Stimulation of the sodium pump by azide and high internal sodium: changes in the number of pumping sites and turnover rate

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.

Excitation-induced activation of the Na(+)-K+ pump in rat skeletal muscle

The stimulating effect of excitation on the Na(+)-K+ pump was characterized in measurements of 22Na efflux, intracellular Na+ content, 86Rb influx, and [3H]ouabain binding in isolated rat soleus muscle. Direct stimulation (10 V, 1 ms, 2 Hz) rapidly increased 22Na efflux and 86Rb influx about twofold. These effects were blocked by tetracaine and ouabain, were not associated with any significant increase in intracellular Na+, and could not be attributed to a rise in extracellular K+. The stimulation of 22Na efflux was unaffected by tubocurarine, dantrolene, trifluoperazine, or bumetanide. Stimulation at 2 Hz increased the rate of [3H]ouabain binding by approximately 120% within 1 min, indicating an early specific activation of the Na(+)-K+ pump. Stimulation at 60 Hz for 10 s increased intracellular Na+ content by 58%. Reextrusion of Na+ was complete in 2 min and could be prevented by ouabain (10(-4) M) or by cooling to 0 degrees C. It is concluded that, in rat soleus muscle, excitation leads to a rapid and pronounced (up to 15-fold) stimulation of the Na(+)-K+ pump, even at modest increases in intracellular Na+. This activation mechanism may be essential for the maintenance of transmembrane Na(+)-K+ gradients and prompt recovery of excitability during contractile activity.

Sodium-azide-evoked noradrenaline and catecholamine release from peripheral sympathetic nerves and chromaffin cells

1. The spontaneous release of [3H]noradrenaline [( 3H]NA) has been measured from rabbit pulmonary arteries and bovine chromaffin cells in the presence of neuronal uptake blocker cocaine (3 x 10(-5) M). 2. The Na+-pump inhibitor sodium-azide (NaN3, 2mM) produced a moderate increase of [3H]NA release from both preparations and relaxed the arteries. The [3H]releasing action of NaN3 was accompanied by a 30% inhibition of 86Rb-uptake into chromaffin cells. 3. In both preparations, ouabain (10(-4) M) markedly increased the release of [3H], contracted the arteries and inhibited the 86Rb-uptake of chromaffin cells by about 75%. A combined application of NaN3 and ouabain produced a similar inhibition of 86Rb-uptake of chromaffin cells and failed to increase further the release of [3H] in comparison to that found in response to ouabain alone. 4. Removal of K+ from the external medium increased both the release of [3H]NA and the tone of pulmonary arteries. NaN3 further increased the transmitter release in "K+-free" solution but relaxed the muscle. In the absence of external K+ and in the presence of azide, ouabain further enhanced the transmitter release but failed to produce significant contraction. 5. Reactivation of the Na+-pump by readmission of K+ (5.9 mM) to the external medium abolished the transmitter releasing action of NaN3 in arteries. 6. It is concluded that in peripheral sympathetic nerves and chromaffin cells, NaN3 inhibits the Na+-pump producing NA and CA release respectively and in nerves even if NA release had already been increased by K+-removal.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantification of the maximum capacity for active sodium-potassium transport in rat skeletal muscle

1. Intact skeletal muscle fibres have been shown to contain a high concentration of [3H]ouabain binding sites (100-800 pmol g wet wt.-1). Under resting conditions, however, it seems that in isolated muscles only 2-6% of the corresponding expected capacity for active Na+-K+ transport is utilized. 2. In order to determine whether all [3H]ouabain binding sites in rat soleus muscle represent functional Na+-K+ pumps, we have measured the maximum rates of the ouabain-suppressible components of isotopic fluxes of Na+ and K+ as well as the net changes in Na+-K+ contents. 3. Experiments with soleus muscles isolated from 4-week-old rats showed that following Na+ loading (I.C. Na+, 126 mmol l-1), the ouabain-suppressible 86Rb+ uptake and 22Na+ efflux as measured during 3 min of exposure to K+-rich buffer were 5800 and 6500 nmol g wet wt.-1 min-1, respectively. 4. These initial high rates of isotopic fluxes were confirmed by flame photometric measurements of Na+-K+ contents. The ouabain-suppressible 86Rb+ uptake had a temperature coefficient of 2.1, was inhibited by 2,4-dinitrophenol, but showed no response to tetracaine, BaCl2, Ca2+-free buffer or tetraethylammonium chloride. 5. In soleus muscles, where the total population of [3H]ouabain binding sites had undergone changes as a result of differentiation, K+ depletion or pre-treatment with thyroid hormone, there was a significant correlation (r = 0.95, P less than 0.005) between the concentration of [3H]ouabain binding sites (260-1170 pmol g wet wt.-1) and the maximum ouabain-suppressible 86Rb+ uptake (2300-10,900 nmol g wet wt.-1 min-1). 6. It is concluded that by the combination of Na+ loading and high extracellular K+, the available Na+-K+ pumps as quantified by the [3H]ouabain binding capacity can be activated to reach a transport rate around 90% of the theoretical maximum at 30 degrees C.