Effect of hydrocortisone and dexamethasone on xylose uptake by isolated rat soleus muscle

To determine the effects of glucocorticoids on sugar uptake, xylose uptake by isolated rat soleus muscle of bilaterally adrenalectomized animals was studied. The results indicate that in vitro addition of 10⁻⁴ M hydrocortusine, dexamethasone or hydrocortisone sodium succinate had no inhibitory effect on basal xylose uptake. In the presence of both low and high medium insulin, the above steroids failed to inhibit insulin-stimulated uptake. When the concentration of hydrocortisone sodium succinate was increased to 10⁻² M, insulin-stimulated uptake was decreased. The results thus indicate that glucocorticoids at concentrations observed under physiological or pathological conditions do not inihibit basal or insulin-stimulated sugar uptake.     

Effect of protein synthesis inhibitors on xylose uptake and 125I-insulin binding by rat soleus muscle

Prolonged exposure (90–180 min) to cycloheximide (0.2 mg/ml), puromycin (0.2 mg/ml) or chloramphenicol (0.1 mg/ml) did not affect ¹²⁵I-insulin binding by rat soleus muscle. Chloramphenicol (2 mg/ml) depressed insulin binding and insulin-stimulated xylose uptake; these effects were attributed to the “toxic” effect of chloramphenicol on muscle ATP levels. Cycloheximide and puromycin inhibited insulin-stimulated xylose uptake without affecting ATP. Puromycin and chloramphenicol, but not cycloheximide, also inhibited basal sugar transport. This difference, and the rapid onset of all these inhibitory effects, suggest that they are not due to the inhibition of protein synthesis, but rather to some more direct effect on sugar transport itself.     

Palmitic acid uptake by the rat soleus muscle in vitro.

Abstract: The rate of fatty acid uptake, oxidation, and deposition in skeletal muscles in relation to total and unbound to albumin fatty acids concentration in the medium were investigated in the incubated rat soleus muscle. An immunohistochemical technique was applied to demonstrate whether the albumin-bound fatty acid complex from the medium penetrates well within all areas of the muscle strips. It was found that the percentage of incorporation of palmitic acid into intramuscular lipids was fairly constant, independently of the fatty acid concentration in the medium, and amounted to 63-72% for triacylglycerols, 7-12% for diacylglycerols-monoacylglycerols, and 19-26% for phospholipids. Both palmitic acid incorporation into the muscle triacylglycerol stores and its oxidation to CO2 closely correlated with an increase in both total and unbound to albumin fatty acid concentrations in the incubation medium. Under conditions of increased total but constant unbound to albumin palmitic acid concentrations, the incorporation of palmitic acid into triacylglycerols and its oxidation to CO2 were also increased, but to a lower extent. This supports the hypothesis that the cellular fatty acid metabolism depends not only on the availability of fatty acids unbound to albumin, but also on the availability of fatty acids complexed to albumin.     

Twitch potentiation and caffeine contractures in isolated rat soleus muscle

1. Electrically-evoked twitch and tetanic tension were measured in isolated rat soleus muscle after exposure to caffeine. 2. Between 0.01 and 2.5 mM caffeine twitch tension was potentiated, reaching a peak of 150% of Resting Tension at 0.5 mM. 3. Biphasic Tension development with relaxation was observed at 2.5 mM caffeine with maximal contractures (110% tetanic tension) occurring at 20 mM. 4. Creatine phosphate and ATP stores were maintained throughout the period of tension development and relaxation. 5. In contrast with amphibian muscle, the isolated soleus is very sensitive to low doses of caffeine and produces biphasic caffeine contractures which relax in the presence of caffeine.     

Insulinomimetic effects of kaempferitrin on glycaemia and on 14C-glucose uptake in rat soleus muscle

Bauhinia forficata is one of the Bauhinia species mostly used as an antidiabetic herbal remedy in Brazil. Kaempferitrin (kaempferol-3,7-O-(alpha)-L-dirhamnoside) is the predominant flavonol glycoside found in the B. forficata leaves. The aim of the present work was to study the long-term effect of kaempferitrin on glycaemia in diabetic rats, as well as the in vitro effect of this compound on 14C-D-glucose uptake and 14C-leucine incorporation into protein in normal rat soleus muscle. Kaempferitrin was found to have an acute lowering effect on blood glucose in diabetic rats and to stimulate the glucose uptake percentile, as efficiently as insulin in muscle from normal rats. This compound did not have any effect on glucosuria or on protein synthesis in muscle from normal and diabetic animals. However, the protein synthesis in the kaempferitrin-treated groups was maintained at the same level as the respective controls. Thus, the hypoglycaemic effect and the prompt efficiency of the kaempferitrin in stimulating [U-14C]-2-deoxi-D-glucose uptake in muscle -constitute the first evidence to indicate that the acute effect of this compound on blood glucose lowering may occur as a consequence of the altered intrinsic activity of the glucose transporter (Vmax or glucose transporters translocation?) not involving directly the synthesis of new carriers.     

Effect of D-Penicillamine on iron uptake by isolated rat hepatocytes

d-penicillamine (β,β-dimethylcysteine) promotes the incorporation of iron into isolated rat hepatocytes. The mechanism for doing this remains unknown. No differences in iron distribution between control and treated cells has been observed. Ferritin appears as the main destination of internalized iron in both cases. Therefore, increasing iron storage may appear as a side effect of the use ofd-penicillamine as a therapeutic agent for several diseases.     

Beta-phenylpyruvate and glucose uptake in isolated mouse soleus muscle and cultured C2C12 muscle cells

Previous investigation demonstrated the potential of beta-phenylpyruvate at high concentration to cause hypoglycemia in mice totally deprived of insulin. For further elucidation of the glucose-lowering mechanism, glucose uptake, and quantity of glucose transporters (GLUT1 and GLUT4) in mouse soleus muscle and C2C12 muscle cell lines were investigated following incubation with beta-phenylpyruvate in various concentrations. A marked enhancement of glucose uptake was demonstrated that peaked at 0.5 and 1.0 mM beta-phenylpyruvate in soleus muscle (P<0.01) and C2C12 cells (P<0.001), respectively. Kinetic analysis in C2C12 cells showed a twofold increase in Vmax compared with controls (P<0.001). In addition, both GLUT1 and GLUT4 levels were increased following exposure to beta-phenylpyruvate. Our findings point to a peripheral hypoglycemic effect of beta-phenylpyruvate.     

GSK3 involvement in amylin signaling in isolated rat soleus muscle

Amylin can evoke insulin resistance by antagonizing insulin in a non-competitive manner. Here, we investigated the glycogenolytic effect of amylin in isolated skeletal muscle and compared it to the effects of a calcitonin gene-related peptide (CGRP). Amylin alone had no statistically significant effect on glucose transport. However, amylin decreased insulin-stimulated glucose transport by about 30%. The involvement of cAMP could not be detected at the concentrations shown to promote glycogenolysis. Previously, it has been shown that increased glycogen synthase kinase 3 (GSK3) activity plays a role in insulin resistance. Here, the ratio of GSK3 :β isoforms in rat soleus was found to be 1.2:1. We found that amylin increased GSK3 activity, which in turn led to increased phosphorylation of glycogen synthase and decreased glycogen synthesis de novo.     

Effect of denervation or unweighting on GLUT-4 protein in rat soleus muscle.

The purpose of this study was to test the hypothesis that the decreased capacity for glucose transport in the denervated rat soleus and the increased capacity for glucose transport in the unweighted rat soleus are related to changes in the expression of the regulatable glucose transporter protein in skeletal muscle (GLUT-4). One day after sciatic nerve sectioning, when decreases in the stimulation of soleus 2-deoxyglucose (2-DG) uptake by insulin (-51%, P less than 0.001), contractions (-29%, P less than 0.05), or insulin and contractions in combination (-40%, P less than 0.001) were observed, there was a slight (-18%, NS) decrease in GLUT-4 protein. By day 3 of denervation, stimulation of 2-DG uptake by insulin (-74%, P less than 0.001), contractions (-31%, P less than 0.001), or the two stimuli in combination (-59%, P less than 0.001), as well as GLUT-4 protein (-52%, P less than 0.001), was further reduced. Soleus muscle from hindlimb-suspended rats, which develops an enhanced capacity for insulin-stimulated glucose transport, showed muscle atrophy similar to denervated soleus but, in contrast, displayed substantial increases in GLUT-4 protein after 3 (+35%, P less than 0.05) and 7 days (+107%, P less than 0.001). These results indicate that altered GLUT-4 expression may be a major contributor to the changes in insulin-stimulated glucose transport that are observed with denervation and unweighting. We conclude that muscle activity is an important factor in the regulation of GLUT-4 expression in skeletal muscle.     

Amylin activates glycogen phosphorylase in the isolated soleus muscle of the rat

The pancreatic beta-cell hormone amylin acts in isolated rat skeletal muscle to decrease insulin-stimulated incorporation of glucose into glycogen. It also increases blood levels of lactate and glucose in fasted rats in vivo. However, it remained uncertain whether amylin exerts direct effects to stimulate muscle glycogenolysis. We now report that amylin caused a dose-dependent increase in activity of muscle glycogen phosphorylase in isolated rat soleus muscle by stimulating phosphorylase a. Insulin inhibited amylin-stimulated activation of phosphorylase. Effects of amylin to stimulate muscle glycogenolysis are consistent with observed effects of amylin in vivo and could be a major mechanism whereby amylin modulates carbohydrate metabolism.     

ATP-dependence of 125I-insulin binding by rat soleus muscle

Muscle ATP levels were lowered by incubating rat soleus muscles under anaerobic conditions, or in the presence of 2:4-dinitrophenol (0.5 mM), EDTA (5 mM) or mannitol (400 mM). 125I-insulin binding, measured under equilibrium conditions at 25 degrees C, was reduced by 49-71% in ATP-depleted muscles. Insulin binding was also determined using two other procedures which minimized internalization of 125I-insulin: these were (a) 5 min at 25 degrees C, and (b) 24 h at 3 degrees C. Under these conditions, 125I-insulin binding was reduced by 28-55% in ATP-depleted muscles. These results confirm that in soleus muscle the effect of ATP-depletion on 125I-insulin binding is actually concerned with the binding step itself and not merely a reflection of ATP-dependent internalization of the bound hormone.