Effects of manipulating tetanic calcium on the curvature of the force-velocity relationship in isolated rat soleus muscles.

AIM: In dynamically contracting muscles, increased curvature of the force-velocity relationship contributes to the loss of power during fatigue. It has been proposed that fatigue-induced reduction in [Ca++ ]i causes this increased curvature. However, earlier studies on single fibres have been conducted at low temperatures. Here, we investigated the hypothesis that curvature is increased by reductions in tetanic [Ca++ ]i in isolated skeletal muscle at near-physiological temperatures. METHODS: Rat soleus muscles were stimulated at 60 Hz in standard Krebs-Ringer buffer, and contraction force and velocity were measured. Tetanic [Ca++ ]i was in some experiments either lowered by addition of 10 µmol/L dantrolene or use of submaximal stimulation (30 Hz) or increased by addition of 2 mmol/L caffeine. Force-velocity curves were constructed by fitting shortening velocity at different loading forces to the Hill equation. Curvature was determined as the ratio a/F0 with increased curvature reflecting decreased a/F0 . RESULTS: Compared to control levels, lowering tetanic [Ca++ ]i with dantrolene or reduced stimulation frequency decreased the curvature slightly as judged from increase in a/F0 of 13 ± 1% (P = < .001) and 20 ± 2% (P = < .001) respectively. In contrast, increasing tetanic [Ca++ ]i with caffeine increased the curvature (a/F0 decreased by 17 ± 1%; P = < .001). CONCLUSION: Contrary to our hypothesis, interventions that reduced tetanic [Ca++ ]i caused a decrease in curvature, while increasing tetanic [Ca++ ]i increased the curvature. These results reject a simple causal relation between [Ca++ ]i and curvature of the force-velocity relation during fatigue.

Time-of-day influences postural balance in older adults.

BACKGROUND: Postural balance assessments are performed in both clinical and basic research settings on a daily basis. During a 24-h time span our physiology and physical performance undergo radical changes as we are influenced by the circadian rhythm. The time-of-day interaction on postural balance is unknown in older adults. The aim of this study was to investigate the time-of-day effect on postural balance in older adults. METHODS: Center of pressure (CoP) excursion was measured (100 Hz) by force plate analysis in 34 older adults during 30 s of narrow quiet bilateral stance. Measurements were performed around 9a.m., 12.30 p.m. and 4 p.m. on the same day. Postural balance was quantified by velocity-moment, confidence ellipse area, total sway area and total sway length. RESULTS: An overall significant time-of-day (between 9 a.m. and 4 p.m.) effect was observed for velocity-moment (mm(2)/s) 57 ± 27-65 ± 29 (p = 0.001), confidence ellipse area (mm(2)) 36 ± 16-44 ± 19 (p < 0.001), total sway area (mm(2)) 548 ± 263-627 ± 285 (p = 0.001) and total sway length (mm) 373 ± 120-379 ± 113 (p = 0.037). The variation of postural balance was mostly pronounced from midday (12.30 p.m.) toward the afternoon (4 p.m.) in all sway parameters. Specifically between 12.30 p.m. and 4 p.m. confidence ellipse area increased by 18.5%, total sway area by 17.1%, velocity-moment by 15.8% and total sway length by 4.6%. No differences were observed between 9 a.m. and 12.30 p.m. in any of the sway parameters. CONCLUSIONS: This study demonstrates that time-of-day influences postural balance in older adults. These findings have important scientific and clinical relevance, as they imply that time-of-day should be a controlled factor when assessing postural balance in older adults.

Effects of calcitonin gene-related peptide on rat soleus muscle excitability: mechanisms and physiological significance.

Intense exercise causes a large loss of K(+) from contracting muscles. The ensuing elevation of extracellular K(+) ([K(+)](o)) has been suggested to cause fatigue by depressing muscle fiber excitability. In isolated muscles, however, repeated contractions confer some protection against this effect of elevated K(+). We hypothesize that this excitation-induced force-recovery is related to the release of the neuropeptide calcitonin gene-related peptide (CGRP), which stimulates the muscular Na(+)-K(+) pumps. Using the specific CGRP antagonist CGRP-(8-37), we evaluated the role of CGRP in the excitation-induced force recovery and examined possible mechanisms. Intact rat soleus muscles were stimulated to evoke short tetani at regular intervals. Increasing extracellular K(+) ([K(+)](o)) from 4 to 11 mM decreased force to approximately 20% of initial force (P < 0.001). Addition of exogenous CGRP (10(-9) M), release of endogenous CGRP with capsaicin, or repeated electrical stimulation recovered force to 50-70% of initial force (P < 0.001). In all cases, force recovery could be almost completely suppressed by CGRP-(8-37). At 11 mM [K(+)](o), CGRP (10(-8) M) did not alter resting membrane potential or conductance but significantly improved action potentials (P < 0.001) and increased the proportion of excitable fibers from 32 to 70% (P < 0.001). CGRP was shown to induce substantial force recovery with only modest Na(+)-K(+) pump stimulation. We conclude that the excitation-induced force recovery is caused by a recovery of excitability, induced by local release of CGRP. The data suggest that the recovery of excitability partly was induced by Na(+)-K(+) pump stimulation and partly by altering Na(+) channel function.

Analysis of exercise-induced Na+-K+ exchange in rat skeletal muscle in vivo.

We aimed to quantify the Na(+)-K(+) exchange occurring during exercise in rat skeletal muscle in vivo. Intracellular Na(+) and K(+) content, Na(+) permeability ((22)Na(+) influx), Na(+)-K(+) pump activity (ouabain-sensitive (86)Rb(+) uptake) and Na(+)-K(+) pump alpha(2) subunit content ([(3)H]ouabain binding) were measured. Six-week-old rats rested (control animals) or performed intermittent running for 10-60 min and were then killed or were killed at 15 or 90 min following 60 min exercise. In the soleus muscle, intracellular Na(+) was 80% higher than in control rats after 60 min exercise, was still elevated (38%) after 15 min rest and returned to control levels after 90 min rest. Intracellular K(+) showed corresponding decreases after 15-60 min exercise, returning to control levels 90 min postexercise. Exercise induced little change in Na(+) and K(+) in the extensor digitorum longus muscle (EDL). In soleus, the exercise-induced rise in Na(+) and reduction in K(+) were augmented by pretreatment with ouabain or by reducing the content of muscular Na(+)-K(+) pumps by prior K(+) depletion of the animals. Fifteen minutes after 60 min exercise, ouabain-sensitive (86)Rb(+) uptake in the soleus was increased by 30% but was unchanged in EDL, and there was no effect of exercise on [(3)H]ouabain binding measured in vitro or in vivo in either muscle. In conclusion, in the soleus, in vivo exercise induces a rise in intracellular Na(+), which reflects the excitation-induced increase in Na(+) influx and leads to augmented Na(+)-K(+) pump activity without apparent change in Na(+)-K(+) pump capacity.

Energy conservation attenuates the loss of skeletal muscle excitability during intense contractions.

High-frequency stimulation of skeletal muscle has long been associated with ionic perturbations, resulting in the loss of membrane excitability, which may prevent action potential propagation and result in skeletal muscle fatigue. Associated with intense skeletal muscle contractions are large changes in muscle metabolites. However, the role of metabolites in the loss of muscle excitability is not clear. The metabolic state of isolated rat extensor digitorum longus muscles at 30 degrees C was manipulated by decreasing energy expenditure and thereby allowed investigation of the effects of energy conservation on skeletal muscle excitability. Muscle ATP utilization was reduced using a combination of the cross-bridge cycling blocker N-benzyl-p-toluene sulfonamide (BTS) and the SR Ca2+ release channel blocker Na-dantrolene, which reduce activity of the myosin ATPase and SR Ca2+-ATPase. Compared with control muscles, the resting metabolites ATP, phosphocreatine, creatine, and lactate, as well as the resting muscle excitability as measured by M-waves, were unaffected by treatment with BTS plus dantrolene. Following 20 or 30 s of continuous 60-Hz stimulation, BTS-plus-dantrolene-treated muscles showed a 25% lower ATP utilization compared with control muscles. Furthermore, the ability of muscles to maintain excitability during high-frequency stimulation was significantly improved in BTS-plus-dantrolene-treated muscles, indicating a strong link between metabolites, energetic state, and the excitability of the muscle.

N-Benzyl-p-toluene sulphonamide allows the recording of trains of intracellular action potentials from nerve-stimulated intact fast-twitch skeletal muscle of the rat.

In skeletal muscle, the intracellular recording of trains of action potentials is difficult owing to the movement of the muscle upon stimulation. A potential tool for the removal of muscle movement is the cross-bridge cycle blocker, N-benzyl-p-toluene sulphonamide (BTS), although the effects of BTS on the passive and active membrane properties of intact muscle fibres are not known. Rat extensor digitorum longus (EDL) muscle was used to show that 50 mum BTS reduced tetanic force to approximately 10% of control force, without markedly altering muscle excitability. Incubation with BTS did not alter intracellular K+ content or Na+-K+ pump activity, but produced minor decreases in intracellular Na+ content (7%), resting 22Na+ influx (14%) and excitation-induced 22Na+ influx (29%). Despite these alterations to Na+ fluxes, BTS did not impair muscle excitability, since membrane conductance, resting membrane potential (RMP), rheobase current and the amplitude, overshoot and maximum rate of depolarization of the action potential were all unaltered. However, BTS did induce a small (8%) decrease in the maximum rate of repolarization of the action potential and an increase in the refractory period. The minor effects of BTS on muscle membrane properties did not compromise the ability of the muscle to propagate action potentials, even during tetanic stimulation. In conclusion, BTS can be used successfully to reduce contractility, allowing the intracellular recording of action potentials during both twitch and tetanic contraction of nerve-stimulated muscle, thus making it an excellent tool for the study of electrophysiology in fast-twitch skeletal muscle.

Na+-K+ pump stimulation restores carbacholine-induced loss of excitability and contractility in rat skeletal muscle.

Intense exercise results in increases in intracellular Na+ and extracellular K+ concentrations, leading to depolarization and a loss of muscle excitability and contractility. Here, we use carbacholine to chronically activate the nicotinic acetylcholine (nACh) receptors to mimic the changes in membrane permeability, chemical Na+ and K+ gradients and membrane potential observed during intense exercise. Intact rat soleus muscles were mounted on force transducers and stimulated electrically to evoke short tetani at regular intervals. Carbacholine produced a 2.6-fold increase in Na+ influx that was tetrodotoxin (TTX) insensitive, but abolished by tubocurarine, resulting in a significant 36% increase in intracellular Na+, and 8% decrease in intracellular K+ content. The mid region, near the motor end plate, had much larger alterations than the more distal regions of the muscle, and showed a larger membrane depolarization from -73 +/- 1 to -60 +/- 1 mV compared with -64 +/- 1 mV. Carbacholine (10(-4) M) significantly reduced tetanic force to 31 +/- 3% of controls, which underwent significant recovery upon application of Na+-K+ pump stimulators: salbutamol (10(-5) M), adrenaline (10(-5) M) and calcitonin gene-related peptide (CGRP; 10(-7) M). The force recovery with salbutamol was accompanied by a recovery of intracellular Na+ and K+ contents, and a small but significant 4-5 mV recovery of membrane potential. Similar results were obtained using succinylcholine (10(-4) M), indicating that Na+-K+ pump stimulation may prevent or restore succinylcholine-induced hyperkalaemia. The stimulation of the Na+-K+ pump allows muscle to partially recover contractility by regaining excitability through electrogenically driven repolarization of the muscle membrane.

Excitability of the T-tubular system in rat skeletal muscle: roles of K+ and Na+ gradients and Na+-K+ pump activity.

Strenuous exercise causes an increase in extracellular [K(+)] and intracellular Na(+) ([Na(+)](i)) of working muscles, which may reduce sarcolemma excitability. The excitability of the sarcolemma is, however, to some extent protected by a concomitant increase in the activity of muscle Na(+)-K(+) pumps. The exercise-induced build-up of extracellular K(+) is most likely larger in the T-tubules than in the interstitium but the significance of the cation shifts and Na(+)-K(+) pump for the excitability of the T-tubular membrane and the voltage sensors is largely unknown. Using mechanically skinned fibres, we here study the role of the Na(+)-K(+) pump in maintaining T-tubular function in fibres with reduced chemical K(+) gradient. The Na(+)-K(+) pump activity was manipulated by changing [Na(+)](i). The responsiveness of the T-tubules was evaluated from the excitation-induced force production of the fibres. Compared to control twitch force in fibres with a close to normal intracellular [K(+)] ([K(+)](i)), a reduction in [K(+)](i) to below 60 mM significantly reduced twitch force. Between 10 and 50 mM Na(+), the reduction in force depended on [Na(+)](i), the twitch force at 40 mM K(+) being 22 +/- 4 and 54 +/- 9% (of control force) at a [Na(+)](i) of 10 and 20 mM, respectively (n= 4). Double pulse stimulation of fibres at low [K(+)](i) showed that although elevated [Na(+)](i) increased the responsiveness to single action potentials, it reduced the capacity of the T-tubules to respond to high frequency stimulation. It is concluded that a reduction in the chemical gradient for K(+), as takes place during intensive exercise, may depress T-tubular function, but that a concomitant exercise-induced increase in [Na(+)](i) protects T-tubular function by stimulating the Na(+)-K(+) pump.

Evidence that the Na+-K+ leak/pump ratio contributes to the difference in endurance between fast- and slow-twitch muscles.

AIM: Muscles containing predominantly fast-twitch (type II) fibres [ext. dig. longus (EDL)] show considerably lower contractile endurance than muscles containing mainly slow-twitch (type I) fibres (soleus). To assess whether differences in Na+-K+ fluxes and excitability might contribute to this phenomenon, we compared excitation-induced Na+-K+ leaks, Na+ channels, Na+-K+ pump capacity, force and compound action potentials (M-waves) in rat EDL and soleus muscles. METHODS: Isolated muscles were mounted for isometric contractions in Krebs-Ringer bicarbonate buffer and exposed to direct or indirect continuous or intermittent electrical stimulation. The time-course of force decline and concomitant changes in Na+-K+ exchange and M-waves were recorded. RESULTS: During continuous stimulation at 60-120 Hz, EDL showed around fivefold faster rate of force decline than soleus. This was associated with a faster loss of excitability as estimated from the area and amplitude of the M-waves. The net uptake of Na+ and the release of K+ per action potential were respectively 6.5- and 6.6-fold larger in EDL than in soleus, which may in part be due to the larger content of Na+ channels in EDL. During intermittent stimulation with 1 s 60 Hz pulse trains, EDL showed eightfold faster rate of force decline than soleus. CONCLUSION: The considerably lower contractile endurance of fast-twitch compared with slow-twitch muscles reflects differences in the rate of excitation-induced loss of excitability. This is attributed to the much larger excitation-induced Na+ influx and K+ efflux, leading to a faster rise in [K+]o in fast-twitch muscles. This may only be partly compensated by the concomitant activation of the Na+-K+ pumps, in particular in fibres showing large passive Na+-K+ leaks or reduced content of Na+-K+ pumps. Thus, endurance depends on the leak/pump ratio for Na+ and K+.

Protective effects of lactic acid on force production in rat skeletal muscle.

1. During strenuous exercise lactic acid accumulates producing a reduction in muscle pH. In addition, exercise causes a loss of muscle K(+) leading to an increased concentration of extracellular K(+) ([K(+)](o)). Individually, reduced pH and increased [K(+)](o) have both been suggested to contribute to muscle fatigue. 2. To study the combined effect of these changes on muscle function, isolated rat soleus muscles were incubated at a [K(+)](o) of 11 mM, which reduced tetanic force by 75 %. Subsequent addition of 20 mM lactic acid led, however, to an almost complete force recovery. A similar recovery was observed if pH was reduced by adding propionic acid or increasing the CO(2) tension. 3. The recovery of force was associated with a recovery of muscle excitability as assessed from compound action potentials. In contrast, acidification had no effect on the membrane potential or the Ca(2+) handling of the muscles. 4. It is concluded that acidification counteracts the depressing effects of elevated [K(+)](o) on muscle excitability and force. Since intense exercise is associated with increased [K(+)](o), this indicates that, in contrast to the often suggested role for acidosis as a cause of muscle fatigue, acidosis may protect against fatigue. Moreover, it suggests that elevated [K(+)](o) is of less importance for fatigue than indicated by previous studies on isolated muscles.

Activity-induced recovery of excitability in K(+)-depressed rat soleus muscle.

Increased extracellular K(+) concentration ([K(+)](o)) can reduce excitability and force in skeletal muscle. Here we examine the effects of muscle activation on compound muscle action potentials (M waves), resting membrane potential, and contractility in isolated rat soleus muscles. In muscles incubated for 60 min at 10 mM K(+), tetanic force and M wave area decreased to 23 and 24%, respectively, of the control value. Subsequently, short (1.5 s) tetanic stimulations given at 1-min intervals induced recovery of force and M wave area to 81 and 90% of control levels, respectively, within 15 min (P < 0.001). The recovery of force and M wave was associated with a partial repolarization of the muscle fibers. Experiments with tubocurarine suggest that the force recovery was related to activation of muscle Na(+)-K(+) pumps caused by the release of some compound from sensory nerves in response to muscle activity. In conclusion, activity produces marked recovery of excitability in K(+)-depressed muscle, and this may protect muscles against fatigue caused by increased [K(+)](o) during exercise.

The Na+/K(+)-pump protects muscle excitability and contractility during exercise.

In skeletal muscle, the concentration of Na+/K(+)-pumps is high and increases through training. In isolated muscles, contractile endurance depends in part on Na+/K(+)-pump concentration. Exercise leads to rundown of Na+/K(+)-gradients, compound action potentials, and force. Early and efficient activation of the Na+/K(+)-pump, however, protects excitability and contractility.

Relations between excitability and contractility in rat soleus muscle: role of the Na+-K+ pump and Na+/K+ gradients.

1. The effects of reduced Na+/K+ gradients and Na+-K+ pump stimulation on compound action potentials (M waves) and contractile force were examined in isolated rat soleus muscles stimulated through the nerve. 2. Exposure of muscles to buffer containing 85 mM Na+ and 9 mM K+ (85 Na+/9 K+ buffer) produced a 54% decrease in M wave area and a 50 % decrease in tetanic force compared with control levels in standard buffer containing 147 mM Na+ and 4 mM K+. Subsequent stimulation of active Na+-K+ transport, using the beta2-adrenoceptor agonist salbutamol, induced a marked recovery of M wave area and tetanic force (to 98 and 87% of the control level, respectively). Similarly, stimulation of active Na+-K+ transport with insulin induced a significant recovery of M wave area and tetanic force. 3. During equilibration with 85 Na+/9 K+ buffer and after addition of salbutamol there was a close linear correlation between M wave area and tetanic force (r = 0.92, P < 0.001). Similar correlations were found in muscles where tetrodotoxin was used to reduce excitability and in muscles fatigued by 120 s of continuous stimulation at a frequency of 30 Hz. 4. These results show a close correlation between excitability and tetanic force. Furthermore, in muscles depressed by a reduction in the Na+/K+ gradients, beta-adrenergic stimulation of the Na+-K+ pump induces a recovery of excitability which can fully explain the previously demonstrated recovery of tetanic force following Na+-K+ pump stimulation. Moreover, the data indicate that loss of excitability is an important factor in fatigue induced by high-frequency (30 Hz) stimulation.

Excitation-induced force recovery in potassium-inhibited rat soleus muscle.

1. Excitation markedly stimulates the Na+-K+ pump in skeletal muscle. The effect of this stimulation on contractility was examined in rat soleus muscles exposed to high extracellular K+ concentration ([K+]o). 2. At a [K+]o of 10 mM, tetanic force declined to 58 % of the force in standard buffer with 5.9 mM K+. Subsequent direct stimulation of the muscle at 1 min intervals with 30 Hz pulse trains of 2 s duration induced a 97 % recovery of force within 14 min. Force recovery could also be elicited by stimulation via the nerve. In muscles exposed to 12.5 mM K+, 30 Hz pulse trains of 2 s duration at 1 min intervals induced a recovery of force from 16 +/- 2 to 62 +/- 4% of the initial control force at a [K+]o of 5.9 mM. 3. The recovery of force was associated with a decrease in intracellular Na+ and was blocked by ouabain. This indicates that the force recovery was secondary to activation of the Na+-K+ pump. 4. Excitation stimulates the release of calcitonin gene-related peptide (CGRP) from nerves in the muscle. Since CGRP stimulates the Na+-K+ pump, this may contribute to the excitation-induced force recovery. Indeed, reducing CGRP content by capsaicin pre-treatment or prior denervation prevented both the excitation-induced force recovery and the drop in intracellular Na+. 5. The data suggest that activation of the Na+-K+ pump in contracting muscles counterbalances the depressing effect of reductions in the chemical gradients for Na+ and K+ on excitability.

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.

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.

Regulation of Na(+)-K+ pump activity in contracting rat muscle.

1. In rat soleus muscle, high frequency electrical stimulation produced a rapid increase in intracellular Na+ (Na+i) content. This was considerably larger in muscles contracting without developing tension than in muscles contracting isometrically. During subsequent rest a net extrusion of Na+ took place at rates which, depending on the frequency and duration of stimulation, approached the maximum transport capacity of the Na(+)-K+ pumps present in the muscle. 2. In isometrically contracting muscles, the net extrusion of Na+ continued for up to 10 min after stimulation, reducing Na+i to values 30% below the resting level (P < 0.001). This undershoot in Na+i, seen in both soleus and extensor digitorum longus muscles, could be maintained for up to 30 min and was blocked by ouabain or cooling to 0 degree C. 3. The undershoot in Na+i could be elicited by direct stimulation as well as by tubocurarine-suppressible stimulation via the motor endplate. It could not be attributed to a decrease in Na+ influx, to effects of noradrenaline or calcitonin gene-related peptide released from nerve endings, to an increase in extracellular K+ or the formation of nitric oxide. 4. The results indicate that excitation rapidly activates the Na(+)-K+ pump, partly via a change in its transport characteristics and partly via an increase in intracellular Na+ concentration. This activation allows an approximately 20-fold increase in the rate of Na+ efflux to take place within 10 s. 5. The excitation-induced activation of the Na(+)-K+ pump may represent a feed-forward mechanism that protects the Na(+)-K+ gradients and the membrane potential in working muscle. Contrary to previous assumptions, the Na(+)-K+ pump seems to play a dynamic role in maintenance of excitability during contractile activity.

Effects of reduced electrochemical Na+ gradient on contractility in skeletal muscle: role of the Na+-K+ pump.

Continued excitation of skeletal muscle may induce a combination of a low extracellular Na+ concentration ([Na+]o) and a high extracellular K+ concentration ([K+]o) in the T-tubular lumen, which may contribute to fatigue. Here, we examine the role of the Na+-K+ pump in the maintenance of contractility in isolated rat soleus muscles when the Na+, K+ gradients have been altered. When [Na+]o is lowered to 25 mM by substituting Na+ with choline, tetanic force is decreased to 30% of the control level after 60 min. Subsequent stimulation of the Na+-K+ pump with insulin or catecholamines induces a decrease in [Na+]i and hyperpolarization. This is associated with a force recovery to 80-90% of the control level which can be abolished by ouabain. This force recovery depends on hyperpolarization and is correlated to the decrease in -Na+-i (r = 0. 93; P<0.001). The inhibitory effect of a low -Na+-o on force development is considerably potentiated by increasing [K+]o. Again, stimulation of the Na+-K+ pump leads to rapid force recovery. The Na+-K+ pump has a large potential for rapid compensation of the excitation-induced rundown of Na+, K+ gradients and contributes, via its electrogenic effect, to the membrane potential. We conclude that these actions of the Na+-K+ pump are essential for the maintenance of excitability and contractile force.

Role of Na(+)-K+ pump and Na+ channel concentrations in the contractility of rat soleus muscle.

The dependence of contractile performance on the leak-to-pump ratio for Na+ has been examined. In isolated rat soleus muscle the concentration of Na(+)-K+ pumps was shown to decrease with age (-57%) or K+ deficiency (-69%), whereas Na+ channel concentration remained constant. This relative increase in the ratio between Na+ channels and Na(+)-K+ pumps was associated with a markedly faster rate of force decline (58 and 97%, respectively; both P < 0.001) during stimulation at 90 Hz and reduced subsequent force recovery (-34 and -38%, respectively; both P < 0.001). Similar effects were elicited by acute inhibition of Na(+)-K+ pump activity with ouabain. Preincubation with aconitine and veratridine, resulting in a 91 and 118% increase in Na+ influx per contraction, respectively (both P < 0.05), significantly hastened the initial rate of force decline (19%; P < 0.05 for aconitine and 69%; P < 0.001 for veratridine) and slowed recovery (-59 and -86%, respectively, both P < 0.001). It is concluded that the ratio between excitation-induced Na+ influx and Na(+)-K+ pump capacity is an important determinant for endurance and rate of recovery of force in skeletal muscle.

Effects of haemoglobin O2 saturation on volume regulation in adrenergically stimulated red blood cells from the trout, Oncorhynchus mykiss.

Adrenergic stimulation of trout red blood cells activates a Na+/H(+)-exchange. If unopposed, the ensuing increase in cell Na+ leads to an isosmotic cell swelling. In this study the effect of the level of haemoglobin O2 saturation on volume regulation has been investigated in adrenergically stimulated red blood cells from trout: at full haemoglobin O2 saturation, net influx of Na+ through the Na+/H(+)-exchanger was balanced by net efflux of K+ and no increases in cell volume took place. In contrast, at low O2 saturation (8-14%) adrenergic stimulation led to a substantial increase in cell Na+, K+ and volume. Moreover, cell volume recovery after adrenergic swelling was incomplete at low O2 saturation, whereas cells at high O2 saturation exhibited a fast and complete cell volume recovery. In cells exposed to alternating high and low O2 saturation, volume regulation was similar to the regulation found in cells maintained at high O2 saturation. In cells at high O2 saturation, extrusion of cellular Na+ by the Na+/K(+)-pump significantly contributed to the volume decrease. It is concluded that trout red blood cells at high or alternating O2 saturations possess a powerful regulatory volume decrease response that is shut off at low O2 saturation. The physiological implications of this regulation is discussed.