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

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.

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.

The A2 isoform of rat Na+,K(+)-adenosine triphosphatase is active and exhibits high ouabain affinity when expressed in transfected fibroblasts

The alpha isoforms of the Na+,K(+)-ATPase (Na+ pump) are expressed with developmental and tissue heterogeneity in rodents and possess different sensitivity to inhibition by ouabain. We directly characterized the ouabain sensitivity of the rat A2 (alpha 2) isoform by transfecting NIH 3T3 cells with rat A2. The treated cells exhibit high affinity (40 nM) ouabain binding with a density of 2 pmol/mg protein. 86Rb+ flux studies confirm that A2 is functional in this system and that A2 is inhibited by submicromolar concentrations of ouabain. These findings are consistent with measurements of ouabain affinity in tissues which express the A2 isoform.

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.

3H-ouabain binding sites in porcine skeletal muscle as influenced by environmental temperature and energy intake

The influence of environmental temperature and energy intake on 3H-ouabain binding sites in skeletal muscle has been investigated in young growing pigs at 8 weeks of age. Animals lived for several weeks at 35 or 10 degrees C on a high (H) or low (L) level of energy intake. The four treatment groups were thus: 35H, 35L, 10H and 10L. The total number of 3H-ouabain binding sites (Bmax) in longissimus dorsi muscle (mean values +/- SEM) were 221 +/- 66, 214 +/- 61, 350 +/- 76 and 486 +/- 114 pmol/g wet weight for the 35H, 35L, 10H and 10L groups respectively. Bmax was significantly greater in those living in the cold than the warm (P less than 0.001). Moreover, at 10 degrees C energy intake had a significant effect, with Bmax being greater in the 10L than the 10H group (P less than 0.005). Level of energy intake had no influence on Bmax at 35 degrees C. The apparent dissociation constant was not affected by either temperature or intake. The elevated Bmax and hence the increase in number of Na+,K+-pumping sites in the cold is probably related to increased muscular activity associated with shivering. However, thyroid status also influences the number of Na+,K+-pumping sites and this may have been a contributory factor in the present study. In addition, the elevated Bmax suggests a greater potential for non-shivering thermogenesis associated with increased Na+,K+-ATPase concentration in the cold. Differences in relative stage of development between the four groups may help to explain the results for Bmax in relation to level of energy intake.