI 4-kinase) activity and that inhibition of this enzyme suppresses glucose-stimulated insulin secretion. Intracellular application of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P[sub 2]] stimulated exocytosis by promoting the priming of secretory granules for release and increasing the number of granules residing in a readily releasable pool. Reducing the cytoplasmic ADP concentration in a way mimicking the effects of glucose stimulation activated PI 4-kinase and increased exocytosis whereas changes of the ATP concentration in the physiological range had little effect. The PI(4,5)P[sub 2]-binding protein Ca[sup 2+]-dependent activator protein for secretion (CAPS) is present in β cells, and neutralization of the protein abolished both Ca[sup 2+]- and PI(4,5)P[sub 2]-induced exocytosis. We conclude that ADPinduced changes in PI 4-kinase activity, via generation of PI(4,5)P[sub 2], represents a metabolic sensor in the β cell by virtue of its capacity to regulate the release competence of the secretory granules. [ABSTRACT FROM AUTHOR]

During ascent to high altitude and pulmonary edema, the alveolar epithelial cells (AEC) are exposed to hypoxic conditions. Hypoxia inhibits alveolar fluid reabsorption and decreases Na,K-ATPase activity in AEC. We report here that exposure of AEC to hypoxia induced a time-dependent decrease of Na,K-ATPase activity and a parallel decrease in the number of Na,K-ATPase alpha(1) subunits at the basolateral membrane (BLM), without changing its total cell protein abundance. These effects were reversible upon reoxygenation and specific, because the plasma membrane protein GLUT1 did not decrease in response to hypoxia. Hypoxia caused an increase in mitochondrial reactive oxygen species (ROS) levels that was inhibited by antioxidants. Antioxidants prevented the hypoxia-mediated decrease in Na,K-ATPase activity and protein abundance at the BLM. Hypoxia-treated AEC deficient in mitochondrial DNA (rho(0) cells) did not have increased levels of ROS, nor was the Na,K-ATPase activity inhibited. Na,K-ATPase alpha(1) subunit was phosphorylated by PKC in hypoxia-treated AEC. In AEC treated with a PKC-zeta antagonist peptide or with the Na,K-ATPase alpha(1) subunit lacking the PKC phosphorylation site (Ser-18), hypoxia failed to decrease Na,K-ATPase abundance and function. Accordingly, we provide evidence that hypoxia decreases Na,K-ATPase activity in AEC by triggering its endocytosis through mitochondrial ROS and PKC-zeta-mediated phosphorylation of the Na,K-ATPase alpha(1) subunit. [ABSTRACT FROM AUTHOR]

1. The present study demonstrates that stimulation of hormonal receptors of proximal tubule cells with the serotonin-agonist 8-hydroxy-2-(di-n-propylamino) tetraline (8-OH-DPAT) induces an augmentation of Na(+),K(+)-ATPase activity that results from the recruitment of enzyme molecules to the plasmalemma. 2. Cells expressing the rodent wild-type Na(+),K(+)-ATPase alpha-subunit had the same basal Na(+),K(+)-ATPase activity as cells expressing the alpha-subunit S11A or S18A mutants, but stimulation of Na(+),K(+)-ATPase activity was completely abolished in either mutant. 3. 8-OH-DPAT treatment of OK cells led to PKC(beta)-dependent phosphorylation of the alpha-subunit Ser-11 and Ser-18 residues, and determination of enzyme activity with the S11A and S18A mutants indicated that both residues are essential for the agonist-dependent stimulation of Na(+),K(+)-ATPase activity. 4. When cells were treated with both dopamine and 8-OH-DPAT, an activation of Na(+),K(+)-ATPase was observed at basal intracellular sodium concentration (approximately 9 mM), and this activation was gradually reduced and became a significant inhibition as the concentration of intracellular sodium gradually increased from 9 to 19 mM. Thus, besides the antagonistic effects of dopamine and 8-OH-DPAT, intracellular sodium modulates whether an activation or an inhibition of Na(+),K(+)-ATPase is produced. [ABSTRACT FROM AUTHOR]

1 The present study demonstrates that stimulation of hormonal receptors of proximal tubule cells with the serotonin-agonist 8-hydroxy-2-(di-n-propylamino) tetraline (8-OH-DPAT) induces an augmentation of Na[SUP+], K[SUP+]-ATPase activity that results from the recruitment of enzyme molecules to the plasmalemma. 2 Cells expressing the rodent wild-type Na[SUP+], K[SUP+]-ATPase α-subunit had the same basal Na[SUP+], K[SUP+]-ATPase activity as cells expressing the α-subunit S11A S18A mutants, but stimulation of Na[SUP+], K[SUP+]-ATPase activity was completely abolished in either mutant. 3 8-OH-DPAT treatment of OK cells led to PKC-dependent phosphorylation of the α-subunit Ser-11 and Ser-18 residues, and determination of enzyme activity with the S11A and S18A mutants indicated that both residues are essential for the agonist-dependent stimulation of Na[SUP+], K[SUP+]-ATPase activity. 4 When cells were treated with both dopamine and 8-OH-DPAT, and activation of Na[SUP+], K[SUP+]-ATPase was observed at basal intracellular sodium concentration (∼9 mM), and this activation was gradually reduced and became a significant inhibition as the concentration of intracellular sodium gradually increased from 9 to 19 mM. Thus, besides the antagonistic effects of dopamine and 8-OH-DPAT, intracellular sodium modulates whether an activation or an inhibition of Na[SUP+], K[SUP+]-ATPase is produced. [ABSTRACT FROM AUTHOR]