Effects of carbamylcholine on membrane potential and Na-K pump activity of cultured rat skeletal myotubes

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.

Veratridine-induced oscillations in membrane potential of cultured rat skeletal muscle: role of the Na-K pump

1. The acute effects of veratridine on membrane potential (Em) and Na-K pump activity in cultured skeletal muscle were examined. 2. At a concentration of 10(-4) M, veratridine caused depolarization of Em and a decrease in Na-K pump activity. At concentrations of 10(-5) and 10(-6) M, veratridine caused oscillations of Em and an increase in Na-K pump activity compared to untreated, control cells. The oscillations consisted of depolarization to about -40 mV followed by hyperpolarization to about -90 mV; the level of hyperpolarization was higher at 37 than at 23 degrees C. 3. Veratridine-induced oscillations could be prevented by pretreatment with tetrodotoxin (10(-6) M) and blocked or prevented by ouabain, which depolarizes Em of cultured myotubes. In contrast, depolarization of Em to -60 mV by excess K+ did not alter the amplitude or frequency of the oscillations. 4. The results demonstrate that veratridine-induced increase in Na influx both depolarizes cultured myotubes and increases the activity of the Na-K pump, which repolarizes Em to levels higher than control. This sequence accounts for veratridine-induced oscillations in Em. High concentrations of veratridine cause only depolarization of Em and inhibition of Na-K pump activity.