Insulin binding in denervated muscle

Phosphorylation of insulin receptor substrate-1 serine 307 correlates with JNK activity in atrophic skeletal muscle

c-Jun NH(2)-terminal kinase (JNK) has been shown to negatively regulate insulin signaling through serine phosphorylation of residue 307 within the insulin receptor substrate-1 (IRS-1) in adipose and liver tissue. Using a rat hindlimb suspension model for muscle disuse atrophy, we found that JNK activity was significantly elevated in atrophic soleus muscle and that IRS-1 was phosphorylated on Ser(307) prior to the degradation of the IRS-1 protein. Moreover, we observed a corresponding reduction in Akt activity, providing biochemical evidence for the development of insulin resistance in atrophic skeletal muscle.

The effect of succinylcholine on energy metabolism studied by 31P-NMR spectroscopy in rat denervated skeletal muscle

BACKGROUND

The goals of this study were: (1) to demonstrate the differences of metabolic changes induced by succinylcholine (SCh) administration between normal and denervated muscle by 31P-NMR spectroscopy: (2) to determine whether three kinds of drugs (vecuronium, midazolam and magnesium sulfate) could prevent these metabolic changes.

METHODS

Following unilateral sciatic nerve section, 20 male Wistar rats were studied at three-week intervals. After SCh 1 mg.kg-1 was administered intravenously, the changes of the inorganic phosphate/phosphocreatine (Pi/PCr) ratio, the beta ATP/(PCr+Pi) ratio, and intracellular pH were measured by 31P-NMR both in normal and denervated hind limb muscles of 5 rats. The other 15 rats were allocated to the pretreatment groups by the following drugs: vecuronium 0.02 mg.kg-1, midazolam 0.1 mg.kg-1 and magnesium sulfate 60 mg.kg-1. After pretreatment 3 min before SCh administration, we measured the same parameters by 31P-NMR.

RESULTS

SCh administration did not change the Pi/PCr ratio in normal muscle, but significantly increased that in denervated muscle (P < 0.05). This increase of the Pi/PCr ratio was also observed in all pretreated groups but was minimal as compared with that in non-pretreatment denervated muscle.

CONCLUSION

These data suggested that SCh administration decreased the level of "energy reserve" in denervated muscle, and that this metabolic change was not totally inhibited by vecuronium, midazolam, or magnesium sulfate.

Metabolic changes in human muscle denervation: topical 31P NMR spectroscopy studies

Metabolic changes in human forearm flexor muscle following partial or complete denervation were studied using 31P nuclear magnetic resonance spectroscopy. In resting muscle, patients with electromyographically verified forearm flexor denervation had a lower ratio of PCr/Pi than healthy controls [4.76 +/- 2.50 (SD) versus 6.50 +/- 1.55 (SD); P less than 0.01] and a higher intracellular pH [7.09 +/- 0.06 versus 7.05 +/- 0.04 (SD); P less than 0.01]. These changes were more apparent when the diseased arm was compared to the contralateral healthy arm in patients with strictly unilateral denervation. Five patients with severe denervation had the lowest PCr/Pi ratio (1.94 +/- 0.70) and the highest pH (7.15 +/- 0.04). Four patients with intact innervation and disuse atrophy as a result of a forearm fracture did not exhibit these changes immediately following removal of the forearm cast. The findings show that denervation leads to a reduction in PCr/Pi within muscle and a rise in intracellular pH, and that these changes correlate with the degree of denervation and improve with recovery. 31P MRS may be useful to follow the course of denervation atrophy.

Hormonal control of muscle growth

In muscle of whole animals, pituitary growth hormone, the thyroid hormones, and insulin are major growth-promoting hormones, and the glucocorticoids have significant catabolic actions. At the cellular level the primary anabolic hormones for cultured myoblasts are the somatomedins (insulin-like growth factors) and fibroblast growth factor. In these cells physiological concentrations of growth hormone, thyroid hormones, and insulin have no growth-promoting effect; some of the reported actions of insulin probably result from cross-reaction with the somatomedin receptor. Results with purified proteins do not support the view that mitogens block myoblast differentiation; transforming growth factor-beta and interferon are nonmitogenic proteins that inhibit differentiation, insulin-like growth factors are mitogens that stimulate differentiation, and fibroblast growth factor is the only purified mitogen that inhibits differentiation. At least six serum-free media have now been devised for the growth of various kinds of muscle cells under closely defined conditions.

Musculotrophic effects of insulin receptors before and after denervation

Insulin binding and metabolic effects have been used to assess properties of the insulin receptor of rodent skeletal muscles before and after denervation. It has been found that the amount of insulin-displaceable insulin binding on both type I (soleus) and type II (extensor digitorum longus) muscles rises slowly for up to 3 weeks after denervation, following a brief period of reduced binding. As estimated from weight loss and unstimulated deoxyglucose uptake, the extra binding sites are not functional. Stimulation in vitro with shocks or excess potassium can temporarily cause a relative increase in sugar uptake in both types of muscle, but an excess of insulin rapidly loses its effect in the soleus muscle. The effects of stimulation are taken to mean either conformational changes in the receptor or enhancement of some postbinding step, or both. It is suggested that the slow increase of binding sites for insulin after denervation may reflect loss of a neural substance normally effective in activating receptor degradation.