Elevated GLUT 1 level in crude muscle membranes from diabetic Zucker rats despite a normal GLUT 1 level in perineurial sheaths

Normal muscle glucose uptake in mice deficient in muscle GLUT4

Skeletal muscle insulin resistance is a major characteristic underpinning type 2 diabetes. Impairments in the insulin responsiveness of the glucose transporter, Glut4 (Slc2a4), have been suggested to be a contributing factor to this disturbance. We have produced muscle-specific Glut4 knockout (KO) mice using Cre/LoxP technology on a C57BL6/J background and shown undetectable levels of GLUT4 in both skeletal muscle and heart. Our aim was to determine whether complete deletion of muscle GLUT4 does in fact lead to perturbations in glucose homoeostasis. Glucose tolerance, glucose turnover and 2-deoxyglucose uptake into muscle and fat under basal and insulin-stimulated conditions were assessed in 12-week-old KO and control mice using the oral glucose tolerance test (OGTT) and hyperinsulinaemic/euglycaemic clamp respectively. KO mice weighed ~17% less and had significantly heavier hearts compared with control mice. Basally, plasma glucose and plasma insulin were significantly lower in the KO compared with control mice, which conferred normal glucose tolerance. Despite the lack of GLUT4 in the KO mouse muscle, glucose uptake was not impaired in skeletal muscle but was reduced in heart under insulin-stimulated conditions. Neither GLUT1 nor GLUT12 protein levels were altered in the skeletal muscle or heart tissue of our KO mice. High-fat feeding did not alter glucose tolerance in the KO mice but led to elevated plasma insulin levels during the glucose tolerance test. Our study demonstrates that deletion of muscle GLUT4 does not adversely affect glucose disposal and glucose tolerance and that compensation from other transporters may contribute to this unaltered homoeostasis of glucose.

Glucose transporter expression in human skeletal muscle fibers

The present study was initiated to investigate GLUT-1 through -5 expression in developing and mature human skeletal muscle. To bypass the problems inherent in techniques using tissue homogenates, we applied an immunocytochemical approach, employing the sensitive enhanced tyramide signal amplification (TSA) technique to detect the localization of glucose transporter expression in human skeletal muscle. We found expression of GLUT-1, GLUT-3, and GLUT-4 in developing human muscle fibers showing a distinct expression pattern. 1) GLUT-1 is expressed in human skeletal muscle cells during gestation, but its expression is markedly reduced around birth and is further reduced to undetectable levels within the first year of life; 2) GLUT-3 protein expression appears at 18 wk of gestation and disappears after birth; and 3) GLUT-4 protein is diffusely expressed in muscle cells throughout gestation, whereas after birth, the characteristic subcellular localization is as seen in adult muscle fibers. Our results show that GLUT-1, GLUT-3, and GLUT-4 seem to be of importance during muscle fiber growth and development. GLUT-5 protein was undetectable in fetal and adult skeletal muscle fibers. In adult muscle fibers, only GLUT-4 was expressed at significant levels. GLUT-1 immunoreactivity was below the detection limit in muscle fibers, indicating that this glucose transporter is of minor importance for muscle glucose supply. Thus we hypothesize that GLUT-4 also mediates basal glucose transport in muscle fibers, possibly through constant exposure to tonal contraction and basal insulin levels.

Effects of metformin treatment on glucose transporter proteins in subcellular fractions of skeletal muscle in (fa/fa) Zucker rats

1. The present study was designed to clarify the cellular mechanism through which the antihyperglycaemic drug, metformin, exerts its effects. For this purpose the contents of glucose transporter protein isoforms GLUT1 and GLUT4 were measured in plasma membrane and intracellular membrane fractions of skeletal muscle obtained from genetically obese, insulin-resistant Zucker rats. 2. Hindlimb muscles were dissected from metformin-treated (300 mg kg-1 day-1, p.o., for 12 days) and control rats in basal treatment state, and after acute stimulation with insulin (22 u kg-1, i.p.). Since metformin treatment reduces food intake, we also used a pair-fed control group to investigate the effects of altered insulinaemia per se. Glucose transporter levels were analysed by Western blot and slot blot-techniques. In addition, 2-deoxy-[14C]-glucose uptake in isolated muscle strips was evaluated. 3. No changes were noted in the contents of GLUT1 proteins in any of the subcellular fractions after metformin treatment. The contents of GLUT4 in subcellular fractions were not altered in the basal treatment state. After acute insulin exposure the content of GLUT4 in the intracellular membrane fraction declined significantly in the metformin-treated group, while no significant effect was seen in the plasma membrane fraction. In agreement with these results, metformin treatment did not alter 2-deoxyglucose uptake into isolated muscle strips. 4. In conclusion, the present study does not support the concept that metformin would enhance translocation of glucose transporter proteins from the intracellular compartment to the plasma membrane in skeletal muscle in vivo.

Increased glucose effectiveness in normoglycemic but insulin-resistant relatives of patients with non-insulin-dependent diabetes mellitus. A novel compensatory mechanism

20 normoglycemic first degree relatives of non-insulin-dependent diabetes mellitus (NIDDM) patients were compared with 20 matched subjects without any family history of diabetes using the intravenous glucose tolerance test with minimal model analysis of glucose disappearance and insulin kinetics. Intravenous glucose tolerance index (Kg) was similar in both groups (1.60 +/- 0.14 vs 1.59 +/- 0.18, x 10(-2) min-1, NS). However, insulin sensitivity (Si) was reduced (3.49 +/- 0.43 vs 4.80 +/- 0.61, x 10(-4) min-1 per mU/liter, P = 0.05), whereas glucose effectiveness (Sg) was increased (1.93 +/- 0.14 vs 1.52 +/- 0.16, x 10(-2) min-1, P < 0.05) in the relatives. Despite insulin resistance neither fasting plasma insulin concentration (7.63 +/- 0.48 vs 6.88 +/- 0.45, mU/liter, NS) nor first phase insulin responsiveness (Phi1) (3.56 +/- 0.53 vs 4.13 +/- 0.62, mU/liter min-1 per mg/dl, NS) were increased in the relatives. Phi1 was reduced for the degree of insulin resistance in the relatives so that the Phi1 x Si index was lower in the relatives (11.5 +/- 2.2 vs 16.7 +/- 2.0, x 10(-4) min-2 per mg/dl, P < 0.05). Importantly, glucose effectiveness correlated with Kg and with basal glucose oxidation but not with total glucose transporter 4 (GLUT4) content in a basal muscle biopsy. In conclusion we confirm the presence of insulin resistance in first degree relatives of NIDDM patients. However, insulin secretion was altered and reduced for the degree of insulin resistance in the relatives, whereas glucose effectiveness was increased. We hypothesize that increased glucose effectiveness maintains glucose tolerance within normal limits in these "normoinsulinemic" relatives of NIDDM patients.