Effects of hypokalemia on the properties and expression of the (Na+,K+)-ATPase of rat skeletal muscle

Inactivity and exercise training differentially regulate abundance of Na+-K+-ATPase in human skeletal muscle

Physical inactivity is a global health risk that can be addressed through application of exercise training suitable for an individual's health and age. People's willingness to participate in physical activity is often limited by an initially poor physical capability and early onset of fatigue. One factor associated with muscle fatigue during intense contractions is an inexcitability of skeletal muscle cells, reflecting impaired transmembrane Na⁺/K⁺ exchange and membrane depolarization, which are regulated via the transmembranous protein Na⁺-K⁺-ATPase (NKA). This short review focuses on the plasticity of NKA in skeletal muscle in humans after periods of altered usage, exploring NKA upregulation with exercise training and downregulation with physical inactivity. In human skeletal muscle, the NKA content quantified by [³H]ouabain binding site content shows robust, yet tightly constrained, upregulation of 8-22% with physical training, across a broad range of exercise training types. Muscle NKA content in humans undergoes extensive downregulation with injury that involves substantial muscular inactivity. Surprisingly, however, no reduction in NKA content was found in the single study that investigated short-term disuse. Despite clear findings that exercise training and injury modulate NKA content, the adaptability of the individual NKA isoforms in muscle (α_1-3 and β_1-3) and of the accessory and regulatory protein FXYD1 are surprisingly inconsistent across studies, for exercise training as well as for injury/disuse. Potential reasons for this are explored. Finally, we provide suggestions for future studies to provide greater understanding of NKA regulation during exercise training and inactivity in humans.

Cyclic stretch stimulates recruitment of active Na⁺/K⁺-ATPase subunits to the plasma membrane of skeletal muscle cells

Cyclic stretch increases Na(+)/K(+)-ATPase activity and abundance in several tissues, including skeletal muscle cells. The present study was undertaken to investigate whether Na(+)/K(+)-ATPase undergoes acute changes in its catalytic activity in response to cyclic stretch. Na(+)/K(+)-ATPase activity increased after continuously stretched for 6 h, and reached the maximum at 24 h. The inhibition of gene transcription (actinomycin D) had no effect on stretch-induced Na(+)/K(+)-ATPase activity. Cyclic stretch also increases the plasma membrane content of α(1)- and α(2)-subunit of Na(+)/K(+)-ATPase. Brefeldin A could completely abolished the stretch-induced recruitment of α-subunits to the plasma membrane and Na(+)/K(+)-ATPase activity. In conclusion, cyclic stretch directly stimulates Na(+)/K(+)-ATPase activity in skeletal muscle cells through post-transcriptional activation, likely by increasing translocation of Na(+)/K(+)-ATPase molecules to plasma membrane.

Aldosterone increases Na+ -K+ -ATPase activity in skeletal muscle of patients with Conn's syndrome

OBJECTIVE

In Conn's syndrome, hypokalaemia normally results from renal potassium loss because of the effect of excess aldosterone on Na(+) -K(+) -ATPase in principal cells. Little is known about the effect of aldosterone on cellular potassium redistribution in skeletal muscle. Our study determined the effect of aldosterone on muscle Na(+) -K(+) -ATPase.

DESIGN

Muscle biopsies were taken from six patients immediately before and 1 month after adrenalectomy. Ten age-matched subjects with normal levels of circulating aldosterone served as controls.

RESULTS

  Average plasma aldosterone was significantly higher in presurgery (235·0 ± 51·1 pg/ml) than postsurgery (64·5 ± 25·1 pg/ml) patients. Similarly, Na(+) -K(+) -ATPase activity, relative mRNA expression of α(2) (not α(1) or α(3) ) and β(1) (not β(2) or β(3) ), and protein abundance of α(2) and β(1) subunits were greater in pre- than postsurgery samples (128·7 ± 12·3 vs 79·4 ± 13·3 nmol·mg/protein/h, 2·45 ± 0·31 vs 1·04 ± 0·17, 1·92 ± 0·22 vs1·02 ± 0·14, 2·17 ± 0·33 vs 0·98 ± 0·09 and 1·70 ± 0·17 vs 0·90 ± 0·17, respectively, all P<0·05). The activity and mRNA expression of the α(2) and β(1) subunits correlated well with plasma aldosterone levels (r = 0·71, r = 0·75 and r = 0·78, respectively, all P < 0·01).

CONCLUSIONS

  Our study provides the first evidence in human skeletal muscle that increased plasma aldosterone leads to increased Na(+) -K(+) -ATPase activity via increases in α(2) and β(1) subunit mRNAs and their protein expressions. The increased activity may contribute in part to the induction of hypokalaemia in patients with Conn's syndrome.

Na+-K+ Pump Regulation and Skeletal Muscle Contractility

Clausen, Torben. Na+-K+ Pump Regulation and Skeletal Muscle Contractility. Physiol Rev 83: 1269-1324, 2003; 10.1152/physrev.00011.2003.—In skeletal muscle, excitation may cause loss of K+, increased extracellular K+ ([K+]o), intracellular Na+ ([Na+]i), and depolarization. Since these events interfere with excitability, the processes of excitation can be self-limiting. During work, therefore, the impending loss of excitability has to be counterbalanced by prompt restoration of Na+-K+ gradients. Since this is the major function of the Na+-K+ pumps, it is crucial that their activity and capacity are adequate. This is achieved in two ways: 1) by acute activation of the Na+-K+ pumps and 2) by long-term regulation of Na+-K+ pump content or capacity. 1) Depending on frequency of stimulation, excitation may activate up to all of the Na+-K+ pumps available within 10 s, causing up to 22-fold increase in Na+ efflux. Activation of the Na+-K+ pumps by hormones is slower and less pronounced. When muscles are inhibited by high [K+]o or low [Na+]o, acute hormone- or excitation-induced activation of the Na+-K+ pumps can restore excitability and contractile force in 10-20 min. Conversely, inhibition of the Na+-K+ pumps by ouabain leads to progressive loss of contractility and endurance. 2) Na+-K+ pump content is upregulated by training, thyroid hormones, insulin, glucocorticoids, and K+ overload. Downregulation is seen during immobilization, K+ deficiency, hypoxia, heart failure, hypothyroidism, starvation, diabetes, alcoholism, myotonic dystrophy, and McArdle disease. Reduced Na+-K+ pump content leads to loss of contractility and endurance, possibly contributing to the fatigue associated with several of these conditions. Increasing excitation-induced Na+ influx by augmenting the open-time or the content of Na+ channels reduces contractile endurance. Excitability and contractility depend on the ratio between passive Na+-K+ leaks and Na+-K+ pump activity, the passive leaks often playing a dominant role. The Na+-K+ pump is a central target for regulation of Na+-K+ distribution and excitability, essential for second-to-second ongoing maintenance of excitability during work.

Training-induced changes in skeletal muscle Na+-K+ pump number and isoform expression in rats with chronic heart failure

The mechanisms responsible for the decrements in exercise performance in chronic heart failure (CHF) remain poorly understood, but it has been suggested that sarcolemmal alterations could contribute to the early onset of muscular fatigue. Previously, our laboratory demonstrated that the maximal number of ouabain binding sites (B(max)) is reduced in the skeletal muscle of rats with CHF (Musch TI, Wolfram S, Hageman KS, and Pickar JG. J Appl Physiol 92: 2326-2334, 2002). These reductions may coincide with changes in the Na(+)-K(+)-ATPase isoform (alpha and beta) expression. In the present study, we tested the hypothesis that reductions in B(max) would coincide with alterations in the alpha- and beta-subunit expression of the sarcolemmal Na(+)-K(+)-ATPase of rats with CHF. Moreover, we tested the hypothesis that exercise training would increase B(max) along with producing significant changes in alpha- and beta-subunit expression. Rats underwent a sham operation (sham; n = 10) or a surgically induced myocardial infarction followed by random assignment to either a control (MI; n = 16) or exercise training group (MI-T; n = 16). The MI-T rats performed exercise training (ET) for 6-8 wk. Hemodynamic indexes demonstrated that MI and MI-T rats suffered from severe left ventricular dysfunction and congestive CHF. Maximal oxygen uptake (Vo(2 max)) and endurance capacity (run time to fatigue) were reduced in MI rats compared with sham. B(max) in the soleus and plantaris muscles and the expression of the alpha(2)-isoform of the Na(+)-K(+)-ATPase in the red portion of the gastrocnemius (gastrocnemius(red)) muscle were reduced in MI rats. After ET, Vo(2 max) and run time to fatigue were increased in the MI-T group of rats. This coincided with increases in soleus and plantaris B(max) and the expression of the alpha(2)-isoform in the gastrocnemius(red) muscle. In addition, the expression of the beta(2)-isoform of the gastrocnemius(red) muscle was increased in the MI-T rats compared with their sedentary counterparts. This study demonstrates that CHF-induced alterations in skeletal muscle Na(+)-K(+)-ATPase, including B(max) and isoform expression, can be partially reversed by ET.

Excitation- and beta(2)-agonist-induced activation of the Na(+)-K(+) pump in rat soleus muscle

In rat skeletal muscle, Na(+)-K(+) pump activity increases dramatically in response to excitation (up to 20-fold) or beta(2)-agonists (2-fold), leading to a reduction in intracellular Na(+). This study examines the time course of these effects and whether they are due to an increased affinity of the Na(+)-K(+) pump for intracellular Na(+). Isolated rat soleus muscles were incubated at 30 (o)C in Krebs-Ringer bicarbonate buffer. The effects of direct electrical stimulation on (86)Rb(+) uptake rate and intracellular Na(+) concentration ([Na(+)](i)) were characterized in the subsequent recovery phase. [Na(+)](i) was varied using monensin or buffers with low Na(+). In the [Na(+)](i) range 21-69 mM, both the beta(2)-agonist salbutamol and electrical stimulation produced a left shift of the curves relating (86)Rb(+) uptake rate to [Na(+)](i). In the first 10 s after 1 or 10 s pulse trains of 60 Hz, [Na(+)](i) showed no increase, but (86)Rb(+) uptake rate increased by 22 and 86 %, respectively. Muscles excited in Na(+)-free Li(+)-substituted buffer and subsequently allowed to rest in standard buffer also showed a significant increase in (86)Rb(+) uptake rate and decrease in [Na(+)](i). Na(+) loading induced by monensin or electroporation also stimulated (86)Rb(+) uptake rate but, contrary to excitation, increased [Na(+)](i). The increase in the rate of (86)Rb(+) uptake elicited by electrical stimulation was abolished by ouabain, but not by bumetanide. The results indicate that excitation (like salbutamol) induces a rapid increase in the affinity of the Na(+)-K(+) pump for intracellular Na(+). This leads to a Na(+)-K(+) pump activation that does not require Na(+) influx, but possibly the generation of action potentials. This improves restoration of the Na(+)-K(+) homeostasis during work and optimizes excitability and contractile performance of the working muscle.

Hyperaldosteronemia in rabbits inhibits the cardiac sarcolemmal Na(+)-K(+) pump

Aldosterone upregulates the Na(+)-K(+) pump in kidney and colon, classical target organs for the hormone. An effect on pump function in the heart is not firmly established. Because the myocardium contains mineralocorticoid receptors, we examined whether aldosterone has an effect on Na(+)-K(+) pump function in cardiac myocytes. Myocytes were isolated from rabbits given aldosterone via osmotic minipumps and from controls. Electrogenic Na(+)-K(+) pump current, arising from the 3:2 Na(+):K(+) exchange ratio, was measured in single myocytes using the whole-cell patch clamp technique. Treatment with aldosterone induced a decrease in pump current measured when myocytes were dialyzed with patch pipette solution containing Na(+) in a concentration of 10 mmol/L, whereas there was no effect measured when the solution contained 80 mmol/L Na(+). Aldosterone had no effect on myocardial Na(+)-K(+) pump concentration evaluated by vanadate-facilitated [(3)H]ouabain binding or by K(+)-dependent paranitrophenylphosphatase activity in crude homogenates. Aldosterone induced an increase in intracellular Na(+) activity. The aldosterone-induced decrease in pump current and increased intracellular Na(+) were prevented by cotreatment with the mineralocorticoid receptor antagonist spironolactone. Our results indicate that hyperaldosteronemia decreases the apparent Na(+) affinity of the Na(+)-K(+) pump, whereas it has no effect on maximal pump capacity.

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.

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+.

Translational regulation of Na,K-ATPase alpha1 and beta1 polypeptide expression in epithelial cells

To investigate the regulation of the Na,K-ATPase, we have studied the expression of the Na,K-ATPase polypeptides in several mammalian cell lines using the vaccinia virus/T7 RNA polymerase expression system. Infection of several fibroblast-like cell lines with viral recombinants containing the Na,K-ATPase alpha and beta isoforms, the glucose transporters, GLUT 1 and GLUT 4, or the capsid protein of the Sindbis virus all result in the production of the appropriate protein products. However, all epithelial cell lines tested fail to synthesize the Na,K-ATPase viral recombinants, yet they efficiently express the other virally directed polypeptides. While Madin-Darby canine kidney (MDCK) epithelial cells infected with the Na,K-ATPase alpha1 or beta1 recombinant viruses produce both mRNAs, the messages are inefficiently translated. Furthermore, the RNA from infected MDCK cells does not direct the in vitro synthesis of the beta1 polypeptide, whereas the message from infected fibroblast-like BSC 40 cells is efficiently translated both in vivo and in vitro. Moreover, the synthesis of the H,K-ATPase alpha subunit is also limited in MDCK cells, although the H,K-ATPase beta subunit is efficiently expressed. Expression of chimeras constructed between the Na+ pump beta1 isoform and the H,K-ATPase beta subunit indicates that sequences in the 5' coding region of the beta1 message have an inhibitory effect; however, the stringent translational regulation of the beta1 isoform in MDCK cells requires the 5' and 3' regions of the coding sequence. The ability of the polarized cell lines to limit the synthesis of the Na+ pump polypeptides while expressing other vaccinia recombinants at high levels suggests that the polarized cells possess a stringent mechanism for the specific translational regulation of a select set of messages.

Cis-elements involved in differential expression of Na(+)-K(+)-ATPase alpha 2 subunit gene in muscle differentiation

The mRNA of the Na(+)-K(+)-ATPase alpha 2 subunit gene (Atpla2) increases during myogenesis in rat following birth. To investigate the mechanism of the transcriptional regulation underlying this process, we examined the promoter activity of Atpla2 by transient transfection assays in myocyte differentiation, using C2C12 murine skeletal myoblast cell line. The promoter activity was increased about 5-fold in the differentiated cells, comparing with that in the growing cells. By analyses of 5'-deletion mutations of the gene, the responsible elements were localized in the region between -108 and +60. We identified a binding factor that exists only in the nuclear extract from the growing cells but not in the extract from the differentiated cells with a probe DNA fragment from +20 to +50 by gel retardation assays. The element to which the factor binds can mediate the inhibition of alpha 2 gene expression in the growing C2C12 cells but not in the differentiated cells. Finally we revealed protein occupancy in this region in the growing cells by in vivo footprinting. We propose a model that the increased expression of Atpla2 during muscle differentiation is mediated by the disappearance of a negative factor.

Tissue-specific modulation of Na, K-ATPase alpha-subunit gene expression in uremic rats

Chronic renal failure in the rat is associated with an impaired extrarenal potassium handling, whereas a renal adaptive mechanism of the remaining nephrons has been described. To understand the molecular basis of potassium homeostasis during renal failure we investigated the in vitro pump activity and the catalytic mRNA transcription in three different tissues: skeletal muscle, isolated adipocytes and kidney. The activity of the sodium pump, as measured by ouabain-sensitive 86Rb/K uptake in isolated adipocytes and skeletal muscle fibers, revealed a significant reduction of the pump activity in uremic rats. The reduction of the Na, K-ATPase activity in adipose tissue was associated with a similar decrement of both catalytic subunits (alpha 1 and alpha 2), whereas in the skeletal muscle tissue was only related to a decrease in the activity of the alpha 1 isoform. The expression of rat Na, K-ATPase catalytic isoforms mRNAs in kidney, muscle and adipose tissue from control and chronic renal failure rats was investigated at the molecular level with cDNA probes specific for the catalytic isoforms (alpha 1 and alpha 2). Northern blot analysis revealed that the respective catalytic mRNAs of uremic rats are regulated in a tissue-specific manner that are in agreement with the sodium-potassium pump activity. Muscle and adipose tissue showed a decrement in the levels of expression for the alpha 1 isoform mRNA. In contrast to these tissues, an increment in alpha 1 mRNA expression was observed in the kidney of rats with chronic renal failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate up-regulates alpha 1 and alpha 2 subunits of the sodium pump in astrocytes of mixed telencephalic cultures but not in pure astrocyte cultures

Prior work employing an in vitro model of the cerebral cortex has shown that sodium pump activity is a critical determinant for neuronal survival of glutamate stimulation. We have hypothesized that up-regulation of total brain sodium pump activity will protect against potential excitotoxins. Increased sodium pump activity could theoretically occur by changes in the reaction rate (short-term) and/or by increased levels of sodium pump protein (long-term) and is potentially complex since the three catalytic (a) subunit isoforms of the sodium pump are distributed in a highly variable, cell-specific pattern in the brain. Short-term regulation (seconds to minutes) has been well studied: brain sodium pump exhibits a large dynamic range. In contrast, the possibility of long-term modulation of sodium pump activity has not been extensively explored. We used isoform specific antibodies and [3H]ouabain binding to determine whether prolonged stimulation of sodium pump activity in rodent telencephalic cultures increased total sodium pump enzyme. Exposure of mixed neuronal-glial cultures to high levels of glutamate (10 mM) for 18 h, which is highly toxic to neurons, was associated with an approximately 80% increase in alpha 1 and alpha 2 subunit expression by glia. Induction of alpha 2 subunit immunoreactivity was also associated with comparable changes in [3H]ouabain binding, suggesting that the up-regulation corresponded to functional alpha 2 protein. Shorter (30 min) glutamate treatments, which also killed neurons, did not produce similar changes in sodium pump expression. In contrast to mixed cultures, pure astrocyte cultures had undetectable alpha 2 and alpha 3 and moderate levels of alpha 1 protein, as confirmed by low levels of [3H]ouabain binding. Glutamate treatment using this protocol was associated with a decrease in alpha 1 sodium pump expression. We conclude that long-term regulation of the sodium pump can be demonstrated in glia which have developed in the presence of neurons. Both alpha 1 and alpha 2 isoforms of the sodium pump are involved in this response to glutamate.

Physiologic rationale for multiple sodium pump isoforms. Differential regulation of alpha 1 vs alpha 2 by ionic stimuli

A number of important themes emerge from our compartmental analyses of Na,K-ATPase biosynthesis in response to ionic stimuli. The ubiquitous alpha 1 beta 1 type sodium pump evolved to generate and maintain transmembrane Na+ and K+ gradients, and there are cell-type specific mechanisms of increasing synthesis and decreasing degradation to control surface expression of this important "housekeeping" enzyme. Expression of alpha 2 beta-type sodium pumps may have evolved in cells designated as K+ storehouses to facilitate maintenance of extracellular K+ in the presence of K+ restriction. Finally, the specialized distribution of Na,K-ATPase (and related E1-E2 type pumps) along the renal epithelia allows for monitoring and fine control of extracellular K+ and Na+ (volume). Many interesting questions remain to be answered, and we now have the probes and techniques needed to answer them.