Effects of adrenaline on excitation-induced stimulation of the sodium-potassium pump in rat skeletal muscle

A century of exercise physiology: effects of muscle contraction and exercise on skeletal muscle Na+,K+-ATPase, Na+ and K+ ions, and on plasma K+ concentration-historical developments

This historical review traces key discoveries regarding K+ and Na+ ions in skeletal muscle at rest and with exercise, including contents and concentrations, Na+,K+-ATPase (NKA) and exercise effects on plasma [K+] in humans. Following initial measures in 1896 of muscle contents in various species, including humans, electrical stimulation of animal muscle showed K+ loss and gains in Na+, Cl- and H20, then subsequently bidirectional muscle K+ and Na+ fluxes. After NKA discovery in 1957, methods were developed to quantify muscle NKA activity via rates of ATP hydrolysis, Na+/K+ radioisotope fluxes, [3H]-ouabain binding and phosphatase activity. Since then, it became clear that NKA plays a central role in Na+/K+ homeostasis and that NKA content and activity are regulated by muscle contractions and numerous hormones. During intense exercise in humans, muscle intracellular [K+] falls by 21 mM (range - 13 to - 39 mM), interstitial [K+] increases to 12-13 mM, and plasma [K+] rises to 6-8 mM, whilst post-exercise plasma [K+] falls rapidly, reflecting increased muscle NKA activity. Contractions were shown to increase NKA activity in proportion to activation frequency in animal intact muscle preparations. In human muscle, [3H]-ouabain-binding content fully quantifies NKA content, whilst the method mainly detects α2 isoforms in rats. Acute or chronic exercise affects human muscle K+, NKA content, activity, isoforms and phospholemman (FXYD1). Numerous hormones, pharmacological and dietary interventions, altered acid-base or redox states, exercise training and physical inactivity modulate plasma [K+] during exercise. Finally, historical research approaches largely excluded female participants and typically used very small sample sizes.

Protection against severe hypokalemia but impaired cardiac repolarization after intense rowing exercise in healthy humans receiving salbutamol

Intense exercise induces pronounced hyperkalemia, followed by transient hypokalemia in recovery. We investigated whether the β₂ agonist salbutamol attenuated the exercise hyperkalemia and exacerbated the postexercise hypokalemia, and whether hypokalemia was associated with impaired cardiac repolarization (QT hysteresis). Eleven healthy adults participated in a randomized, counterbalanced, double-blind trial receiving either 1,000 µg salbutamol (SAL) or placebo (PLAC) by inhalation. Arterial plasma potassium concentration ([K⁺]_a) was measured at rest, during 3 min of intense rowing exercise, and during 60 min of recovery. QT hysteresis was calculated from ECG ( n = 8). [K⁺]_a increased above baseline during exercise (rest, 3.72 ± 0.7 vs. end-exercise, 6.81 ± 1.4 mM, P < 0.001, mean ± SD) and decreased rapidly during early recovery to below baseline; restoration was incomplete at 60 min postexercise ( P < 0.05). [K⁺]_a was less during SAL than PLAC (4.39 ± 0.13 vs. 4.73 ± 0.19 mM, pooled across all times, P = 0.001, treatment main effect). [K⁺]_a was lower after SAL than PLAC, from 2 min preexercise until 2.5 min during exercise, and at 50 and 60 min postexercise ( P < 0.05). The postexercise decline in [K⁺]_a was correlated with QT hysteresis ( r = 0.343, n = 112, pooled data, P = 0.001). Therefore, the decrease in [K⁺]_a from end-exercise by ~4 mM was associated with reduced QT hysteresis by ~75 ms. Although salbutamol lowered [K⁺]_a during exercise, no additive hypokalemic effects occurred in early recovery, suggesting there may be a protective mechanism against severe or prolonged hypokalemia after exercise when treated by salbutamol. This is important because postexercise hypokalemia impaired cardiac repolarization, which could potentially trigger arrhythmias and sudden cardiac death in susceptible individuals with preexisting hypokalemia and/or heart disease. NEW & NOTEWORTHY Intense rowing exercise induced a marked increase in arterial potassium, followed by a pronounced decline to hypokalemic levels. The β₂ agonist salbutamol lowered potassium during exercise and late recovery but not during early postexercise, suggesting a protective effect against severe hypokalemia. The decreased potassium in recovery was associated with impaired cardiac QT hysteresis, suggesting a link between postexercise potassium and the heart, with implications for increased risk of cardiac arrhythmias and, potentially, sudden cardiac death.

Properties of single FDB fibers following a collagenase digestion for studying contractility, fatigue, and pCa-sarcomere shortening relationship

The objective of this study was to optimize the approach to obtain viable single flexor digitorum brevis (FDB) fibers following a collagenase digestion. A first aim was to determine the culture medium conditions for the collagenase digestion. The MEM yielded better fibers in terms of morphology and contractility than the DMEM. The addition of FBS to culture media was crucial to prevent fiber supercontraction. The addition of FBS to the physiological solution used during an experiment was also beneficial, especially during fatigue. Optimum FBS concentration in MEM was 10% (vol/vol), and for the physiological solution, it ranged between 0.2 and 1.0%. A second aim was to document the stability of single FDB fibers. If tested the day of the preparation, most fibers (∼80%) had stable contractions for up to 3 h, normal stimulus duration strength to elicit contractions, and normal and stable resting membrane potential during prolonged microelectrode penetration. A third aim was to document their fatigue kinetics. Major differences in fatigue resistance were observed between fibers as expected from the FDB fiber-type composition. All sarcoplasmic [Ca(2+)] and sarcomere length parameters returned to their prefatigue levels after a short recovery. The pCa-sarcomere shortening relationship of unfatigued fibers is very similar to the pCa-force curve reported in other studies. The pCa-sarcomere shortening from fatigue data is complicated by large decreases in sarcomere length between contractions. It is concluded that isolation of single fibers by a collagenase digestion is a viable preparation to study contractility and fatigue kinetics.

Effects of salbutamol on the contractile properties of human skeletal muscle before and after fatigue

AIM

The study examined the effects of an oral acute administration of the β2-agonist salbutamol (Sal) (6 mg) vs. placebo on muscle strength and fatigability in 12 non-asthmatic recreational male athletes in a randomized double-blind protocol.

METHODS

Contractile properties of the right quadriceps muscle were measured during electrical stimulations, i.e. twitch, 1-s pulse trains at 20 (P(20) ) and 80 Hz (P(80) ) and during maximal voluntary isometric contraction (MVIC) before (PRE) and after (POST) a fatigue-producing protocol set by an electromyostimulation (30 contractions, frequency: 75 Hz, on-off ratio: 6.25-20s). In addition, the level of muscle voluntary activation was measured.

RESULTS

In PRE and POST conditions, the peak torque (PT) of twitch, P(80) and MVIC were not modified by the treatment. The PT in POST P(20) was slightly, although not significantly, less affected by fatigue in Sal compared with placebo condition. Moreover, twitch half-relaxation time at PRE was smaller under Sal than under placebo (P < 0.05). No significant changes in the degree of voluntary activation were observed with Sal treatment in PRE or POST condition.

CONCLUSION

  Although these findings did not exclude completely an effect of Sal on peripheral factors of human skeletal muscle, oral acute administration of the β2-agonist Sal seems to be without any relevant ergogenic effect on muscle contractility and fatigability in non-asthmatic recreational male athletes.

Effects of lactic acid and catecholamines on contractility in fast-twitch muscles exposed to hyperkalemia

Intensive exercise is associated with a pronounced increase in extracellular K+ ([K+]o). Because of the ensuing depolarization and loss of excitability, this contributes to muscle fatigue. Intensive exercise also increases the level of circulating catecholamines and lactic acid, which both have been shown to alleviate the depressing effect of hyperkalemia in slow-twitch muscles. Because of their larger exercise-induced loss of K+, fast-twitch muscles are more prone to fatigue caused by increased [K+]o than slow-twitch muscles. Fast-twitch muscles also produce more lactic acid. We therefore compared the effects of catecholamines and lactic acid on the maintenance of contractility in rat fast-twitch [extensor digitorum longus (EDL)] and slow-twitch (soleus) muscles. Intact muscles were mounted on force transducers and stimulated electrically to evoke short isometric tetani. Elevated [K+]o (11 and 13 mM) was used to reduce force to approximately 20% of control force at 4 mM K+. In EDL, the beta2-agonist salbutamol (10(-5) M) restored tetanic force to 83 +/- 2% of control force, whereas in soleus salbutamol restored tetanic force to 93 +/- 1%. In both muscles, salbutamol induced hyperpolarization (5-8 mV), reduced intracellular Na+ content and increased Na+-K+ pump activity, leading to an increased K+ tolerance. Lactic acid (24 mM) restored force from 22 +/- 4% to 58 +/- 2% of control force in EDL, an effect that was significantly lower than in soleus muscle. These results amplify and generalize the concept that the exercise-induced acidification and increase in plasma catecholamines counterbalance fatigue arising from rundown of Na+ and K+ gradients.

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.

The sodium pump keeps us going

This invited lecture reviews recent evidence that, in skeletal muscle, excitability and contractility depend on the transmembrane distribution of Na(+) and K(+) and the membrane potential, which in turn are determined by the operation of the Na(+)-K(+) pump. Action potentials are elicited by passive fluxes of Na(+) and K(+). Because of their size and sudden onset, these transport events constitute the major challenge for the Na(+)-K(+) pumps. When the Na(+)-K(+) pumps cannot readily restore the Na(+)-K(+) gradients, working muscle cells often undergo net loss of K(+) and gain of Na(+). This leads to loss of excitability and force, in particular, in muscles where excitation-induced passive Na(+)-K(+) fluxes are large. Thus, excitability depends on the leak/pump ratio for Na(+) and K(+). When this ratio is increased by inhibition or downregulation of the Na(+)-K(+) pumps, the force decline seen during continued stimulation is accelerated. This effect is highly significant already within the first seconds of electrical stimulation. Fortunately, electrical stimulation also increases Na(+)-K(+) pumping rate within seconds. Thus, maximum increase (20-fold above the resting level) may be reached in 10 seconds, with utilization of all available Na(+)-K(+) pumps. In muscles, where excitability was inhibited by exposure to high [K(+)](o) (10-12.5 mM), activation of the Na(+)-K(+) pumps by hormones or electrical stimulation restored excitability and contractile force. In working muscles, the Na(+)-K(+) pumps, because of rapid activation of their large transport capacity, play a dynamic regulatory role in the second-to-second ongoing restoration and maintenance of excitability and force. The Na(+)-K(+) pumps become a limiting factor for contractile endurance, in particular, if their capacity is reduced by inactivity or disease.

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.

Reduced activity of muscle Na(+)-K(+)-ATPase after prolonged running in rats

The purpose of this study was to investigate the hypothesis that Na(+)-K(+)-ATPase activity is reduced in muscle of different fiber composition after a single session of aerobic exercise in rats. In one experiment, untrained female Sprague-Dawley rats (weight 275 +/- 21 g; means +/- SE; n = 30) were run (Run) on a treadmill at 21 m/min and 8% grade until fatigue, or to a maximum of 2 h, which served as control (Con), or performed an additional 45 min of low-intensity exercise at 10 m/min (Run+). In a second experiment, utilizing rats of similar characteristics (weight 258 +/- 18 g; n = 32), Run was followed by passive recovery (Rec). Directly after exercise, rats were anesthetized, and tissue was extracted from Soleus (Sol), red vastus lateralis (RV), white vastus lateralis (WV), and extensor digitorum longus (EDL) and frozen for later analysis. 3-O-methylfluorescein phosphatase activity (3-O-MFPase) was determined as an indicator of Na(+)-K(+)-ATPase activity, and glycogen depletion identified recruitment of each muscle during exercise. 3-O-MFPase was decreased (P < 0.05) at Run+ by an average of 12% from Con in all muscles (P < 0.05). No difference was found between Con and Run. Glycogen was lower (P < 0.05) by 65, 57, 44, and 33% (Sol, EDL, RV, and WV, respectively) at Run, and there was no further depletion during the continued low-intensity exercise period. No differences in Na(+)-K(+)-ATPase activity was observed between Con and Rec. The results of this study indicate that inactivation of Na(+)-K(+)-ATPase can be induced by aerobic exercise in a volume-dependent manner and that the inactivation that occurs is not specific to muscles of different fiber-type composition. Inactivation of Na(+)-K(+)-ATPase suggests intrinsic structural modifications by mechanisms that are unclear.

Insulin-independent, MAPK-dependent stimulation of NKCC activity in skeletal muscle

Na(+)-K(+)-Cl(-) cotransporter (NKCC) activity in quiescent skeletal muscle is modest. However, ex vivo stimulation of muscle for as little as 18 contractions (1 min, 0.3 Hz) dramatically increased the activity of the cotransporter, measured as the bumetanide-sensitive (86)Rb influx, in both soleus and plantaris muscles. This activation of cotransporter activity remained relatively constant for up to 10-Hz stimulation for 1 min, falling off at higher frequencies (30-Hz stimulation for 1 min). Similarly, stimulation of skeletal muscle with adrenergic receptor agonists phenylephrine, isoproterenol, or epinephrine produced a dramatic stimulation of NKCC activity. It did not appear that stimulation of NKCC activity was a reflection of increased Na(+)-K(+)-ATPase activity because insulin treatment did not stimulate NKCC activity, despite insulin's well-known stimulation of Na(+)-K(+)-ATPase activity. Stimulation of NKCC activity could be blocked by pretreatment with inhibitors of mitogen-activated protein kinase (MAPK) kinase 1/2 (MEK1/2) activity, indicating that activation of the extracellular signal-regulated kinase 1/2 (ERK1/2) MAPKs may be required. These data indicate a regulated NKCC activity in skeletal muscle that may provide a significant pathway for potassium transport into skeletal muscle fibers.

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.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

K+ transport in resting rat hind-limb skeletal muscle in response to paraxanthine, a caffeine metabolite

This study tested the hypothesis that paraxanthine, a caffeine metabolite, stimulates skeletal muscle potassium (K+) transport by an increase in Na+-K+ ATPase activity. The unidirectional transport of K+ into muscle (JinK) was studied using a perfused rat hind limb technique. Using 12 hind limbs, we examined the response to 20 min of paraxanthine perfusion (0.1 mM), followed by 20 min perfusion with 0.1 mM paraxanthine and 5 mM ouabain (n = 5) to irreversibly inhibit Na+-K+ ATPase activity. Paraxanthine stimulated JinK by 23 ± 5% within 20 min. Ouabain abolished the paraxanthine-induced stimulation of JinK, suggesting the increase in K+ uptake was due to activation of the Na+-K+ ATPase. To confirm the role of the Na+-K+ ATPase, 14 hind limbs were perfused for 20 min with 5 mM ouabain prior to 20 min perfusion with 0.1 mM paraxanthine and 5 mM ouabain (n = 6). Ouabain alone resulted in a 41 ± 7% decrease in JinK within 15 min. Inhibition of ouabain-sensitive JinK prevented the paraxanthine-induced increase in JinK. Hind limbs (n = 3) were also perfused with 0.1 mM paraxanthine for 60 min to examine the response to longer duration paraxanthine perfusion. The paraxanthine-induced increase in JinK continued for the entire 60 min. In another series, hind limbs were perfused with 0.01 (n = 9), 0.1 (n = 9), or 0.5 (n = 6) mM paraxanthine for 15 min. There was no concentration-dependent relationship between JinK and paraxanthine concentration, and 0.01, 0.1, and 0.5 mM paraxanthine increased JinK similarly (25 ± 5, 22 ± 4, and 27 ± 6%, respectively). The effect of paraxanthine on JinK could not be reversed by subsequent perfusion with paraxanthine-free perfusate. Caffeine (0.05-1.0 mM) had no effect on K+ transport. It is concluded that paraxanthine increases JinK in resting skeletal muscle by stimulating ouabain-sensitive Na+-K+ ATPase activity.Key words: caffeine, methylxanthine, ouabain, potassium transport, sodium pump, Na-K ATPase, VO2, glycolysis.

Alterations in heart failure of cyclic AMP-dependent inotropic and lusitropic properties of cardiac and skeletal muscle

A central working hypothesis in our laboratory is that deficient cellular cyclic AMP concentrations may be responsible, at least in part, for striated muscle dysfunction, both cardiac and skeletal, in heart failure. These results suggest that therapy aimed at restoring cyclic AMP to normal levels may be effective with regard to improving systolic and diastolic function in the heart and may decrease the development of fatigue in skeletal muscle of patients with failure. The use of cyclic AMP-dependent drugs in clinical practice has been limited by side effects associated with raising total cellular content of this cyclic nucleotide. However, evidence suggesting that separate pools of cyclic AMP may exist within the cell raises the possibility that those pools associated with excitation/contraction coupling could serve as more specific therapeutic targets.

The regulation of the Na+,K+ pump in contracting skeletal muscle

Increased passive Na+,K+ fluxes necessitate an efficient activation of the Na+,K+ pump in working muscles to limit the rundown of the Na+,K+ chemical gradients and ensuing loss of excitability. Several studies have demonstrated an increase in Na+,K+-pump rate in working muscles, and in electrically stimulated muscles up to a 22-fold increase in active Na+,K+ transport has been observed. Excitation-induced increase in intracellular Na+ is believed to be the primary stimulus for Na+,K+ pumping in a contracting muscle. In muscles recovering from electrical stimulation, however, the activity of the pump may stay elevated even after intracellular Na+ has been reduced to below the resting level. Moreover, in rat soleus muscles 10-s stimulation at 60 Hz induced a 5-fold increase in the activity of the Na+,K+ pump although mean intracellular [Na+] was unchanged. These findings strongly suggest that a substantial part of the excitation-induced increase in Na+,K+-pump activity is caused by mechanisms other than increased intracellular [Na+]. The mechanism behind this activation is not clear, but may involve a change in the affinity of the Na+,K+ pump for intracellular Na+. In addition to intracellular [Na+], the Na+,K+ pump may be stimulated in contracting muscles by other factors such as catecholamines, calcitonin gene-related peptide (CGRP), free fatty acids and cytoskeletal links. Together, this activation may form a feed forward mechanism protecting muscles from loss of excitability during periods of contraction by increasing Na+,K+-pump activity prior to erosion of the Na+,K+ chemical gradients. During exercise of high intensity, however, intracellular [Na+] increases substantially constituting an additional stimulus for the pump.

The Na+,K+ pump and muscle excitability

In most types of mammalian skeletal muscles the total concentration of Na+,K+ pumps is 0.2-0.8 nmol g wet wt(-1). At rest, only around 5% of these Na+,K+ pumps are active, but during high-frequency stimulation, virtually all Na+,K+ pumps may be called into action within a few seconds. Despite this large capacity for active Na+,K+ transport, excitation often induces a net loss of K+, a net gain of Na+, depolarization and ensuing loss of excitability. In muscles exposed to high [K+]o or low [Na+]o, alone or combined, excitability is reduced. Under these conditions, hormonal or excitation-induced stimulation of the Na+,K+ pump leads to considerable force recovery. This recovery can be blocked by ouabain and seems to be the result of Na+,K+ pump induced hyperpolarization and restoration of Na+,K+ gradients. In muscles where the capacity of the Na+,K+ pump is reduced, the decline in the force developing during continuous electrical stimulation (30-90 Hz) is accelerated and the subsequent force recovery considerably delayed. The loss of endurance is significant within a few seconds after the onset of stimulation. Increased concentration of Na+ channels or open-time of Na+ channels is also associated with reduced endurance and impairment of force recovery. This indicates that during contractile activity, excitability is acutely dependent on the ratio between Na+ entry and Na+,K+ pump capacity. Contrary to previous assumptions, the Na+,K+ pump, due to rapid activation of its large transport capacity seems to play a dynamic role in the from second to second ongoing restoration and maintenance of excitability in working skeletal muscle.

Deficient cellular cyclic AMP may cause both cardiac and skeletal muscle dysfunction in heart failure

Deficient myocardial cyclic AMP concentrations contribute to abnormal Ca2+ handling and systolic and diastolic dysfunction in chronic heart failure (CHF). We tested the hypothesis that decreased cyclic AMP in skeletal muscle of animals with failure may contribute to the weakness and easy fatiguability also common in patients with CHF. We compared intracellular Ca2+ signaling and contractility in skeletal muscle preparations from rats 6 weeks after myocardial infarction-induced CHF versus sham-operated controls. Bundles of 100 to 200 cells were dissected from the extensor digitorum longus (EDL) muscle of control and CHF rats. Muscles from CHF rats exhibited depressed tension development compared with control muscles during twitches. Treatment with 2mM dibutyryl cyclic AMP returned tension and Ca2+ towards normal levels. There was no evidence of cellular atrophy in the CHF rats. In conclusion, EDL skeletal muscle from rats with CHF had intrinsic abnormalities in excitation-contraction coupling that could be reversed with cyclic AMP supplementation as previously reported for the heart. This suggests that deficient cyclic AMP levels may contribute to both cardiac and skeletal muscle dysfunction in CHF.

Insulin-induced translocation of Na+-K+-ATPase subunits to the plasma membrane is muscle fiber type specific

We have previously shown that an acute insulin treatment induces redistribution of the alpha 2- and beta 1- isoforms of the Na+-K+-ATPase from intracellular membranes to plasma membranes detected on subcellular fractionation of mixed muscles and immunoblotting with isoform-specific antibodies (H. S. Hundal et al. J. Biol. Chem. 267: 5040-5043, 1992). In the present study we give both biochemical and morphological evidence that this insulin effect is operative in muscles composed mostly of oxidative (red) fibers but not in muscles composed mostly of glycolytic (white) fibers. The redistribution of the Na+-K+-ATPase alpha 2- and beta 1-isoforms after insulin injection was detected in membranes isolated from and muscles (soleus, red gastrocnemius, red rectus femoris, and red vastus lateralis) but not in membranes from white muscles (white gastrocnemius, tensor fasciae latae, white rectus femoris, and white vastus lateralis). After insulin injection, the potassium-dependent 3-O-methylfluorescein phosphatase activity of the enzyme was higher by 22% in the plasma membrane-enriched fraction and lower by 15% in the internal membrane fraction isolated from red but not from white muscles. Quantitative immunoelectron microscopy of ultrathin muscle cryosections showed that in vivo insulin stimulation augmented the density of Na+-K+-ATPase alpha 2- and beta 1- isoforms at the plasma membrane of soleus muscle by 80 and 124%, respectively, with no change in white gastrocnemius muscle. The effect of insulin to increase the content of Na+-K+-ATPase alpha 2- and beta 1-subunits in isolated plasma membranes was still observed when glycemia was prevented from dropping by using hyperinsulinemic-euglycemic clamps. We conclude that the insulin-induced redistribution of the alpha 2- and beta 1-isoforms of the Na+-K+-ATPase from an intracellular pool to the plasma membrane in restricted to oxidative fiber-type skeletal muscles. This may be related to the selective expression of beta 1-subunits in these fibers and implies that the beta 2-subunit, typical of glycolytic muscles, does not sustain translocation of alpha 2 beta 2-complexes.

Potentiation and depression of the M wave in human biceps brachii

1. The effects of repeated excitation on the compound action potential, or M wave, of mammalian muscle fibres have been investigated in the human biceps brachii. 2. During continuous indirect stimulation at 10 and 20 Hz the mean voltage-time area of the M wave doubled within the first minute, while the mean peak-to-peak amplitude increased by approximately half. The enlargement of the M wave was sustained during stimulation at 10 Hz but not at 20 Hz. Stimulation at 3 Hz caused a small increase which was significant for M wave amplitude only. 3. When the 20 Hz stimulation was performed under ischaemic conditions, the M wave first enlarged and then gradually declined. After 20 Hz stimulation was discontinued, the M wave increased in size; in the ischaemic experiments the release of the cuff produced a further, rapid augmentation. In both the ischaemic and non-ischaemic experiments, the amplitudes and areas of the M waves during the recovery period became significantly larger than the resting values (range, 15-60% at the endplate zone). 4. The mean muscle fibre impulse conduction velocity decreased to less than half the resting value during 20 Hz stimulation, with or without ischaemia, and then increased above the resting value during recovery. 5. On the basis of previous experiments in animals, the augmentation of the M wave was attributed to enhanced electrogenic Na(+)-K+ pumping, and the biceps brachii appeared to be an excellent preparation for studying the time course of this enhancement.

High-frequency fatigue in rat skeletal muscle: role of extracellular ion concentrations

High-frequency fatigue (HFF), the decline of force during continuous tetanic stimulation (lasting 4-40 s), was studied in isolated bundles of rat skeletal muscle fibers. HFF was slower in slow-twitch soleus fibers than in fast-twitch red or white sternomastoid fibers; denervation accelerated fatigue in soleus. Maximal 200-mmol/L potassium contractures of normal amplitude were induced in fatigued fibers, suggesting that crossbridge cycling and the voltage activation of excitation-contraction coupling could still occur maximally, but that activation by action potentials was impaired. An increase in [Na+]o slowed HFF, while a small increase in [K+]o or reduction in [Cl(-)]o accelerated HFF. Increasing the tetanic stimulation frequency exacerbated fatigue. Recovery from HFF proceeded rapidly since force increased markedly within a few seconds when stimulation ceased. These results support the hypothesis that a redistribution of Na+, K+, and Cl- across the transverse tubular membranes during repeated action potential activity induces fatigue by reducing the amplitude and conduction of action potentials.

Kinetics of plasma potassium concentrations during exhausting exercise in trained and untrained men

The purpose of this study was to examine the time course of changes in plasma potassium concentration during high intensity exercise and recovery in trained and untrained men. The subjects performed two exercise protocols, an incremental test and a sprint, on a cycle ergometer. A polyethylene catheter was inserted into the antecubital vein to obtain blood samples for the analysis of plasma electrolyte concentrations and acid-base parameters, during and after exercise. During both tests, venous plasma sodium, potassium and chloride concentrations increased in all the subjects, although the largest relative increase was detected in potassium concentration--35% and 31% over rest in the progressive test and 61% and 37.7% in the sprint test, for cyclists and controls, respectively. After exercise plasma potassium concentration decreased exponentially to below resting values. There was a linear correlation between the amount of potassium accumulated in plasma during exercise and the amount eliminated from plasma when the exercise ceased. We found that, although plasma potassium accumulation occurred in both forms of exercise in the trained and nontrained subjects, the time constant of potassium decrease following exercise was shorter in the trained subjects. Thus, the trained subjects exhibited a better capacity to recover to resting concentrations of plasma potassium. We propose that the extracellular potassium accumulation acts as a negative feedback signal for sarcolemma excitability depending on the muscle metabolic rate.

Role of interstitial potassium

Interstitial potassium concentration, [K+], is modulated during muscle activity due to a number of different mechanisms: diffusion and active transport of K+ in combination with water fluxes. The relative significance of the various mechanisms for muscle function is quantified. The effect of interstitial [K+] locally on the single muscle fiber is discussed along with its effect on the cardiovascular and respiratory systems and its role in motor control. It is concluded that K+ may play a significant role in the prevention as well as the development of fatigue.

Intracellular Na+ and K+ shifts induced by contractile activities of rat skeletal muscles

The effects of direct and indirect electrical stimulation on intracellular potassium and sodium contents ([K]i and [Na]i, respectively) in rat soleus muscle (SOL) and extensor digitorum longus muscle (EDL) were investigated under in vivo conditions. The changes of [K]i and [Na]i contents in both muscles which were stimulated indirectly reached respective values at 30 min or 1 hr after the beginning of stimulation, whereas those of EDL stimulated with 60 Hz changed gradually through 2 hr stimulation. The shifts of [K]i and [Na]i in EDL occurred during the twitch contraction at considerably lower frequency stimulation (0.5-10 Hz), whereas those in SOL were observed during the tetanus contraction at high frequency stimulation (10-40 Hz). The difference of change in cationic shifts between EDL and SOL under low frequency stimulation was reduced by ouabain treatment, though the difference was still significant. When the muscles were indirectly stimulated 6000 times at 1, 5, 10 and 20 Hz, the cationic shifts in EDL were greater than those in SOL, extending over all frequencies. It was concluded that such a difference in ionic shift between contracting EDL and SOL may be primarily due to the difference in unidirectional ionic fluxes per stimulation and, secondly, to the difference in Na(+)-K+ pump activity.

The Na+,K(+)-pump and muscle contractility

In skeletal muscle, the excitation induced influx of Na+ and efflux of K+ may be sufficient to exceed the activity or even the capacity of the available Na+,K(+)-pumps. This leads to a rise in intracellular Na+ and extracellular K+. Both events interfere with excitability and may present important limitations for the continuation of contractile activity. Furthermore, inhibition of the Na+,K(+)-pump or reduction of the concentration of functional Na+,K(+)-pumps decrease excitability and the maintenance of force during continued stimulation. Conversely, in muscles where contractile force is inhibited by exposure to high extracellular K+, acute stimulation of the Na+,K(+)-pump with catecholamines, CGRP or insulin leads to a rapid recovery of force. The large passive fluxes of Na+ and K+ associated with excitation constitute the major drive on the activity of the Na+,K(+)-pump, giving rise to up to 20-fold stimulation of the transport rate. In keeping with this, training induces an upregulation of the total concentration of Na+,K(+)-pumps in skeletal muscle. The activity and the capacity of the Na+,K(+)-pump are important limiting factors determining the maintenance of excitability and contractile performance.

Pseudofacilitation: a misleading term

The possible causes of the transient enlargement of muscle compound action potentials during repetitive stimulation ("pseudofacilitation") are considered. The phenomenon cannot be due to mechanical artefact, while hypersynchronization of the muscle fiber action potentials, the usual explanation, can only make a minor contribution. A more convincing explanation, for which there is now experimental evidence, is that the muscle fibers undergo hyperpolarization, due to the intramuscular release of norepinephrine and consequent stimulation of the electrogenic Na+,K(+)-pump. Defective phosphorylation of the Na+,K(+)-pump is a possible cause of the transient weakness and myotonia in myotonic dystrophy.

K+ shifts of skeletal muscle during stepwise bicycle exercise with and without beta-adrenoceptor blockade

1. K+ efflux rate and control of K+ reuptake rate in exercising muscle cells was examined in six healthy female volunteers. 2. A K(+)-selective electrode in the femoral vein continuously monitored K+ concentration ([K+]fv) during bicycling. Power was increased stepwise 5-6 times by 30-40 W every fourth minute until exhaustion before and after I.V. administration of propranolol. Leg blood flow was measured by bolus injections of Cardiogreen. 3. [K+]fv increased from about 4.3 to 6.8 mmol l-1 at exhaustion both before and after propranolol administration, but after drug infusion endurance was reduced from 22.2 +/- 0.6 to 19.7 +/- 1.1 min, so [K+]fv rose more rapidly. 4. The exercise-induced efflux rate of K+ from the muscle cells was estimated to be about 11 mumol kg-1s-1 at exhaustion both before and after propranolol administration. 5. As an indicator of rate of net loss of K+ from the leg, veno-arterial concentration differences ([K+]fv-a) during first, fourth and fifth power increments were high after 15 and 40 s, but declined toward the end of each power step. Propranolol accentuated [K+]fv-a only after 15 and 40 s of the first and fourth increments. 6. The exercise-induced increase in reuptake rate of K+ in the muscle, estimated at exhaustion, was not significantly changed by propranolol and was about 10 mumol kg-1s-1, corresponding to about 15% of maximum Na(+)-K+ pump capacity in man. 7. Extracellular accumulation and loss of K+ from muscle during bicycle exercise is due to Na(+)-K+ pump lag. The higher [K+]fv during propranolol is mainly due to impaired redistribution outside the exercising muscles. In addition at low powers, beta-adrenoceptor blockade caused a transiently increased net loss due to an accentuated Na(+)-K+ pump lag.

Beta-adrenoceptor activation shows high-frequency fatigue in skeletal muscle fibers of the rat

The effect of terbutaline (a beta 2-adrenergic agonist) on high-frequency fatigue (HFF) was studied in small bundles of rat soleus muscle fibers. HFF, the decline in force during continuous stimulation (50 Hz for 20 s), was reduced by 10-20% with 10 microM terbutaline. A similar reduction in HFF with 2 mM dibutyryl-adenosine 3',5'-cyclic monophosphate (DBcAMP) implicated adenosine 3',5'-cyclic monophosphate (cAMP) as the second messenger in the terbutaline effect. Sodium (Na-K)-pump inhibition with 1 mM ouabain depressed peak tetanic force but did not significantly alter either the subsequent fatigue or the effect of terbutaline on fatigue. This suggested that the pump was neither rate limiting in HFF nor involved in the terbutaline effect. Nevertheless, a significant hyperpolarization recorded with terbutaline implied that beta 2-adrenoceptor activation stimulated the Na-K pump at rest. Caffeine (1 mM) slowed HFF and prevented additional effects with terbutaline. Caffeine is known to potentiate Ca2+ release from the sarcoplasmic reticulum (SR), and we suggest that terbutaline, acting via cAMP, facilitates Ca2+ release from the SR to better maintain myoplasmic Ca2+ concentration during continuous tetanic stimulation.

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.

Na(+)-K+ pump stimulation elicits recovery of contractility in K(+)-paralysed rat muscle

1. This study explores the role of active electrogenic Na(+)-K+ transport in restoring contractility in isolated rat soleus muscles exposed to high extracellular potassium concentration ([K+]o). This was done using agents (catecholamines and insulin) known to stimulate the Na(+)-K+ pump via different mechanisms. 2. When exposed to Krebs-Ringer bicarbonate buffer containing 10 mM K+, the isometric twitch and tetanic force of intact muscles decreased by 40-69%. The major part of this decline could be prevented by the addition of salbutamol (10(-5) M). In the presence of 10 mM K+, force could be restored almost completely within 5-10 min by the addition of salbutamol or adrenaline and partly by insulin. 3. In muscles exposed to 12.5 mM K+, force declined by 96%. Salbutamol (10(-5) M), adrenaline (10(-6) M) and insulin (100 mU ml-1) produced 57-71, 61-71 and 38-47% recovery of force within 10-20 min, respectively. The effects of these supramaximal concentrations of salbutamol and insulin on force recovery were additive. Salbutamol and adrenaline produced significant recovery of contractility at concentrations down to 10(-8) M (P < 0.005). 4. In soleus, the same agents stimulated 86Rb+ uptake and decreased intracellular Na+. These actions reflect stimulation of active Na(+)-K+ transport and both showed a highly significant correlation to the recovery of twitch as well as tetanic force (r = 0.80-0.88; P < 0.001). 5. The force recovery induced by salbutamol, adrenaline and insulin was suppressed by pre-exposure to ouabain (10(-5) M for 10 min or 10(-3) M for 1 min) as well as by tetrodotoxin (10(-6) M). 6. The observations support the conclusion that the inhibitory effect of high [K+]o on contractility in skeletal muscle can be counterbalanced by stimulation of active electrogenic Na(+)-K+ transport, the ensuing increase in the clearance of extracellular K+ and in the transmembrane electrochemical gradient for Na+.

Regulation of hepatic glucose production during exercise in humans: role of sympathoadrenergic activity

To investigate the role of sympathoadrenergic activity on glucose production (Ra) during exercise, eight healthy males bicycled 20 min at 41 +/- 2 and 74 +/- 4% maximal O2 uptake (VO2max; mean +/- SE) either without (control; Co) or with blockade of sympathetic nerve activity to liver and adrenal medulla by local anesthesia of the celiac ganglion (Bl). Epinephrine (Epi) was in some experiments infused during blockade to match (normal Epi) or exceed (high Epi) Epi levels during Co. A constant infusion of somatostatin and glucagon was given before and during exercise. At rest, insulin was infused at a rate maintaining euglycemia. During intense exercise, insulin infusion was halved to mimic physiological conditions. During exercise, Ra increased in Co from 14.4 +/- 1.0 to 27.8 +/- 3.0 mumol.min-1.kg-1 (41% VO2max) and to 42.3 +/- 5.2 (74% VO2max; P < 0.05). At 41% VO2max, plasma glucose decreased, whereas it increased during 74% VO2max. Ra was not influenced by Bl. In high Epi, Ra rose more markedly compared with control (P < 0.05), and plasma glucose did not fall during mild exercise and increased more during intense exercise (P < 0.05). Free fatty acid and glycerol concentrations were always lower during exercise with than without celiac blockade. We conclude that high physiological concentrations of Epi can enhance Ra in exercising humans, but normally Epi is not a major stimulus. The study suggests that neither sympathetic liver nerve activity is a major stimulus for Ra during exercise. The Ra response is enhanced by a decrease in insulin and probably by unknown stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in K+, Na+ and calcium contents during in vivo stimulation of rat skeletal muscle

The effects of in vivo stimulation via the sciatic nerve on Na+, K+ and calcium contents in slow-twitch and fast-twitch muscles were compared. Whereas intermittent stimulation for 24 h at 20 Hz caused only minor changes in soleus (SOL), a considerable loss of K+ (around 24%) and gain of Na+ (around 84%) was observed in extensor digitorum longus (EDL) and tibialis anterior (TA) muscles. These changes could be detected within 0.5 h and a plateau was maintained from 2 to 24 h. Total calcium content increased progressively, reaching values 245 and 382% above the control level in EDL and TA muscle, respectively, after 24 h of 20 Hz stimulation. Whereas the Na+ and K+ content recovered within a few hours, calcium content did not return towards control level until after 48 h of rest. In a pilot study performed with continuous stimulation at 10 Hz, the changes in Na+ and K+ contents in SOL, EDL and TA muscle were comparable to those at 20 Hz. The concentration of the Na(+)-K+ pumps was highest in the fast-twitch EDL and TA muscles and was unaffected by 10 Hz stimulation. It is concluded that a stimulation pattern leading to a rise in intracellular Na+ and a loss of K+ may cause a marked accumulation of calcium. These events seem to be related to insufficient activation of the Na-K+ pump rather than to variations in the total Na(+)-K+ pump capacity.

Correlation between magnesium and potassium contents in muscle: role of Na(+)-K+ pump

In young rats fed a Mg(2+)-deficient diet for 3 wk, Mg2+ and K+ contents in soleus and extensor digitorum longus muscles were significantly reduced and closely correlated. In isolated soleus muscles, Mg2+ depletion induced an even more pronounced loss of K+, and Mg2+ and K+ contents were correlated over a wide range (r = 0.95, P < 0.001). Extracellular Mg2+ (0-1.2 mM) caused no change in total or ouabain-suppressible 86Rb influx. After long-term incubation in Ca(2+)-Mg(2+)-free buffer with EDTA and EGTA, cellular Mg2+ and K+ contents were reduced by 35 and 15%, respectively, without any reduction in ATP and total or ouabain-suppressible 86Rb influx. In Mg(2+)-depleted muscles 42K efflux was increased by up to 42%, and repletion with Mg2+ produced a graded decrease. We conclude that Mg2+ and K+ contents are closely correlated in muscles Mg2+ depleted in vivo or in vitro and that neither extracellular nor moderate intracellular Mg2+ depletion affects total or Na(+)-K+ pump-mediated K+ influx. The reduced K+ content may rather be related to increased K+ efflux from the muscles.

Na+ current densities and voltage dependence in human intercostal muscle fibres

1. Voltage-clamp Na+ currents (INa) were studied in human intercostal muscle fibres using the loose-patch-clamp technique. 2. The fibres could be divided into two groups based upon the properties of INa. The two groups of fibres were called type 1 and type 2. 3. Both type 1 and type 2 fibres demonstrated fast and slow inactivation of INa. 4. Type 1 fibres had lower INa on the endplate border and extrajunctional membrane than type 2 fibres and required larger membrane depolarizations to inactivate Na+ channels by fast or slow inactivation of INa. 5. Type 2 fibres had a higher ratio of INa at the endplate border compared to extrajunctional membrane than Type 1 fibres. 6. Measurement of membrane capacitance suggested that the increase in INa at the endplate border was due to increased Na+ channel density. 7. Histochemical staining of some fibres suggested that type 1 fibres were slow twitch and type 2 fibres were fast twitch. 8. Differences in the properties of Na+ channels between fast- and slow-twitch fibres may contribute to the ability of fast-twitch fibres to operate at high firing frequencies and slow-twitch fibres to be tonically active.

Transient hyperpolarization of non-contracting muscle fibres in anaesthetized rats

1. Following laminectomy, the L5 ventral roots of anaesthetized rats were split and approximately half of the nerve fibres were stimulated at 40 Hz. Resting membrane potentials were then measured in previously contracting and non-contracting soleus muscle fibres. 2. In the non-contracting soleus fibres there was a post-tetanic increase in mean resting potential (from -82.2 +/- 7.1 (S.D.) to -91.6 +/- 8.7 mV) which was similar to that in contracting fibres (from -83.02 +/- 6.1 to -91.3 +/- 7.3 mV). In both types of fibres the hyperpolarizing responses (HRs) were evidently due to increased sodium pump activity since they could be abolished by the addition of ouabain (1 x 10(-4) M) to the bathing fluid. 3. The beta-adrenergic antagonist, propranolol, completely suppressed HRs in the non-contracting fibres and produced moderate reductions in the contracting ones. The alpha-adrenergic blocking agent, phentolamine, had no effect on the contracting fibres and only a modest, inhibitory, one on the non-contracting fibres. 4. On the basis of the above drug actions it appeared that catecholamines were necessary for the full development of HRs in contracting and non-contracting fibres; noradrenaline, released from intramuscular sympathetic nerve endings, may have been involved. 5. The increased sodium pump activity in the non-contracting fibres would serve to moderate the rise in interstitial [K+] caused by K+ efflux from the contracting fibres. By preventing passive depolarization, due to the rise in interstitial [K+], the sodium pump would also maintain the availability of non-contracting fibres for subsequent recruitment.

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.

Na current density at and away from end plates on rat fast- and slow-twitch skeletal muscle fibers

Na current density and membrane capacitance were studied with the loose patch voltage clamp technique on rat fast- and slow-twitch skeletal muscle fibers at three different regions on the fibers: 1) the end plate border, 2) greater than 200 microns from the end plate (extrajunctional), and 3) on the end plate postsynaptic membrane. Fibers were treated with collagenase to improve visualization of the end plate and to enzymatically remove the nerve terminal. The capacitance of membrane patches was similar on fast- and slow-twitch fibers and patches of membrane on the end plate had twice the capacitance of patches elsewhere. For fast- and slow-twitch fibers, the sizes of the Na current normalized to the area of the patch were as follows: end plate greater than end plate border greater than extrajunctional. For both types of fibers, the amplitudes of the Na current normalized to the capacitance of the membrane patch were as follows: end plate approximately end plate border greater than extrajunctional. At each of the three regions, the Na current densities were larger on fast-twitch fibers and fast-twitch fibers had a larger increase in Na current density at the end plate border compared with extrajunctional membrane.

Effects of selective beta 2-adrenoceptor blockade on serum potassium and exercise performance in normal men

1. The differential effects of beta-adrenoceptor subtypes on potassium fluxes and exercise capacity were compared in eight healthy young men using single oral doses of the selective beta 2-adrenoceptor antagonist ICI-118551, the selective beta 1-adrenoceptor antagonist atenolol or the non-selective beta-adrenoceptor antagonist propranolol. The study was randomized, double-blind and placebo controlled. 2. Potassium in the venous effluent from the exercising muscles increased progressively with increasing exercise intensity. This response was augmented by propranolol, whereas neither atenolol nor ICI-118551 modified the response. After exercise potassium concentration fell exponentially with no difference between the treatment regimens. 3. Cumulative work was significantly reduced by ICI-118551 (6.4%, P = 0.04) and by propranolol (12.4%, P less than 0.01), whereas the reduction with atenolol (5.6%) did not reach statistical significance. 4. Atenolol and propranolol reduced peak heart rate by 23% and 29%, and peak systolic blood pressure by 9% and 11% respectively during maximal exercise. ICI-118551 caused a non-significant reduction in heart rate during submaximal exercise, with a significant reduction at maximum exercise (6% reduction), whereas systolic blood pressure was not different from placebo. Diastolic blood pressures were similar across all treatment regimens. 5. Similar glucose concentrations were obtained at baseline and at exhaustion during all treatment regimens. Lactate concentrations were comparable for any given exercise intensity irrespective of treatment regimens. Propranolol reduced lactate concentrations from the exercising muscles at maximum exercise in proportion to the reduction of maximal exercise capacity. 6. The subjective perception of fatigue was not affected by either beta 1- or beta 2-adrenoceptor blockade.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium regulation during exercise and recovery

The concentrations of extracellular and intracellular potassium (K+) in skeletal muscle influence muscle cell function and are also important determinants of cardiovascular and respiratory function. Several studies over the years have shown that exercise results in a release of K+ ions from contracting muscles which produces a decrease in intracellular K+ concentrations and an increase in plasma K+ concentrations. Following exercise there is a recovery of intracellular K+ concentrations in previously contracting muscle and plasma K+ concentrations rapidly return to resting values. The cardiovascular and respiratory responses to K+ released by contracting muscle produce some changes which aid exercise performance. Increases in the interstitial K+ concentrations of contracting muscles stimulate CIII and CIV afferents to directly stimulate heart rate and the rate of ventilation. Localised K+ release causes a vasodilatation of the vascular bed within contracting muscle. This, together with the increase in cardiac output (through increased heart rate), results in an increase in blood flow to isometrically contracted muscle upon cessation of contraction and to dynamically contracting muscle. This exercise hyperaemia aids in the delivery of metabolic substrates to, and in the removal of metabolic endproducts from, contracting and recovering muscle tissues. In contrast to the beneficial respiratory and cardiovascular effects of elevations in interstitial and plasma K+ concentrations, the responses of contracting muscle to decreases in intracellular K+ concentrations and increases in intracellular Na+ concentrations and extracellular K+ concentrations contribute to a reduction in the strength of muscular contraction. Muscle K+ loss has thus been cited as a major factor associated with or contributing to muscle fatigue. The sarcolemma, because of changes in intracellular and extracellular K+ concentrations and Na+ concentrations on the membrane potential and cell excitability, contributes to a fatigue 'safety mechanism'. The purpose of this safety mechanism would be to prevent the muscle cell from the self-destruction which is evident upon overload (metabolic insufficiency) of the tissues. The net loss of K+ and associated net gain of Na+ by contracting muscles may contribute to the pain and degenerative changes seen with prolonged exercise. During exercise, mechanisms are brought into play which serve to regulate cellular and whole body K+ homeostasis. Increased rates of uptake of K+ by contracting muscles and inactive tissues through activation of the Na(+)-K+ pump serve to restore active muscle intracellular K+ concentrations towards precontraction levels and to prevent plasma K+ concentrations from rising to toxic levels. These effects are at least partially mediated by exercise-induced increases in plasma catecholamines, particularly adrenaline.(ABSTRACT TRUNCATED AT 400 WORDS)

Plasma potassium changes with high intensity exercise

1. Exercise seems to change the extracellular potassium concentration far beyond the narrow limits seen in resting subjects. To examine alterations in plasma potassium concentration during exercise, twenty healthy, well-trained men ran on the treadmill at 6 deg inclination with catheters inserted in the femoral vein and artery. 2. During 1 min exhausting exercise plasma potassium concentration rose in parallel in the vein and artery, reaching peak post-exercise values of 8.34 +/- 0.23 mmol l-1 and 8.17 +/- 0.29 mmol l-1. After 3 min recovery the potassium concentration was 0.50 +/- 0.05 mmol l-1 below pre-exercise values. Both the rise of plasma potassium concentration during exercise and the decline during recovery followed exponential time courses with a half-time of 25 s. 3. Exercise at reduced intensity showed that the peak post-exercise potassium concentration was linearly related to the exercise intensity. Individual resting, peak and nadir values were proportionally related. 4. The increased potassium concentration during exercise can be explained in full by the electrical activity in the exercising muscles. Repeated 1 min exhausting exercise bouts revealed no relationship between potassium concentration and plasma pH nor glycogen break-down. 5. All of the observations fit a simple model of potassium efflux from active muscle and elimination from blood with the following characteristics: the efflux increases (decreases) stepwise at the onset (end) of exercise, and the efflux rate during exercise increases with exercise intensity. Potassium is eliminated from blood by a proportional regulator which may be the Na(+)-K+ pump of the exercising muscle. Extracellular potassium is indirectly linked to the pump stimulus, and the rate of reuptake is proportional to the extracellular accumulation. Thus no limited maximal power for potassium uptake was found. The post-exercise undershoot of 0.5 mmol l-1 can be explained by a higher gain of the pump after exercise. 6. The large, rapid changes in the plasma potassium concentration during and after exercise is due to the first order kinetics of the reuptake mechanism rather than to a limited power to take up potassium.