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The relative abundance and subcellular distribution of the GLUT-1 and GLUT-4 glucose transporter isoforms were determined in basal and insulin-stimulated adipose cells from wheel cage exercise-trained rats and compared with both age-matched sedentary controls and young cell size-matched sedentary controls. Exercise training increased total estimated GLUT-4 by 67 and 54% compared with age-matched and young controls, respectively. Total estimated GLUT-1 per cell was not significantly different among the three groups. Expressed per cell, plasma membrane GLUT-4 protein in basal adipose cells from exercise-trained and age-matched control rats was 2.5-fold greater than in young controls (P < 0.05) and was associated with higher basal rates of glucose transport in these cells (P < 0.02). In insulin-stimulated cells, plasma membrane GLUT-4 was 67% greater in the exercise-trained animals than young controls (P < 0.01), and 31% greater than in age-matched controls. Rates of glucose transport were correspondingly higher. In basal cells, low-density microsomal GLUT-4 from exercise-trained rats was approximately twofold greater than from age-matched controls and young controls. With insulin stimulation, GLUT-4 in low-density microsomes decreased to similar levels in all groups. We conclude that the total amount of GLUT-4 protein, but not GLUT-1, is increased in adipose cells by exercise training and that this increase in GLUT-4 is due primarily to an increase in intracellular GLUT-4.(ABSTRACT TRUNCATED AT 250 WORDS)

Skeletal muscle contractile activity results in increased rates of glucose transport that are associated with an increase in the number and activity of plasma membrane glucose transporters. In the current study it was determined whether exercise causes a translocation of glucose transporters from an intracellular pool to the plasma membrane and whether exercise and insulin stimulate the same glucose transporter protein. Plasma membrane glucose transporter number, measured by cytochalasin B binding, increased from 10.1 +/- 0.73 to 15.0 +/- 1.4 pmol/mg protein (P less than 0.01) in muscle of exercised rats, whereas microsomal membrane transporters decreased significantly from 6.0 +/- 0.7 to 4.2 +/- 0.4 pmol/mg protein (P less than 0.05). Western blot analysis using the monoclonal antibody mAb 1F8 (specific for GLUT-4) demonstrated a 45% increase in plasma membrane GLUT-4 from exercised skeletal muscle compared with controls, whereas microsomal membranes from the exercised muscle had a concomitant 25% decrease in GLUT-4 protein. These data suggest that exercise recruits transporters to the plasma membrane from an intracellular microsomal pool, similar to the translocation of transporters that occurs with insulin stimulation. Furthermore, both exercise and insulin stimulate the translocation of GLUT-4 in skeletal muscle, while GLUT-1 is not altered.

To study the interactions between insulin and contraction on the skeletal muscle glucose transport system, the hindquarters of male rats were perfused in the absence of insulin, in the presence of insulin (30 mU/ml), during contractions induced by sciatic nerve stimulation, or during contractions plus insulin. Compared with control preparations, rates of glucose uptake in the perfused hindquarter were increased by 2.5- and 2.6-fold in the insulin and insulin plus contraction groups, respectively, but not significantly increased in the contraction only preparations. After perfusion, soleus and red and white gastrocnemius muscles from the hindquarter were pooled and used for the preparation of plasma membranes. Skeletal muscle plasma membrane vesicle glucose transport rates were 2.2 +/- 0.5, 7.9 +/- 1.7, 9.0 +/- 2.2, and 10.8 +/- 2.0 nmol.mg protein-1.s-1 (40 mM glucose), and plasma membrane glucose transporter numbers were 4.7 +/- 0.5, 8.1 +/- 0.9, 9.1 +/- 1.0, and 8.6 +/- 0.6 pmol/mg protein in the control, contraction, insulin, and insulin plus contraction groups, respectively. The transport-transporter ratio, an indication of plasma membrane glucose transporter intrinsic activity, was increased by contraction, insulin, and insulin plus contraction. These results demonstrate that contractile activity in the absence of insulin increases muscle plasma membrane glucose transport by increasing transporter number and intrinsic activity. In addition, under these experimental conditions, the effects of insulin and contraction to increase muscle glucose transport are not additive.

Skeletal muscle responds to exercise by increasing the rate of glucose uptake. Recent studies have indicated that these changes occur via mechanisms modulating the number of transporters in the plasma membrane and/or transporter intrinsic activity. In the present study, a protocol was developed for measuring the initial rate of glucose uptake by rat hindlimb skeletal muscle plasma membrane vesicles. Membranes were isolated from sedentary (control) and acutely exercised rats, and the initial rates of D- and L-glucose influx were assayed under equilibrium exchange conditions to obtain the kinetic constants for carrier-mediated transport. These values were compared with the values for transporter number measured by cytochalasin B binding, and the carrier turnover numbers were calculated. The maximum velocity (Vmax) for carrier-mediated glucose influx was increased 3.7-fold by exercise, from 3.5 nmol.mg protein-1.s-1 for the membranes from control rats to 13 nmol.mg protein-1.s-1 for the membranes from exercised animals. The mean affinity constant (K0.5; approximately 20 mM) was not different between the two groups. The number of transporters in the plasma membrane was increased to a lesser degree, 5.4 to 9.4 pmol/mg protein. As a result, the average carrier turnover number was increased almost twofold by exercise, 719 s-1 in the controls vs. 1,380 s-1 in the exercised rats. These data indicate that the response of glucose transport to exercise involves an increase in the average carrier intrinsic activity as well as a recruitment of transporters to the plasma membrane. Whether the increase in carrier turnover number is due to activation of the transporters or recruitment of a more “active” form of the carrier is unknown.

We studied the mechanism for the increase in glucose transport activity that occurs in adipose cells of exercise-trained rats. Glucose transport activity, glucose metabolism, and the subcellular distribution of glucose transporters were measured in adipose cells from rats raised in wheel cages for 6 wk (mean total exercise 350 km/rat), age-matched sedentary controls, and young sedentary controls matched for adipose cell size. Basal rates of glucose transport and metabolism were greater in cells from exercise-trained rats compared with young controls, and insulin-stimulated rates were greater in the exercise-trained rats compared with both age-matched and young controls. The numbers of plasma membrane glucose transporters were not different among groups in the basal state; however, with insulin stimulation, cells from exercise-trained animals had significantly more plasma membrane transporters than young controls or age-matched controls. Exercise-trained rats also had more low-density microsomal transporters than control rats in the basal state. When the total number of glucose transporters/cell was calculated, the exercise-trained rats had 42% more transporters than did either control group. These studies demonstrate that the increased glucose transport and metabolism observed in insulin-stimulated adipose cells from exercise-trained rats is due, primarily, to an increase in the number of plasma membrane glucose transporters translocated from an enlarged intracellular pool.

The effects of 6-wk treadmill training program on the metabolism of isolated adipose cells from obese (fa/fa) and lean (Fa/?) Zucker rats were studied. Glucose metabolism and transport, insulin binding, and lipolysis were measured in adipose cells prepared from sedentary control and exercise-trained (ET) lean and/or obese rats. Two- to threefold increases in glucose metabolism were observed in cells from lean and obese ET rats compared with their respective controls. However, the insulin concentrations giving half-maximal stimulation (measuring insulin sensitivity) did not change (approximately 8 microunits/ml in lean and approximately 45 microunits/ml in obese rats). In lean ET rats, glucose transport and maximal glucose metabolic capacity (transport not rate-limiting) were increased twofold and sensitivity of lipolysis to epinephrine was increased three- to fourfold. These were not measured in obese rats. The results suggest that training of both lean and obese Zucker rats increases glucose utilization in adipose cells by increasing both glucose transport and intracellular glucose metabolism. Increased triglyceride turnover is also suggested by the increased sensitivity of lipolysis to epinephrine.

We studied whole body and regional fuel metabolism 1-4 h after cycle exercise [70% maximum O2 consumption (VO2max)], using the insulin clamp technique (40 mU.M-2.min-1) with indirect calorimetry. Substrate fluxes were determined in nonexercised (forearm) muscle tissues. Total glucose utilization was not increased by exercise, either in the preinsulin or insulin-stimulated state. Glucose oxidation tended to decrease, and lipid oxidation was increased after exercise. Forearm glucose uptake (FGU) was increased 5 times by insulin in the resting state, due largely to increased fractional extraction (P less than 0.05). After exercise, FGU was not increased by insulin. Forearm alanine and lactate release was doubled 2 h after exercise. Branched-chain amino acid (BCAA) concentrations were increasing after exercise (P less than 0.01) at a time when forearm muscle was taking up these amino acids. Insulin infusion suppressed the elevated release of gluconeogenic precursors from the forearm and suppressed the elevated concentrations of BCAAs, free fatty acids, and glycerol present after exercise. In summary, basal and insulin-stimulated glucose utilization is not augmented by prior high-intensity exercise, partly because nonexercised muscle is insulin resistant. Insulin infusion attenuates the altered metabolic milieu seen during early recovery.