Complete quantification of the total concentration of rat skeletal-muscle Na+ + K+-dependent ATPase by measurements of [3H]ouabain binding

Specificity and reversibility of the training effects on the concentration of Na+, K+-ATPase in foal skeletal muscle

The purpose of the present study was to determine whether training and detraining affect the Na+,K+-ATPase concentration in horse skeletal muscles, and whether these effects are specific for the muscles involved in the training programme. Twenty-four Dutch Warmblood foals age 7 days were assigned randomly to 3 groups: Box (box-rest without training), Training (box-rest with training: short-sprint) and Pasture (pasture without training). Exercise regimens were carried out for 5 months and were followed by 6 months of detraining. Five of the foals in each group were subjected to euthanasia at age 5 months and the remaining foals at 11 months. Muscle samples were collected from the deep part of the gluteus medius, semitendinosus and masseter muscles. The Na+,K+-ATPase concentration was quantified by [3H]ouabain binding. In the Training group, the concentration of Na+,K+-ATPase in gluteus medius and semitendinosus muscle, but not in masseter muscle, showed a relative increase of 20% (P<0.05) as compared to Box foals. After detraining for the subsequent 6 months, the concentration of Na+,K+-ATPase in semitendinosus muscle remained the same, while that in gluteus medius muscle was reduced by 10%. It is concluded that: 1) short-sprint training for 5 months induced an increase of the Na+,K+-ATPase concentration in gluteus medius and semitendinosus muscles of the foal. Interestingly, this effect persisted during the 6 months of the detraining period. Whether the higher Na+,K+-ATPase concentration due to training of young foals leads to a better athletic performance when they become mature still needs to be established; 2) the factors that initiate an increase in Na+,K+-ATPase concentration following training are likely to be located in the muscle itself and 3) the training effect may last for several months after returning to normal activity, especially in muscles containing a high percentage of fast-twitch fibres.

Glucose supplements increase human muscle in vitro Na+-K+-ATPase activity during prolonged exercise

Regulation of maximal Na+-K+-ATPase activity in vastus lateralis muscle was investigated in response to prolonged exercise with (G) and without (NG) oral glucose supplements. Fifteen untrained volunteers (14 males and 1 female) with a peak aerobic power (V̇o2peak) of 44.8 ± 1.9 ml·kg−1·min−1; mean ± SE cycled at ∼57% V̇o2peak to fatigue during both NG (artificial sweeteners) and G (6.13 ± 0.09% glucose) in randomized order. Consumption of beverage began at 30 min and continued every 15 min until fatigue. Time to fatigue was increased ( P < 0.05) in G compared with NG (137 ± 7 vs. 115 ± 6 min). Maximal Na+-K+-ATPase activity (Vmax) as measured by the 3- O-methylfluorescein phosphatase assay (nmol·mg−1·h−1) was not different between conditions prior to exercise (85.2 ± 3.3 or 86.0 ± 3.9), at 30 min (91.4 ± 4.7 vs. 91.9 ± 4.1) and at fatigue (92.8 ± 4.3 vs. 100 ± 5.0) but was higher ( P < 0.05) in G at 90 min (86.7 ± 4.2 vs. 109 ± 4.1). Na+-K+-ATPase content (βmax) measured by the vanadate facilitated [3H]ouabain-binding technique (pmol/g wet wt) although elevated ( P < 0.05) by exercise (0<30, 90, and fatigue) was not different between NG and G. At 60 and 90 min of exercise, blood glucose was higher ( P < 0.05) in G compared with NG. The G condition also resulted in higher ( P < 0.05) serum insulin at similar time points to glucose and lower ( P < 0.05) plasma epinephrine and norepinephrine at 90 min of exercise and at fatigue. These results suggest that G results in an increase in Vmax by mechanisms that are unclear.

Membrane excitability, weakness, and fatigue

A failure in membrane excitability, defined as an inability of the sarcolemma and T-tubule to translate the neural discharge command into repetitive action potentials, represents an inviting cause of mechanical disfunction in both health and disease. A failure at this level would precipitate a disturbance in signal transmission between the T-tubule and the calcium release channels of the sarcoplasmic reticulum, resulting in reduced release of Ca2+, lower cytosolic free Ca2+ levels, and depressed myofibrillar activation and force generation. The ability of the sarcolemma and T-tubules to conduct repetitive action potentials is intimately dependent on active transport of Na+ and K+ following an action potential. The active transport of these cations is mediated by the Na+-K+-ATPase, an integral membrane protein that uses the energy from the hydrolysis of 1 ATP to transport 3 Na+ out of the cell and 2 K+ into the cell. A failure to recruit sufficient Na+-K+-ATPase activity during contractile activity could result in a rundown of the transmembrane gradients for Na+ and K+, leading to a loss of membrane excitability. The Na+-K+-ATPase activity depends on the amount and isoform composition of the protein, substrate availability, and acute regulatory factors. Each of these factors is examined as a potential cause of altered activation of the Na+-K+-ATPase activity and loss of membrane excitability in fatigue. Regular exercise represents a potent stimulus for upregulating Na+-K+-ATPase levels and for increasing the ability for cation transport across the sarcolemma and T-tubule membrane. As such, training may be a valuable tool in the management of fatigue in health and disease.

Potassium depletion improves myocardial potassium uptake in vivo

Potassium depletion (KD) is a very common clinical entity often associated with adverse cardiac effects. KD is generally considered to reduce muscular Na-K-ATPase density and secondarily reduce K uptake capacity. In KD rats we evaluated myocardial Na-K-ATPase density, ion content, and myocardial K reuptake. KD for 2 wk reduced plasma K to 1.8 +/- 0.1 vs. 3.5 +/- 0.2 mM in controls (P < 0.01, n = 7), myocardial K to 80 +/- 1 vs. 86 +/- 1 micromol/g wet wt (P < 0.05, n = 7), increased Mg, and induced a tendency to increased Na. Myocardial Na-K-ATPase alpha(2)-subunit abundance was reduced by approximately 30%, whereas increases in alpha(1)- and K-dependent pNPPase activity of 24% (n = 6) and 13% (n = 6), respectively, were seen. This indicates an overall upregulation of the myocardial Na-K pump pool. KD rats tolerated a higher intravenous KCl dose. KCl infusion until animals died increased myocardial K by 34% in KD rats and 18% in controls (P < 0.05, n = 6 for both) but did not induce different net K uptake rates between groups. However, clamping plasma K at approximately 5.5 mM by KCl infusion caused a higher net K uptake rate in KD rats (0.22 +/- 0.04 vs. 0.10 +/- 0.03 micromol x g wet wt(-1) x min(-1); P < 0.05, n = 8). In conclusion, a minor KD-induced decrease in myocardial K increased Na-K pump density and in vivo increased K tolerance and net myocardial K uptake rate during K repletion. Thus the heart is protected from major K losses and accumulates considerable amounts of K during exposure to high plasma K. This is of clinical interest, because a therapeutically induced rise in myocardial K may affect contractility and impulse generation-propagation and may attenuate increased myocardial Na, the hallmark of heart failure.

Na+-K+-ATPase in rat skeletal muscle: content, isoform, and activity characteristics

The purpose of this study was to investigate the hypothesis that muscle Na+-K+-ATPase activity is directly related to Na+-K+-ATPase content and the content of the alpha2-catalytic isoform in muscles of different fiber-type composition. To investigate this hypothesis, tissue was sampled from soleus (Sol), red gastrocnemius (RG), white gastrocnemius (WG), and extensor digitorum longus (EDL) muscles at rest from 38 male Wistar rats weighing 413 +/- 6.0 g (mean +/- SE). Na+-K+-ATPase activity was determined in homogenates (Hom) and isolated crude membranes (CM) by the regenerating ouabain-inhibitable hydrolytic activity assay (ATPase) and the 3-O-methylfluorescein K+-stimulated phosphatase (3-O-MFPase) assay in vitro. In addition, Na+-K+-ATPase content (Bmax) and the distribution of alpha1-, alpha2-, beta1-, and beta2-isoforms were determined by [3H]ouabain binding and Western blot, respectively. For the ATPase assay, differences (P < 0.05) in enzyme activity between muscles were observed in Hom (EDL > WG) and in CM (Sol > EDL = WG). For the 3-O-MFPase assay, differences (P < 0.05) were also found for Hom (Sol > RG = EDL > WG) and CM (Sol = WG > RG). For Bmax, differences in the order of RG = EDL > Sol = WG (P < 0.05) were observed. Isoform distribution was similar between Hom and CM and indicated in CM, a greater density (P < 0.05) of alpha1 in Sol than WG and EDL (P < 0.05), but more equal distribution of alpha2 between muscles. The beta1 was greater (P < 0.05) in Sol and RG, and the beta2 was greater in EDL and WG (P < 0.05). Over all muscles, the correlation (r) between Hom 3-O-MFPase and Bmax was 0.45 (P < 0.05) and between Hom alpha2 and Bmax, 0.59 (P < 0.05). The alpha1 distribution correlated to Hom 3-O-MFPase (r = 0.79, P < 0.05) CM ATPase (r = 0.69, P < 0.005) and CM 3-O-MFPase activity (r = 0.32, P < 0.05). The alpha2 distribution was not correlated with any of the Na+-K+-ATPase activity measurements. The results indicate generally poor relationships between activity and total pump content and alpha2 isoform content of the Na+-K+-ATPase. Several factors, including the type of preparation and the type of assay, appear important in this regard.

Severe intracellular magnesium and potassium depletion in patients after treatment with cisplatin

The purpose of this study is (1) to evaluate skeletal muscle magnesium (Mg) and potassium (K) during treatment with cisplatin; (2) to evaluate the predictive value of plasma (P)-Mg for intracellular Mg during cisplatin treatment; and (3) to evaluate whether changes in intracellular K influence skeletal muscle Na,K-ATPase. In all, 65 patients had a needle muscle biopsy obtained before and 26 patients both before and after cisplatin treatment. Biopsies were analysed for Mg, K, and Na,K-ATPase concentrations, and P-Mg and P-K determined. Treatment with a total dose of ≈500 mg (270 mg m⁻² surface area) cisplatin over 80 days was associated with reductions in muscle [Mg] (95% CI) (8.95 (8.23–9.63) to 7.76 (7.34–8.18) μmol g⁻¹ wet wt. (P<0.01), and muscle [K] (90.81 (83.29–98.34) to 82.87 (78.74–87.00) μmol g⁻¹ wet wt. (P<0.05), as well as in P-Mg 0.82 (0.80–0.85) to 0.68 (0.64–0.73) mmol l⁻¹ (P<0.01 but not in P-K (4.0 (3.8–4.1) vs 3.8 (3.7–4.0) mmol l⁻¹). No simple correlations were observed between P-Mg and muscle [Mg], or between P-K and muscle [K], either before (n=65) or after (n=26) treatment with cisplatin. The changes in [Mg] and [K] were not associated with changes in the muscle Na,K-ATPase concentration. Following treatment with cisplatin, an ≈15% decline in P-Mg was accompanied by an ≈15% loss of muscle [Mg], as well as an ≈10% reduction of muscle [K] and fatigue and muscle weakness previously ascribed to hypomagnesaemia may therefore also be well explained by muscle K depletion observed despite normal levels of P-K. There was no correlation between P-Mg and SM-Mg or between P-K and SM-K. Thus, P-Mg and P-K are not reliable indicators for Mg and K depletion during treatment with cisplatin. However, the majority of patients will present Mg and K depletion after cisplatin therapy and of these only very few patients will present a low P-Mg or P-K. Therefore, routine supplementation should be considered in all patients receiving cisplatin.

Effect of glucose/insulin infusion and magnesium supplementation on serum and muscle sodium and potassium and muscle [3H]ouabain binding capacity in Type 1 diabetes mellitus

Glucose has recently been found to decrease muscle potassium content. The aim of this study was to evaluate the effect of the infusion of glucose and insulin and the effect of magnesium supplementation on serum and muscle sodium and potassium and muscle [3H]ouabain binding capacity in patients with Type 1 diabetes mellitus and in controls. Muscle potassium and sodium content, muscle [3H]ouabain binding capacity and serum potassium and sodium concentrations were determined in 10 patients with Type 1 diabetes mellitus and in 5 controls before and after an euglycaemic, hyperinsulinaemic clamp, and after an intravenous magnesium load test. Nine of the patients with Type 1 diabetes mellitus were restudied after 24 weeks of oral magnesium oxide supplementation. Basic serum and muscle sodium and potassium and muscle [3H]ouabain binding capacity did not differ between groups. The infusion of glucose and insulin reduced muscle potassium content, whereas muscle sodium content was unchanged. There were no differences between groups. Oral magnesium oxide supplementation increased muscle potassium content by 6%. Muscle [3H]ouabain binding capacity was unchanged. In patients with Type 1 diabetes mellitus, the intravenous infusion of magnesium increased serum potassium concentration before but not after oral magnesium oxide supplementation. In controls, the infusion of magnesium did not affect serum potassium concentration. It was found that intravenous infusion of glucose and insulin decreases muscle potassium content, probably by shifting potassium from the muscle cells to the splanchnic organs. Oral magnesium oxide supplementation increases muscle potassium content in patients with Type 1 diabetes mellitus. The increase in serum potassium concentration owing to the intravenous infusion of magnesium could be used in the evaluation of magnesium status in patients with Type 1 diabetes mellitus. This, however, requires further investigation.

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.

Potassium depletion increases potassium clearance capacity in skeletal muscles in vivo during acute repletion

Muscular K uptake depends on skeletal muscle Na-K-ATPase concentration and activity. Reduced K uptake is observed in vitro in K-depleted rats. We evaluated skeletal muscle K clearance capacity in vivo in rats K depleted for 14 days. [(3)H]ouabain binding, alpha(1) and alpha(2) Na-K-ATPase isoform abundance, and K, Na, and Mg content were measured in skeletal muscles. Skeletal muscle K, Na, and Mg and plasma K were measured in relation to intravenous KCl infusion that continued until animals died, i.e., maximum KCl dose was administered. In soleus, extensor digitorum longus (EDL), and gastrocnemius muscles K depletion significantly reduced K content by 18%, 15%, and 19%, [(3)H]ouabain binding by 36%, 41%, and 68%, and alpha(2) isoform abundance by 34%, 44%, and 70%, respectively. No significant change was observed in alpha(1) isoform abundance. In EDL and gastrocnemius muscles K depletion significantly increased Na (48% and 59%) and Mg (10% and 17%) content, but only tendencies to increase were observed in soleus muscle. K-depleted rats tolerated up to a fourfold higher KCl dose. This was associated with a reduced rate of increase in plasma K and increases in soleus, EDL, and gastrocnemius muscle K of 56%, 42%, and 41%, respectively, but only tendencies to increase in controls. However, whereas K uptake was highest in K-depleted animals, the K uptake rate was highest in controls. In vivo K depletion is associated with markedly increased K tolerance and K clearance despite significantly reduced skeletal muscle Na-K-ATPase concentration. The concern of an increased risk for K intoxication during K repletion seems unwarranted.

Age, fatigue, and excitation-contraction coupling in masseter muscles of rats

The purpose of this study was to determine if masseter muscle endurance changes with increasing age and, if so, to examine mechanisms of fatigue. Characteristics of fatigue were measured under isometric conditions using high-frequency stimulation of anterior deep masseter (ADM) muscles of male Fischer 344 rats, 5 to 24 months old, and fed a hard (HD) or a soft (SD) diet. Potentiating effects of caffeine on ADM muscle performance in vitro were also examined. Fatigability increased by 48% with age in muscles of HD rats. Muscles of SD rats were highly fatigable at all ages. Increased HD fatigability was associated with significantly decreased concentrations of Na+/K+-adenosine triphosphatase (22%) and decreased responsiveness to caffeine postfatigue (29%). The pH levels decreased similarly in fatigued muscles of all groups. We conclude that the age-related increase in fatigability is associated with alterations in excitation-contraction coupling mechanisms. However, differences between SD and HD on ADM muscles represent possible fiber-type transitions.

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.

Skeletal muscle Na(+)-K(+)-ATPase and K+ homeostasis during exercise: effects of short-term training

The objective of this study was to determine the effects of 10 consecutive days of moderate intensity training on 1) the concentration of middle gluteal muscle Na(+)-K(+)-ATPase as determined by vanadate-facilitated 3H[ouabain binding; and 2) plasma potassium regulation before, during and after exercise at 100% of the pre-training maximum rate of oxygen consumption (VO2max). Six mature, unfit Thoroughbred horses completed both incremental (for determination of VO2max) and high-intensity exercise protocols before (HI1) and after (HI2) training. There additional horses undertook no training or exercise tests and served as controls for determination of middle gluteal muscle Na(+)-K(+)-ATPase concentration. Training consisted of 10 consecutive days of running at 55% VO2max for 60 min per day (13-14 km/day). For each high intensity exercise protocol, horses completed a 10 min warm-up at 50% VO2max, followed by exercise at 100% of pre-training VO2max (6 degrees incline, mean speed 9.8 m/s) until fatigue. Training resulted in a 13.8% increase in resting plasma volume (pre: 20.9 +/- 0.8 l; post: 23.8 +/- 0.9 l; P = 0.03), and an 8.9% increase in VO2max (pre: 142 +/- 4 ml/kg/min; post: 155 +/- 4 ml/kg/min; P = 0.004) during HI. Peak values for plasma potassium concentration and content during exercise decreased by 13% (P = 0.02) and 7% (P = 0.0002), respectively, after training whereas training had no effect on increases in packed cell volume, plasma total solids, and erythrocyte K+ concentration and content during exercise. Following training, there was also a significant (23%) increase in Na(+)-K(+)-ATPase concentration in biopsies of middle gluteal muscle, as measured by vanadate-facilitated 3H[ouabain binding. We conclude that 10 days of moderate intensity exercise results in increases in skeletal muscle Na(+)-K(+)-ATPase and attenuation in the elevation in plasma K+ during high intensity exercise at the same absolute workload. The increase in middle gluteal muscle Na(+)-K(+)-ATPase concentration is consistent with decreases in K+ efflux from working muscle during exercise.

Methodological aspects of measuring human skeletal muscle electrolyte content and ouabain binding capacity

The aim of the study was to evaluate the use of freeze-dried and dissected small muscle biopsy specimens ("dry") for the determination of human muscle electrolyte content and ouabain binding capacity, compared with an easier method, without this freeze-drying step ("wet"). Freeze-drying and dissection of muscle biopsy specimens reduced the variation in the determination of muscle potassium and magnesium content. The total coefficient of variation was 8.6% in the dry determination of muscle potassium content and 13.5% in the wet determination (P < 0.05). In the determination of muscle magnesium content, the total coefficient of variation was 7.4% in the dry determination and 13.7% when determined wet (P < 0.005). Muscle sodium content had a very large coefficient of variation, independent of the method used. The content of dry solids was too high in biopsies which were incubated in Tris-vanadate buffer (31.9%), compared to biopsies which were not incubated in Tris-vanadate buffer (24.9%, P < 0.001). Hereby, the measured ouabain binding capacity became too high when measured wet. In conclusion, muscle electrolyte content and ouabain binding capacity should be determined after drying and microdissection of the biopsies, because this method confers the least variation and the highest accuracy.

Thyroid hormone-induced upregulation of Na+ channels and Na(+)-K+ pumps: implications for contractility

We have previously observed in rat soleus muscle that endurance is a function of the ratio between the concentration of Na+ channels and Na(+)-K+ pumps [Harrison, A. P., O. B. Nielsen, and T. Clausen. Am. J. Physiol. 272 (Regulatory Integrative Comp. Physiol. 41): R1402-R1408, 1997]. In this study we explore this relationship further by comparing the changes in Na+ channel and Na(+)-K+ pump concentrations induced by injections of 3,5,3'-triiodothyronine (T3) with endurance. T3 induced upregulation of the concentration of Na+ channels and Na(+)-K+ pumps, which was associated with a progressive loss of contractile endurance. The increase in Na+ channels preceded that of the Na(+)-K+ pumps and amounted to 49 and 52% (both P < 0.01) after 48 and 72 h of T3 treatment, respectively. Concomitantly, during 90-Hz stimulation, the initial rate of force decline increased by 42 and 45% after 48 and 72 h of T3 treatment, respectively (both P < 0.001). These observations are important for an understanding of the fatigue associated with hyperthyroidism and add further support to the hypothesis that muscle endurance depends on the leak-to-pump ratio for Na+.

Early functional and biochemical adaptations to low-frequency stimulation of rabbit fast-twitch muscle

To examine mechanisms underlying force reduction after the onset of chronic low-frequency (10 Hz) stimulation (CLFS), we exposed rabbit tibialis anterior muscles to various durations of CLFS. To follow changes in isometric contractile properties and electromyographic (EMG) activity, we studied stimulated and contralateral muscles during a terminal test at 10 Hz for 10 min. In addition, activities and protein amounts of the sarcoplasmic reticulum Ca(2+)-ATPase, content of Na(+)-K(+)-ATPase, and expression patterns of triad junction components were examined. Force output and EMG amplitude declined abruptly soon after the onset of stimulation, suggesting refractoriness of a large fiber population. Although twitch force and to a lesser extent EMG activity gradually recovered after stimulation for 6 days and longer, the muscles exhibited profoundly altered properties, i.e., enhanced fatigue resistance, absence of twitch potentiation, and prolonged contraction and relaxation times. These changes were associated with significant increases in Na(+)-K(+)-ATPase concentration and significant decreases in Ca(2+)-ATPase, ryanodine receptor, dihydropyridine receptor, and triadin concentrations over the course of the 20 days of stimulation. Alterations in excitability, Ca2+ handling, and excitation-contraction coupling prior to changes in myofibrillar protein isoforms may thus be responsible for early functional alterations.

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.

K+ supplementation increases muscle [Na+-K+-ATPase] and improves extrarenal K+ homeostasis in rats

Effects of K+ supplementation (approximately 200 mmol KCl/100 g chow) on plasma K+, K+ content, and Na+-K+-adeonsinetriphosphatase (ATPase) concentration ([Na+-K+-ATPase]) in skeletal muscles as well as on extrarenal K+ clearance were evaluated in rats. After 2 days of K+ supplementation, hyperkalemia prevailed (K+-supplemented vs. weight-matched control animals) [5.1 +/- 0.2 (SE) vs. 3.2 +/- 0.1 mmol/l, P < 0.05, n = 5-6], and after 4 days a significant increase in K+ content was observed in gastrocnemius muscle (104 +/- 2 vs. 97 +/- 1 micromol/g wet wt, P < 0.05, n = 5-6). After 7 days of K+ supplementation, a significant increase in [3H] ouabain binding site concentration (344 +/- 5 vs. 239 +/- 8 pmol/g wet wt, P < 0.05, n = 4) was observed in gastrocnemius muscle. After 2 wk, increases in plasma K+, K+ content, and [3H]ouabain binding site concentration in gastrocnemius muscle amounted to 40, 8, and 68% (P < 0.05) above values observed in weight-matched control animals, respectively. The latter change was confirmed by K+-dependent p-nitrophenyl phosphatase activity measurements. Fasting for 1 day reduced plasma K+ and K+ content in gastrocnemius muscle in rats that had been K+ supplemented for 2 wk by 3.1 +/- 0.3 mmol/l (P < 0.05, n = 5) and 15 +/- 2 micromol/g wet wt (P < 0.05, n = 5), respectively. After induction of anesthesia, arterial plasma K+ was measured during intravenous KCl infusion (0.75 mmol KCl x 100 g body wt(-1) x h(-1)). The K+-supplemented fasted group demonstrated a 42% (P < 0.05) lower plasma K+ rise, associated with a significantly higher increase in K+ content in gastrocnemius muscle of 7 micromol/g wet wt (P < 0.05, n = 5) compared with their control animals. In conclusion, K+ supplementation increases plasma K+, K+ content, and [Na+-K+-ATPase] in skeletal muscles and improves extrarenal K+ clearance capacity.

Postoperative arrhythmias and myocardial electrolytes in patients undergoing coronary artery bypass grafting

Electrolyte changes in right atrial and skeletal muscle pre- intra- and postoperatively, and their relationship to the development of postoperative atrial fibrillation or flutter were evaluated in 31 patients with coronary artery bypass grafting (CABG). Such postoperative arrhythmias occurred in 14 patients (45%). Before CABG the skeletal muscle potassium concentration was lower in these patients than in the others: median 261.4 (range 148.2-329.5) vs 298.6 (167.1-416.4) mumol/g dry weight, p = 0.017. The right atrial potassium concentration was normal, but sodium levels were higher in the patients with, than in those without postoperative arrhythmias: median 340.3 (263.7-454.9) vs 296.3 (203.9-355.0) mumol/g dry weight, p = 0.008, indicating disturbed transmembrane electrolyte transfer. During CABG the potassium levels fell and sodium increased in both right atrium and skeletal muscle, and on postoperative day 2 the potassium content in skeletal muscle was not yet restored. Magnesium levels showed no changes in right atrium or skeletal muscle, but serum magnesium declined postoperatively. As the observed electrolyte derangements may be important in the development of postoperative arrhythmias, concomitant potassium and magnesium supplement postoperatively may be beneficial in restoring cellular potassium concentration.

Quantification in crude homogenates of rat myocardial Na+, K(+)- and Ca(2+)-ATPase by K+ and Ca(2+)-dependent pNPPase. Age-dependent changes

Assays for complete quantification of Na+, K(+)- and Ca(2+)-ATPase in crude homogenates of rat ventricular myocardium by determination of K(+)- and Ca(2+)-dependent p-nitrophenyl phosphatase (pNPPase) activities were evaluated and optimized. Using these assays the total K(+)- and Ca(2+)-dependent pNPPase activities in ventricular myocardium of 11-12 week-old rats were found to be 2.98 +/- 0.10 and 0.29 +/- 0.02 mumol x min-1 x g-1 wet wt. (mean +/- SEM) (n = 5), respectively. Coefficient of variance of interindividual determinations was 7 and 12%, respectively. The total Na+, K(+)- and Ca(2+)-ATPase concentrations were estimated to 2 and 10 nmol x g-1 wet wt., respectively. Evaluation of a putative developmental variation revealed a biphasic age-related change in the rat myocardial Ca(2+)-dependent pNPPase activity with an increase from birth to around the third week of life followed by a decrease. By contrast, the K(+)-dependent pNPPase activity of the rat myocardium showed a decrease from birth to adulthood. It was excluded that the changes were simple outcome of variations in water and protein content of myocardium. Expressed per heart, the K(+)- and Ca(2+)-dependent pNPPase activity gradually increased to a plateau. The present assay for Na+, K(+)-ATPase quantification has the advantage over [3H] ouabain binding of being applicable on the ouabain-resistant rat myocardium, and is more simple and rapid than measurements of K(+)-dependent 3-O-methylfluorescein phosphatase (3-O-MFPase) in crude tissue homogenates. Furthermore, with few modifications the pNPPase assay allows quantification of Ca(2+)-ATPase on crude myocardial homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional distributions of hippocampal Na+,K(+)-ATPase, cytochrome oxidase, and total protein in temporal lobe epilepsy

Na+,K(+)-ATPase (the sodium pump) is a ubiquitous enzyme that consumes ATP to maintain an adequate neuronal transmembrane electrical potential necessary for brain function and to dissipate ionic transients. Reductions in sodium pump function augment the sensitivity of neurons to glutamate, increasing excitability and neuronal damage in vitro. Temporal lobe epilepsy (TLE) is one disease characterized by hyperexcitability and marked hippocampal neuronal losses that could depend in part, on impaired sodium pump capacity secondary to changes in sodium pump levels and/or insufficient ATP supply. To assess whether abnormalities in the sodium pump occur in this disease, we used [3H]ouabain to determine the density of Na+,K(+)-ATPase for each anatomic region of hippocampus by in vitro autoradiography. Tissues were surgically obtained from epileptic patients with hippocampal sclerosis and compared with specimens from patients with seizures originating from temporal lobe tumors and autopsy controls. Changes in cellular population arising from neuronal losses or gliosis were assessed by protein densities derived from quantitative computerized densitometry of Coomassie-stained tissue sections. We estimated regional differences in capacity for ATP generation by determining cytochrome c oxidase (CO) activity. Principal neurons of hippocampus exhibit high levels of sodium pump enzyme. Both epilepsy groups exhibited slight but significant increases in sodium pump density/unit mass of protein in the dentate molecular layer, CA2, and subiculum as compared with autopsy controls. Greater hilar sodium pump density was also observed in sclerotic hippocampi. In contrast, CO activity was reduced in both epilepsy types throughout hippocampus. Results suggest that although sodium pump protein in surviving neurons appears to be upregulated in epilepsy, sodium pump capacity may be limited by the reduced levels of CO activity. Functional reduction in sodium pump capacity may be an important factor in hyperexcitability and neuronal death.

Changes in Na+, K(+)-adenosinetriphosphatase, citrate synthase and K+ in sheep skeletal muscle during immobilization and remobilization

The K+ balance and muscle activity seem to interact in a complex way with regard to regulating the muscle density of Na(+)-K+ pumps. The effect of immobilization was examined in ten sheep that had low muscle K+ content. Three additional sheep served as untreated controls. After being brought from pasture to sheep stalls one hindlimb was immobilized in a plaster splint for 9 weeks, and in five of the animals remobilization was carried out for a further 9 weeks. The weight bearing of the leg in plaster was recorded by a force plate. Open muscle biopsies from the vastus lateralis muscle were obtained before the study, after 9 weeks of immobilization, and after another 9 weeks of remobilization. The Na(+)-K+ pump density was measured as [3H]-ouabain binding to intact tissue, and citrate synthase activity was measured in tissue homogenate. The tissue content of K+ was measured in fat-free dried tissue. Muscle K+ content increased linearly by almost 70% through the 18-week period independent of intervention. Immobilization reduced thigh circumference by 8% (P < 0.05). A slight decrease in the area of type I fibres at 9 weeks and a slight increase at 18-weeks was found. The [3H]-ouabain binding was reduced by 39% and 22% in the immobilized and control legs, respectively, whereas citrate synthase activity was reduced by about 30% in both legs after 9 weeks of immobilization. During remobilization both the [3H]-ouabain binding and the citrate synthase activity increased to the same level as in the control animals. The plaster cast significantly reduced mass bearing of the immobilized leg, and a corresponding reduction in muscle activity must be assumed to have occurred in both legs as judged from citrate synthase activity. We concluded from this study that the reduction in the [3H]-ouabain binding during immobilization independent of an increase in muscle K+ content points to muscle activity as a strong stimulus for control of Na(+)-K+ pump density.

Effects of intensified endurance training on the concentration of Na,K-ATPase and Ca-ATPase in human skeletal muscle

Thirty-nine moderately endurance trained males increased their normal training programme of 2.2 h week-1 with an average training intensity of 65% of maximum heart rate (HRmax) to 2.7 h week-1 and a mean intensity of 78% of HRmax. Performance tests and measurements of the total concentrations of Na,K-ATPase (3H-ouabain binding) and Ca-ATPase, fibre type distribution and fibre area were performed in biopsies from the vastus lateralis muscle before and after increased training. The 6 weeks of training elevated VO2max from 54.9 +/- 3.1 to 58.3 +/- 3.0 ml O2 min-1 kg-1 (P < 0.0001). Exercise time to exhaustion at 86% of VO2max (pre-training) increased from 35 +/- 8 to 61 +/- 17 min (P < 0.0001). The concentration of Ca-ATPase was unaffected by the intensified training (6.74 +/- 1.03 vs. 6.68 +/- 1.07 nmol g wet wt-1), but the concentration of Na,K-ATPase increased from 307 +/- 43 to 354 +/- 59 pmol g wet wt-1 (P < 0.0001). The relative distribution of FT-fibres was correlated with the concentration of Ca-ATPase (r = 0.72, P < 0.0001). The data support the view that intensive training induces an upregulation of the concentration of skeletal muscle Na,K-ATPase, but no change in the total capacity for reaccumulation of Ca2+ into the SR. There was no correlation between the concentrations of Na,K-ATPase, Ca-ATPase and indices of endurance performance.

Na,K-ATPase is decreased in hippocampus of kainate-lesioned rats

The effects of intraventricular injection of kainic acid on the Na,K-ATPase (Na,K pump) were examined in discrete pyramidal cell regions of rat hippocampus. [3H]Ouabain binding was used to quantitate Na,K-ATPase catalytic subunits and in situ hybridization was used to determine Na,K-ATPase mRNA levels. Large decreases were found in both [3H]ouabain binding and alpha 3 isoform mRNA in hippocampus areas, especially in the CA3 pyramidal cell layer, which sustains heavy cell losses as a result of bilateral, intraventricular injection of kainic acid. Substantial decreases in the high affinity component of ouabain binding and in the alpha 3 isoform mRNA (but not isoforms for other Na,K-ATPase subunits) were also observed in the CA1 region of hippocampus, an area preserved in this model. High affinity [3H]ouabain binding was decreased 25-33% in the stratum pyramidale and stratum radiatum after treatment with kainic acid, and alpha 3 mRNA was decreased by 26-50%. To further characterize the decrease in alpha 3 mRNA, animals were killed at 1, 2, and 3 weeks after injection of kainate and results show a large decrease in alpha 3 mRNA only at 2 weeks recovery time. While the pathology underlying temporal lobe epilepsy is unclear, changes in the Na,K-ATPase may be involved in abnormal firing characteristics of cells in epileptic tissue.

Increases in human skeletal muscle Na(+)-K(+)-ATPase concentration with short-term training

To investigate the effect of short-term training on Na(+)-K(+)-adenosine triphosphatase (ATPase) concentration in skeletal muscle and on plasma K+ homeostasis during exercise, 9 subjects performed cycle exercise for 2 h per day for 6 consecutive days at 65% of maximal aerobic power (VO2 max). Na(+)-K(+)-ATPase concentration determined from biopsies obtained from the vastus lateralis muscle using the [3H]ouabain-binding technique increased 13.6% (P < 0.05) as a result of the training (339 +/- 16 vs. 385 +/- 19 pmol/g wet wt, means +/- SE). Increases in Na(+)-K(+)-ATPase concentration were accompanied by a small but significant increase in VO2 max (3.36 +/- 0.16 vs. 3.58 +/- 0.13 l/min). The increase in arterialized plasma K+ concentration and plasma K+ content determined during continuous exercise at three different intensities (60, 79, and 94% VO2 max) was depressed (P < 0.05) following training. These results indicate that not only is training capable of inducing an upregulation in sarcolemmal Na(+)-K(+)-ATPase concentration in humans, but provided that the exercise is of sufficient intensity and duration, the upregulation can occur within the first week of training. Moreover, our findings are consistent with the notion that the increase in Na(+)-K(+)-ATPase pump concentration attenuates the loss of K+ from the working muscle.

Quantification of rat cerebral cortex Na+,K(+)-ATPase: effect of age and potassium depletion

Na+,K(+)-ATPase concentration in rat cerebral cortex was studied by vanadate-facilitated [3H]ouabain binding to intact samples and by K(+)-dependent 3-O-methylfluorescein phosphatase activity determinations in crude homogenates. Methodological errors of both methods were evaluated. [3H]Ouabain binding to cerebral cortex obtained from 12-week-old rats measured incubating samples in buffer containing [3H]ouabain, and ouabain at a final concentration of 1 x 10(-6) mol/L gave a value of 11,351 +/- 177 (n = 5) pmol/g wet weight (mean +/- SEM) without any significant variation between the lobes. Evaluation of affinity for ouabain was in agreement with a heterogeneous population of [3H]ouabain binding sites. K(+)-dependent 3-O-methylfluorescein phosphatase activity in crude cerebral homogenates of age-matched rats was 7.24 +/- 0.14 (n = 5) mumol/min/g wet weight, corresponding to a Na+,K(+)-ATPase concentration of 12,209 +/- 236 pmol/g wet weight. It was concluded that the present methods were suitable for quantitative studies of cerebral cortex Na+,K(+)-ATPase. The concentration of rat cerebral cortex Na+,K(+)-ATPase showed approximately 10-fold increase within the first 4 weeks of life to reach a plateau of approximately 11,000-12,000 pmol/g wet weight, indicating a larger synthesis of Na+,K+ pumps than tissue mass in rat cerebral cortex during the first 4 weeks of development. K+ depletion induced by K(+)-deficient fodder for 2 weeks resulted in a slight tendency toward a reduction in K+ content (6%, p > 0.5) and Na+,K(+)-ATPase concentration (3%, p > 0.4) in cerebral cortex, whereas soleus muscle K+ content and Na+,K(+)-ATPase concentration were decreased by 30 (p < 0.02) and 32% (p < 0.001), respectively. Hence, during K+ depletion, cerebral cortex can maintain almost normal K+ homeostasis, whereas K+ as well as Na+,K+ pumps are lost from skeletal muscles.

Time-dependent increases in Na+,K(+)-ATPase content of low-frequency-stimulated rabbit muscle

Chronic low-frequency stimulation of rabbit fast-twitch muscle induced time-dependent increases in the concentration of the sarcolemmal Na+,K(+)-ATPase and in mitochondrial citrate synthase activity. The almost twofold increase in Na+,K(+)-ATPase preceded the rise in citrate synthase and was complete after 10 days of stimulation. We suggest that the increase in Na+,K(+)-ATPase enhances resistance to fatigue of low-frequency-stimulated muscle prior to elevations in aerobic-oxidative capacity.

Effects of fasting, refeeding, and fasting with T3 administration on Na-K,ATPase in rat skeletal muscle

It is known that Na-K,adenosine triphosphatase (ATPase) in cell membranes represents an important consumer of cellular energy, eg, adenosine triphosphate (ATP), and that the concentration and activity of this enzyme change in a dose-dependent manner with serum thyroid hormone levels. To examine the hypothesis that low triiodothyronine (T3) syndrome represents a cellular adaptation in generalized severe illnesses that saves tissue energy expenditure, we measured the muscle Na-K,ATPase concentration and its activity in rats that led to low T3 syndrome induced by fasting. The Na-K,ATPase concentration was measured by 3H-ouabain binding to soleus muscle, and its activity was measured by 42K uptake in the contralateral soleus muscle. The effects of refeeding or T3 administration on Na-K,ATPase in soleus muscle in fasted rats were also examined. Na-K,ATPase concentration and activity were both increased in hyperthyroid rats and decreased in hypothyroid rats. In the fasting state, they were decreased to as low as the levels seen in hypothyroidism. Furthermore, with fasting + refeeding or fasting + T3 administration, Na-K,ATPase in soleus muscle returned to the normal level. These results suggest that tissue energy expenditure, as assessed by Na-K,ATPase, in skeletal muscles of fasted rats with low T3 syndrome is actually decreased to levels seen in hypothyroidism, due at least partly to the decrease in serum T3 concentrations, and that there exist some adaptation mechanisms in the peripheral tissues for the accommodation of energy metabolism in the body through decreased thyroxine (T4) to T3 conversion.

Inhibition of α2/α3 sodium pump isoforms potentiates glutamate neurotoxicity

Excessive stimulation of neurons by glutamic acid initiates a destructive cascade of ion fluxes, cellular swelling, and death. Homeostatic mechanisms which rectify these disturbances depend largely upon transmembrane ion gradients maintained by Na+,K(+)-ATPase (NaP). We proposed that the neurotoxicity of glutamate is enhanced when the NaP capacity is exceeded, and therefore, that the degree of neuronal death varies inversely with endogenous NaP activity. To test this concept, we directly reduced NaP activity in cultured rat telencephalic cells using either the specific inhibitor ouabain, or dcAMP, and assessed whether these treatments increased glutamate-induced neuronal death. Since rodent NaP catalytic subunits possess both low (alpha 1) and high (alpha 2/alpha 3) affinity for ouabain, we were able to inhibit selectively the alpha 2 (principally glial) and alpha 3 (neuronal) catalytic subunits without affecting the alpha 1 isoform. Brief exposures (5-60 min) to high ouabain concentrations (1-10 mM), which blocks the activity of all three catalytic subunits, killed differentiated neurons but spared glia. In contrast, differential inhibition of the alpha 2/alpha 3 isoforms (by 1 microM ouabain) was not of itself toxic, but produced a supersensitivity to glutamate. [3H]Ouabain binding studies confirmed that the glutamate neurotoxicity observed varied inversely with the degree of NaP inhibition. Further, this relationship was not absolutely dependent upon ouabain, since reductions in alpha 2/alpha 3 pump activity induced by dcAMP also amplified glutamate toxicity. We conclude that inhibition of neuronal NaP with high affinity for ouabain is not lethal to unstimulated cells, but markedly increases susceptibility to glutamate excitotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of the Na-K pump by intracellular Na in rat slow- and fast-twitch muscle

Experiments were performed on isolated rat soleus (slow-twitch) and extensor digitorum longus (EDL) (fast-twitch) muscle of 4-week-old rats. In soleus muscle, electrical simulation at 2 Hz for 5 min increased the ouabain-suppressible 86Rb+ uptake by 138%, without significant changes in intracellular Na+ content or Na+/K+ ratio. In EDL muscle, the ouabain-suppressible 86Rb+ uptake was stimulated by only 58%, whereas intracellular Na+ content and Na+/K+ ratio were increased by around 70%. Na(+)-loading of the muscles by exposure to K(+)-free or K(+)-Ca(2-)-Mg(2+)-free buffer stimulated the ouabain-suppressible 86Rb+ uptake in the two muscles to roughly the same extent, but in EDL muscle this was associated with a more than twofold larger increase in Na+/K+ ratio. When the Na+ influx was increased by exposure to veratridine similar results were obtained. Graded variation in intracellular Na+ content was achieved by exposure to monensin. In soleus muscle, a 25% increase in intracellular Na+/K+ ratio resulted in a doubling of the ouabain-suppressible 86Rb+ uptake, whereas a doubling of the Na(+)-K+ transport rate in EDL muscle required a 140% increase in Na+/K+ ratio. The results indicate that in soleus muscle the Na+/K+ pump is much more sensitive to changes in intracellular Na+ content than in EDL muscle. This might explain the larger activation of the Na(+)-K+ pump in slow-twitch muscle during electrical stimulation and might be of significance for the activation of the Na(+)-K+ pump in vivo during work.

Exercise-induced hyperkalaemia can be reduced in human subjects by moderate training without change in skeletal muscle Na,K-ATPase concentration

In 15 conscripts, venous plasma potassium was followed during exercise on a training bicycle before and after 10 weeks of moderate physical training and a putative relationship with skeletal muscle Na,K-ATPase was evaluated. Peak plasma potassium concentration obtained at exhaustion was 6.1 +/- 0.2 and 5.6 +/- 0.2 mmol l-1 (mean +/- SEM, n = 14, P less than 0.05) before and after training, respectively. Throughout the exercise period and within the first minutes of rest plasma potassium concentration was 0.2-0.5 mmol l-1 higher before than after training. Neither peak values nor peak rises in plasma potassium concentration before nor after training were correlated to the 3H-ouabain binding site (Na,K-ATPase) concentration in vastus lateralis muscle. The results indicate that net loss of potassium from the skeletal muscle pool during exercise is reduced after training, that the heart during exercise may be exposed to a smaller rise in plasma potassium concentration after training than before, and that moderate improvement of capacity to clear extracellular potassium during exercise may be due to increased activity of existing Na,K-pumps in resting skeletal muscle fibres. This may reduce muscle fatigue, increase physical performance and explain the paradoxical observation that, despite an increased catecholamine response, there is a reduced risk of cardiac events after training.

Na(+)-K+ pump in rat muscle: effects of hypophysectomy, growth hormone, and thyroid hormone

We investigated the effects of growth hormone and thyroid hormone on the synthesis of Na(+)-K+ pumps in rat soleus muscle. Hypophysectomized rats were treated for 11 days with saline, human growth hormone (hGH; 140 micrograms/day), thyroxine (T4; 3 micrograms/day), or hGH plus T4. Age-matched nonhypophysectomized control rats received no treatment. The concentration of Na(+)-K+ pumps was reduced by 75% in the hypophysectomized rats. Treatment with hGH alone or combined with T4 restored the growth rate, whereas T4 alone did not stimulate growth. In contrast, the synthesis of Na(+)-K+ pumps was only responsive to T4 treatment when given alone or in combination with hGH. The concentration of Na(+)-K+ pumps increased around threefold in the T4-treated groups and showed full normalization to the control level after 11 days of treatment. It is concluded that growth hormone does not play any major role in the de novo synthesis of Na(+)-K+ pumps in skeletal muscle. More important, thyroid hormone, also at physiological doses, seems to be the major endocrine factor determining the concentration of Na(+)-K+ pumps in skeletal muscle.

Quantification of the total Na,K-ATPase concentration in atria and ventricles from mammalian species by measuring 3H-ouabain binding to intact myocardial samples. Stability to short term ischemia reperfusion

Na,K-ATPase concentration was measured by vanadate facilitated 3H-ouabain binding to intact samples taken from various parts of porcine and canine myocardium. In porcine and canine heart 3H-ouabain binding site concentration in ventricles was 1.4-2.5 times larger than in atria. Evaluation of 3H-ouabain binding kinetics revealed no major difference between atria and ventricles: Equilibrium was obtained after the same incubation time in right atrium (RA) as in left ventricle (LV), both in porcine and canine heart. Unspecific uptake and retention of 3H-ouabain was for porcine heart RA and LV 1.5 and 1.4, respectively, and for canine heart RA and LV, both 1.2% filling (i.e., volume (ml) of incubation medium 3H-radioactivity taken up per mass unit (g wet wt.) of tissue multiplied by 100). The apparent dissociation constant (KD) was 1.4 x 10(-8) and 1.9 x 10(-8) in porcine RA and LV and 2.6 x 10(-8) and 6.1 x 10(-8) mol/l in canine RA and LV. Loss of specifically bound 3H-ouabain during the washout procedure occurred with a half-life time (T1/2) of 16.7 and 28.6 in RA and LV of porcine heart and 91.2 and 151.6 h in RA and LV of canine heart. Duly corrected for these errors of the method--factor 1.16 and 1.13, respectively, for porcine RA and LV, and factor 1.11 and 1.13 for canine RA and LV, total 3H-ouabain binding site concentration was found to be 553 +/- 74 and 1037 +/- 45 pmol/g wet wt. (means +/- SEM, n = 5) in porcine RA and LV, and 569 +/- 37 and 1410 +/- 40 pmol/g wet wt. (means +/- SEM, n = 5) in the canine RA and LV. These values were confirmed by measurements of 3H-digoxin binding to the porcine heart. The present quantification of myocardial Na,K-ATPase gives values up to 154 times higher than measurements based upon Na,K-ATPase activities in membrane fractions where the recovery of Na,K-ATPase may be less than 1% due to loss during purification. A higher Na,K-ATPase concentration is found in small animals than in large animals. A relationship between higher concentration of Na,K-ATPase and larger pressure work in ventricles compared to atria is suggested. Myocardial 3H-ouabain binding sites were found to be stable for 20 min of ischemia, followed by 1 h of reperfusion, supporting the concept that myocyte injury induced by short term ischemia may be reversible and that reperfusion may result in normalization.

The concentration of ouabain binding sites in biopsies of uterine muscle

The concentration of Na, K-ATPase in biopsies of uterine muscle was determined by measurement of [3H]ouabain binding in the presence of vanadate. For this purpose a method previously described for skeletal muscle (Nørgaard et al. 1983) was modified. Biopsies were obtained from uterine muscle from pregnant women (during caesarian section), non-pregnant women (during hysterectomy) and from adult, non-pregnant guinea-pigs and rats. The ouabain binding site concentration in uterine muscle of the pregnant women averaged 72 +/- 2 pmol g-1 wet wt (n = 8), with an apparent dissociation constant (KD) for ouabain of 3 x 10(-9) mol l-1. The ouabain-binding capacity in uterine muscle of the non-pregnant women amounted to 83 +/- 9 pmol g-1 wet wt (n = 8). In uterine muscle of the guinea-pig, two populations of ouabain binding sites were observed: one with a maximum binding capacity of 230 pmol g-1 wet wt and an apparent KD of 1.6 x 10(-6) mol l-1, and one with a maximum capacity of 62 pmol g-1 wet wt and an apparent KD of 5 x 10(-8) mol l-1. Immediate freezing of the biopsies in liquid N2 and storage at -60 degrees C for up to 6 weeks caused no change in ouabain-binding capacity. The dry weight/wet weight ratio of the samples from different subjects showed values of around 20%. It is concluded that the concentration of Na-K pumps in human uterine muscle can be quantified by [3H]ouabain binding using samples weighing 5-10 mg.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin sensitivity of rat muscle sodium pump

The aim of the present study was to examine the possible physiological responsiveness of the sodium pump to insulin in rat muscle, an effect that has never been convincingly demonstrated. The insulin stimulation of the sodium pump was estimated by two well-established parameters: ouabain binding to pieces of soleus muscle, and Na/K-ATPase activity of purified plasma membranes. For both parameters the dose dependence of the insulin effect on the sodium pump shows the characteristic bell-shaped stimulation pattern, with a maximum in the physiological hormone concentration range. This result has not been observed in previous studies where insulin concentrations two to three orders of magnitude higher were used. It can be concluded that an effect of insulin on the regulation of the Na pump in muscle might well be operating in vivo.

Abnormal cation transport in uremia. Mechanisms in adipocytes and skeletal muscle from uremic rats

The cause of the abnormal active cation transport in erythrocytes of some uremic patients is unknown. In isolated adipocytes and skeletal muscle from chronically uremic chronic renal failure rats, basal sodium pump activity was decreased by 36 and 30%, and intracellular sodium was increased by 90 and 50%, respectively, compared with pair-fed control rats; insulin-stimulated sodium pump activity was preserved in both tissues. Lower basal NaK-ATPase activity in adipocytes was due to a proportionate decline in [3H]ouabain binding, while in muscle, [3H]ouabain binding was not changed, indicating that the NaK-ATPase turnover rate was decreased. Normal muscle, but not normal adipocytes, acquired defective Na pump activity when incubated in uremic sera. Thus, the mechanism for defective active cation transport in CRF is multifactorial and tissue specific. Sodium-dependent amino acid transport in adipocytes closely paralleled diminished Na pump activity (r = 0.91), indicating the importance of this defect to abnormal cellular metabolism in uremia.

Inhibition of the electrogenic Na,K pump and Na,K-ATPase activity by tetraethylammonium, tetrabutylammonium, and apamin

The K+-induced hyperpolarization of Na-loaded mouse diaphragm muscle, enzymatic activity of Na,K-ATPase and 3H-ouabain binding to rat brain microsomes was measured in the presence of K+ channel blockers tetraethylammonium (TEA), tetrabutylammonium (TBA) and apamin. TBA, and to a lesser extent TEA in millimolar concentrations, inhibited the electrogenic effect of the Na,K pump, Na,K-ATPase activity, and 3H-ouabain binding. The inhibition of 3H-ouabain binding by TEA or TBA was more evident in the presence of ATP and Na+ ions. Apamin in nanomolar concentrations inhibited the electrogenic effect of Na,K pump and Na,K-ATPase but not the 3H-ouabain binding. The hyperpolarizing effects of insulin and NADH, but not that of noradrenaline, were also prevented by apamin. The inhibition of Na,K pump by TEA and TBA is apparently due to both competition with K+ for a binding site on the Na,K-ATPase and a reduction in the number of transporting sites. The site of action of apamin on Na,K-ATPase is different from that of tetra-alkylammonium compounds; it apparently decreases the turnover rate of the enzyme.

Homogeneity of [3H]ouabain-binding sites in rat soleus muscle

Homogeneity or heterogeneity of rat soleus-muscle Na,K-ATPase (Na+ + K+-dependent ATPase) with respect to affinity for [3H]ouabain was evaluated. Since the standard method for measuring specific [3H]ouabain binding to rat skeletal-muscle samples includes subtraction of a value for non-specific [3H]ouabain uptake and retention, and a wash-out in the cold to remove [3H]ouabain from the extracellular phase, it was possible that these procedures could hide a class of [3H]ouabain-binding sites either with low affinity or with a rapid dissociation of [3H]ouabain. However, measurements of [3H]ouabain uptake and retention over the range 0.1-5 mM, as well as the omission of wash-out, gave no evidence for heterogeneity of [3H]ouabain-binding sites in rat soleus muscle. Furthermore, the observation of agreement between the uptake and retention of non-specific [3H]ouabain and of [14C]sucrose gave no evidence for the existence of a major pool of [3H]ouabain-binding sites with low affinity for [3H]ouabain. Assuming homogeneity, the total concentration of [3H]ouabain binding sites in soleus-muscle samples from 12-week-old rats is 278-359 pmol/g wet wt.

Quantification of rat sciatic nerve (Na+,K+)-ATPase by measurements of [3H]ouabain binding in intact nerve samples

The (Na+,K+)-ATPase concentration in rat sciatic nerve was quantified by measurements of [3H]ouabain binding to intact nerve samples in a vanadate-Tris buffer. This gave values of 145-172 pmol/g wet weight in mature female rats. The methodological errors of the [3H]ouabain binding assay was identified, quantified and corrected for. The [3H]ouabain binding sites were homogeneous with respect to [3H]ouabain affinity with an apparent dissociation constant of 5 X 10(-8) mol/l, i.e. in the same range as in samples of skeletal muscles incubated at similar conditions. Post-mortem the [3H]ouabain binding site capacity in rat sciatic nerve decreased with a half-life time of 5 days, allowing measurements to be performed within 6 h after death with a reduction in [3H]ouabain binding sites of only 3%. When nerve samples were frozen the [3H]ouabain binding site concentration remained constant. An increase in sciatic nerve [3H]ouabain binding site concentration of 122% was seen from the 2nd to the 4th week of life followed by a decrease of 53% to the 12th week of life. In mature males the [3H]ouabain binding site concentration was 1.8 times that in nerves from age-matched female rats. Denervation caused a reduction of 61%, whereas K+ depletion or thyroid status has no effect on sciatic nerve [3H]ouabain binding site concentration.