Training-induced changes in skeletal muscle Na+-K+ pump number and isoform expression in rats with chronic heart failure

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

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

Animal exercise studies in cardiovascular research: Current knowledge and optimal design-A position paper of the Committee on Cardiac Rehabilitation, Chinese Medical Doctors' Association

Growing evidence has demonstrated exercise as an effective way to promote cardiovascular health and protect against cardiovascular diseases However, the underlying mechanisms of the beneficial effects of exercise have yet to be elucidated. Animal exercise studies are widely used to investigate the key mechanisms of exercise-induced cardiovascular protection. However, standardized procedures and well-established evaluation indicators for animal exercise models are needed to guide researchers in carrying out effective, high-quality animal studies using exercise to prevent and treat cardiovascular diseases. In our review, we present the commonly used animal exercise models in cardiovascular research and propose a set of standard procedures for exercise training, emphasizing the appropriate measurements and analysis in these chronic exercise models. We also provide recommendations for optimal design of animal exercise studies in cardiovascular research, including the choice of exercise models, control of exercise protocols, exercise at different stages of disease, and other considerations, such as age, sex, and genetic background. We hope that this position paper will promote basic research on exercise-induced cardiovascular protection and pave the way for successful translation of exercise studies from bench to bedside in the prevention and treatment of cardiovascular diseases.

Guidelines for animal exercise and training protocols for cardiovascular studies

Whole body exercise tolerance is the consummate example of integrative physiological function among the metabolic, neuromuscular, cardiovascular, and respiratory systems. Depending on the animal selected, the energetic demands and flux through the oxygen transport system can increase two orders of magnitude from rest to maximal exercise. Thus, animal models in health and disease present the scientist with flexible, powerful, and, in some instances, purpose-built tools to explore the mechanistic bases for physiological function and help unveil the causes for pathological or age-related exercise intolerance. Elegant experimental designs and analyses of kinetic parameters and steady-state responses permit acute and chronic exercise paradigms to identify therapeutic targets for drug development in disease and also present the opportunity to test the efficacy of pharmacological and behavioral countermeasures during aging, for example. However, for this promise to be fully realized, the correct or optimal animal model must be selected in conjunction with reproducible tests of physiological function (e.g., exercise capacity and maximal oxygen uptake) that can be compared equitably across laboratories, clinics, and other proving grounds. Rigorously controlled animal exercise and training studies constitute the foundation of translational research. This review presents the most commonly selected animal models with guidelines for their use and obtaining reproducible results and, crucially, translates state-of-the-art techniques and procedures developed on humans to those animal models.

Pivotal role of α2 Na+ pumps and their high affinity ouabain binding site in cardiovascular health and disease

Reduced smooth muscle (SM)-specific α2 Na⁺ pump expression elevates basal blood pressure (BP) and increases BP sensitivity to angiotensin II (Ang II) and dietary NaCl, whilst SM-α2 overexpression lowers basal BP and decreases Ang II/salt sensitivity. Prolonged ouabain infusion induces hypertension in rodents, and ouabain-resistant mutation of the α2 ouabain binding site (α2^R/R mice) confers resistance to several forms of hypertension. Pressure overload-induced heart hypertrophy and failure are attenuated in cardio-specific α2 knockout, cardio-specific α2 overexpression and α2^R/R mice. We propose a unifying hypothesis that reconciles these apparently disparate findings: brain mechanisms, activated by Ang II and high NaCl, regulate sympathetic drive and a novel neurohumoral pathway mediated by both brain and circulating endogenous ouabain (EO). Circulating EO modulates ouabain-sensitive α2 Na⁺ pump activity and Ca²⁺ transporter expression and, via Na⁺ /Ca²⁺ exchange, Ca²⁺ homeostasis. This regulates sensitivity to sympathetic activity, Ca²⁺ signalling and arterial and cardiac contraction.

Dynamic exercise training prevents exercise pressor reflex overactivity in spontaneously hypertensive rats

Cardiovascular responses to exercise are exaggerated in hypertension. We previously demonstrated that this heightened cardiovascular response to exercise is mediated by an abnormal skeletal muscle exercise pressor reflex (EPR) with important contributions from its mechanically and chemically sensitive components. Exercise training attenuates exercise pressor reflex function in healthy subjects as well as in heart failure rats. However, whether exercise training has similar physiological benefits in hypertension remains to be elucidated. Thus we tested the hypothesis that the EPR overactivity manifest in hypertension is mitigated by exercise training. Changes in mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) in response to muscle contraction, passive muscle stretch, and hindlimb intra-arterial capsaicin administration were examined in untrained normotensive Wistar-Kyoto rats (WKYUT; n = 6), exercise-trained WKY (WKYET; n = 7), untrained spontaneously hypertensive rats (SHRUT; n = 8), and exercise-trained SHR (SHRET; n = 7). Baseline MAP after decerebration was significantly decreased by 3 mo of wheel running in SHRET (104 ± 9 mmHg) compared with SHRUT (125 ± 10 mmHg). As previously reported, the pressor and renal sympathetic responses to muscle contraction, stretch, and capsaicin administration were significantly higher in SHRUT than WKYUT. Exercise training significantly attenuated the enhanced contraction-induced elevations in MAP (SHRUT: 53 ± 11 mmHg; SHRET: 19 ± 3 mmHg) and RSNA (SHRUT: 145 ± 32%; SHRET: 57 ± 11%). Training produced similar attenuating effects in SHR during passive stretch and capsaicin administration. These data demonstrate that the abnormally exaggerated EPR function that develops in hypertensive rats is significantly diminished by exercise training.

Exercise training improves functional sympatholysis in spontaneously hypertensive rats through a nitric oxide-dependent mechanism

Functional sympatholysis is impaired in hypertensive animals and patients. Exercise training (ET) improves functional sympatholysis through a nitric oxide (NO)-dependent mechanism in normotensive rats. However, whether ET has similar physiological benefits in hypertension remains to be elucidated. Thus we tested the hypothesis that the impairment in functional sympatholysis in hypertension is reversed by ET through a NO-dependent mechanism. In untrained normotensive Wistar-Kyoto rats (WKYUT; n = 13), untrained spontaneously hypertensive rats (SHRUT; n = 13), and exercise-trained SHR (SHRET; n = 6), changes in femoral vascular conductance (FVC) were examined during lumbar sympathetic nerve stimulation (1, 2.5, and 5 Hz) at rest and during muscle contraction. The magnitude of functional sympatholysis (Δ%FVC = Δ%FVC muscle contraction - Δ%FVC rest) in SHRUT was significantly lower than WKYUT (1 Hz: -2 ± 4 vs. 13 ± 3%; 2.5 Hz: 9 ± 3 vs. 21 ± 3%; and 5 Hz: 12 ± 3 vs. 26 ± 3%, respectively; P < 0.05). Three months of voluntary wheel running significantly increased maximal oxygen uptake in SHRET compared with nontrained SHRUT (78 ± 6 vs. 62 ± 4 ml·kg(-1)·min(-1), respectively; P < 0.05) and restored the magnitude of functional sympatholysis in SHRET (1 Hz: 9 ± 2%; 2.5 Hz: 20 ± 4%; and 5 Hz: 34 ± 5%). Blockade of NO synthase (NOS) by N(G)-nitro-l-arginine methyl ester attenuated functional sympatholysis in WKYUT but not SHRUT. Furthermore, NOS inhibition significantly diminished the improvements in functional sympatholysis in SHRET. These data demonstrate that impairments in functional sympatholysis are normalized via a NO mechanism by voluntary wheel running in hypertensive rats.

Exercise and suspension hypokinesia-induced alterations in mechanical properties of rat fast and slow-twitch skeletal muscles

Physical activity has a modulatory role on regulatory steps of excitation-contraction coupling (ECC) determining skeletal muscle contractility. We evaluated and compared the contractile responsiveness and caffeine-induced contractures of fast (extensor digitorum longus; EDL) and slow-twitch (soleus; SOL) muscles in suspension hypokinesia (SH) and exercised rats. After SH or low intensity exercise, EDL and SOL were isolated, twitch and tetanic contractions and caffeine (10 mM) contractures were recorded. Twitch and tetanic contractions of EDL increased by 60% in exercised rats (p <0.05) while no alteration was observed after SH. Exercise did not alter twitch and tetanic contractions of SOL, while SH depressed contractions (p <0.05). Caffeine contractures were diminished in exercised rat EDL (P <0.05). In SH-rat EDL, contractures increased in amplitude (p <0.01) with a rapid time course (p <0.05). Contractures did not change in SOL after exercise or SH. We concluded that SH and exercise exerted diverse modulatory effects on skeletal muscle contractility. Contractile improvement due to exercise was prominent in EDL. Our results suggest that the muscle-type specific adaptations are related to a change in ECC due to the differences in the regulatory steps, particularly in the intracellular Ca(2+) handling mechanisms.

Exercise training restores impaired dilator responses of cerebral arterioles during chronic exposure to nicotine

Our goal was to determine whether exercise training (ExT) alleviates impaired nitric oxide synthase (NOS)-dependent dilation of pial arterioles during chronic exposure to nicotine. We measured dilation of cerebral (pial) arterioles in sedentary and exercised control and nicotine-treated (2 mg·kg(-1)·day(-1) for 4 wk via an osmotic minipump) rats to an endothelial NOS (eNOS)-dependent (ADP), a neuronal NOS (nNOS)-dependent [N-methyl-D-aspartic acid (NMDA)], and a NOS-independent (nitroglycerin) agonist. In addition, we harvested brain tissue from sedentary and exercised control and nicotine-treated rats to measure the production of superoxide anion and measured superoxide dismutase-1 (SOD-1) protein in cerebral microvessels using Western blot. We found that eNOS-and nNOS-dependent, but not NOS-independent, vasodilation was impaired in nicotine-treated compared with control rats. In addition, the production of superoxide anion (lucigenin chemiluminescence) was increased, and SOD-1 protein decreased, in rats treated with nicotine compared with control rats. Further, although ExT did not significantly affect eNOS- or nNOS-dependent vasodilation in control rats, ExT restored impaired eNOS- and nNOS-dependent responses in nicotine-treated rats. In addition, the increase in superoxide anion production observed in nicotine-treated rats was reduced by ExT, and SOD-1 protein was increased in nicotine-treated rats by ExT. We suggest that ExT restores impaired NOS-dependent dilation of pial arterioles during chronic exposure to nicotine by a mechanism related to the formation of superoxide anion.

Effects of exercise training on dendritic morphology in the cardiorespiratory and locomotor centers of the mature rat brain

It has been shown that dendritic branching in neural cardiorespiratory and locomotor centers can be attenuated with exercise training (ET) initiated immediately after weaning. The purpose of this study was to determine whether neuroplastic changes occur within cardiorespiratory and locomotor centers due to ET after maturation. Male Sprague-Dawley rats (21 days old, n=28) were individually housed in standard cages. At 91 days of age, animals were divided into two groups: untrained (UN; n=14) and trained (TR; n=14). The TR group exercised spontaneously for 50 days on running wheels. ET indexes were obtained, including maximal O2 consumption, percent body fat, resting heart rate, and heart weight-to-body weight ratios. The brain was processed with a modified Golgi-Cox procedure. Impregnated neurons from the periaqueductal gray (PAG), posterior hypothalamic area (PH), nucleus of the tractus solitarius (NTS), cuneiform nucleus (CnF), rostral ventrolateral medulla, nucleus cuneatus, and cerebral cortex were examined. Neurons were traced and analyzed using the Sholl concentric ring analysis of dendritic branching. The mean total number of dendritic intersections with the concentric rings per neuron per animal were compared between UN and TR groups. There were significant differences between UN and TR groups in the PH, PAG, CnF, and NTS in the total number of intersections per animal. In some areas, the effect size was smaller when ET was initiated in mature animals, possibly related to their relatively reduced activity levels. In conclusion, the adult rat brain remains dynamic and adapts to chronic ET. However, some brain areas appear to be more affected if ET is initiated in early postnatal development.

Muscle Na+-K+-ATPase response during 16 h of heavy intermittent cycle exercise

This study investigated the effects of a 16-h protocol of heavy intermittent exercise on the intrinsic activity and protein and isoform content of skeletal muscle Na(+)-K(+)-ATPase. The protocol consisted of 6 min of exercise performed once per hour at approximately 91% peak aerobic power (Vo(2 peak)) with tissue sampling from vastus lateralis before (B) and immediately after repetitions 1 (R1), 2 (R2), 9 (R9), and 16 (R16). Eleven untrained volunteers with a Vo(2 peak) of 44.3 +/- 2.3 ml x kg(-1) x min(-1) participated in the study. Maximal Na(+)-K(+)-ATPase activity (V(max), in nmol x mg protein(-1) x h(-1)) as measured by the 3-O-methylfluorescein K(+)-stimulated phosphatase assay was reduced (P < 0.05) by approximately 15% with exercise regardless of the number of repetitions performed. In addition, V(max) at R9 and R16 was lower (P < 0.05) than at R1 and R2. Vanadate-facilitated [(3)H]ouabain determination of Na(+)-K(+)-ATPase content (maximum binding capacity, pmol/g wet wt), although unaltered by exercise, increased (P < 0.05) 8.3% by R9 with no further increase observed at R16. Assessment of relative changes in isoform abundance measured at B as determined by quantitative immunoblotting showed a 26% increase (P < 0.05) in the alpha(2)-isoform by R2 and a 29% increase in alpha(3) by R9. At R16, beta(3) was lower (P < 0.05) than at R2 and R9. No changes were observed in alpha(1), beta(1), or beta(2). It is concluded that repeated sessions of heavy exercise, although resulting in increases in the alpha(2)- and alpha(3)-isoforms and decreases in beta(3)-isoform, also result in depression in maximal catalytic activity.

Increased interleukin-6 receptor expression in the paraventricular nucleus of rats with heart failure

Activation of the hypothalamic-pituitary-adrenal (HPA) axis and augmented plasma and tissue levels of IL-6 are hallmarks of heart failure (HF). Within the forebrain, cardiovascular homeostasis is mediated in part by the paraventricular nucleus (PVN) of the hypothalamus. IL-6, via binding to the IL-6 receptor (IL-6R)/glycoprotein 130 (gp130) complex influences cellular and physiological responses. Thus, in the current study, we hypothesized that PVN IL-6R protein and gene expression are upregulated in HF vs. sham-operated rats, whereas gp130 levels in the same tissues remain stable. Six weeks after coronary ligation surgery, hemodynamic measurements were obtained, and HF rats were divided into moderate noncongestive and severe chronic congestive groups based on cardiac indices. Plasma IL-6 levels were determined and changes in gene and protein expression of IL-6R and gp130 between sham-operated and HF rats were determined via real-time PCR and Western blot analyses, respectively. Plasma levels of IL-6 were elevated in rats with severe, but not moderate, HF compared with sham-operated controls. In both moderate and severe HF rats, protein but not gene expression of IL-6R was significantly increased in PVN tissue but not in non-PVN tissue, compared with sham-operated controls. Gene and protein levels of the gp130 subunit were not altered by HF in either tissue analyzed. Collectively, these data suggest that within the brain of HF rats, IL-6R expression is not a global change. Rather the increased IL-6 levels characteristic of HF may alter PVN-mediated physiological responses via enhanced expression of the IL-6R.

Exercise training reverses downregulation of HSP70 and antioxidant enzymes in porcine skeletal muscle after chronic coronary artery occlusion

Oxidative stress is associated with muscle fatigue and weakness in skeletal muscle of ischemic heart disease patients. Recently, it was found that endurance training elevates protective heat shock proteins (HSPs) and antioxidant enzymes in skeletal muscle in healthy subjects and antioxidant enzymes in heart failure patients. However, it is unknown whether coronary ischemia and mild infarct without heart failure contributes to impairment of stress proteins and whether exercise training reverses those effects. We tested the hypothesis that exercise training would reverse alterations in muscle TNF-α, oxidative stress, HSP70, SOD (Mn-SOD, Cu,Zn-SOD), glutathione peroxidase (GPX), and catalase (CAT) due to chronic coronary occlusion of the left circumflex (CCO). Yucatan swine were divided into three groups ( n = 6 each): sedentary with CCO (SCO); 12 wk of treadmill exercise training following CCO (ECO); and sham surgery controls (sham). Forelimb muscle mass-to-body mass ratio decreased by 27% with SCO but recovered with ECO. Exercise training reduced muscle TNF-α and oxidative stress (4-hydroxynonenal adducts) caused by CCO. HSP70 levels decreased with CCO (−45%), but were higher with exercise training (+348%). Mn-SOD activity, Mn-SOD protein expression, and Cu,Zn-SOD activity levels were higher in ECO than SCO by 72, 82, and 112%, respectively. GPX activity was 177% greater in ECO than in SCO. CAT trended higher ( P = 0.059) in ECO compared with SCO. These data indicate that exercise training following onset of coronary artery occlusion results in recovery of critical stress proteins and reduces oxidative stress.

Reversibility of exercise-induced dendritic attenuation in brain cardiorespiratory and locomotor areas following exercise detraining

It has been shown previously that dendritic branching in cardiorespiratory and locomotor brain areas can be attenuated with exercise training (ET). It was not known whether this process was reversible. Twenty-three (n = 23) male Sprague-Dawley rats were individually caged and divided into two groups: untrained (UN; n = 11) and detrained (DTR; n = 12). DTR were provided with a running wheel at 21 days of age and exercised spontaneously. After 120 days (70 days of ET followed by 50 days of detraining), ET indexes were obtained, including maximal oxygen uptake, percent body fat, resting heart rate, and heart weight-to-body weight ratios. The brain was processed according to a modified Golgi-Cox procedure. Impregnated neurons from the periaqueductal gray (PAG), posterior hypothalamic area (PH), nucleus of the tractus solitarius (NTS), and cuneiform nucleus (CfN) were examined in coronal sections. Neurons were traced using a camera lucida technique and analyzed using the Sholl concentric ring analysis of dendritic branching. t-Tests compared the mean number of intersections per neuron by grouping inner rings, outer rings, and total number of intersections per animal. There were no significant differences between UN and DTR in PH, PAG, CfN, and NTS in the inner rings, outer rings, and total number of intersections per animal. A separate group of animals was used to show that a training effect in the CfN and NTS was present at 56 days of ET. Our results show that dendritic attenuation resulting from 70 days of ET in PH, PAG, CfN, and NTS is completely reversed with 50 days of detraining.

Neuroplastic adaptations to exercise: neuronal remodeling in cardiorespiratory and locomotor areas

Neuronal activity has been shown to be attenuated in cardiorespiratory and locomotor centers of the brain in response to a single bout of exercise in trained (TR) vs. untrained (UN) animals, but the mechanisms remain obscure. Based on this finding, dendritic branching patterns of seven brain areas associated with cardiorespiratory and locomotor activity were examined in TR and UN animals. Twenty-eight male Sprague-Dawley rats were kept in individual cages and divided into TR and UN. TR were provided with a running wheel and exercised spontaneously. After 85 or 120 days, exercise training indexes were obtained, including maximal oxygen consumption, percent body fat, resting heart rate, and heart weight-to-body weight ratios. The brain was removed and processed according to a modified Golgi-Cox procedure. Impregnated neurons from seven brain areas were examined in coronal sections: the periaqueductal gray, posterior hypothalamic area, nucleus of the tractus solitarius, rostral ventrolateral medulla, cuneiform nucleus, nucleus cuneatus, and cerebral cortex. Neurons were traced using a camera lucida technique and analyzed using the Sholl analysis of dendritic branching. t-tests were conducted to compare the mean number of intersections per neuron by grouping inner rings and outer rings and also comparing the total number of intersections per animal. There were significant differences between groups in the posterior hypothalamic area, periaqueductal gray, cuneiform nucleus, and nucleus of the tractus solitarius in the inner rings, outer rings, and the total number of intersections per animal. Our results show that dendritic fields of neurons in important cardiorespiratory and locomotor centers of the brain are attenuated in TR animals.

Na+-K+-ATPase properties in rat heart and skeletal muscle 3 mo after coronary artery ligation

This study was designed to determine whether chronic heart failure (CHF) results in changes in Na(+)-K(+)-ATPase properties in heart and skeletal muscles of different fiber-type composition. Adult rats were randomly assigned to a control (Con; n = 8) or CHF (n = 8) group. CHF was induced by ligation of the left main coronary artery. Examination of Na(+)-K(+)-ATPase activity (means +/- SE) 12 wk after the ligation measured, using the 3-O-methylfluorescein phosphatase assay (3-O-MFPase), indicated higher (P < 0.05) levels in soleus (Sol) (250 +/- 13 vs. 179 +/- 18 nmol.mg protein(-1).h(-1)) and lower (P < 0.05) levels in diaphragm (Dia) (200 +/- 12 vs. 272 +/- 27 nmol.mg protein(-1).h(-1)) and left ventricle (LV) (760 +/- 62 vs. 992 +/- 16 nmol.mg protein(-1).h(-1)) in CHF compared with Con, respectively. Na(+)-K(+)-ATPase protein content, measured by the [(3)H]ouabain binding technique, was higher (P < 0.05) in white gastrocnemius (WG) (166 +/- 12 vs. 135 +/- 7.6 pmol/g wet wt) and lower (P < 0.05) in Sol (193 +/- 20 vs. 260 +/- 8.6 pmol/g wet wt) and LV (159 +/- 10 vs. 221 +/- 10 pmol/g wet wt) in CHF compared with Con, respectively. Isoform content in CHF, measured by Western blot techniques, showed both increases (WG; P < 0.05) and decreases (Sol; P < 0.05) in alpha(1). For alpha(2), only increases [red gastrocnemius (RG), Sol, and Dia; P < 0.05] occurred. The beta(2)-isoform was decreased (LV, Sol, RG, and WG; P < 0.05) in CHF, whereas the beta(1) was both increased (WG and Dia; P < 0.05) and decreased (Sol and LV; P < 0.05). For beta(3), decreases (P < 0.05) in RG were observed in CHF, whereas no differences were found in Sol and WG between CHF and Con. It is concluded that CHF results in alterations in Na(+)-K(+)-ATPase that are muscle specific and property specific. Although decreases in Na(+)-K(+)-ATPase content would appear to explain the lower 3-O-MFPase in the LV, such does not appear to be the case in skeletal muscles where a dissociation between these properties was observed.

Impact of aging on muscle blood flow in chronic heart failure

Chronic heart failure (CHF) is manifested principally in the elderly population. Therefore, to understand the causes of exercise intolerance in CHF patients, it is imperative to resolve the effects of aging on muscle blood flow (BF) in CHF. To address this issue, we determined the muscle BF response to submaximal treadmill exercise (20 m/min, 5% grade) in young (Y(CHF): 6-8 mo, 412 +/- 11 g, n = 11) and old (O(CHF): 27-29 mo, 494 +/- 10 g, n = 8) Fischer 344 x Brown Norway rats with similar degrees of myocardial infarction-induced left ventricular (LV) dysfunction [resting LV end-diastolic pressure: Y(CHF) = 24 +/- 2, O(CHF) = 22 +/- 2 mmHg; derivative of LV pressure over time: Y(CHF) = 5,168 +/- 285; O(CHF) = 5,050 +/- 165 mmHg/s; lung weight normalized to body weight: Y(CHF) = 9.14 +/- 0.72; O(CHF) = 8.21 +/- 0.29 mg/g (all P > 0.05)]. The exercising heart rate response was blunted in O(CHF) compared with Y(CHF) rats (Y(CHF) = 454 +/- 8, O(CHF) = 395 +/- 9 beats/min; P < 0.05). BF (radiolabeled microspheres) to the total hindlimb musculature and to each of the 28 individual muscles examined was similar between Y(CHF) and O(CHF) rats under resting conditions. During exercise, BF to five of the hindlimb muscles that normally possess a majority of slow-twitch oxidative and fast-twitch oxidative glycolytic muscle fibers increased significantly less (-25 to -42%) for O(CHF) compared with Y(CHF) rats. In contrast, BF to 14 of the hindlimb muscles that normally possess a majority of fast-twitch glycolytic muscle fibers was increased (+22 to +337%) for O(CHF) vs. Y(CHF) rats, which contributed to a greater mass-specific total hindlimb BF response in O(CHF) rats (Y(CHF) = 78 +/- 5, O(CHF) = 100 +/- 11 ml.min(-1).100 g(-1); P < 0.05) and coincided with greater reductions in BF to the kidneys and splanchnic organs during exercise in O(CHF) vs. Y(CHF). In conclusion, there appears to be a profound age-related redistribution of BF from the highly oxidative to the highly glycolytic muscles of the hindlimb during exercise in O(CHF) compared with Y(CHF) rats. This phenomenon is qualitatively similar to that reported previously for healthy young and old rats.

Malleability of human skeletal muscle Na+-K+-ATPase pump with short-term training

To investigate the hypothesis that short-term submaximal training would result in changes in Na+-K+-ATPase content, activity, and isoform distribution in skeletal muscle, seven healthy, untrained men [peak aerobic power (peak oxygen consumption; V̇o2 peak) = 45.6 ml·kg−1·min−1 (SE 5.4)] cycled for 2 h/day at 60–65% V̇o2 peak for 6 days. Muscle tissue, sampled from the vastus lateralis before training (0 days) and after 3 and 6 days of training and analyzed for Na+-K+-ATPase content, as assessed by the vanadate facilitated [3H]ouabain-binding technique, was increased ( P < 0.05) at 3 days (294 ± 8.6 pmol/g wet wt) and 6 days (308 ± 15 pmol/g wet wt) of training compared with 0 days (272 ± 9.7 pmol/g wet wt). Maximal Na+-K+-ATPase activity as evaluated by the 3- O-methylfluorescein phosphatase assay was increased ( P < 0.05) by 6 days (53.4 ± 5.9 nmol·h−1·mg protein−1) but not by 3 days (35.9 ± 4.5 nmol·h−1·mg protein−1) compared with 0 days (37.8 ± 3.7 nmol·h−1·mg protein−1) of training. Relative isoform distribution, measured by Western blot techniques, indicated increases ( P < 0.05) in α2-content by 3 days and β1-content by 6 days of training. These results indicate that prolonged aerobic exercise represents a potent stimulus for the rapid adaptation of Na+-K+-ATPase content, isoform, and activity characteristics.

Changes in skeletal muscle SR Ca2+ pump in congestive heart failure due to myocardial infarction are prevented by angiotensin II blockade

In order to understand the mechanisms of exercise intolerance and muscle fatigue, which are commonly observed in congestive heart failure, we studied sarcoplasmic reticulum (SR) Ca2+-transport in the hind-leg skeletal muscle of rats subjected to myocardial infarction (MI). Sham-operated animals were used for comparison. On one hand, the maximal velocities (Vmax) for both SR Ca2+-uptake and Ca2+-stimulated ATPase activities in skeletal muscle of rats at 8 weeks of MI were higher than those of controls. On the other hand, the Vmax values for both SR Ca2+-uptake and Ca2+-stimulated ATPase activities were decreased significantly at 16 weeks of MI when compared with controls. These alterations in Ca2+-transport activities were not associated with any change in the affinity (1/Ka) of the SR Ca2+-pump for Ca2+. Furthermore, the stimulation of SR Ca2+-stimulated ATPase activity by cyclic AMP-dependent protein kinase was not altered at 8 or 16 weeks of MI when compared with the respective control values. Treatment of 3-week infarcted animals with angiotensin-converting enzyme (ACE) inhibitors such as captopril, imidapril, and enalapril or an angiotensin receptor (AT1R) antagonist, losartan, for a period of 13 weeks not only attenuated changes in left ventricular function but also prevented defects in SR Ca2+-pump in skeletal muscle. These results indicate that the skeletal muscle SR Ca2+-transport is altered in a biphasic manner in heart failure due to MI. It is suggested that the initial increase in SR Ca2+-pump activity in skeletal muscle may be compensatory whereas the depression at late stages of MI may play a role in exercise intolerance and muscle fatigue in congestive heart failure. Furthermore, the improvements in the skeletal muscle SR Ca2+-transport by ACE inhibitors may be due to the decreased activity of renin-angiotensin system in congestive heart failure.Key words: skeletal muscle, sarcoplasmic reticulum, Ca2+-transport, SR Ca2+-pump, congestive heart failure, renin-angiotensin system.

Influence of exercise on dilatation of the basilar artery during diabetes mellitus

Our goal was to examine whether exercise training alleviates impaired nitric oxide synthase (NOS)-dependent dilatation of the basilar artery in Type 1 diabetic rats. To test this hypothesis, we measured in vivo diameter of the basilar artery in sedentary and exercised nondiabetic and diabetic rats in response to NOS-dependent (acetylcholine) and -independent (nitroglycerin) agonists. To determine the potential role for nitric oxide in vasodilatation in sedentary and exercised nondiabetic and diabetic rats, we examined responses after NG-monomethyl-l-arginine (l-NMMA). We found that acetylcholine produced dilatation of the basilar artery that was similar in sedentary and exercised nondiabetic rats. Acetylcholine produced only minimal vasodilatation in sedentary diabetic rats. However, exercise alleviated impaired acetylcholine-induced vasodilatation in diabetic rats. Nitroglycerin produced dilatation of the basilar artery that was similar in sedentary and exercised nondiabetic and diabetic rats. l-NMMA produced similar inhibition of acetylcholine-induced dilatation of the basilar artery in sedentary and exercised nondiabetic and diabetic rats. Finally, we found that endothelial NOS (eNOS) protein in the basilar artery was higher in diabetic compared with nondiabetic rats and that exercise increased eNOS protein in the basilar artery of nondiabetic and diabetic rats. We conclude that 1) exercise can alleviate impaired NOS-dependent dilatation of the basilar artery during diabetes mellitus, 2) the synthesis and release of nitric oxide accounts for dilatation of the basilar artery to acetylcholine in sedentary and exercised nondiabetic and diabetic rats, and 3) exercise may exert its affect on cerebrovascular reactivity during diabetes by altering levels of eNOS protein in the basilar artery.

Exercise training differentially modifies age-associated alteration in expression of Na+-K+-ATPase subunit isoforms in rat skeletal muscles

The present study tests the hypothesis that endurance exercise training (ETr) reverses age-associated alterations in expression of Na+-K+-ATPase subunit isoforms in rat skeletal muscles. Expression of the isoforms was examined in 16-mo-old sedentary middle-aged, 29-mo-old sedentary senescent, and 29-mo-old treadmill exercise-trained senescent Fischer 344 x Brown Norway rats. Levels of the alpha1-isoform increased with age in red gastrocnemius (GR), white gastrocnemius (GW), and extensor digitorum longus (EDL) muscles, and ETr further increased its levels. Levels of the alpha2-isoform were unchanged in GR, had a strong trend for a decrease in GW, and decreased significantly in EDL. ETr increased expression of the alpha2-isoform in all three muscle groups. There was no increase in expression of the beta1-isoform in GR, GW, or EDL with age, whereas ETr markedly increased its levels in the muscles. There was a marked decrease with age in expression of the beta2-isoform in the muscle groups that was not reversed by ETr. By contrast, beta3-isoform levels increased with age in GR and GW, and ETr was able to reverse this increase. Na+-K+-ATPase enzyme activity was unchanged with age in GR and GW but increased in EDL. ETr increased enzyme activity in GR and GW and did not change in EDL. Myosin heavy chain isoforms in the muscle groups did not change significantly with age; ETr caused a general shift toward more oxidative fibers. Thus ETr differentially modifies age-associated alterations in expression of Na+-K+-ATPase subunit isoforms, and a mechanism(s) other than physical inactivity appears to play significant role in some of the age-associated changes.