Endurance training, not acute exercise, differentially alters beta-receptors and cyclase in skeletal fiber types

Adrenal stress hormone action in skeletal muscle during exercise training: An old dog with new tricks?

Exercise is a key component of a healthy lifestyle as it helps maintain a healthy body weight and reduces the risk of various morbidities and co-morbidities. Exercise is an acute physiological stress that initiates a multitude of processes that attempt to restore physiological homeostasis and promote adaptation. A component of the stress response to exercise is the rapid release of hormones from the adrenal gland including glucocorticoids, the catecholamines and aldosterone. While each hormone targets several tissues throughout the body, skeletal muscle is of interest as it is central to physical function and various metabolic processes. Indeed, adrenal stress hormones have been shown to elicit specific performance benefits on the muscle. However, how the acute, short-lived release of these stress hormones during exercise influences adaptations of skeletal muscle to long-term training remains largely unknown. Thus, the objective of this review was to briefly highlight the known impact of adrenal stress hormones on skeletal muscle metabolism and function (Old Dog), and critically examine the current evidence supporting a role for these endogenous hormones in mediating long-term training adaptations in skeletal muscle (New Tricks).

Disruption of adenylyl cyclase type 5 mimics exercise training

Exercise training is key to healthful longevity. Since exercise training compliance is difficult, it would be useful to have a therapeutic substitute that mimicked exercise training. We compared the effects of exercise training in wild-type (WT) littermates with adenylyl cyclase type 5 knock out (AC5 KO) mice, a model of enhanced exercise performance. Exercise performance, measured by maximal distance and work to exhaustion, was increased in exercise-trained WT to levels already attained in untrained AC5 KO. Exercise training in AC5 KO further enhanced their exercise performance. The key difference in untrained AC5 KO and exercise-trained WT was the β-adrenergic receptor signaling, which was decreased in untrained AC5 KO compared to untrained WT but was increased in WT with exercise training. Despite this key difference, untrained AC5 KO and exercise-trained WT mice shared similar gene expression, determined by deep sequencing, in their gastrocnemius muscle with 183 genes commonly up or down-regulated, mainly involving muscle contraction, metabolism and mitochondrial function. The SIRT1/PGC-1α pathway partially mediated the enhanced exercise in both AC5 KO and exercise-trained WT mice, as reflected in the reduced exercise responses after administering a SIRT1 inhibitor, but did not abolish the enhanced exercise performance in the AC5 KO compared to untrained WT. Increasing oxidative stress with paraquat attenuated exercise performance more in untrained WT than untrained AC5 KO, reflecting the augmented oxidative stress protection in AC5 KO. Blocking nitric oxide actually reduced the enhanced exercise performance in untrained AC5 KO and trained WT to levels below untrained WT, demonstrating the importance of this mechanism. These results suggest that AC5 KO mice, without exercise training, share similar mechanisms responsible for enhanced exercise capacity with chronic exercise training, most importantly increased nitric oxide, and demonstrate more reserve with the addition of exercise training. A novel feature of the enhanced exercise performance in untrained AC5 KO mice is their decreased sympathetic tone, which is also beneficial to patients with cardiovascular disease.

Effects of intermittent weight-bearing and clenbuterol on disuse atrophy of rat hindlimb muscles

The present study was undertaken to evaluate the effects of intermittent weight-bearing (IWB) combined with β 2-agonist clenbuterol (Cb) medication for suppressing muscle atrophy during progressive disuse atrophy. Male Wistar rats (age: 8weeks, body weight: 232 ± 14 g) were divided into a control group (CON) and an experimental group. The experimental group was further subdivided into a Cb medication group under normal conditions and a hindlimb unweighting (HU) treatment group. The HU treatment group was composed of four groups: HU treatment-only, HU treatment + IWB, HU treatment + Cb medication and HU treatment + IWB + Cb medication. IWB was performed by temporarily removing the suspension device for one hour daily. On Day 14, bilateral soleus muscle (SOL) and extensor digitorum longus muscle (EDL) were extracted. Muscles from the right side were used for the measurement of contractile properties (physiological functional evaluations). Muscles from the left side were used for histochemical and biochemical analysis. During HU, IWB combined with Cb medication worked to preserve the wet weight and relative weight of SOL as compared to CON. Its contractile properties were affected by weight-bearing, while the cross-sectional area of type I fiber and protein concentration were affected by Cb. This combined therapy had marked effects on the morphology of EDL, particularly on the cross-sectional area of type II fiber. The protein concentration and contractile properties of EDL were unaffected by this combined therapy. The effect of a combination of IWB and Cb medication was specific to fiber-type and region. The data suggested that 1) IWB was effective on functional aspects such as contractile properties and useful for physical therapy, 2) Cb medication exerted the atrophy-suppressive effect in morphological parameters and manifested less effect on functional aspects. The results in this study indicated the possibility of elevating the efficacy of IWB by Cb medication in SOL.

cAMP signaling in skeletal muscle adaptation: hypertrophy, metabolism, and regeneration

Among organ systems, skeletal muscle is perhaps the most structurally specialized. The remarkable subcellular architecture of this tissue allows it to empower movement with instructions from motor neurons. Despite this high degree of specialization, skeletal muscle also has intrinsic signaling mechanisms that allow adaptation to long-term changes in demand and regeneration after acute damage. The second messenger adenosine 3',5'-monophosphate (cAMP) not only elicits acute changes within myofibers during exercise but also contributes to myofiber size and metabolic phenotype in the long term. Strikingly, sustained activation of cAMP signaling leads to pronounced hypertrophic responses in skeletal myofibers through largely elusive molecular mechanisms. These pathways can promote hypertrophy and combat atrophy in animal models of disorders including muscular dystrophy, age-related atrophy, denervation injury, disuse atrophy, cancer cachexia, and sepsis. cAMP also participates in muscle development and regeneration mediated by muscle precursor cells; thus, downstream signaling pathways may potentially be harnessed to promote muscle regeneration in patients with acute damage or muscular dystrophy. In this review, we summarize studies implicating cAMP signaling in skeletal muscle adaptation. We also highlight ligands that induce cAMP signaling and downstream effectors that are promising pharmacological targets.

Muscle plasticity and β₂-adrenergic receptors: adaptive responses of β₂-adrenergic receptor expression to muscle hypertrophy and atrophy

We discuss the functional roles of β₂-adrenergic receptors in skeletal muscle hypertrophy and atrophy as well as the adaptive responses of β₂-adrenergic receptor expression to anabolic and catabolic conditions. β₂-Adrenergic receptor stimulation using anabolic drugs increases muscle mass by promoting muscle protein synthesis and/or attenuating protein degradation. These effects are prevented by the downregulation of the receptor. Endurance training improves oxidative performance partly by increasing β₂-adrenergic receptor density in exercise-recruited slow-twitch muscles. However, excessive stimulation of β₂-adrenergic receptors negates their beneficial effects. Although the preventive effects of β₂-adrenergic receptor stimulation on atrophy induced by muscle disuse and catabolic hormones or drugs are observed, these catabolic conditions decrease β₂-adrenergic receptor expression in slow-twitch muscles. These findings present evidence against the use of β₂-adrenergic agonists in therapy for muscle wasting and weakness. Thus, β₂-adrenergic receptors in the skeletal muscles play an important physiological role in the regulation of protein and energy balance.

Impaired skeletal muscle beta-adrenergic activation and lipolysis are associated with whole-body insulin resistance in rats bred for low intrinsic exercise capacity

Rats selectively bred for high endurance running capacity (HCR) have higher insulin sensitivity and improved metabolic health compared with those bred for low endurance capacity (LCR). We investigated several skeletal muscle characteristics, in vitro and in vivo, that could contribute to the metabolic phenotypes observed in sedentary LCR and HCR rats. After 16 generations of selective breeding, HCR had approximately 400% higher running capacity (P < 0.001), improved insulin sensitivity (P < 0.001), and lower fasting plasma glucose and triglycerides (P < 0.05) compared with LCR. Skeletal muscle ceramide and diacylglycerol content, basal AMP-activated protein kinase (AMPK) activity, and basal lipolysis were similar between LCR and HCR. However, the stimulation of lipolysis in response to 10 mum isoproterenol was 70% higher in HCR (P = 0.004). Impaired isoproterenol sensitivity in LCR was associated with lower basal triacylglycerol lipase activity, Ser660 phosphorylation of HSL, and beta2-adrenergic receptor protein content in skeletal muscle. Expression of the orphan nuclear receptor Nur77, which is induced by beta-adrenergic signaling and is associated with insulin sensitivity, was lower in LCR (P < 0.05). Muscle protein content of Nur77 target genes, including uncoupling protein 3, fatty acid translocase/CD36, and the AMPK gamma3 subunit were also lower in LCR (P < 0.05). Our investigation associates whole-body insulin resistance with impaired beta-adrenergic response and reduced expression of genes that are critical regulators of glucose and lipid metabolism in skeletal muscle. We identify impaired beta-adrenergic signal transduction as a potential mechanism for impaired metabolic health after artificial selection for low intrinsic exercise capacity.

Catecholamines and the effects of exercise, training and gender

Stress hormones, adrenaline (epinephrine) and noradrenaline (norepinephrine), are responsible for many adaptations both at rest and during exercise. Since their discovery, thousands of studies have focused on these two catecholamines and their importance in many adaptive processes to different stressors such as exercise, hypoglycaemia, hypoxia and heat exposure, and these studies are now well acknowledged. In fact, since adrenaline and noradrenaline are the main hormones whose concentrations increase markedly during exercise, many researchers have worked on the effect of exercise on these amines and reported 1.5 to >20 times basal concentrations depending on exercise characteristics (e.g. duration and intensity). Similarly, several studies have shown that adrenaline and noradrenaline are involved in cardiovascular and respiratory adjustments and in substrate mobilization and utilization. Thus, many studies have focused on physical training and gender effects on catecholamine response to exercise in an effort to verify if significant differences in catecholamine responses to exercise could be partly responsible for the different performances observed between trained and untrained subjects and/or men and women. In fact, previous studies conducted in men have used different types of exercise to compare trained and untrained subjects in response to exercise at the same absolute or relative intensity. Their results were conflicting for a while. As research progressed, parameters such as age, nutritional and emotional state have been found to influence catecholamine concentrations. As a result, most of the recent studies have taken into account all these parameters. Those studies also used very well trained subjects and/or more intense exercise, which is known to have a greater effect on catecholamine response so that differences between trained and untrained subjects are more likely to appear. Most findings then reported a higher adrenaline response to exercise in endurance-trained compared with untrained subjects in response to intense exercise at the same relative intensity as all-out exercise. This phenomenon is referred to as the 'sports adrenal medulla'. This higher capacity to secrete adrenaline was observed both in response to physical exercise and to other stimuli such as hypoglycaemia and hypoxia. For some authors, this phenomenon can partly explain the higher physical performance observed in trained compared with untrained subjects. More recently, these findings have also been reported in anaerobic-trained subjects in response to supramaximal exercise. In women, studies remain scarce; the results are more conflicting than in men and the physical training type (aerobic or anaerobic) effects on catecholamine response remain to be specified. Conversely, the works undertaken in animals are more unanimous and suggest that physical training can increase the capacity to secrete adrenaline via an increase of the adrenal gland volume and adrenaline content.

Increased thermogenic responsiveness to intravenous beta-adrenergic stimulation in habitually exercising humans is not related to skeletal muscle beta2-adrenergic receptor density

Habitually exercising adults demonstrate greater thermogenic responsiveness to beta-adrenergic receptor (beta-AR) stimulation compared with their sedentary peers, but the molecular mechanisms involved are unknown. To determine the possible role of increased beta-AR density, we studied 32 healthy adults: 17 habitual aerobic exercisers (age 45 +/- 5 years, 11 males) and 15 sedentary (49 +/- 5 years, 7 males). Maximal oxygen uptake (43.7 +/- 2.5 versus 31.6 +/- 2.9 ml kg(-1) min(-1), P = 0.002, mean +/- S.E.M.) and vastus lateralis muscle maximal citrate synthase activity (1.70 +/- 0.36 versus 0.58 +/- 0.11 micromol min(-1) g(-1), P = 0.008) were higher in the habitually exercising subjects. Resting energy expenditure (EE) adjusted for fat-free mass (FFM) was similar in the habitually exercising (5903 +/- 280 kJ day(-1)) and sedentary adults (6054 +/- 289 kJ day(-1), P = 0.43). The percentage increase in EE (DeltaEE%; indirect calorimetry, ventilated hood) above resting EE in response to beta-AR stimulation (intravenous isoproterenol at 6, 12 and 24 ng (kg FFM)(-1) min(-1)) was greater (7.1 +/- 1.2, 13.7 +/- 1.0, 20.7 +/- 1.3 versus 5.9 +/- 0.9, 9.9 +/- 1.4, 15.9 +/- 1.70%, respectively, P = 0.04), and the dose of isoproterenol required to increase EE by 10% above resting EE was lower (8.2 +/- 1.5 versus 17.1 +/- 4.1 ng (kg FFM)(-1) min(-1), P = 0.03) in the habitually exercising adults. In contrast, vastus lateralis muscle beta(2)-AR density was similar in the habitually exercising and sedentary subjects (7.46 +/- 0.29 versus 7.44 +/- 0.60 fmol (mg dry weight muscle)(-1), P = 0.98), and was not related to DeltaEE% (r = 0.02, P = 0.94) or to the isoproterenol dose required to increase EE by 10% above resting EE (r = -0.06, P = 0.76). These findings indicate that increased beta(2)-AR density is not a mechanism contributing to the greater thermogenic responsiveness to beta-AR stimulation in adult humans who regularly perform aerobic exercise.

The effect of previous weight training and concurrent weight training on endurance for functional electrical stimulation cycle ergometry

Forty-five paraplegic subjects participated in three series of experiments to examine the interrelationships between previous weight training, concurrent weight training and muscle strength and endurance during cycle ergometry elicited by functional electrical stimulation (FES). When subjects only underwent isokinetic weight training (series 1) three times per week on the quadriceps, hamstring and gluteus maximus groups for 12 weeks, strength increased linearly with time for all three muscle groups from an initial average of 17 N to 269 N at the end of training, a 15-fold increase. In the second series of experiments, different groups of subjects either underwent no strength training prior to cycle ergometry or underwent strength training of these same three muscle groups for 2 weeks, four weeks, or 6 weeks prior to cycle ergometry. Any strength training was effective in increasing endurance for cycle ergometry. However, the rate of increase in endurance during cycle ergometry with no prior strength training was only 5 min per week, whereas the rate of increase in cycle endurance during ergometry was 14.6, 25.0, and 33.3 min per week increase in endurance after strength training for 2.4 and 6 weeks, respectively. When weight training was accomplished during FES cycle ergometry (concurrently) in a third series of experiments, there was an additional increase in endurance during cycling if strength training was concurrently accomplished. With no weight training, endurance increased 23 min per week, whereas with concurrent weight training at three times per week, endurance increased during cycling by 41.6 min per week. The results of these experiments seem to show a clear advantage of weight training concurrently and before FES cycle ergometry. Results are given as mean (SD).

Effect of electrical stimulation on beta-adrenergic receptor population and cyclic amp production in chicken and rat skeletal muscle cell cultures

Expression of the beta-adrenergic receptor (betaAR) and its coupling to cyclic AMP (cAMP) synthesis are important components of the signaling system that controls muscle atrophy and hypertrophy, and the goal of this study was to determine if electrical stimulation in a pattern simulating slow muscle contraction would alter the betaAR response in primary cultures of avian and mammalian skeletal muscle cells. Specifically, chicken skeletal muscle cells and rat skeletal muscle cells that had been grown for 7 d in culture were subjected to electrical stimulation for an additional 2 d at a pulse frequency of 0.5 pulses/sec and a pulse duration of 200 msec. In chicken skeletal muscle cells, the betaAR population was not significantly affected by electrical stimulation; however, the ability of these cells to synthesize cyclic AMP was reduced by approximately one-half. In contrast, the betaAR population in rat muscle cells was increased slightly but not significantly by electrical stimulation, and the ability of these cells to synthesize cyclic AMP was increased by almost twofold. The basal levels of intracellular cyclic AMP in neither rat muscle cells nor chicken muscle cells were affected by electrical stimulation.

Exercise reverses peripheral insulin resistance in trained L-NAME-hypertensive rats

Several studies have demonstrated an increase in peripheral resistance to insulin associated with hypertension. To assess the hemodynamic and metabolic effects of exercise training, normotensive and N(omega)-nitro-L-arginine methyl ester (L-NAME)-hypertensive male Wistar rats were submitted to low-intensity treadmill exercise training for 10 weeks and compared with their sedentary controls. Blood pressure signals were obtained and processed with a data acquisition system (CODAS, 1 kHz) to evaluate mean arterial pressure, heart rate, autonomic control of heart rate, and baroreflex sensitivity. Exercise training induced a nonsignificant 6.5-mm Hg decrease in mean arterial pressure in trained hypertensive rats (163+/-9 mm Hg) compared with sedentary hypertensive rats (169.5+/-5. 5 mm Hg). The hypertensive groups showed impairment of baroreflex function in response to changes in arterial pressure compared with sedentary controls. Furthermore, exercise training improved the tachycardic response to decreasing arterial pressure and reduced intrinsic heart rate in trained control rats compared with all other groups. Sedentary hypertensive rats presented a decrease in body weight compared with normotensive animals. Basal evaluation of the glucose/insulin ratio showed increased insulin resistance in sedentary (28.4+/-3) and trained (23.5+/-2.7) hypertensive rats compared with sedentary control rats (40.5+/-3). However, the glucose/insulin ratio evaluated during the exercise session in trained rats showed an improvement in insulin resistance (54.5+/-5 for control rats and 44+/-9 for hypertensive rats). In conclusion, L-NAME-induced hypertension is accompanied by an increase in insulin resistance in rats. The improvement in peripheral insulin sensitivity during exercise and the body weight gain observed in trained hypertensive rats may support the positive role of physical activity in the management of hypertension.

Peripheral and central adrenoceptor modulation of the behavioural effects of clozapine in the paw test

1. In rats, the atypical neuroleptic, clozapine, has been found to increase the hindlimb retraction time but not the forelimb retraction time, in the paw test. These parameters have predictive validity for the antipsychotic efficacy and extrapyramidal side-effects of drugs, respectively. The present study analysed to what extent drugs acting on adrenoceptors affect the behavioural effect of clozapine in the paw test. 2. The alpha 1-adrenoceptor agonist, ST 587 but not the peripherally working alpha 1-agonist, methoxamine, decreased the effect of clozapine on the hindlimb retraction time. The alpha 1-antagonist phenoxybenzamine increased this effect of clozapine, and blocked the effect of ST 587 on clozapine at low doses. Only the combination of phenoxybenzamine with clozapine produced an increase in forelimb retraction time. 3. The alpha 2-adrenoceptor agonist, clonidine, decreased the effect of clozapine on the hindlimb retraction time. This effect was neither antagonized by the alpha 2-antagonist rauwolscine nor by the alpha 1-antagonist phenoxybenzamine. Rauwolscine or the peripherally working alpha 2-antagonist L-659,066 did not influence the effect of clozapine on the hindlimb retraction time. The forelimb retraction time was not affected by any of the drug combinations. 4. In contrast to the beta 2-adrenoceptor agonist, clenbuterol, which was ineffective, the peripherally acting beta-agonist, (-)-isoprenaline, increased the effects of clozapine on the hindlimb retraction time. The beta-antagonist, (-)-propranolol as well as the peripherally acting beta-antagonist, nadolol decreased this effect of clozapine. Low doses of the peripherally acting beta 1-antagonist, atenolol, as well as low doses of the beta2-antagonist, ICI-118,551, decreased the effect of clozapine. A low dose of nadolol blocked the effect of (-)-isoprenaline on clozapine. Only the combination of clenbuterol with clozapine produced an increase in forelimb retraction time.5. It is concluded that blockade of central alpha l-adrenoceptors plays an important role in the effect of clozapine on the hindlimb retraction time. Furthermore, the effect of clozapine on the hindlimb retraction time is strongly modulated by peripheral beta 1- and/or beta 2-adrenoceptors. Given the predictive validity of the paw test, the presented data suggest that the alpha 1-adrenoceptor antagonist properties of clozapine are important for its therapeutic effects, but not for its lack of extrapyramidal side-effects.