G. L. Dohm

Hypoxia Stimulates Glucose Transport in Insulin-Resistant Human Skeletal Muscle

Insulin and muscle contraction stimulate glucose transport into muscle cells by separate signaling pathways, and hypoxia has been shown to operate via the contraction signaling pathway. To elucidate the mechanism of insulin resistance in human skeletal muscle, strips of rectus abdominis muscle from lean (body mass index [BMI] < 25), obese (BMI > 30), and obese non-insulin-dependent diabetes mellitus (NIDDM) (BMI > 30) patients were incubated under basal and insulin-, hypoxia-, and hypoxia + insulin-stimulated conditions. Insulin significantly stimulated 2-deoxyglucose transport twofold in muscle from lean (P < 0.05) patients, but not in muscle from obese or obese NIDDM patients. Furthermore, maximally insulin-stimulated transport rates in muscle from obese and diabetic patients were significantly lower than rates in muscle from lean patients (P < 0.05). Hypoxia significantly stimulated glucose transport in muscle from lean and obese patients. There were no significant differences in hypoxia-stimulated glucose transport rates among lean, obese, and obese NIDDM groups. Hypoxia + insulin significantly stimulated glucose transport in lean, obese, and diabetic muscle. The results of the present study suggest that the glucose transport effector system is intact in diabetic human muscle when stimulated by hypoxia.

IGF-I–Stimulated Glucose Transport in Human Skeletal Muscle and IGF-I Resistance in Obesity and NIDDM

Based on the observation that insulinlike growth factor I (IGF-I) can stimulate glucose utilization in nondiabetic subjects and that the action of the IGF-I receptor is normal in the skeletal muscle of patients with noninsulin-dependent diabetes mellitus (NIDDM), it seems possible that IGF-I might provide an effective acute treatment for the hyperglycemia of NIDDM. Using our recently developed in vitro human muscle preparation, we investigated the hypothesis that IGF-I might be an effective alternative to insulin in stimulating glucose transport in diabetic muscle. Abdominal muscle samples from nonobese nondiabetic, obese nondiabetic, and obese NIDDM patients were obtained during elective abdominal surgery. Plasma levels of IGF-I in diabetic patients were lower than those in either of the nondiabetic groups. Binding studies with wheat-germ–agglutinin–chromatography–purified receptors demonstrated the presence of IGF-I receptors in human muscle, with IGF-I binding being ∼24% that of insulin. There was no change in IGF-I binding in muscle from obese or diabetic subjects, and the structural characteristics of the IGF-I receptor were not altered, as determined by electrophoretic mobility. IGF-I stimulated glucose transport approximately twofold in incubated muscle from control subjects, but there was no IGF-I stimulation of transport in muscle from obese subjects with or without NIDDM. These results confirm a previous report that human muscle contains receptors for IGF-I and demonstrate for the first time that IGF-I can stimulate glucose transport in human muscle. However, muscle from obese subjects with or without NIDDM is resistant to the action of IGF-I.

Skeletal Muscle GLUT4 Protein Concentration and Aging in Humans

The insulin resistance of aging has been attributed to a postreceptor defect in skeletal muscle. The present study examined whether a reduction in the concentration of the insulin-stimulated glucose transporter (GLUT4) in skeletal muscle was associated with advancing age in men (n = 55) and women (n = 29). Insulin sensitivity (minimal model) was negatively associated (P > 0.001) with age (range, 18–80 years) in men (r = −0.44) and women (r = −0.58). GLUT4 protein concentration in the vastus lateralis was also negatively associated (P < 0.05) with age (men, r = −0.28; women, r = −0.51). There was no relation (P > 0.15) between GLUT4 content in the gastrocnemius and age. GLUT4 concentration in the vastus lateralis was positively associated (P < 0.01) with insulin sensitivity in both sexes (r = 0.42); this relationship persisted in the men after adjusting for overall adiposity, regional adiposity, and cardiorespiratory fitness. These findings suggest that a decrement in GLUT4 protein concentration in skeletal muscle may at least partially contribute to the insulin resistance of aging in humans.

Immunolocalization of Glucose Transporter GLUT4 Within Human Skeletal Muscle

To investigate the cellular and subcellular distributioi of glucose transporters in skeletal muscle, the gluco transporter isoform GLUT4 was localized in human muscle by electron microscopy via immunogold labeling with monoclonal (1F8) or COOH-terminal peptide polyclonal (ECU4) antibody and in isolated rat membranes by Western blot. There was no labeling c GLUT4 in endothelial cells of the capillaries. There also was no labeling of GLUT4 on the surface plasms membrane (sarcolemma) under either basal or insulin-stimulated conditions. Specific labeling for GLUT4 was clearly observed in two compartments: within the trie (on terminal cisternae and transverse tubules) and on an intracellular compartment, possibly sarcoplasmic tubules. Isolated triad membranes from rat muscle al contained substantial quantities of GLUT4 transporte but there was no detectable GLUT4 protein in isolate sarcolemmal membranes. These data suggest a possible mechanism that involves glucose transport across the muscle cell at the transverse tubule membrane, not the sarcolemma.

Okadaic Acid, Vanadate, and Phenylarsine Oxide Stimulate 2-Deoxyglucose Transport in Insulin-Resistant Human Skeletal Muscle

In response to insulin, several proteins are phosphorylated on tyrosine and on serine/threonine residues. Decreased phosphorylation of signaling peptides by a defective insulin receptor kinase may be a cause of insulin resistance. Accordingly, inhibition of the appropriate phosphatases might increase the phosphorylation state of these signaling peptides and thereby elicit increased glucose transport. The purpose of this study was to examine the effect of the serine/threonine phosphatase inhibitor okadaic acid and the tyrosine phosphatase inhibitors phenylarsine oxide and vanadate on 2-deoxyglucose transport in insulin-resistant human skeletal muscle. All three phosphatase inhibitors stimulated 2-deoxyglucose transport in insulin-resistant skeletal muscle. These data suggest that these compounds have bypassed a defect in at least one of the signaling pathways leading to glucose transport. Furthermore, maximal transport rates induced by the simultaneous presence of insulin and phosphatase inhibitor in insulin-resistant muscle were equal to insulin-stimulated rates in lean control subjects. However, both vanadate alone and vanadate plus insulin stimulated 2-deoxyglucose transport significantly more in insulin-sensitive tissue than in insulin-resistant tissue. These results demonstrate that although vanadate is able to stimulate glucose transport in insulin-resistant muscle, it is not able to normalize transport to the same rate achieved in insulin-sensitive muscle.