Total and myofibrillar protein breakdown in different types of rat skeletal muscle: Effects of sepsis and regulation by insulin

Abstract

Proteolysis is increased in sepsis, but it is not known whether myofibrillar and non-myofibrillar proteins are broken down in the same fashion, or respond to the same regulatory forces as in non-septic muscle. In this study, therefore, the effect of sepsis on total and myofibrillar protein breakdown in incubated rat extensor digitorum longus (EDL) and soleus (SOL) muscles was determined, and the response in vitro to different concentrations of insulin (10 to 10 5 μU/mL) of protein degradation was studied in incubated EDL muscles from control and septic rats. Sepsis was induced in rats weighing 40 to 60 g by cecal ligation and puncture (CLP). Control animals were sham operated. Sixteen hours after CLP or sham operation, intact EDL and SOL muscles were incubated for two hours in oxygenated Krebs-Henseleit bicarbonate buffer containing glucose (10 mmol/L) and cycloheximide (0.5 mmol/L), and total and myofibrillar protein breakdown was assessed from release into incubation medium of tyrosine and 3-methylhistidine (3-MH), respectively. Tyrosine and 3-MH were determined fluorometrically by high performance liquid chromatography (HPLC). Tissue levels of tyrosine and 3-MH remained stable both in control and septic muscles during incubation for two hours. The rate of tyrosine release was increased during sepsis by 58% ( P < .001) and 15% (NS) in EDL and SOL muscle, respectively. The corresponding figures for 3-MH were 103% ( P < .001) and 21% (NS). Tyrosine release was reduced by insulin at a concentration of 10 3 μU/mL in control muscle and at a concentration of 10 4 μU/mL in septic muscle. 3-MH release by incubated EDL muscle from control rats was not affected by insulin at any concentration tested, while in septic muscle 3-MH release was reduced by 11% to 25% by insulin at concentrations ranging from 10 3 to 10 5 μU/mL. The present results suggest that myofibrillar proteins are more sensitive than non-myofibrillar proteins to the effects of sepsis, and that protein breakdown in white muscle is more susceptible to sepsis than proteolysis in red muscle. The results also indicate that myofibrillar and total proteins are regulated independently by insulin and that the response to insulin is altered during sepsis.