Responses of beta-adrenoceptor in rat soleus to phosphorus compound levels and/or unloading

β-guanidinopropionic acid and metformin differentially impact autophagy, mitochondria and cellular morphology in developing C2C12 muscle cells

The serine/threonine kinase AMP-activated protein kinase (AMPK) is a drug target for the treatment of obesity and type 2 diabetes (T2D). Metformin, a widely prescribed anti-hyperglycemic agent, and β-guanidinopropionic acid (β-GPA), a dietary supplement and creatine analog, have been shown to increase activity of AMPK. Macroautophagy is an intracellular degradation pathway for aggregated proteins and dysfunctional organelles, which can be mediated by AMPK. The present study sought to elucidate how metformin and β-GPA affect cell morphology, AMPK activity, autophagy and mitochondrial morphology and function in developing C2C12 myotubes. β-GPA reduced myotube diameter and increased length throughout differentiation, while metformin increased myotube diameter only at the 48 h time point. β-GPA treatment enhanced AMPK signaling and expression of autophagy-related proteins. β-GPA treatment also increased the density of autophagosomes, autolysosomes, and lysosomes. Metformin also increased activation of AMPK after 48 h, but in contrast to β-GPA, led to a dramatic reduction in the density of autophagosomes and lysosomes. Both metformin and β-GPA reduced the mitochondrial oxygen consumption rate, and differentially altered mitochondrial morphology. Obesity and T2D have been shown to increase mitochondrial dysfunction and reduce autophagic flux in skeletal muscle cells. Therefore, β-GPA may help to alleviate the effects of metabolic disease by increasing autophagic flux in skeletal muscle cells. In contrast, the reduction of autophagy by metformin may lead to dysregulation of mitochondrial maintenance, as well as muscle development.

Elevated p70S6K phosphorylation in rat soleus muscle during the early stage of unloading: Causes and consequences

Currently, there is a lack of investigation into the initial signaling events underlying the development of disuse muscle atrophy. The study was aimed to (i) identify an assumed relationship between AMPK dephosphorylation and p70S6K hyperphosphorylation in the initial period of hindlimb unloading (HS), and (ii) assess the signaling consequences of p70S6K hyperphosphorylation following 24-h HS. For experiment 1, rats were treated with AMPK activator (AICAR) for 6 d before HS as well as during 24-h HS. For experiment 2, rats were treated with mTORC1 inhibitor rapamycin during 24-h HS. The key signaling markers implicated in protein turnover were assessed using WB and RT-PCR. One-day HS resulted in a significant upregulation of MuRF-1 and MAFbx expression, increase in p70S6K (Thr389) and IRS-1 (Ser639) phosphorylation and a significant decrease in phosphorylated AMPK, AKT, FOXO3, total IRS-1 content, and HDAC5 nuclear content. AMPK and p70S6K phosphorylation did not differ from control in AICAR-treated unloaded rats. Rapamycin treatment during unloading abolished p70S6K and E3 ligases upregulation and increased HDAC5 nuclear accumulation. The results of the study suggest that mTORC-1/p70S6K signaling pathway in rat soleus muscle is activated following 24-h mechanical unloading. This activation is facilitated by a decrease in AMPK phosphorylation. Increased p70S6K activity at the initial stage of hindlimb unloading could lead to the upregulation of E3 ligases MAFbx/atrogin-1 and MuRF-1 via nuclear export of HDAC5.

Responses of skeletal muscles to gravitational unloading and/or reloading

Adaptation of morphological, metabolic, and contractile properties of skeletal muscles to inhibition of antigravity activities by exposure to a microgravity environment or by simulation models, such as chronic bedrest in humans or hindlimb suspension in rodents, has been well reported. Such physiological adaptations are generally detrimental in daily life on earth. Since the development of suitable countermeasure(s) is essential to prevent or inhibit these adaptations, effects of neural, mechanical, and metabolic factors on these properties in both humans and animals were reviewed. Special attention was paid to the roles of the motoneurons (both efferent and afferent neurograms) and electromyogram activities as the neural factors, force development, and/or length of sarcomeres as the mechanical factors and mitochondrial bioenergetics as the metabolic factors.

Effects of gravitational loading levels on protein expression related to metabolic and/or morphologic properties of mouse neck muscles

The effects of 3 months of spaceflight (SF), hindlimb suspension, or exposure to 2G on the characteristics of neck muscle in mice were studied. Three 8‐week‐old male C57BL/10J wild‐type mice were exposed to microgravity on the International Space Station in mouse drawer system (MDS) project, although only one mouse returned to the Earth alive. Housing of mice in a small MDS cage (11.6 × 9.8‐cm and 8.4‐cm height) and/or in a regular vivarium cage was also performed as the ground controls. Furthermore, ground‐based hindlimb suspension and 2G exposure by using animal centrifuge (n = 5 each group) were performed. SF‐related shift of fiber phenotype from type I to II and atrophy of type I fibers were noted. Shift of fiber phenotype was related to downregulation of mitochondrial proteins and upregulation of glycolytic proteins, suggesting a shift from oxidative to glycolytic metabolism. The responses of proteins related to calcium handling, myofibrillar structure, and heat stress were also closely related to the shift of muscular properties toward fast‐twitch type. Surprisingly, responses of proteins to 2G exposure and hindlimb suspension were similar to SF, although the shift of fiber types and atrophy were not statistically significant. These phenomena may be related to the behavior of mice that the relaxed posture without lifting their head up was maintained after about 2 weeks. It was suggested that inhibition of normal muscular activities associated with gravitational unloading causes significant changes in the protein expression related to metabolic and/or morphological properties in mouse neck muscle.

Inhibition of gravitational loading in space and on the Earth for 3 months caused similar responses of protein expression in mouse neck muscle. Downregulation of mitochondrial proteins and upregulation of glycolytic proteins were induced, suggesting a shift from oxidative to glycolytic metabolism. Furthermore, the responses of proteins, involved in calcium handling, myofibrillar structure, and heat stress, related to the shift of muscular properties toward fast‐twitch type were also noted. It was suggested that inhibition of normal muscular activities associated with gravitational unloading caused significant changes in the protein expression related to metabolic and/or morphological properties in mouse neck muscle.

Region-specific responses of adductor longus muscle to gravitational load-dependent activity in Wistar Hannover rats

Response of adductor longus (AL) muscle to gravitational unloading and reloading was studied. Male Wistar Hannover rats (5-wk old) were hindlimb-unloaded for 16 days with or without 16-day ambulation recovery. The electromyogram (EMG) activity in AL decreased after acute unloading, but that in the rostral region was even elevated during continuous unloading. The EMG levels in the caudal region gradually increased up to 6th day, but decreased again. Approximately 97% of fibers in the caudal region were pure type I at the beginning of experiment. Mean percentage of type I fibers in the rostral region was 61% and that of type I+II and II fiber was 14 and 25%, respectively. The percent type I fibers decreased and de novo appearance of type I+II was noted after unloading. But the fiber phenotype in caudal, not rostral and middle, region was normalized after 16-day ambulation. Pronounced atrophy after unloading and re-growth following ambulation was noted in type I fibers of the caudal region. Sarcomere length in the caudal region was passively shortened during unloading, but that in the rostral region was unchanged or even stretched slightly. Growth-associated increase of myonuclear number seen in the caudal region of control rats was inhibited by unloading. Number of mitotic active satellite cells decreased after unloading only in the caudal region. It was indicated that the responses of fiber properties in AL to unloading and reloading were closely related to the region-specific neural and mechanical activities, being the caudal region more responsive.

Role(s) of gravitational loading during developing period on the growth of rat soleus muscle fibers

Effects of gravitational loading or unloading on the gain of the characteristics in soleus muscle fibers were studied in rats. The tail suspension was performed in newborn rats from postnatal day 4 to month 3, and the reloading was allowed for 3 mo in some rats. Single expression of type I myosin heavy chain (MHC) was observed in approximately 82% of fibers in 3-mo-old controls, but the fibers expressing multiple MHC isoforms were noted in the unloaded rats. Although 97% of fibers in 3-mo-old controls had a single neuromuscular junction at the central region of fiber, fibers with multiple nerve endplates were seen in the unloaded group. Faster contraction speed and lower maximal tension development, even after normalization with fiber size, were observed in the unloaded pure type I MHC fibers. These parameters generally returned to the age-matched control levels after reloading. It was suggested that antigravity-related tonic activity plays an important role in the gain of single neural innervation and of slow contractile properties and phenotype in soleus muscle fibers.

Metabolic modulation of muscle fiber properties unrelated to mechanical stimuli

The effects of chronically increasing (creatine-fed) or decreasing (beta-guanidinopropionic acid [beta-GPA]-fed) high-energy phosphates for up to 8 weeks on daily voluntary activity levels, swimming endurance capacity, electromyogram (EMG) activity, and the morphological and metabolic properties of single fibers in the soleus and extensor digitorum longus (EDL) muscles in young rats were determined. High-energy phosphate, voluntary activity, and soleus-integrated EMG levels were lower in beta-GPA-fed rats than in control rats. Endurance capacity was higher at a relatively low intensity of swimming and lower at a relatively high intensity in beta-GPA-fed rats than in control rats. Muscle mass and fiber size were smaller, and the percentage of slow fibers was higher in the soleus and EDL of beta-GPA-fed rats than in control rats. Succinate dehydrogenase activity was higher in both the fast and slow fibers of the EDL of beta-GPA-fed rats than in control rats. Thus, a reduction in high-energy phosphates transformed some fast fibers toward a slow phenotype. Creatine supplementation had minimal effects: The only significant change was an increase in alpha-glycerophosphate dehydrogenase activity in the fast fibers of the EDL. These results indicate that the metabolic environment of a muscle fiber can influence the prominence of a given muscle fiber independent of the activity level of muscle.

Evaluation of creatine transport using Caco-2 monolayers as an in vitro model for intestinal absorption

Creatine is a nutraceutical that has gained popularity in both well-trained and casual athletes for its performance-enhancing or ergogenic properties. The major disadvantages of creatine monohydrate formulations are poor solubility and oral bioavailability. In the present study, creatine transport was examined using Caco-2 monolayers as an in vitro model for intestinal absorption. Confluent monolayers of Caco-2 cells (passage 25-35) were used for the permeability studies. Monolayers were placed in side-by-side diffusion chambers. (14)C-Creatine (0.1-0.5 microCi/mL) was added to either the apical or basolateral side, and the transport of the creatine across the Caco-2 monolayer was measured over a 90-min period. The apical to basolateral transport of (14)C-creatine was small, ranging from 0.2-3% of the original amount appearing on the receiver side in a 90-min period. Interestingly, the basolateral to apical permeability of radiolabeled creatine was substantially greater than that observed in the apical to basolateral direction. Studies with drug efflux transport inhibitors indicate that neither the P-glycoprotein nor multidrug resistance-associated protein is involved in the enhanced basolateral to apical transport of creatine.

Effect of the beta(2)-agonist clenbuterol on the locomotor activity of rat submitted to a 14-day period of hypodynamia-hypokinesia

The beta(2)-adrenergic agonist clenbuterol is known for its anabolic action on normal and atrophied muscles. The aim of this work was to evaluate if chronic clenbuterol administration could prevent alterations in the locomotor activity induced by hindlimb suspension. The effects of clenbuterol were evaluated in three studies: muscle morphological characteristics, observation of locomotor movement and electromyographic activity of soleus and gastrocnemius muscles. Rats were divided into four groups: control (CON, morphological study only), hindlimb suspended (HS), clenbuterol administered (CB, 2 mg kg(-1) per day in drinking water), and hindlimb suspended+clenbuterol administered (HSCB). The soleus muscle weight was reduced in the two suspended groups (HS and HSCB) but did not change after clenbuterol treatment. By contrast, the gastrocnemius weight was not affected by suspension but was increased by clenbuterol (CB and HSCB). Some locomotor deficits were always observed in HS rats (unstable gait, ankle hyperextension, ellipsis). Clenbuterol administration did not prevent these perturbations. Cycle duration and soleus burst duration were increased in the three groups. Soleus mean EMG (burst area/duration) was decreased in HS rats, but not in the two other groups. For the gastrocnemius, burst duration was increased in CB rats, decreased in HSCB rats and unchanged in HS ones; mean EMG did not change. In conclusion, clenbuterol cannot be used as a countermeasure to reduce the alteration in locomotor performance. Moreover, our results suggest that this alteration is specifically related to changes in neuronal properties.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Neuromuscular adaptation to microgravity environment

Morphological and/or functional char-acteristics of skeletal muscles have a greater adaptability in response to changes in environmental stimuli. For example, an atrophy associated with a shift of fiber characteristics toward fast-twitch type is a common adaptation of antigravity muscle to a microgravity environment. Neuromuscular responses and possible mechanisms of both neural and muscular adaptations to a microgravity environment are discussed in this article. Responses of morphological, metabolic, and contractile properties, as well as fiber phenotype, of muscles are briefly reviewed. Discussion is further extended to the patterns of electromyogram and tension development of muscle, responses of postural stability and locomotion, and/or motoneurons in order to study the mechanism for muscular adaptation to microgravity.

Physical activity, skeletal muscle beta-adrenoceptor changes and oxidative metabolism in experimental chronic heart failure

In chronic heart failure (CHF), changes in sympathetic nervous activity and skeletal muscle metabolism contribute to a limitation in the capacity for exercise. The aim of this study was to investigate the potential relationships between physical deconditioning, skeletal muscle beta-adrenoceptor (beta-AR) characteristics and muscle metabolic changes in rats with coronary ligation-induced experimental CHF. Muscle beta-AR and norepinephrine levels were assessed in rats with CHF that had been treated with propranolol at 28 mg/kg/day and compared with rats with CHF that had not been treated and those that had undergone sham operations. The soleus muscle was investigated because of its predominantly oxidative fibre-type composition. Measurements of spontaneous locomotion activity were carried out using telemetry. After 85 days, muscle energetic phosphate levels were assessed using 31P-magnetic resonance spectroscopy. The phosphocreatine resynthesis rate was decreased in the untreated CHF rats (15 +/- 3 vs 33 +/- 5 mmol L-1 min-1 in the sham-operated rats, p < 0.05), but this had been partially reversed in the rats given propranolol (22 +/- 3 mmol L-1 min-1, non-significant (NS) when compared with the sham-operated rats). Spontaneous activity did not differ among the three groups of animals. Soleus beta-adrenoceptor density was decreased in rats with CHF (8.8 +/- 3.0 fM/mg of protein vs 22.0 +/- 7.0 fM/mg of protein in the sham-operated rats, p < 0.05) and normalized in the propranolol-treated rats (31.9 +/- 7.0 fM/mg of protein, NS vs the sham-operated rats; p < 0.05 vs the untreated rats with CHF). Unchanged spontaneous activity in the rats with CHF suggests that physical deconditioning could not account for the muscle metabolic changes. Changes in skeletal muscle energy metabolism were accompanied by changes in beta-AR density, occurring in typically oxidative beta-AR-rich muscles, reversible after beta-blocker therapy and therefore suggestive of beta-AR downregulation.

Improved fatigue resistance not associated with maximum oxygen consumption in creatine-depleted rats

Effects of feeding of either creatine or its analog beta-guanidinopropionic acid (beta-GPA) on endurance work capacity and oxygen consumption were studied in rats. Resting high-energy phosphate contents in hindlimb muscles were lower in the beta-GPA group and higher in the creatine group than in controls. The glycogen contents in resting hindlimb muscles of rats fed beta-GPA were significantly higher than those in controls. The endurance run and swimming times to exhaustion were significantly greater (32-70%) in the beta-GPA group than in the control and creatine groups. However, there were no beneficial effects on the maximum oxygen consumption (VO2max) and oxygen transport capacity of blood by the feeding of beta-GPA. None of these parameters were significantly influenced by creatine supply. Both maximum exercise time and VO2max in the beta-GPA group were not changed by normalization of glycogen levels. The activities of mitochondrial enzymes in skeletal muscles were higher in the beta-GPA group than in the controls. Thus endurance capacity is improved if the respiratory capacity of muscles is increased, even when the contents of high-energy phosphates in muscles are lower. Increased endurance capacity was not directly associated with the elevated levels of muscle glycogen, oxygen transport capacity of blood, or VO2max.

Increased growth of brown adipose tissue but its reduced thermogenic activity in creatine-depleted rats fed beta-guanidinopropionic acid

To study the responses of thermogenic activity in brown adipose tissue (BAT) to creatine depletion, male Wistar rats were fed creatine analogue beta-guanidinopropionic acid (beta-GPA) for about 10 weeks. Compared to control rats, a marked decrease in the levels of high-energy phosphates, such as phosphocreatine and ATP, was noted in BAT of beta-GPA rats. Conversely, upward trends in other chemical components (DNA, glycogen, and total protein) in BAT as well as an increase in BAT mass were observed in beta-GPA rats, suggesting a tendency to hyperplasia of the BAT. The thermogenic activity (which was assessed by guanosine 5'-diphosphate binding to BAT mitochondria) in the mitochondria recovered from BAT of beta-GPA rats, however, was not increased in response to such changes but rather decreased. Moreover, uncoupling protein (UCP) content in the mitochondrial fraction of beta-GPA rats was significantly lower than that in control rats (the relative amounts were 77 +/- 6 and 100 +/- 4%, respectively). Nevertheless, surprisingly, the level of UCP mRNA was remarkably greater in beta-GPA rats than in control rats. These observations indicate that there is a discordance between BAT growth and activity in beta-GPA rats, thereby suggesting that a failure on and after UCP translation may be involved in the impairment of BAT thermogenic activity with creatine depletion. The impairment of BAT thermogenic activity, that is, UCP activity may indicate that uncoupling or heat production was inhibited in order to increase the ATP synthesis in BAT of beta-GPA rats in compensation for a reduction in the levels of high-energy phosphates (including ATP), with resultant hypothermia.

Effects of creatine and beta-guanidinopropionic acid on the growth of Ehrlich ascites tumor cells: i.p. injection and culture study

Growth of Ehrlich ascites tumor (EAT) cells in the abdominal space of mice or in cell culture was studied in response to i.p. injection or addition, respectively, of creatine or creatine analogue beta-guanidinopropionic acid (beta-GPA). The increase in body weight of the mice due to cancer growth was less in the beta-GPA-injected than in the creatine- or sham-injected group. The volume of abdominal ascites and total cell counts at 11th day after implantation of EAT cells was significantly less in the beta-GPA than in the other groups. The proliferation rate of EAT cells in the beta-GPA group was 27% and 35% of the creatine- and sham-injected groups, respectively. Supplementation of creatine tended to enhance the growth of EAT cells. The creatine concentration in ascites fluid was approximately 4-times greater than in blood plasma of sham-injected control mice. But the creatine content in EAT cells was significantly reduced to approximately 50% in response to beta-GPA injection. Cell culture without creatine caused a significant decrease in viability. The viability was improved, however, by addition of either creatine or serum into the medium. By contrast, it was not significantly increased by addition of serum alone which caused only a minor elevation of the creatine level (23 microM). It is suggested that EAT cell growth is inhibited by lowering the availability of creatine in association with some unknown factors in serum or ascites fluid.