Protein metabolism in skeletal muscle during sepsis : with special reference to mediators and mechanisms of increased protein breakdown / Marianne Hall Angerås.

Tese (doutor) - University of Göteborg, 1990

Increased protein breakdown in skeletal muscle is a characteristic response to sepsis. The mechanism and mediators are not known. Total and myofibrillar protein breakdown was characterized in an in vitro muscle preparation, i.e., incubated extensor digitorum longus (EDL) and soleus (SOL) muscles of young rats. The effects of elevated temperature on protein turnover rates were studied in EDL and SOL muscles from septic rats. Sepsis was induced by by cecal ligation and puncture (CLP). The activity of myofibrillar proteinase, cathepsin B, total and myofibrillar breakdown rates were studied in septic EDL muscle, before and after administration in vivo of the mast cell degranulating substance 48/80 or the lysosonal protease inhibitor leupeptin. Protein turnover rates were studied in EDL muscles of rats treated with tumor necrosis factor (TNF), corticosterone, or a combination. Corticosterone alone or in combination with TNF was administered to adrenalectomized rats and protein breakdown rate in incubated EDL muscle was studied. The effect administration of corticosterone to rats in two differents vehicles on plasma levels of corticosterone and protein breakdown were studied. Plasma levels of corticosterone, norepinephrine, epinephrine, and protein turnover rates in incubated EDL were studied in septic rats treated with the glucocorticoid receptor anatogonist RU 38486. Total and myofibrillar protein breakdown rates were assessed by net production by the incubated muscles of tyrosine and 3-methylhistidine (3-MH), repsectively, determined by high performance liquid chromatography (HPLC). Protein synthesis rate was determined by measuring incorporation of 14C-phenylalanine into protein. Both total and myofibrillar protein breakdown rates were higher in SOL than in EDL muscles and were reduced by incubating the muscles maintained at resting length. Myofibrillar protein degradation was markedely increased by fasting in both muscles. Total protein brakdown was increased in EDL muscles. Total protein breakdown was increased at 40ºC in EDL and SOL from control rats. In muscles from septic rats, total and myofibrillar protein breakdown was increased at 40ºC in EDL but was not altered in SOL. Activity of myofibrillar proteinase and cathepsin B was increased in septic EDL. Total and myofibrillar protein breakdown rates in septic EDL were elevated. Treatment of septic rats with the mast cell degranulating substance 48/80 or the lysosomal proteinase inhibitos leupeptin, reduced myofibrillar proteinase and cathepsin B activities, but did not affect protein breakdown rates. Administration of TNF alone did not affect muscle protein turnover. Corticosterone inhibited muscle protein synthesis and stimulated total and myofibrillar protein breakdown. When TNF was administered together with corticosterone, total and myofibrillar protein breakdown rates were further increased. Plasma corticosterone levels were higher in rats treated with both TNF and corticosterone than in animals treated with corticosterone alone. In adrenalectomized rats TNF did not elevate plasma corticosterone levels or muscle protein breakdown rates when administered in combination with corticosterone. When administration of corticosteron resulted in plasma levels similar to those observed in septic rats, total or myofibrillar protein breakdown rates were not altered. Administration of corticosterone resulting in plasma levels almost four times higher than the hormone levels observed in septic rats, stimulated muscle proteolysis with a predominant increase in myofibrillar protein breakdown. Treatment with RU 38486 resulted in a reduction of total and especially myofibrillar protein breakdown in septic EDL muscle The results support the concepts of individual regulation of myofibrillar and non-myofibrillar proteins and of different effects of various conditions on protein breakdown in different types of skeletal muscle. The results confirm that total and in particular myofibrillar protein breakdown rate is increased in septic EDL. Elevated temperature does not seem to be the primary mechanism of increased muscle protein breakdown in sepsis, but increased temperature may potentiate protein breakdown which is already stimulated. Increased muscle protein breakdown in septic muscle seem to be associated with, but not caused by, myofibrillar proteinase or cathepsin B activity. Muscle catabolism induced by TNF may be mediated by glucocorticoids. The results support a role of glucocorticoids in accelarated muscle proteolysis during sepsis, probably in interaction with other substances.