Extracellular K+ concentration and K+ balance of the gastrocnemius muscle of the dog during exercise

KIR channel activation links local vasodilatation with muscle fibre recruitment during exercise in humans

KEY POINTS

During exercise, blood flow to working skeletal muscle increases in parallel with contractile activity such that oxygen delivery is sufficient to meet metabolic demand. K⁺ released from active skeletal muscle fibres could facilitate vasodilatation in proportion to the degree of muscle fibre recruitment. Once released, K⁺ stimulates inwardly rectifying K⁺ (K_IR ) channels on the vasculature to elicit an increase in blood flow. In the present study, we demonstrate that K_IR channels mediate the rapid vasodilatory response to an increase in exercise intensity. We also show that K_IR channels augment vasodilatation during exercise which demands greater muscle fibre recruitment independent of the total amount of work performed. These results suggest that K⁺ plays a key role in coupling the magnitude of vasodilatation to the degree of contractile activity. Ultimately, the findings from this study help us understand the signalling mechanisms that regulate muscle blood flow in humans.

ABSTRACT

Blood flow to active skeletal muscle is augmented with greater muscle fibre recruitment. We tested whether activation of inwardly rectifying potassium (K_IR ) channels underlies vasodilatation with elevated muscle fibre recruitment when work rate is increased (Protocol 1) or held constant (Protocol 2). We assessed forearm vascular conductance (FVC) during rhythmic handgrip exercise under control conditions and during local inhibition of K_IR channels (intra-arterial BaCl₂ ). In Protocol 1, healthy volunteers performed mild handgrip exercise for 3 min, then transitioned to moderate intensity for 30 s. BaCl₂ eliminated vasodilatation during the first contraction at the moderate workload (ΔFVC, BaCl₂ : -1 ± 17 vs. control: 30 ± 28 ml min^-1  100 mmHg^-1 ; n = 9; P = 0.004) and attenuated the 30 s area under the curve by 56 ± 14% (n = 9; P < 0.0001). In Protocol 2, participants performed two exercise bouts in which muscle fibre recruitment was manipulated while total contractile work was held constant via reciprocal changes in contraction frequency: (1) low fibre recruitment, with contractions at 12.5% maximal voluntary contraction once every 4 s and (2) high fibre recruitment, with contractions at 25% maximal voluntary contraction once every 8 s. Under control conditions, steady-state FVC was augmented in high vs. low fibre recruitment (211 ± 90 vs. 166 ± 73 ml min^-1 ⋅100 mmHg^-1 ; n = 10; P = 0.0006), whereas BaCl₂ abolished the difference between high and low fibre recruitment (134 ± 59 vs. 134 ± 63 ml min^-1  100 mmHg^-1 ; n = 10; P = 0.85). These findings demonstrate that K_IR channel activation is a key mechanism linking local vasodilatation with muscle fibre recruitment during exercise.

Elevated extracellular potassium prior to muscle contraction reduces onset and steady-state exercise hyperemia in humans

The increase in interstitial potassium (K⁺) during muscle contractions is thought to be a vasodilatory signal that contributes to exercise hyperemia. To determine the role of extracellular K⁺ in exercise hyperemia, we perfused skeletal muscle with K⁺ before contractions, such that the effect of any endogenously-released K⁺ would be minimized. We tested the hypothesis that local, intra-arterial infusion of potassium chloride (KCl) at rest would impair vasodilation in response to subsequent rhythmic handgrip exercise in humans. In 11 young adults, we determined forearm blood flow (FBF) (Doppler ultrasound) and forearm vascular conductance (FVC) (FBF/mean arterial pressure) during 4 min of rhythmic handgrip exercise at 10% of maximal voluntary contraction during 1) control conditions, 2) infusion of KCl before the initiation of exercise, and 3) infusion of sodium nitroprusside (SNP) as a control vasodilator. Infusion of KCl or SNP elevated resting FVC similarly before the onset of exercise (control: 39 ± 6 vs. KCl: 81 ± 12 and SNP: 82 ± 13 ml·min^-1·100 mmHg^-1; both P < 0.05 vs. control). Infusion of KCl at rest diminished the hyperemic (ΔFBF) and vasodilatory (ΔFVC) response to subsequent exercise by 22 ± 5% and 30 ± 5%, respectively (both P < 0.05 vs. control), whereas SNP did not affect the change in FBF ( P = 0.74 vs. control) or FVC ( P = 0.61 vs. control) from rest to steady-state exercise. These findings implicate the K⁺ ion as an essential vasodilator substance contributing to exercise hyperemia in humans. NEW & NOTEWORTHY Our findings support a significant and obligatory role for potassium signaling in the local vasodilatory and hyperemic response to exercise in humans.

Impact of age on haematological markers pre- and post-marathon running

This study investigated whether haematological markers differ between young and masters marathon participants, running at similar performance levels. Nine young (31.89 ± 4.96 years) and eight masters (63.13 ± 4.61 years) runners participated. At five time points (pre-race through 54 h post-race), a complete blood cell count, basic metabolic panel and creatine kinase (CK) isoenzyme panel were assessed. Race performance was standardised using the World Masters Association Age Grading Performance Tables. Total CK levels were elevated for all participants at all time points post-race (P < 0.001). The CK-isoenzyme MB% was elevated across groups at 6, 30 and 54 h post-race (P < 0.01, P < 0.01 and P < 0.05), with masters runners having a higher CK-MB% at 30 and 54 h (P < 0.05, P < 0.05). Total white blood cell and neutrophil counts were elevated through 6 h post-race (P < 0.001), with higher levels found in younger runners (P < 0.001). When considering all blood work, masters runners had a higher number of abnormal values at 6, 30 and 54 h post-race (P < 0.05, P < 0.01 and P < 0.05). In conclusion, masters runners demonstrated sustained CK-MB elevation, which may suggest greater cardiac stress. However, future studies using additional cardiac markers should be completed to confirm these findings. In addition, masters runners showed an increased number of laboratory values outside normal range, indicating the body's reduced capacity to respond to marathon running.

Plasma potassium and phosphate concentrations--influence by adrenaline infusion, beta-blockade and physical exercise

Infusion of adrenaline into healthy male subjects reduced the plasma concentrations of both potassium and phosphate to a similar extent, in a dose-dependent manner, an effect which was prevented by the administration of propranolol. Ergometer bicycling until exhaustion, which caused marked accumulation of lactic acid in the blood and reduction of pH, induced great elevations of both plasma potassium and phosphate with close relationships between the raised plasma concentrations and the reduction in pH, also during beta-blockade. However, longer-term aerobic exercise, without acidosis, also caused some rise of the potassium and phosphate concentrations. During recovery from anaerobic, but not from aerobic, exercise there was a rapid decrease of the plasma potassium levels while the phosphate values normalized gradually together with pH. From measurements of the ion concentrations both in the femoral effluent of one leg, which carried out maximal isokinetic work, and in the opposite antecubital vein it could be calculated that there was for potassium, but not for phosphate, a post-exercise uptake both in the exercised muscle and in the entire organism, indicating the participation of systemic factors.

The cardiac glycoside binding site on the Na,K-ATPase alpha2 isoform plays a role in the dynamic regulation of active transport in skeletal muscle

The physiological significance of the cardiac glycoside-binding site on the Na,K-ATPase remains incompletely understood. This study used a gene-targeted mouse (alpha2(R/R)) which expresses a ouabain-insensitive alpha2 isoform of the Na,K-ATPase to investigate whether the cardiac glycoside-binding site plays any physiological role in active Na(+)/K(+) transport in skeletal muscles or in exercise performance. Skeletal muscles express the Na,K-ATPase alpha2 isoform at high abundance and regulate its transport over a wide dynamic range under control of muscle activity. Na,K-ATPase active transport in the isolated extensor digitorum longus (EDL) muscle of alpha2(R/R) mice was lower at rest and significantly enhanced after muscle contraction, compared with WT. During the first 60 s after a 30-s contraction, the EDL of alpha2(R/R) mice transported 70.0 nmol/g.min more (86)Rb than WT. Acute sequestration of endogenous ligand(s) in WT mice infused with Digibind to sequester endogenous cardiac glycoside(s) produced similar effects on both resting and contraction-induced (86)Rb transport. Additionally, the alpha2(R/R) mice exhibit an enhanced ability to perform physical exercise, showing a 2.1- to 2.8-fold lower failure rate than WT within minutes of the onset of moderate-intensity treadmill running. Their enhanced exercise performance is consistent with their enhanced contraction-induced Na,K-ATPase transport in the skeletal muscles. These results demonstrate that the Na,K-ATPase alpha2 isozyme in skeletal muscle is regulated dynamically by a mechanism that utilizes the cardiac glycoside-binding site and an endogenous ligand(s) and that its cardiac glycoside-binding site can play a physiological role in the dynamic adaptations to exercise.

Role of Na+,K+-pumps and transmembrane Na+,K+-distribution in muscle function. The FEPS lecture - Bratislava 2007

Na(+),K(+)-ATPase situated in the plasma membrane mediates active extrusion of Na(+) and intracellular accumulation of K(+). This transport system the Na(+),K(+)-pump is the major regulator of the transmembrane distribution of Na(+) and K(+), and is itself subject to regulation by a wide variety of factors in skeletal muscles. The excitation of skeletal muscles is elicited by a rapid influx of Na(+), followed by an equivalent efflux of K(+) across sarcolemmal and t-tubular membranes. Due to their size and sudden onset, these events constitute the major transport challenge for the Na(+),K(+)-pumps. Skeletal muscles contain the largest single pool of K(+) in the organism. During intense exercise, the Na(+),K(+)-pumps cannot readily reaccumulate K(+) into the muscle cells. Therefore, the working muscles undergo a net loss of K(+), causing up to a doubling of the K(+) concentration in the arterial blood plasma in less than 1 min and even larger increases in interstitial K(+). This may induce depolarization, loss of excitability and force, in particular in muscles, where the excitation-induced passive Na(+),K(+)-fluxes are large. During continuous stimulation of isolated rat muscles, there is a highly significant correlation between the rise in extracellular K(+) and the rate of force decline. Fortunately, excitation increases the Na(+),K(+)-pumping rate within seconds. Thus, maximum activation of up to 20-fold above the resting transport rate may be reached in 10 s, with utilization of all available Na(+),K(+)-pumps. In muscles, where excitability is reduced by pre-exposure to high [K(+)]o, acute activation of the Na(+),K(+)-pumps by hormones or intermittent electrical stimulation restores excitability and contractility. In working muscles, the Na(+),K(+)-pumps, due to rapid activation of their large transport capacity, play a dynamic regulatory role in the from second to second ongoing restoration and maintenance of excitability and force. Excitation is a self-limiting process that depends on the leak/pump ratio for Na(+) and K(+). Acute inhibition of the Na(+),K(+)-pumps with ouabain or downregulation of the Na(+),K(+)-pump capacity clearly reduces contractile endurance in isolated muscles. The Na(+),K(+)-pumps are a limiting factor for contractile force and endurance. This is in particular noted if their capacity is reduced because of inactivity or disease. For these reasons, tight regulation of the Na(+),K(+)-pumps is crucial for the maintenance of plasma K(+), membrane potential and excitability in skeletal muscle. This is achieved by: (1) acute activation of the Na(+),K(+)-pumps elicited by excitation, catecholamines, insulin, insulin-like growth factor I, calcitonins and amylin; and (2) long-term regulation of the content of Na(+),K(+)-pumps exerted by thyroid hormones, adrenal steroids, insulin, training, inactivity, fasting, K(+)-deficiency or K(+)-overload. In conclusion, the Na(+),K(+)-pump is a central target for regulation of Na(+),K(+)-distribution, important for the contractile performance of skeletal muscles, the pathophysiology of several diseases and for therapeutic intervention.

Regulation of Na+–K+ homeostasis and excitability in contracting muscles: implications for fatigue

The performance of skeletal muscles depends on their ability to initiate and propagate action potentials along their outer membranes in response to motor signals from the central nervous system. This excitability of muscle fibres is related to the function of Na+ and K+ and Cl– channels and to steep chemical gradients for the ions across the cell membranes, i.e., the sarcolemma and T-tubular membranes. At rest, the chemical gradients for Na+ and K+ are maintained within close limits by the action of the Na+–K+ pump. During contractile activity, however, the muscles lose K+, which causes an increase in the concentration of K+ in the extracellular compartments of the body, the magnitude of which depends on the intensity of the exercise and the size of the muscle groups involved. Since the ensuing reduction in the chemical K+ gradient can have adverse effects on muscle excitability, it has repeatedly been suggested that, during intense exercise, the loss of K+ from muscle fibres can contribute to the complex set of mechanisms that leads to the development of muscle fatigue. In this review, aspects of the regulation of Na+–K+ homeostasis and excitability in contracting muscles is discussed within this context, together with the implications for the contractile function of skeletal muscles.

Experimental therapies in renal replacement: the effect of two different potassium acetate-free biofiltration protocols on striated muscle fibers

Dramatic removal of potassium during hemodialysis sessions can induce changes in the electrical properties of nerve cells or muscle fibers, which may underlie neuromuscular symptoms referred by end-stage renal disease patients. The primary aim of our study was to investigate the effects of acetate-free biofiltration (AFB) on the amplitude of compound motor action potential (cMAP) obtained after stimulation of the ulnar nerve at the wrist. The secondary aim was to compare the effect of two different potassium removal modalities on cMAP amplitude and to analyze the effects on muscular force by specific dynamometric tests. Twenty-eight patients received dialysis for 4 h, 3 times per week, first with standard AFB with constant potassium (AFB) and then with AFB with a variable concentration of potassium in the dialysis bath (AFB(K)). The amplitude of cMAP was determined after ulnar nerve stimulation at the wrist at different time intervals: at the start of dialysis; at 15, 45, 90, and 120 min after beginning the session; and at the end of treatment. At the same time intervals, muscle force generation was determined using a dynamometer. Finally, we measured plasma electrolytes, intraerythrocytic potassium, and the electrical membrane potential at rest (REMP) of the erythrocytic membrane. The main finding of this study was a significant reduction of cMAP amplitude in the first 45 min after AFB, which paralleled the reduction in serum potassium levels. Moreover, there was a reduction of muscular strength determined with dynamometric measurements. Potassium removal induced by the two different modalities of AFB may significantly affect myocardial and fibromuscular cells by modulating the electrochemical balance of cell membranes. The transient alteration of the electrical properties on voluntary striated muscle fibers may contribute to the brief reduction in muscular strength we detected in patients who underwent AFB. AFB(K) can minimize the negative effects of standard AFB treatment on neuromuscular excitability, most likely through a more gentle variation of potassium levels during dialysis.

Respiratory and cardiovascular responses evoked by tibialis anterior muscle afferent fibers in rats

The muscle metaboreflex is thought to be one of the neural mechanisms involved in the cardiovascular and respiratory adjustments to muscular activity. The afferent arm of the reflex is composed of thinly myelinated group III and unmyelinated group IV sensitive fibers. Such reflex arc had been extensively described in cats, dogs, rabbits and humans. However, results obtained in rats are controversial and the role of the afferent fibers from the tibialis anterior skeletal muscle has never been shown. The purpose of the present experiments was to study the responses of both respiratory and cardiovascular systems following activation of the metabosensitive fibers originating from tibialis anterior muscle in non decerebrated and non vagotomized barbituric anesthetized adult rats. Mean arterial blood pressure, mean arterial blood flow, heart rate and phrenic nerve activity (frequency and amplitude) were monitored during electrically induced fatigue or after intramuscular injection of potassium chloride or lactic acid (specific stimuli of the group III and IV afferent fibers). The experiments were performed under normal condition, then after regional circulatory occlusion, which isolated and maintained the neural drive and abolished humoral communication and after section of the peroneal nerve innervating the tibialis anterior muscle. We showed that cardiorespiratory parameters were increased significantly in response to stimuli under normal conditions and after venous outflow occlusion excluding any participation of central chemoception. No change was observed after nerve section. Our data indicate that changes occurring in rat hindlimb muscle such as the tibialis anterior are sufficient to regulate the cardiorespiratory function via metabosensitive fiber activation.

The relationships between plasma potassium, muscle excitability and fatigue during voluntary exercise in humans

The relationships between extracellular potassium elevation and EMG variables in relation to muscle fatigue were investigated during handgrip exercise in humans. Acid-base state, lactate, potassium ([K+](v)) and sodium in venous plasma, as well as variables of surface voluntary and evoked (M-wave) EMG were determined during repeated dynamic (DE) and static (SE) exercise (1 min exercise, 4 min rest). The different rises of [K+](v) were induced by randomly varied workloads. After 15 min of warming up, the M-wave area increased to 124.9 +/- 19.6% (P < 0.001) in comparison with the control value. Simultaneously, the [K+](v) decreased from 4.1 +/- 0.3 to 3.6 +/- 0.3 mmol l(-1) (P < 0.01). During both SE and DE, there were marked intensity-dependent signs of fatigue. The [K+](v) correlated with changes of the integrated EMG (r = 0.87, P < 0.001 for both DE and SE). Changes in the M-wave area during the exercise bouts correlated inversely with the [K+](v) (r = -0.73, P < 0.001). The M-wave area did not decrease below the control value at any intensity. The median frequency of the EMG decreased during exercise, depending on the exercise intensity (r = -0.73 for SE, r = -0.47 for DE, P < 0.001) with a maximal decrease to about 80% after SE with the maximal workload. The muscle action potential propagation velocity changed in the range of about +/-2%. For the first time, a negative relationship between venous potassium and M-wave area was shown during voluntary exercise. However, there was no evidence that the decrease in muscle performance was mainly caused by a decrease in sarcolemmal excitability resulting from a high extracellular [K+].

Potassium initiates vasodilatation induced by a single skeletal muscle contraction in hamster cremaster muscle

The rapid onset of vasodilatation within seconds of a single contraction suggests that the vasodilators involved may be products of skeletal muscle activation, such as potassium (K(+)). To test the hypothesis that K(+) was in part responsible for the rapid dilatation produced by muscle contraction we stimulated four to five skeletal muscle fibres in the anaesthetized hamster cremaster preparation in situ and measured the change in diameter of arterioles at a site of overlap with the stimulated muscle fibres before and after a single contraction stimulated over a range of stimulus frequencies (4, 10, 20, 30, 40, 60 and 80 Hz; 250 ms train duration). Muscle fibres were stimulated in the absence and presence of an inhibitor of a source of K(+), the voltage dependent K(+) channel inhibitor 3,4-diaminopyridine (DAP, 3 x 10(-4) M) and inhibitors of the K(+) dilatory signal transduction pathway, either a Na(+) K(+)-ATPase inhibitor (ouabain; 10(-4) M) or an inward rectifying K(+) channel inhibitor (barium chloride, BaCl(2); 5 x 10(-5) M). We observed significant inhibitions of the rapid dilatation at all stimulus frequencies with each inhibitor. The dilatory event at 4 s was significantly inhibited at all stimulus frequencies by an average of 65.7 +/- 3.6%, 58.8 +/- 6.1% and 64.4 +/- 2.1% in the presence DAP, ouabain and BaCl(2), respectively. These levels of inhibition did not correlate with non-specific changes in force generation by skeletal muscle measured in vitro. Therefore, our data support that K(+) is involved in the rapid dilatation in response to a single muscle contraction across a wide range of stimulus frequencies.

Re-evaluation of muscle wisdom in the human adductor pollicis using physiological rates of stimulation

Motor unit discharge rates decline by about 50 % over 60 s of a sustained maximum voluntary contraction (MVC). It has been suggested that this decline in discharge rate serves to maintain force by protecting against conduction failure and by optimizing the input to motor units as their contractile properties change. This hypothesis, known as muscle wisdom, is based in part on studies in which muscle force was shown to decline more rapidly when stimulation was maintained at a high rate than when stimulus rate was reduced over time. The stimulus rates used in those studies, however, were higher than those normally encountered during MVCs. The purpose of this study was to compare force loss under constant and declining stimulus rate conditions using rates similar to those that occur during voluntary effort. Isometric force and surface EMG signals were recorded from human adductor pollicis muscles in response to supramaximal stimuli delivered to the ulnar nerve at the elbow. Three fatigue protocols, each 60 s in duration, were carried out on separate days on each of 10 subjects: (1) continuous stimulation at 30 Hz, (2) stimulation at progressively decreasing rates from 30 to 15 Hz and (3) sustained MVC. The relative force-time integral (endurance index) was significantly smaller for the sustained MVC (0.75 +/- 0.08) and decreasing stimulus rate conditions (0.76 +/- 0.16) compared to the condition in which stimulus rate was maintained at 30 Hz (0.90 +/- 0.13). These findings suggest that decreases in discharge rate may contribute to force decline during a sustained MVC.

Interpretation of EMG changes with fatigue: facts, pitfalls, and fallacies

Failure to maintain the required or expected force, defined as muscle fatigue, is accompanied by changes in muscle electrical activity. Although studied for a long time, reasons for EMG changes in time and frequency domain have not been clear until now. Many authors considered that theory predicted linear relation between the characteristic frequencies and muscle fibre propagation velocity (MFPV), irrespective of the fact that spectral characteristics can drop even without any changes in MFPV, or in proportion exceeding the MFPV changes. The amplitude changes seem to be more complicated and contradictory since data on increased, almost unchanged, and decreased amplitude characteristics of the EMG, M-wave or motor unit potential (MUP) during fatigue can be found in literature. Moreover, simultaneous decrease and increase in amplitude of MUP and M-wave, detected with indwelling and surface electrodes, were referred to as paradoxical. In spite of this, EMG amplitude characteristics are predominantly used when causes for fatigue are analysed. We aimed to demonstrate theoretical grounds for pitfalls and fallacies in analysis of experimental results if changes in intracellular action potential (IAP), i.e. in peripheral factors of muscle fatigue, were not taken into consideration. We based on convolution model of potentials produced by a motor unit and detected by a point or rectangular plate electrode in a homogeneous anisotropic infinite volume conductor. Presentation of MUP in the convolution form gave us a chance to consider power spectrum (PS) of MUP as a product of two terms. The first one, PS of the input signal, represented PS of the first temporal derivative of intracellular action potential (IAP). The second term, PS of the impulse response, took into account MFPV, differences in instants of activation of each fibre, MU anatomy, and MU position in the volume conductor in respect to the detecting electrode. PS presentation through product means that not only changes in MFPV could be responsible for PS shift as is usually assumed. Changes in IAP duration and IAP after-potential magnitude, affecting the first term of the product, influence the product and thus MUP PS. Moreover, the interrelations between the two spectra and thus sensitivity of spectrum to different parameters change with MU-electrode distance because the second term depends on it. Thus, we have demonstrated that theory does not predict a linear relation between the characteristic frequencies (maximum, mean and median) and MFPV. IAP duration and after-potential magnitude are among parameters affecting MUP or M-wave PS and thus, EMG PS detected by monopolar and bipolar electrodes. Usage of single fibre action potential models instead of MUP ones can result in false dependencies of frequency characteristics. The MUP amplitude characteristics are determined not only by amplitude of IAP, but also by the length of the IAP profile and source-electrode distance. Due to the IAP profile lengthening and an increase in the negative after-potential, surface detected EMG amplitude characteristics can increase even when IAP amplitude decreases considerably during fatigue. Increase in surface detected MUP or M-wave amplitude should not be attributed to a weaker attenuation of the low-frequency components with distance. Simultaneous decrease and increase in amplitude of MUP and M-wave detected with indwelling and surface electrodes are regular, not paradoxical. Corner frequency of the high pass filter should be 0.5 or 1 Hz when muscle fatigue is analyzed. The area of MUP or M-wave normalized in respect of the amplitude of the terminal phase (that is produced during extinction of the depolarized zones at the ends of the fibres) could be useful as a fatigue index. Analysing literature data on IAP changes due to Ca(2+) increasing, we hypothesised that the ability of muscle fibres to uptake Ca(2+) back into the sarcoplasmic reticulum could be the limiting site for fatigue. If this hypothesis is valid, IAP changes are not a cause of fatigue; they are due to it.

Aquaporin-4 deficiency in skeletal muscle and brain of dystrophic mdx mice

We report a detailed study of AQP4 expression in the neuromuscular system of mdx mice. Immunocytochemical analysis performed by double immunostaining revealed that mdx mice manifest a progressive reduction in AQP4 at the sarcolemmal level of skeletal muscle fast fibers and that type IIB fibers are the first to manifest this reduction in AQP4 expression. No labeling was observed in the cytoplasm of muscle fibers, indicating that the reduction in sarcolemma staining is not associated with an intracellular compartmentalization of mistargeted protein. By Western blot and RT-PCR analysis, we found that whereas the total content of AQP4 protein decreased (by 90% in adult mdx mice), mRNA levels for AQP4 remained unchanged. A similar age-related reduction in AQP4 expression was found in brain astrocytic end-feet surrounding capillaries of mdx mice. Morphometric analysis performed after immunogold electron microscopy indicated a reduction of approximately 85% in gold particles (32+/-2/microm vs. 4.7+/-0.61/microm). Western blot experiments conducted using membrane fractions from brain cortex revealed a strong reduction (of 70%) in AQP4 protein in adult mdx mice, and RT-PCR experiments demonstrated that the reduction was not at transcription level. More interesting was the finding that AQP4 reduction was associated with swelling of astrocytic perivascular processes whose ultrastructural modifications are commonly indicated as an important and early event in the development of brain edema. No apparent reduction in AQP4 was found in mdx stomach and kidney. Our data provide evidence that dystrophin deficiency in mdx mice leads to disturbances in AQP4 assembly in the plasma membrane of fast skeletal muscle fibers and brain astrocytic end-feet, suggesting that changes in the osmotic equilibrium of the neuromuscular apparatus may be involved in the pathology of muscular dystrophy.

Interstitial and arterial-venous [K+] in human calf muscle during dynamic exercise: effect of ischaemia and relation to muscle pain

Changes in the concentration of interstitial K+ surrounding skeletal muscle fibres ([K+]I) probably play some role in the regulation of cardiovascular adjustments to muscular activity, as well as in the aetiology of muscle pain and fatigue during high-intensity exercise. However, there is very little information on the response of [K+]I to exercise in human skeletal muscle. Five young healthy subjects performed plantar flexion exercise for four 5 min periods at increasing power outputs ( approximately 1-6 W) with 10 min intervening recovery periods, as well as for two 5 min periods with ischaemia at approximately 1 and approximately 3 W. Microdialysis probes were inserted into the gastrocnemius medialis muscle of the right leg to measure [K+]I, and K+ release from the plantar flexors during and after incremental exercise was calculated from plasma flow and arterial-venous differences for K+. Calf muscle pain was assessed using a visual analogue scale. On average, [K+]I was 4.4 mmol l(-1) at rest and increased during minutes 3-5 of incremental exercise by approximately 1-7 mmol l(-1) as a positive function of power output. K+ release also increased as a function of exercise intensity, although there was a progressive increase by approximately 1-6 mmol l-1 in the [K+] gradient between the interstitium and arterial-venous plasma. [K+]I was lower during ischaemic exercise than control exercise. In contrast to this effect of ischaemia on [K+]I, muscle pain was relatively higher during ischaemic exercise, which demonstrates that factors other than changes in [K+]I are responsible for ischaemic muscle pain. In conclusion, this study has demonstrated that during 5 min of dynamic exercise, [K+]I increases during the later period of exercise as a positive function of exercise intensity, ischaemia reduces [K+]I during rest and exercise, and the increase in [K+]I is not responsible for muscle pain during ischaemic exercise.

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.

The role of the gamma-motor system in increasing muscle tone and muscle pain syndromes: a review of the Johansson/Sojka hypothesis

OBJECTIVES

To review literature that pertained to the Johansson/Sojka hypothesis that positive feedback loops in the gamma-motor system are responsible for chronic muscle pain and increases in muscle tone.

DATA SOURCES

Articles were selected from MEDLINE searches and from manual library searches.

RESULTS

Normal, static, and ischemic muscle contractions and/or chemical mediators of inflammation excite intramuscular groups III and IV chemonociceptors. In groups III and IV, afferent firing stimulates gamma-motorneurons, which causes the firing of Ia and II muscle spindle afferents and increased extrafusal resistance to stretch (muscle tone). Some criticism of the involvement of the gamma-motor system in muscle tone was found to be dated or based on data from noncomparable research. Most of these studies (pro and con) were performed on prepared test animals, and the results may or may not translate to human subjects.

CONCLUSIONS

There exists a sizable body of research that establishes a link between the activation of intramuscular chemonociceptors, increased gamma-motor activity, and increased Ia and II spindle output, as proposed by the hypothesis of Johansson and Sojka. However, because of the lack of sufficient data on human subjects, their hypothesis should not be considered proved. Further research into the effects of metabolites of muscle contraction and their effects on muscle tone is recommended. Research into subluxation/joint dysfunction in light of the Johansson/Sojka hypothesis is recommended.

Do changing patterns of heat and humidity influence thermoregulation and endurance performance?

The purpose of this project was to determine whether changing patterns of temperature and humidity, as expected in the morning versus afternoon, had a differential effect on thermoregulation and endurance performance. Eight male distance runners each participated in two heat pattern tests consisting of two hours treadmill running at 70%-maximum oxygen consumption. The mean heat load for each test was identical (22.2 degrees C wet bulb temperature) but either dry bulb temperature increased (24 to 27.5 degrees C) or decreased (27.5 to 24 degrees C) over the course of the two hour heat stress test. Whole body sweat rate was 10.7% higher (p<0.05) and there was greater plasma volume loss (2.7 versus 1.6%, p<0.05) in the cooling versus warming pattern test. Mean skin and body temperature changed in a significantly different (p<0.05) manner between the two patterns and closely followed ambient dry bulb temperature change. The thermoregulatory variables of heart rate and rectal temperature were not affected and performance did not differ between pattern tests. Ratings of perceived exertion (RPE) and oxygen consumption were also not significantly different between cooling and warming test. In summary, although some minor differences were noted, thermal homeostasis was maintained equally well during either warming or cooling for wet bulb temperatures between 24 and 27 degrees C. The mean heat load is therefore more important than changing patterns of temperature and humidity in determining an individual's physiological response to exercise in a warm environment.

Microdialysis and the measurement of muscle interstitial K+ during rest and exercise in humans

The purpose of this study was to examine whether microdialysis and the internal reference thallium-201 ((201)Tl) could accurately measure muscle interstitial K+ (Ki+) before, during, and after exercise. The relative loss of (201)Tl and simultaneous relative recovery of K+ were measured in vitro for 12 microdialysis probes that were bathed in Ringer acetate medium and perfused at various flows (3-10 microl/min). (201)Tl loss was linearly related to K+ recovery, and their level of agreement was not different from zero. Microdialysis and (201)Tl were then used to measure Ki+ in the gastrocnemius medialis muscle of four humans during rest and static plantar flexion exercise. At rest, Ki+ was 3.9-4.3 mmol/l when the perfusate flow was 2 or 5 microl/min. During exercise, Ki+ increased from 6.9 +/- 0.4 to 7.5 +/- 0.3 mmol/l at low to high intensity and declined to 5.2 +/- 0.3 mmol/l after exercise. These results suggest that large changes in Ki+ in human skeletal muscle can be accurately measured by using microdialysis and (201)Tl.

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.

Expression of aquaporin-4 in fast-twitch fibers of mammalian skeletal muscle

In this study we analyzed the expression of aquaporin-4 (AQP4) in mammalian skeletal muscle. Immunohistochemical experiments revealed that affinity-purified AQP4 antibodies stained selectively the sarcolemma of fast-twitch fibers. By immunogold electron microscopy, little or no intracellular labeling was detected. Western blot analysis showed the presence of two immunopositive bands with apparent molecular masses of 30 and 32 kD specifically present in membrane fraction of a fast-twitch rat skeletal muscle (extensor digitorum longus, EDL) and not revealed in a slow-twitch muscle (soleus). PCR Southern blot experiments resulted in a selective amplification in EDL of a 960-bp cDNA fragment encoding for the full-length rat form of AQP4. Functional experiments carried out on isolated skeletal muscle bundle fibers demonstrated that the osmotic response is faster in EDL than in soleus fibers isolated from the same rat. These results provide for the first time evidence for the expression of an aquaporin in skeletal muscle correlated to a specific fiber-type metabolism. Furthermore, we have analyzed AQP4 expression in skeletal muscle of mdx mice in which a decreased density of orthogonal arrays of particles, a typical morphological feature of AQP4, has been reported. Immunofluorescence experiments showed a marked reduction of AQP4 expression suggesting a critical role in the membrane alteration of Duchenne muscular dystrophy.

Red blood cells do not contribute to removal of K+ released from exhaustively working forearm muscle

K+ released from exercising muscle via K+ channels needs to be removed from the interstitium into the blood to maintain high muscle cell membrane potential and allow normal muscle contractility. Uptake by red blood cells has been discussed as one mechanism that would also serve to regulate red blood cell volume, which was found to be constant despite increased plasma osmolality and K+ concentration ([K+pl]). We evaluated exercise-related changes in [K+pl], pH, osmolality, mean cellular Hb concentration, cell water, and red blood cell K+ concentration during exhaustive handgrip exercise. Unidirectional 86Rb+ (K+) uptake by red blood cells was measured in media with elevated extracellular K+, osmolarity, and catecholamines to simulate particularly those exercise-related changes in plasma composition that are known to stimulate K+ uptake. During exercise [K+pl] increased from 4.4 +/- 0.7 to 7.1 +/- 0.5 mmol/l plasma water and red blood cell K+ concentration increased from 137.2 +/- 6.0 to 144.6 +/- 4.6 mmol/l cell water (P </= 0.05), but the intracellular K+-to-mean cellular Hb concentration ratio did not change. 86Rb+ uptake by red blood cells was increased by approximately 20% on stimulation, caused by activation of the Na+-K+ pump and Na+-K+-2Cl- cotransport. Results indicate the K+ content of red blood cells did not change as cells passed the exhaustively exercising forearm muscle despite the elevated [K+pl]. The tendency for an increase in intracellular K+ concentration was due to a slight, although statistically not significant, decrease in red blood cell volume. K+ uptake, although elevated, was too small to move significant amounts of K+ into red blood cells. Our results suggest that red blood cells do not contribute to the removal of K+ released from muscle and do not regulate their volume by K+ uptake during exhaustive forearm exercise.

Reversal of muscle fatigue in intact rabbits by intravenous potassium chloride

PURPOSE

Skeletal muscle fatigue has been associated with potassium efflux from the myocytes, resulting in endogenous increases in blood potassium concentration ([K+]). Conversely, exogenous increases in extracellular [K+] potentiates contraction in isolated muscle preparations. The mechanisms responsible for these contradictory effects of [K+] on skeletal muscle function are unknown. Moreover, little is known about the effect of exogenous increases in [K+] on force generation by intact animals, given potassium's deleterious effect on cardiac function.

METHODS

We compared the response to exogenous increases in blood [K+] in rabbits given an infusion of potassium chloride (KCl) intravenously (IV) (0.2 mol/L; KCl group; n = 7) to a group given 0.9% sodium chloride (NaCl) (control; n = 7). The rabbits underwent low-frequency, isometric twitch stimulation of the left hindlimb (square wave pulses 100 microseconds, 40V, 0.25 Hz) throughout the experiment. Both groups received 0.9% NaCl (25 mL/h) during the first hour of twitch stimulation and experienced similar decreases in hindlimb forces to 70% of initial force. A continuous infusion of KCl or of saline (60 mL/h) was started, and hindlimb stimulation continued for 2 hours.

RESULTS

There were no changes in [K+] in the control group, and twitch forces progressively declined during the next 2 hours (369 +/- 47 g to 279 +/- 34 g, P < .01). Arterial [K+] increased in the KCl group from 2.6 +/- 0.1 to 10.1 +/- 0.5 mmol/L (P < .01), and hindlimb twitch forces almost doubled (418 +/- 49 g to 756 +/- 55 g, P < .01). Force frequency curves showed improved contractility in the KCl group at stimulation frequencies below 30 Hz.

CONCLUSIONS

Exogenous increases in blood [K+] potentiate skeletal muscle contraction in intact animals and reverse low-frequency twitch fatigue. A possible mechanism may be the maintenance of intracellular [K+] by hindering K+ efflux from skeletal muscle cells.

The relationship between plasma potassium, muscle membrane excitability and force following quadriceps fatigue

To examine the simultaneous changes in plasma [K+], muscle excitability and force during fatigue, ten male adults (mean age = 22 +/- 0.5 years) held an isometric contraction of their right quadriceps muscle at an intensity of 30% maximum voluntary contraction (MVC) for 3 min. Femoral venous and brachial arterial [K+] were determined from serial samples drawn before, during, and for 15 min following the 3-min contraction. Each blood sample was synchronized with a maximal stimulation of the right femoral nerve to evoke a twitch and compound muscle action potential (M-wave). Immediately post-exercise, twitch torque was only 42% of baseline and femoral venous plasma [K+] had increased significantly from 4.02 +/- 0.08 mmol/l to 5.9 +/- 0.22 mmol/l. Femoral venous plasma lactate rose to a peak level of 10.0 +/- 0.8 mmol/l at 1 min post exercise. The recovery of the twitch torque was exponentially related to the recovery of femoral venous plasma [K+] (r2 = 0.93, P < 0.01). There was no evidence for any loss of muscle membrane excitability during the period of increased extracellular [K+], in fact, the M-waves tended to be potentiated in the early phases of the recovery period. These results suggest that muscle membrane excitability is maintained in spite of increased extracellular [K+] following fatigue induced by a sustained submaximal quadriceps contraction. However, the strong relationship between twitch torque and femoral venous plasma [K+] suggests that K+ may be exerting its effect distal to surface membrane action potential propagation, most likely in the T-tubular region.

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.

The role of the sarcolemma action potential in fatigue

A prevalent feature of neuromuscular fatigue is a decline in the extracellularly recorded myoelectric signal. One factor that could underlie this change is a decrease in the amplitude of the sarcolemmal action potential. Based on observed reductions in action potential amplitude without effect on force, it has been argued that changes in the action potential during sustained activity would be unlikely to contribute to fatigue. However, those observations were primarily from experiments in which 1) high frequency stimulation may have caused signal cancellation due to action potential overlap; or 2) sustained membrane depolarization may have directly activated excitation-contraction coupling. The relatively low and narrow range of membrane depolarization required for full activation of amphibian and slow-twitch mammalian fibers makes them resistant to incomplete activation if action potentials are depressed during fatigue. Mammalian fast-twitch fibers, on the other hand, require greater depolarization for full activation and also exhibit a greater decrease in action potential amplitude with fatigue. Therefore, it seems probable that fatigue-related decline in action potential amplitude in these fibers leads to incomplete activation and loss of force.

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.

The effect of K+ on the recovery of the twitch and tetanic force following fatigue in the sartorius muscle of the frog, Rana pipiens

The goal of this study was to investigate how an increase in the extracellular K+ (K0+) concentration immediately after fatigue affects the recovery of the resting potential, the twitch and tetanic contraction of frog sartorius muscle to further understand the role of K+ in the mechanism of fatigue. Resting potentials were measured with conventional microelectrodes. Twitch and tetanic contractions were elicited by field stimulation. All muscles were fatigued with tetanic contractions at a rate of one contraction per second for 3 min while being exposed to 3 mmole l-1 K0+. During fatigue development the resting potential decreased by 16 mV (control group and pH0 7.2, extracellular pH), while the decrease in the twitch force was 32.8%, compared to 79.3% for the tetanic force, and 84.6% for the maximum rate of force development of the tetanus. Fatigued muscles were also unable to maintain a plateau phase during a tetanus: force declined by 14.8% during this phase. During the recovery period under control conditions (3 mmole l-1 K0+), all four parameters returned to their pre-fatigue values, the recovery of the plateau phase was the fastest (10 min), while that of the twitch force was the slowest (80 min). When K0+ was increased to 7.5 or 9.5 mmole l-1 immediately after fatigue, the recovery rate of the tetanic force and plateau phase was reduced. The maximum rate of force development of the tetanus, however, recovered at a faster rate than control muscles. The recovery of the twitch force was also increased above that of control when K0+ was increased to 9.0 mmole l-1 (a concentration which maximally potentiates the twitch force of unfatigued muscle). Frog sartorius muscles were also tested at pH0 6.4, a pH0 which inhibits force recovery. At that pH0 the effects of K0+ were similar to those observed at pH0 7.2. It is concluded that the role of K+ in muscle fatigue is more complex and may not involve just a contribution to the decrease in force during fatigue development, but may also contribute to an increase in force development under some conditions.

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.

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.

The effect of glibenclamide on frog skeletal muscle: evidence for K+ATP channel activation during fatigue

1. The purpose of this study was to determine whether ATP-sensitive K+ (K+ATP) channels are activated and contribute to the decrease in force during fatigue development in the sartorius muscle of the frog, Rana pipiens. Tetanic force (elicited by field stimulation), action potential and membrane conductance (using conventional microelectrodes), were measured in the presence and absence of glibenclamide, a K+ATP channel antagonist. Experiments were performed in bicarbonate-buffered solutions at pH 7.2. 2. In unfatigued muscle 100 mumol l-1 glibenclamide had no effect on the resting potential, the overshoot, the half-depolarization time or the maximum rate of depolarization of action potentials, while the mean half-repolarization time increased by 19 +/- 4% (+/- S.E.M.) and the maximum rate of repolarization decreased by 17 +/- 5%. 3. Fatigue was elicited using 100 ms tetanic contractions every 1 s for 3 min. In the absence of glibenclamide the mean half-repolarization time increased from 0.57 +/- 0.05 to 0.89 +/- 0.05 ms during fatigue. The mean half-repolarization times after fatigue, when muscle fibres were exposed to 100 mumol l-1 glibenclamide either 60 min prior to fatigue or 60 s before the end of fatigue, were 1.16 +/- 0.08 and 1.17 +/- 0.07 ms respectively. Application of 100 mumol l-1 glibenclamide after 5 min of recovery did not increase the half-repolarization time, but decreased the rate of recovery compared to control values. 4. In unfatigued muscles, 100 mumol l-1 glibenclamide did not affect the tetanic contraction. In the absence of glibenclamide, the mean tetanic force after fatigue was 11.0 +/- 0.9% of prefatigue values. Application of 100 mumol l-1 glibenclamide 60 min before fatigue increased the rate of fatigue development as the mean tetanic force was 4.8 +/- 0.8% after 3 min of stimulation. The addition of 100 mumol l-1 glibenclamide 60 s before the end of fatigue had no effect on tetanic force during this time compared to control. 5. In the absence of glibenclamide, muscles recovered 90.1 +/- 1.6% of their tetanic force after 100 min. Addition of 100 mumol l-1 glibenclamide 60 min prior to fatigue significantly reduced the capacity of muscles to recover their tetanic force: after 100 min of recovery tetanic force was only 47.3 +/- 9.4% of the pre-fatigue value.(ABSTRACT TRUNCATED AT 400 WORDS)

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+.

Time-dependent increases in Na+,K(+)-ATPase content of low-frequency-stimulated rabbit muscle

Chronic low-frequency stimulation of rabbit fast-twitch muscle induced time-dependent increases in the concentration of the sarcolemmal Na+,K(+)-ATPase and in mitochondrial citrate synthase activity. The almost twofold increase in Na+,K(+)-ATPase preceded the rise in citrate synthase and was complete after 10 days of stimulation. We suggest that the increase in Na+,K(+)-ATPase enhances resistance to fatigue of low-frequency-stimulated muscle prior to elevations in aerobic-oxidative capacity.

Plasma potassium and lactate concentrations in thoroughbred horses during exercise of varying intensity

To investigate the effect of moderate to high intensity exercise of up to 6 min duration on plasma potassium and lactate concentrations, 6 Thoroughbred horses were studied using a treadmill at a 5 degree incline. Each test consisted of an 8-min standardised warm-up followed by an exercise bout at 8, 9, 10 or 12 m/sec. The horses were galloped at each speed for up to a maximum of 6 min or until signs of fatigue were present. The horses were then walked at 0 degree incline. Carotid arterial blood samples were taken during and after the exercise. At 8, 9 and 10 m/sec there was a general pattern of an initial rise in potassium to a peak around 1.5 min of exercise with the concentration then slowly decreasing. At 12 m/sec there was a continuous rise to a peak at the end of exercise in all horses. Immediately after exercise there was a rapid return (within 3-4 min) to the potassium concentrations recorded at the end of the warm-up period. Plasma lactate peaked around the end of exercise at all speeds. At the highest intensity of exercise the mechanisms for the re-uptake of potassium did not appear to be able to match the rate of efflux. In contrast, at less intense work loads, the rate of re-uptake appeared to be similar to or slightly greater than the rate of efflux. It is possible that a disturbance in this balance between efflux and re-uptake could result in a disturbance in normal neuromuscular function during exercise.

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)

Pathophysiological mechanisms involved in genesis and spread of muscular tension in occupational muscle pain and in chronic musculoskeletal pain syndromes: a hypothesis

This paper introduces a pathophysiological model for the cause of muscular tension and pain in occupational pain syndromes and chronic muskuloskeletal pain syndromes, which also might clarify why these conditions have a tendency to perpetuate themselves and spread from one muscle to another. The model can briefly be described as follows. Metabolites produced by (static) muscle contractions stimulate group III and IV muscle afferents, which activate gamma-motoneurones projecting to both homonymous and heteronymous muscles. The gamma-motoneurones influence the stretch sensitivity and discharges of secondary and primary spindle afferents. Increased activity in the primary muscle spindle afferents enhances the muscle stiffness, which leads to further production of metabolites in both homo- and heteronymous muscles. Increased activity in secondary spindle afferents, which project back to the gamma system, constitutes a 'built in' second positive feedback loop which may perpetuate the condition with less 'support' from activity in group III and IV muscle afferents.

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)

Physiological changes in the running greyhound (Canis domesticus): influence of race length

1. Racing greyhounds were allowed to run 402 m, 503 m, and 704 m to determine the influence of increased race length on physiological changes during and after exercise. 2. Plasma and whole blood variables that changed significantly with increased race length were [K+], [total protein], [lactate], hematocrit, and arterial pH. 3. Those variables that changed with increased race distance showed no indication of reaching a plateau; thus, the maximum changes that these variables might undergo with longer exercise duration remains unknown.

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.

Excessive plasma K+ increase after ischemic exercise in myotonic muscular dystrophy

Changes in plasma electrolyte levels upon ischemic forearm exercise were studied in myotonic muscular dystrophy (MyD) patients, disease control groups, and healthy volunteers. Significant differences were observed in the pH and the concentrations of creatine kinase and Na+ before exercise between healthy volunteers and MyD patients. In comparison with healthy volunteers a lower pH and higher concentrations of both CK and Na+ were found in MyD patients. The concentrations of K+, inorganic phosphate, lactate, and ammonia increase upon exercise in all groups. The mean increase in plasma K+ for healthy volunteers amounted to 0.8 mM (= 23%). In MyD patients a significantly higher increase in plasma K+ was found [mean 2.2 mM (= 65%)]. No abnormal release of K+ from muscular tissue was found in the disease control groups. Data on the postexercise increase in the concentration of other muscular constituents such as creatine kinase, inorganic phosphate, or creatine exclude the possibility of a generally increased membrane permeability in MyD. The abnormally high increase of plasma K+ upon muscular exercise seems to be specific for MyD and may be related to the biochemical defect in this disease.

Interrelationship between pH, plasma potassium concentration and ventilation during intense continuous exercise in man

During resting conditions plasma hydrogen ion concentration ([H+]P) is known to influence ventilation (VE), whereas the control of plasma potassium concentration ([K+]P) at rest and of both [K+]P and VE during exercise are controversial issues. To obtain more information about these variables during muscular work, eight trained men performed two successive intense continuous cycle-ergometer tests, the first (test I) during metabolic acidosis, the second (test II) with an alkalotic pH. No correlation was found between [H+]P and [K+]P or VE in the direction of change of these variables in test I. Furthermore, no correlation between [H+]P and [K+]P in test I and II was seen. Instead [K+]P and VE changed in relation to the exercise intensity. We suggest that the results confirm [K+]P as an indicator of muscular stress. In addition, the similar behaviour of relative values of [K+]P and VE changes in test I (r = 0.9, m = 1.0, where m is the slope of the regression curve) supports the hypothesis that extracellular potassium controls VE and thereby [H+]P also.

Effect of prolonged physical exercise on intra-erythrocyte and plasma potassium

The intracellular concentrations of sodium [Na+] and potassium [K+] and the water content in human erythrocytes were investigated in 21 male runners before and after a marathon. From 2 to 5 min after the race, the intra-erythrocyte [K+] was significantly decreased (p less than 0.001) by 7% whereas the plasma [K+], intra-erythrocyte [Na+] and the erythrocyte water content were unchanged. The change in the intra-erythrocyte [K+] observed immediately after the marathon, was negatively correlated with the race time (r = -0.44; p less than 0.05). Furthermore, the change in the plasma [K+] (r = -0.64; p less than 0.001) and the amount of K+ excreted in the urine during the race (r = 0.54; p less than 0.05) were also, respectively, negatively and positively correlated with the race time. It is concluded that during prolonged physical exercise the erythrocytes could serve as a kind of K+ reservoir that is drained with increasing magnitude of body K+ loss. This might explain why in the faster marathon runners, in whom the urinary K+ loss is smaller and the K+ intake is greater than in the slower runners during race, the intra-erythrocyte [K+] is unchanged after a marathon whereas in the slower runners it is decreased.

The importance of potassium and lactate for maximal exercise performance during beta blockade

Changes in femoral vein pH, lactate, glucose and potassium were studied in a double-blind randomized, short-term, dynamic cycle ergometry exercise test on six healthy male subjects after administration of non-selective (timolol), beta-1-selective (atenolol) beta blocker or placebo. The exercise intensity was increased in steps of 200 kpm/min every 2 min until exhaustion. During submaximal exercise, potassium concentrations in blood from the exercising leg muscles increased progressively with increasing exercise intensity, and was significantly higher for any given exercise level following timolol as compared to placebo administration. The potassium concentrations following atenolol were in-between those of timolol and placebo. Despite reduced working capacity after non-selective beta blockade, almost identical potassium concentrations were reached at exhaustion irrespective of treatment regimens (placebo: 6.3, range 5.8-6.8 mmol/l; atenolol: 6.5, range 6.1-7.3 mmol/l and timolol: 6.4, range 6.2-6.8 mmol/l). The increase in s-lactate concentrations was similar across all treatments, and rose in proportion to the increase in the exercise intensity. A biphasic increase in lactate was observed with identical breaking points (anaerobic threshold) irrespective of treatment regimens. There was no difference in glucose concentrations between the treatment regimens. The marked increase in serum potassium during maximal exercise coincides with leg muscle fatigue and may, by its effect on the muscle cell membrane potential, limit the maximal working capacity following beta blockers. The rise in serum potassium may curtail the use of maximal exercise test as an index of cardiac performance in healthy young subjects.

Increased sodium pump activity following repetitive stimulation of rat soleus muscles

1. Soleus muscles of anaesthetized rats were stimulated tetanically (4 s at 20 Hz every 5 s for 5 min), following which the resting and action potentials were measured in surface fibres. 2. At the end of the stimulation period, the mean resting potential was found to have increased from a control value of -79.5 +/- 4.8 mV (mean +/- S.D.) to -90.5 +/- 6.3 mV. The hyperpolarization started to decline after 9 min but was still present at 15 min. 3. Associated with the membrane hyperpolarization was an increase in the mean amplitude of the muscle fibre action potential, from 82.2 +/- 10.8 to 96.8 +/- 10.0 mV. 4. Both the hyperpolarization and the enlargement of the muscle fibre action potential were abolished by 1.25 X 10(-4) M-ouabain, cooling the bathing fluid to 19 degrees C or removing K+ from the bathing fluid. 5. The results are explained in terms of an increase in electrogenic sodium pump activity resulting from tetanic stimulation. When the bathing fluid contained 20 mM-K+, the mean resting potential of stimulated fibres was approximately -30 mV greater than that calculated from the Goldman-Hodgkin-Katz equation. 6. The increase in sodium pumping not only acts to restore the concentrations of Na+ and K+ on either side of the muscle fibre membrane, but, through its electrogenic effect, enables fibres to remain excitable during continuous contractile activity.

The effect of beta 2-adrenoceptor activation on ion-shifts and fatigue in mouse soleus muscles stimulated in vitro

The resting membrane potential (RMP), the intracellular free Na+ concentration ([Na+]i) and the intracellular free K+ concentration ([K+]i), were measured with double-barrelled ion-selective microelectrodes in mouse soleus muscles in vitro. In addition, the Na+ contents and K+ contents have been measured with the flame photometric technique. At rest the beta 2-selective adrenoceptor agonist terbutaline (10(-5) M) increased the membrane potential and [K+]i, and decreased [Na+]i when compared with control muscles. During a 5 min stimulation period the muscles, which were incubated with the beta 2-adrenoceptor agonist, showed a smaller depolarization, a smaller decrease in [K+]i and a smaller increase in [Na+]i than stimulated control muscles. This difference was probably associated with an increased rate of Na-K-pumping in the beta 2-adrenoceptor stimulated muscles. The beta 2-agonist treated muscles were more resistant to fatigue than control muscles. This effect was significant with 10(-6) M terbutaline (25 degrees C). A depolarization obtained by increasing [K+]o was shown to reduce the maximal tension. It is postulated, that the K+ shifts, which are responsible for the depolarization during muscle activity, are one of the mechanisms underlying muscle fatigue.

Muscle energy metabolism and electrolyte shifts during low-level prolonged static contraction in man

Seven men performed one-legged isometric knee extension at 5% MVC for 1 h. Total body oxygen uptake amounted to 451 (420-471) ml min-1 and oxygen uptake over the contracting leg to 200 (172-216) ml min-1, with no changes occurring during the 1 h contraction. Venous O2 tension decreased from 29.4 mmHg at rest to 23.1 mmHg with contraction and CO2 tension tended to increase from a resting value of 50.5 mmHg to 57.2 mmHg (n.s.). No similar changes occurred in arterial O2 and CO2 tensions. There was a small but continuous glucose uptake at both rest and throughout the contraction, whereas a lactate release occurred only in the early phase (2 min) of contraction. Muscle glycogen content was 312 mmol kg-1 dry wt at rest, no significant changes had occurred following 30 min or 1 h of contraction. Arterial and venous Hct and Hb values indicated that a flux of water occurred from the vascular bed to the contracting muscle, in which H2O increased from 3.06 l kg-1 dry wt at rest to 3.30 l kg-1 dry wt after 1 h at 5% MVC. Simultaneously potassium (K), was released from the muscle throughout contraction with a mean venous-arterial difference of 0.25 mmol l-1. With a plasma flow of 335 ml min-1 kg-1 wet wt the K loss amounted to 5 mmol kg-1 wet wt or roughly 5% of the total muscle K content.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiorespiratory reflex responses to static contraction of vascularly isolated hindleg muscles of the rat

One hindleg of an anasthetized rat (n = 15) was isolated from systemic blood circulation. The preparation was connected to the body only by nerve and bone. A. and V. femorales were cannulated and perfused with normoxic (PO2 = 530 mm Hg) or hypoxic (PO2 = 60 mm Hg) Tyrode solutions. Static contractions of the muscle were elicited by electrical stimulation on the sciatic nerve (2 x motor threshold, 400-800 mV, 50 s-1). A 1 s stimulus was followed by a 2 s rest period. Total test time amounted to 40 min. It was proceeded and succeeded by 20 min periods of control perfusions without stimulation. Heart rate (HR) and respiratory rate (f) were measured and cross correlated with the following outflow parameters from V. femoralis of the experimental muscle: [K+], [Na+], PO2, PCO2, pH and [lactate]. During the test period HR and f increased significantly within 20 min of the start of stimulation: HR 5.8% (p less than 0.005) and f 24.3% (p less than 0.005) for hypoxic perfusion (n = 6) and HR 3.2% (p less than 0.005) and f (p less than 0.001, ANOVA) for normoxic perfusion (n = 3). The dynamic changes of several outflow parameters were nearly simultaneous with the cardiorespiratory responses. Cross correlation analyses revealed an excellent temporal relationship between HR and PO2 or [lactate] and between f and PO2 or [lactate]. In addition PCO2 and pH correlated well with HR as well as with f. Comparison of the threshold of the cardiorespiratory response revealed an optimal relationship to pH, a good one to PCO2 and lactate concentration but no correlation to PO2.(ABSTRACT TRUNCATED AT 250 WORDS)