Metabolic and structural effects of insulin-like growth factor-I and high-protein diet on dystrophic hamster skeletal muscle

Consequences of thyroxine treatment on diaphragm and EDL of normal and dystrophic hamsters

Previously administration of thyroxine (T4) to dystrophic hamsters improved ventilation and slowed the progression of the disease. We hypothesized that the normalization of ventilation in these animals was due to T4 improving structural and functional characteristics of the diaphragm. In the present study, contractile characteristics of the diaphragm and the extensor digitorum longus (EDL) from normal and dystrophic hamsters were evaluated after two months of T4 treatment. Compared to their placebo-treated counterparts, diaphragms and EDLs of T4-treated normal hamsters showed increased optimal muscle lengths and twitch tension, decreased contraction times and increased fatigability. T4-treatment in dystrophic hamsters showed only an increase in diaphragmatic twitch tension development. Force-frequency curves before treatment were generally higher for the normal compared to dystrophic diaphragms and EDLs. T4 administration only increased the force in normal diaphragms at the lower frequencies and in the EDLs at the higher frequencies. Although T4 serum levels were increased in both T4-treated groups, triiodothyronine (T3) was much lower in the dystrophic compared to normal hamsters, suggesting that conversion of T4 to T3 was reduced in dystrophic hamsters. We conclude that the limited functional changes in the diaphragms of T4-treated dystrophic hamsters cannot account for the marked improvement in ventilation previously reported.Key words: dystrophy, thyroid hormones, skeletal muscles, diaphragm.

Effects of insulin-like growth factor-1/binding protein-3 complex on muscle atrophy in rats

Muscle atrophy and wasting is a serious problem that occurs in patients with prolonged debilitating illness, burn injury, spinal injury, as well as with space flight. Current treatment for such atrophy, which often relies on nutritional supplementation and physical therapy, is of limited value in preventing the muscle wasting that occurs. Considerable recent attention has focused on the use of anabolic growth factors such as insulin-like growth factor (IGF-1) in preventing muscle atrophy during limb disuse or with various catabolic conditions. However, potential side effects such as hypoglycemia appear to be limiting factors in the usefulness of IGF-1 for clinical treatment of muscle wasting conditions. The formulation of IGF-1 used in this study (IGF-1/BP3) is already bound to its endogenous-binding protein (BP3) and, as a result, has a greater specificity of action and significantly less hypoglycemic effect. Using a rat model of hind limb suspension (HLS) for 10 days, we induced marked muscle atrophy that was accompanied by enhanced muscle proteolysis and reduced muscle protein content. When HLS rats were treated with IGF-1/BP3 (50 mg/kg, b.i.d.), they retained greater body and muscle mass. Muscle protein degradation was significantly reduced and muscle protein content was preserved. The rate of protein synthesis, although somewhat reduced in HLS muscle, was not significantly elevated by IGF-1/BP3 treatment. Volume density of HLS-treated muscles were increased compared to untreated HLS rats and the actual number of fibers per area of muscle was likewise increased. The results of the current study suggest that IGF-1/BP3 might be useful for inhibiting muscle proteolysis in catabolic conditions and thus preserving muscle protein content and mass.

Induction of stearoyl CoA desaturase is associated with high-level induction of emerin RNA

The genes encoding enzymes involved in fatty acid metabolism are regulated by sterols. Stearoyl CoA desaturase, a key enzyme in the synthesis of unsaturated fatty acyl-CoAs is transcriptionally regulated by fat-free diet and sterols. To identify other genes that are induced in rat liver by fat-free diet we performed an in vivo gene expression profile analysis using DNA microarrays. Here we report that among the genes highly expressed is emerin, an integral protein of the inner nuclear membrane. Mutated or nonexpressed emerin occurs in patients with muscular dystrophy. Sterol regulatory element binding proteins activate the transcription of several sterol regulated genes. To investigate whether sterol regulatory element binding proteins or indirectly cholesterol activates the transcription of stearoyl CoA desaturase and emerin, we cultured Chinese hamster ovary (CHO), either in cholesterol-rich or cholesterol-depleted mediums. We also transiently transfected the cell culture with plasmid encoding sterol regulatory element binding proteins in cholesterol-rich medium. Our data show that cholesterol-supplemented media as well as the transient transfection induced the expression of stearoyl CoA desaturase RNA 3.5- and 7-fold respectively. However, the RNA level of emerin was not altered under these conditions, implying that the parallel induction of emerin is independent of the sterol regulatory element binding regulation pathway.

IGF-I treatment improves the functional properties of fast- and slow-twitch skeletal muscles from dystrophic mice

Although insulin-like growth factor-I (IGF-I) has been proposed for use by patients suffering from muscle wasting conditions, few studies have investigated the functional properties of dystrophic skeletal muscle following IGF-I treatment. 129P1 ReJ-Lama2(dy) (129 ReJ dy/dy) dystrophic mice suffer from a deficiency in the structural protein, laminin, and exhibit severe muscle wasting and weakness. We tested the hypothesis that 4 weeks of IGF-I treatment ( approximately 2 mg/kg body mass, 50 g/h via mini-osmotic pump, subcutaneously) would increase the mass and force producing capacity of skeletal muscles from dystrophic mice. IGF-I treatment increased the mass of the extensor digitorum longus (EDL) and soleus muscles of dystrophic mice by 20 and 29%, respectively, compared with untreated dystrophic mice (administered saline-vehicle only). Absolute maximum force (P(o)) of the EDL and soleus muscle was increased by 40 and 32%, respectively, following IGF-I treatment. Specific P(o) (sP(o)) was increased by 23% in the EDL muscles of treated compared with untreated mice, but in the soleus muscle sP(o) was unchanged. IGF-I treatment increased the proportion of type IIB and type IIA fibres and decreased the proportion of type I fibres in the EDL muscles of dystrophic mice. In the soleus muscles of dystrophic mice, IGF-I treatment increased the proportion of type IIA fibres and decreased the proportion of type I fibres. Average fibre cross-sectional area was increased in the EDL and soleus muscles of treated compared with untreated mice. We conclude that IGF-I treatment ameliorates muscle wasting and improves the functional properties of skeletal muscles of dystrophic mice. The findings have important implications for the role of IGF-I in ameliorating muscle wasting associated with the muscular dystrophies.

Higher content of insulin-like growth factor-I in dystrophic mdx mouse: potential role in the spontaneous regeneration through an electrophysiological investigation of muscle function

Insulin-like growth factor-I (IGF-I) is known to promote proliferation and differentiation of muscle cells during growth and regeneration. Both these conditions are characterized by acquisition of specialized muscle functions, such as a large macroscopic chloride conductance (GCl), a parameter that is a target of growth hormone (GH)/IGF-I axis action on skeletal muscle. The present study has been aimed at evaluating the role of IGF-I in the spontaneous regeneration occurring in hind limb muscle of dystrophic mdx mouse. IGF-I levels have been measured in hind limb muscles, plasma and liver of mdx and control mice of 8-10 weeks and 5 months of age by radioimmunoassay. In parallel the biophysical and pharmacological properties of muscle chloride channels of extensor digitorum longus (EDL) muscle fibers of mice belonging to the same age-group have been measured electrophysiologically in vitro. At 8-10 weeks of age, significantly greater amounts of IGF-I were found in plasma and hind limb muscles of mdx mice with respect to controls. Such a difference was only just detectable and no longer statistically significant at 5 months of age. No differences were found in hepatic IGF-I levels at either age. The EDL muscle fibers of mdx mice at 8-10 weeks of age were characterized by higher GCl values and by a different pharmacological sensitivity to the enantiomers of 2-(p-chlorophenoxy)-propionic acid (CPP), specific chloride channel ligands, with respect to age-matched controls. However, these differences were no longer detected at 5 months of age. Our results suggest a role of IGF-I in the high regenerative potential of muscles from mdx mice and support the hypothesis that the biophysical and pharmacological properties of chloride channels of EDL muscle fibers are sensitive indices of the action of regeneration-promoting factors on muscle function.

Growth hormone and insulin-like growth factor-I as anabolic agents

The reduced growth hormone and insulin-like growth factor-I concentrations in growth hormone deficiency and normal ageing are associated with reduced muscle mass and strength, and slower muscle protein synthesis. Recent research has addressed the hypothesis that growth hormone and insulin-like growth factor-I have an anabolic effect in adults, including the elderly. These hormones stimulate whole-body and muscle protein synthesis, at least under some conditions. There is increasing evidence to justify long-term administration of growth hormone to promote muscle growth in growth hormone deficient adults. However, the long-term effects on muscle mass and function in the elderly do not seem beneficial enough to justify widespread hormone replacement therapy. These hormones may be useful anabolic agents to counteract muscle wasting under other conditions, including surgical stress, renal failure, muscular dystrophy, glucocorticoid administration and HIV infection, but more clinical trials are needed to determine the functional significance of the protein anabolic effects under these conditions.