Effects of beta 1- vs. beta 1- beta 2-blockade on training adaptations in rat skeletal muscle

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.

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.

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.

Reduced glutathione inhibits rabbit and rat skeletal muscle lactate dehydrogenase and prevents dinitrophenol induced extracellular acidification by an epithelial cell line

Glutathione (GSH) is a tripeptide synthesised enzymatically from its components amino-acids by unicellular and multicellular organisms. GSH acts as a cellular anti-oxidant, protects the structural configuration of some enzymes, is involved in erythrocyte function and plays a role as co-enzyme in several reactions. We have found that GSH inhibits purified lactate dehydrogenase (1.56 U LDH/ml) from rabbit skeletal muscle after 6 min pre-incubation with an ED50 of about 5.4 microM. The inhibition is time dependent with a maximum after 45 minutes pre-incubation. Buffer (5 x 10(-2) M TRIZMA hydrochloride, pH 7.4) and a chelator (2 x 10(-3) M EDTA) in the pre-incubation solution did not prevent the inhibition. Prolonged dialysis was almost without effect on GSH-inhibited LDH activity solution, indicating either an irreversible or a very tight binding inhibition. Kinetic analysis showed that this inhibition is of a very tight binding and at the same time of the uncompetitive type. GSH also inhibits LDH activity of rat M. soleus and M. gastrocnemius homogenates. This effect is probably unrelated to the reducing property of GSH since dithioerythritol (0.17-1.34 mM) does not mimic it. Loading of MDCK cells with glutathione ethylester completely prevented the acidification induced by 2,4-dinitrophenol, suggesting that GSH may influence the glycolytic pathway in vivo.

Interaction of exercise training and clenbuterol on GLUT-4 protein in muscle of obese Zucker rats

Chronic administration of clenbuterol, a beta 2-adrenergic agonist, attenuates the exercise training-induced improvement in muscle insulin resistance of the obese Zucker rat. The present study was conducted to determine whether clenbuterol also attenuates the increase in muscle GLUT-4 protein that occurs with exercise training and whether the action of clenbuterol is related to its ability to downregulate the beta-adrenergic receptors. Female obese Zucker rats were randomly assigned to one of the following four groups: control (CON, n = 7), clenbuterol (CL, n = 8), exercise training (TR, n = 8), and clenbuterol with exercise training (CL+TR, n = 8). Rats assigned to the training groups were run on a rodent motor-driven treadmill for 6-7 wk. Rats receiving clenbuterol were intubated with 0.8 mg/kg body weight 30 min before running each day. Red quadriceps (RQ) and white quadriceps (WQ) GLUT-4 protein concentrations of TR rats were significantly greater than those of CON and CL+TR rats. The RQ GLUT-4 protein concentration of the CL+TR rats was significantly greater than that of CON rats, but this difference did not occur in the WQ. GLUT-4 protein concentrations were not different between the CON and CL rats. The patterns of RQ and WQ GLUT-4 mRNA were similar to those of their respective GLUT-4 proteins. Rats receiving daily injections of propranolol (30 mg/kg body wt), a beta-adrenergic receptor antagonist, demonstrated no increase in GLUT-4 protein in RQ or WQ after 6 wk of exercise training. These results indicate that 1) clenbuterol can attenuate the increase in muscle GLUT-4 protein associated with exercise training and 2) this effect is likely mediated by a downregulation of the beta-adrenergic receptors.

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

Responses of beta-adrenoceptor (beta-AR) in rat soleus to gravitational unloading and/or changes in the levels of phosphorus compounds by feeding either creatine or its analogue beta-guanidinopropionic acid (beta-GPA) were studied. A decrease in the density of beta-AR (about -35%) was induced by 10 days of hindlimb suspension, but the affinity of the receptor was unaffected. Suspension unloading tended to increase the levels of adenosine triphosphate and phosphocreatine and decrease inorganic phosphate. Even without unloading, the beta-AR density decreased after an oral creatine supplementation (about -20%), which also tended to elevate the high-energy phosphate levels in muscle. However, an elevation of beta-AR density was induced (about +36%) after chronic depletion of high-energy phosphates by feeding beta-GPA (about +125%). Data suggest that the density of beta-AR in muscle is elevated if the high-energy phosphate contents are chronically decreased and vice versa. However, it may not be directly related to the degree of muscle contractile activity.

Dynamic cardiomyoplasty in chronic left ventricular failure: an experimental model

Dynamic cardiomyoplasty continues to attract interest as a therapeutic option in the management of heart failure. In a large animal model of ischemic heart failure, we have compared dynamic cardiomyoplasty with both adynamic cardiomyoplasty and a control group. Heart failure was induced by coronary artery ligation in sheep, and under the same anesthetic dynamic cardiomyoplasty (n = 5), adynamic cardiomyoplasty (n = 4), or no further procedure was performed (n = 5). After recovery the animals were housed for a further 3 months. The dynamic cardiomyoplasty underwent a recognized muscle transformation protocol during this period. At terminal studies, the animals were hemodynamically assessed, both under baseline conditions and after colloid volume loading. The data at baseline were compared with unpaired t tests, and the function curves created by volume loading were compared by analysis of variance. Although the changes at baseline were small, there were highly significant improvements in the function curves in the dynamic cardiomyoplasty group when the stimulators were turned on compared with stimulators off (p = 0.005) for cardiac output; p = 0.035 for left ventricular end-diastolic pressure; p = 0.002 for pulmonary artery capillary wedge pressure; p = 0.004 for stroke volume; and p = 0.003 for cardiac power). There were also significant improvements in indices of cardiac performance when the dynamic cardiomyoplasty group was compared with both the control and adynamic cardiomyoplasty groups. We conclude that there is experimental evidence that cardiomyoplasty augments cardiac function in a model of chronic left ventricular failure.

Effects of beta-adrenergic blockade on training-induced structural adaptations in rat left ventricle

The study was designed to evaluate the effects of eight weeks of exercise training or training-beta-adrenergic blockade combination on gross and microscopic alterations of rat cardiac muscle and microvascular bed. Rats were randomly assigned to either sedentary control (C), trained (T), metoprolol-trained (MT), or propranolol-trained (PT) groups. The training protocol involved treadmill running for 8 weeks at 0.5 ms-1, 20% grade. Earlier experiments by us showed this training protocol to be effective in producing significant changes in selected skeletal muscle enzyme activities in all trained groups. In the current study an absolute reduction in left ventricular (LV) weight was observed in the PT compared to the C group (0.91 +/- 0.02 vs. 1.04 +/- 0.04 g, P less than 0.05). LV weight in the T and MT groups was no different from C so that LV to BW ratio (mg.g-1) was significantly increased (P less than 0.05) due to a similar reduction in body weight (BW) in all three training groups. Morphometric analysis of LV myocardium revealed no significant differences in myocyte mean cross-sectional area (micron 2) in any of the groups (289 +/- 16-C, 332 +/- 20-T, 281 +/- 44-MT, and 273 +/- 12-PT). Capillary density independently calculated by light and electron microscopy was unchanged by training or training-beta-blockade combination. It was concluded that training of sufficient intensity and duration to produce skeletal muscle enzyme adaptations does not necessarily produce myocyte hypertrophy or alter LV capillarity. Additionally functioning beta-adrenergic receptors appear to play a role in both the central and peripheral adaptations to endurance exercise training.