Glucose transporters and in vivo glucose uptake in skeletal and cardiac muscle: fasting, insulin stimulation and immunoisolation studies of GLUT1 and GLUT4

Abstract

Our aim was to study glucose transporters GLUT1 and GLUT4 in relation to in vivo glucose uptake in rat cardiac and skeletal muscle. The levels of both transporters were of a similar order of magnitude in whole muscle tissue (GLUT1/GLUT4 ratio varied from 0.1 to 0.6), suggesting that both may have an important physiological role in regulating muscle glucose metabolism. GLUT4 correlated very strongly (r2 = 0.97) with maximal insulin-stimulated glucose uptake (Rg' max., estimated using the glucose clamp plus 2-deoxy[3H]glucose bolus technique) in six skeletal muscles and heart. A distinct difference in regulation of the two transporters was evident in heart: in 5 h-fasted rats, basal glucose uptake and GLUT1 levels in heart were very high and both were reduced, by 90 and 60% respectively, by 48 h fasting. However, in heart (and in red skeletal muscle), neither GLUT4 levels nor Rg' max. were reduced by 48 h fasting. GLUT1 was shown to be specifically expressed in cardiac myocytes, because intracellular vesicles enriched in GLUT4 contained significant levels of GLUT1. In conclusion, the high association of muscle GLUT4 content with insulin responsiveness in different muscles, and the preservation of both with fasting, supports a predominant role of GLUT4 in insulin-mediated glucose uptake. GLUT1 may play an important role in mediating cardiac muscle glucose uptake in the basal metabolic state. Marked changes in GLUT1 expression with alterations in the metabolic state, such as prolonged fasting, may play an important role in cardiac glucose metabolism.

Insulin-induced Recruitment of Glucose Transporter 4 (GLUT4) and GLUT1 in Isolated Rat Cardiac Myocytes

“…On the other hand, we found a high degree of GLUT1 expression in cardiomyocytes, as determined by quantitative Western blot analysis, which confirms qualitative and semiquantitative data obtained by others in heart tissue (24,25). This high level of GLUT1 expression appears to be a unique property of the heart among insulin-sensitive tissues.…”

“…Beads were collected by a 6-s spin in a Microfuge and washed in PBS. LDM preparations (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) g of proteins) were incubated with 1F8-or 3F8-agarose overnight at 4°C in the absence of detergents (0.1% bovine serum albumin and 1 mM EDTA in PBS; final volume, 200 l). The agarose beads and vesicles bound to them were collected by a 6-s spin in a Microfuge.…”

“…In heart and skeletal muscle, the content of GLUT1 mRNA and protein was shown to largely decrease during postnatal development (21,22), whereas the reverse is true for GLUT4 (21)(22)(23). However, the adult heart still appears to contain considerable amounts of GLUT1, in contrast to skeletal muscles (24,25). Moreover, recent studies have shown that in heart muscle cells GLUT1 is recruited to the plasma membrane by several * This work was supported in part by Grant Fi 551/1-2 from the Deutsche Forschungsgemeinschaft, Grant PB92/0805 from the Dirección General de Investigación Científica y Técnica, Grant GRQ94-1040 from Generalitat de Catalunya, Spain, and Grants HA94-125 and HA95-125 from the Deutscher Akademischer Austauschdienst in conjunction with Acciones Integradas Hispano-Alemañ as.…”

“…Although the results presented above point to GLUT4 as the major transporter responsible for insulin-dependent glucose uptake, reports indicating a relatively high expression of GLUT1 in cardiac tissue (24,25) prompted us to directly compare the total amounts of GLUT4 and GLUT1 in isolated cardiomyocytes by Western blot analysis. For this purpose, nonstimulated cardiomyocytes were homogenized as described under "Materials and Methods," and their content in GLUT4 and GLUT1 was estimated by comparing the cell samples with known standards obtained from intracellular adipocyte membranes (GLUT4 standard) or erythrocyte membranes (GLUT1 standard).…”

Mathematical modelling of hepatic lipid metabolism

“…k gi 2.632 × 10 7 mmol −1 Approximated using data from Kelley and Kraegen simultaneously [30,35] See kg k gl 0.9277l/min Liver to blood glucose scalar In combination with k gl2 this confers steady state concentrations of liver (8mmol/l) to blood (5mmol/l).…”