Downregulation of Na+-K+-ATPase pumps in skeletal muscle with training in normobaric hypoxia

Molecular adaptations in human skeletal muscle to endurance training under simulated hypoxic conditions

This study was performed to explore changes in gene expression as a consequence of exercise training at two levels of intensity under normoxic and normobaric hypoxic conditions (corresponding to an altitude of 3,850 m). Four groups of human subjects trained five times a week for a total of 6 wk on a bicycle ergometer. Muscle biopsies were taken, and performance tests were carried out before and after the training period. Similar increases in maximal O(2) uptake (8.3-13.1%) and maximal power output (11.4-20.8%) were found in all groups. RT-PCR revealed elevated mRNA concentrations of the alpha-subunit of hypoxia-inducible factor 1 (HIF-1) after both high- (+82.4%) and low (+78.4%)-intensity training under hypoxic conditions. The mRNA of HIF-1alpha(736), a splice variant of HIF-1alpha newly detected in human skeletal muscle, was shown to be changed in a similar pattern as HIF-1alpha. Increased mRNA contents of myoglobin (+72.2%) and vascular endothelial growth factor (+52.4%) were evoked only after high-intensity training in hypoxia. Augmented mRNA levels of oxidative enzymes, phosphofructokinase, and heat shock protein 70 were found after high-intensity training under both hypoxic and normoxic conditions. Our findings suggest that HIF-1 is specifically involved in the regulation of muscle adaptations after hypoxia training. Fine-tuning of the training response is recognized at the molecular level, and with less sensitivity also at the structural level, but not at global functional responses like maximal O(2) uptake or maximal power output.

Reduced neuromuscular activity and force generation during prolonged cycling

We examined neuromuscular activity during stochastic (variable intensity) 100-km cycling time trials (TT) and the effect of dietary carbohydrate manipulation. Seven endurance-trained cyclists performed two 100-km TT that included five 1-km and four 4-km high-intensity epochs (HIE) during which power output, electromyogram (EMG), and muscle glycogen data were analyzed. The mean power output of the 4-km HIE decreased significantly throughout the trial from 319 +/- 48 W for the first 4-km HIE to 278 +/- 39 W for the last 4-km HIE (P < 0.01). The mean integrated EMG (IEMG) activity during the first 4-km HIE was 16.4 +/- 9.8% of the value attained during the pretrial maximal voluntary contraction (MVC). IEMG decreased significantly throughout the trial, reaching 11.1 +/- 5.6% during the last 4-km HIE (P < 0.01). The study establishes that neuromuscular activity in peripheral skeletal muscle falls parallel with reduction in power output during bouts of high-intensity exercise. These changes occurred when <20% of available muscle was recruited and suggest the presence of a central neural governor that reduces the active muscle recruited during prolonged exercise.

Effects of a 21-day expedition to 6,194 m on human skeletal muscle SR Ca2+-ATPase

We investigated the effects of a 21-day expedition to the summit of Mount Denali, Alaska (6,194 m) on selected Ca2+ sequestration properties of sarcoplasmic reticulum (SR) calcium pump in vastus lateralis muscle. Muscle samples were obtained by biopsy from 5 male climbers (peak oxygen consumption, VO2peak = 52.3 +/- 2.1 mL.kg(-1).min(-1)) approximately 7 days prior to (PRE) and 4 days following (POST) the expedition. A comparison of PRE versus POST measures of maximal Ca2+-ATPase activities (117 +/- 8.5 vs. 97.6 +/- 5.6 nmol.mg protein(-1).min(-1)) and Ca2+-uptake (204 +/- 15 vs. 161 +/- 11 nmol.mg protein(-1).min(-1)) measured in crude homogenates obtained from pre-exercised muscle, indicated only an effect (p < 0.05) of the expedition on Ca2+-uptake. The reduction in Ca2+-ATPase activity, representing 16.6%, was not significant (p = 0.089). The sarco endoplasmic reticulum calcium (SERCA)-ATPase isoforms, measured using Western blotting techniques, revealed a small reduction (p < 0.05) in SERCA 1 (-4.6 +/- 1.9%), but not in SERCA 2a (+2.0 +/- 1.4%). Prior to the expedition, both Ca2+-ATPase activity and Ca2+-uptake were reduced (p < 0.05) by approximately 34 and 18%, respectively, following 40 min of a two-step continuous cycling task (20 min at 59% VO2peak and 20 min at 74% VO2peak). The exercise-induced reduction in Ca2+-ATPase activity was independent of fiber type. Only in the case of Ca2+-uptake was a lower exercise response (p < 0.05) observed following the expedition, an effect that was due to the lower resting value. It is concluded that acclimatization as experienced during a mountaineering expedition induces changes in the properties of the SR Ca2+-pump, and particularly to Ca2+-sequestering function.

Effects of acute hypoxemia on force and surface EMG during sustained handgrip

Data on the consequences of acute hypoxemia on the strength of contraction are often contradictory. In healthy subjects, we tested the effects of hypoxemia (PaO(2) = 56 mmHg), maintained for a 30-min period, on static handgrip elicited by voluntary effort or direct electrical muscle stimulation, in order to separate the consequences of hypoxemia on central or peripheral factors, respectively. Force was measured during maximal voluntary contractions (MVCs), 60% MVCs sustained until exhaustion, and 1-min periods of electrical muscle stimulation at 60 HZ. The evoked compound muscle action potential (M wave) was recorded in resting muscle and after each period of 60-HZ stimulation or sustained 60% MVC. Power spectrum analysis of surface electromyogram (EMG) was performed during sustained 60% MVC. Compared to normoxemia, acute hypoxemia lowered MVC (-12%, P < 0.01) but enhanced (+38%, P < 0.01) the peak force elicited by electrical muscle stimulation. In resting muscle, hypoxemia had no influence on the M-wave amplitude but lengthened the neuromuscular transmission time(+740 micros, P < 0.05). Hypoxemia did not alter the M wave measured after 60 HZ stimulation and 60% MVC. During sustained 60% MVC, hypoxemia markedly depressed the EMG changes, abolishing the leftward shift of power spectra. These data show that acute hypoxemia reduces MVC through depression of the central drive, whereas it improves the peripheral muscle response to electrical stimulation. In addition, hypoxemia reduces the recruitment of slow firing motor unit, which are highly oxygen-dependent. This could constitute an adaptative muscle response to a reduced oxygen supply.

Normal skeletal muscle Na(+)-K(+) pump concentration in patients with chronic heart failure

Intrinsic changes in skeletal muscle are being increasingly suspected as part of the underlying cause of exercise intolerance in patients with chronic heart failure (CHF). The objective of the present study was to determine whether differences existed between CHF patients and age-matched healthy controls in the concentration of skeletal muscle Na(+)-K(+)-ATPase (adenosine triphosphatase), a cation pump that functions to restore Na(+)-K(+) gradients and protect membrane excitability. Moreover, given the potency for physical activity in altering long-term regulation of the pump, an additional objective was to examine the role of activity level in pump expression in CHF patients. Na(+)-K(+)-ATPase concentration (pmol/g wet wt) determined in the vastus lateralis muscle of 27 CHF males (ejection fraction, 23 +/- 1.6%), using the vanadate facilitated [(3)H] ouabain binding technique, was not different (264 +/- 10) from 10 sedentary controls (268 +/- 19,P > 0.05). Similarly, no differences (P > 0.05) could be found between female patients (228 +/- 16, n = 7) and controls (243 +/- 13, n = 9). Differences between untrained control (294 +/- 20, n = 7), chronically active (251 +/- 20, n = 9), and trained (252 +/- 16, n = 6) CHF groups in Na(+)-K(+) pump expression were also insignificant. This study indicates that long-term regulation of Na(+)-K(+)-ATPase concentration is not altered in moderate CHF patients, regardless of the history of regular activity. However, the positive correlations (P < 0.05) that were observed between peak aerobic power (VO(2) peak) and Na(+)-K(+)-ATPase (r = 0.422) and VO(2) peak and maximal citrate synthase activity (r = 0.404) suggests a role for the skeletal muscle in explaining exercise intolerance in CHF patients.

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.

Altitude acclimatization, training and performance

Exposure to altitude results in a reduction in partial pressure of oxygen in the arterial blood and a reduction in oxygen content. In an attempt to maintain aerobic metabolism during increased effort, a series of acclimatization responses occur. Among the most conspicuous of these responses is an increase in hemoglobin (Hb) concentration. The increase in Hb has been construed as the fundamental adaptation enabling increases in aerobic power and performance to occur on return to sea-level. However, the use of altitude to boost training adaptations and improve elite sea-level performance, although tantalizing, is largely unproven. The reasons appear to be many, ranging from the poor experimental designs employed, to the numerous strategies designed to manipulate the altitude experience and the large inter-individual differences in response patterns. However, other factors may also be important. Acclimatization has also been shown to induce alteration in selected properties of the muscle cell, some of which may be counterproductive. The processes involved in cation cycling, as an example, appear to be down-regulated. Changes in these processes could impair certain types of performance.

Downregulation in muscle Na(+)-K(+)-ATPase following a 21-day expedition to 6,194 m

To investigate the hypothesis that acclimatization to altitude would result in a downregulation in muscle Na(+)-K(+)-ATPase pump concentration, tissue samples were obtained from the vastus lateralis muscle of six volunteers (5 males and 1 female), ranging in age from 24 to 35 yr, both before and within 3 days after a 21-day expedition to the summit of Mount Denali, Alaska (6,194 m). Na(+)-K(+)-ATPase, measured by the [(3)H]ouabain-binding technique, decreased by 13.8% [348 +/- 12 vs. 300 +/- 7.6 (SE) pmol/g wet wt; P < 0.05]. No changes were found in the maximal activities (mol. kg protein(-1). h(-1)) of the mitochondrial enzymes, succinic dehydrogenase (3.63 +/- 0.20 vs. 3.25 +/- 0.23), citrate synthase (4. 76 +/- 0.44 vs. 4.94 +/- 0.44), and malate dehydrogenase (12.6 +/- 1. 8 vs. 12.7 +/- 1.2). Similarly, the expedition had no effect on any of the histochemical properties examined, namely fiber-type distribution (types I, IIA, IIB, IC, IIC, IIAB), area, capillarization, and succinic dehydrogenase activity. Peak aerobic power (52.3 +/- 2.1 vs. 50.6 +/- 1.9 ml. kg(-1). min(-1)) and body mass (76.9 +/- 3.7 vs. 75.5 +/- 2.9 kg) were also unaffected. We concluded that acclimatization to altitude results in a downregulation in muscle Na(+)-K(+)-ATPase pump concentration, which occurs without changes in oxidative potential and other fiber-type histochemical properties.