Inhibition of insulin-stimulated xylose uptake in rat soleus muscle by cycloheximide

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

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) microU/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 less than .001) and 15% (NS) in EDL and SOL muscle, respectively. The corresponding figures for 3-MH were 103% (P less than .001) and 21% (NS). Tyrosine release was reduced by insulin at a concentration of 10(3) microU/mL in control muscle and at a concentration of 10(4) microU/mL in septic muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin-stimulated alpha-(methyl)aminoisobutyric acid uptake in skeletal muscle. Evidence for a short-term activation of uptake independent of Na+ electrochemical gradient and protein synthesis

1. The present study was designed to explore the mechanisms by which insulin stimulates system A of amino acid transport in extensor digitorum longus (EDL) muscles, by using a system A analogue, alpha-(methyl)aminoisobutyric acid (MeAIB). 2. Insulin stimulation of MeAIB uptake was noted after only 30 min of incubation and was maximal at 60 min. Kinetics of the insulin effect on MeAIB uptake were characterized by an increased Vmax. without modification of Km for MeAIB. 3. Incubation of EDL muscles with cycloheximide for 90 min did not modify MeAIB uptake in either the presence or the absence of insulin, indicating the independence of insulin action from protein synthesis de novo. Incubations for 180 min with cycloheximide caused a decrease in basal MeAIB uptake; however, the percentage stimulation of amino acid transport by insulin was unaltered. Basal MeAIB uptake was increased by incubation for 180 min, but under these conditions no change in the percentage effect of insulin was found. 4. Ouabain, gramicidin D, or both, markedly decreased basal MeAIB uptake by EDL muscle, but the percentage effect of insulin was unaltered. 5. We conclude that insulin action on amino acid transport through system A in muscle is rapid, is characterized by an increased Vmax., and is independent of protein synthesis de novo and the Na+ electrochemical gradient. Our data are compatible with insulin acting directly on the system A transporter.

Induction of sugar uptake response to insulin by serum depletion in fusing L6 myoblasts

The L6 muscle cell line is proposed as an excellent cell culture system for studying glucose transport and its regulation by serum and insulin throughout myogenesis. The rate of hexose uptake decreased after cell fusion. Fetal calf serum (5 h) stimulated hexose uptake, and serum deprivation depressed it. Both effects were prevented by cycloheximide. Insulin stimulated the uptake of either 2-deoxy-D-glucose or 3-O-methyl-D-glucose provided that the basal rate was previously depressed by serum deprivation. The response to the hormone was significant only after the onset of cell fusion. Stimulation of transport was observed within the first 10 min after insulin administration and was insensitive to cycloheximide. Half-maximal stimulation of uptake was obtained with 5 X 10(-8) M insulin in 1% albumin solutions. Basal and insulin-stimulated uptake showed equal sensitivity to cytochalasin B. However, they differed in their sensitivity to extracellular Ca2+: basal transport was depressed, whereas insulin-stimulated transport was further enhanced in Ca2+-free media. Advantages of the cell culture system are discussed.