A paired-tracer dilution method for characterizing membrane transport in the perfused rat hindlimb. Effects of insulin, feeding and fasting on the kinetics of sugar transport

Effects of glucose on contractile function, [Ca2+]i, and glycogen in isolated mouse skeletal muscle

Extensor digitorum longus muscles were stimulated to contract to fatigue and allowed to recover for 2 h in the absence or presence of 5.5 or 11 mM extracellular glucose. This was followed by a second fatigue run, which ended when the absolute force was the same as at the end of the first run. During the first fatigue run, the fluorescence ratio for indo 1 increased [reflecting an increase in myoplasmic free Ca2+ concentration ([Ca2+]i)] during the initial tetani, peaking at approximately 115% of the first tetanic value, followed by a continuous decrease to approximately 90% at fatigue. During the first fatigue run, myofibrillar Ca2+ sensitivity was significantly decreased. During the second run, the number of tetani was 57 +/- 6% of initial force in muscles that recovered in the absence of glucose and 110 +/- 6 and 119 +/- 2% of initial force in muscles that recovered in 5.5 and 11 mM glucose, respectively. Fluorescence ratios during the first, peak, and last tetani did not differ significantly between the first and second fatigue runs during any of the three conditions. Glycogen decreased by almost 50% during the first fatigue run and did not change further after recovery in the absence of glucose. After recovery in the presence of 5.5 and 11 mM glucose, glycogen increased 32 and 42% above the nonstimulated control value (P < 0.01). These data demonstrate that extracellular glucose delays the decrease of tetanic force and [Ca2+]i during fatiguing stimulation and that glycogen supercompensation following contraction can occur in the absence of insulin.

Functional limitations to glucose uptake in muscles comprised of different fiber types

Skeletal muscle glucose uptake requires delivery of glucose to the sarcolemma, transport across the sarcolemma, and the irreversible phosphorylation of glucose by hexokinase (HK) inside the cell. Here, a novel method was used in the conscious rat to address the roles of these three steps in controlling the rate of glucose uptake in soleus, a muscle comprised of type I fibers, and two muscles comprised of type II fibers. Experiments were performed on conscious rats under basal conditions or during hyperinsulinemic euglycemic clamps. Rats received primed, constant infusions of 3-O-methyl-[3H]glucose (3-O-MG) and [1-14C]mannitol. Total muscle glucose concentration and the steady-state ratio of intracellular to extracellular 3-O-MG concentration, which distributes based on the transsarcolemmal glucose gradient (TSGG), were used to calculate glucose concentrations at the inner and outer sarcolemmal surfaces ([G](im) and [G](om), respectively) in muscle. Muscle glucose uptake was much lower in muscle comprised of type II fibers than in soleus under both basal and insulin-stimulated conditions. Under all conditions, the TSGG in type II muscle exceeded that in soleus, indicating that glucose transport plays a more important role to limit glucose uptake in type II muscle. Although hyperinsulinemia increased [G](im) in soleus, indicating that phosphorylation was a limiting factor, type II muscle was limited primarily by glucose delivery and glucose transport. In conclusion, the relative importance of glucose delivery, transport, and phosphorylation in controlling the rate of insulin-stimulated muscle glucose uptake varies between muscle fiber types, with glucose delivery and transport being the primary limiting factors in type II muscle.

Limitations to basal and insulin-stimulated skeletal muscle glucose uptake in the high-fat-fed rat

Rats fed a high-fat diet display blunted insulin-stimulated skeletal muscle glucose uptake. It is not clear whether this is due solely to a defect in glucose transport, or if glucose delivery and phosphorylation are also impaired. To determine this, rats were fed standard chow (control rats) or a high-fat diet (HF rats) for 4 wk. Experiments were then performed on conscious rats under basal conditions or during hyperinsulinemic euglycemic clamps. Rats received primed constant infusions of 3-O-methyl-[(3)H]glucose (3-O-MG) and [1-(14)C]mannitol. Total muscle glucose concentration and the steady-state ratio of intracellular to extracellular 3-O-MG concentration [which distributes based on the transsarcolemmal glucose gradient (TSGG)] were used to calculate glucose concentrations at the inner and outer sarcolemmal surfaces ([G](im) and [G](om), respectively) in soleus. Total muscle glucose was also measured in two fast-twitch muscles. Muscle glucose uptake was markedly decreased in HF rats. In control rats, hyperinsulinemia resulted in a decrease in soleus TSGG compared with basal, due to increased [G](im). In HF rats during hyperinsulinemia, [G](im) also exceeded zero. Hyperinsulinemia also decreased muscle glucose in HF rats, implicating impaired glucose delivery. In conclusion, defects in extracellular and intracellular components of muscle glucose uptake are of major functional significance in this model of insulin resistance.

Limitations to exercise- and maximal insulin-stimulated muscle glucose uptake

The hypothesis of this investigation was that insulin and muscle contraction, by increasing the rate of skeletal muscle glucose transport, would bias control so that glucose delivery to the sarcolemma (and t tubule) and phosphorylation of glucose intracellularly would exert more influence over glucose uptake. Because of the substantial increases in blood flow (and hence glucose delivery) that accompany exercise, we predicted that glucose phosphorylation would become more rate determining during exercise. The transsarcolemmal glucose gradient (TSGG; the glucose concentration difference across the membrane) is inversely related to the degree to which glucose transport determines the rate of glucose uptake. The TSGG was determined by using isotopic methods in conscious rats during euglycemic hyperinsulinemia [Ins; 20 mU/(kg. min); n = 7], during treadmill exercise (Ex, n = 6), and in sedentary, saline-infused rats (Bas, n = 13). Rats received primed, constant intravenous infusions of trace 3-O-[3H]methyl-D-glucose and [U-14C]mannitol. Then 2-deoxy-[3H]glucose was infused for the calculation of a glucose metabolic index (Rg). At the end of experiments, rats were anesthetized, and soleus muscles were excised. Total soleus glucose concentration and the steady-state ratio of intracellular to extracellular 3-O-[3H]methyl-D-glucose (which distributes on the basis of the TSGG) were used to calculate ranges of possible glucose concentrations ([G]) at the inner and outer sarcolemmal surfaces ([G]im and [G]om, respectively). Soleus Rg was increased in Ins and further increased in Ex. In Ins, total soleus glucose, [G]om, and the TSGG were decreased compared with Bas, while [G]im remained near 0. In Ex, total soleus glucose and [G]im were increased compared with Bas, and there was not a decrease in [G]om as was observed in Ins. In addition, accumulation of intracellular free 2-deoxy-[3H]glucose occurred in soleus in both Ex and Ins. Taken together, these data indicate that, in Ex, glucose phosphorylation becomes an important limitation to soleus glucose uptake. In Ins, both glucose delivery and glucose phosphorylation influence the rate of soleus glucose uptake more than under basal conditions.

Kinetics of 2-deoxyglucose transport in skeletal muscle: effects of insulin and contractions

There is some controversy regarding whether insulin or contractile activity alters the affinity of skeletal muscle glucose transporters for glucose and its analogues. The effects of insulin and contractions on the kinetics of glucose transport were therefore reexamined in isolated rat skeletal muscles. Concentration-dependent rates of 2-deoxyglucose (2-DG) transport were measured in the absence or presence of insulin (2 mU/ml) in the epitrochlearis and split soleus muscles. The apparent half-maximal saturating substrate concentration (Km) for basal 2-DG transport (approximately 12 mM) was similar for the split soleus and epitrochlearis, and the apparent Km was not changed by insulin in either muscle type. The presence of 2 mU/ml insulin increased the maximal transport velocity (Vmax) approximately fourfold in the epitrochlearis and approximately eightfold in the split soleus. In the epitrochlearis, in vitro muscle contractions also resulted in an approximately fourfold increases in Vmax with no change in apparent Km. The combined effects of insulin and contractions on Vmax were completely additive, but the apparent Km was not different from insulin alone. The apparent Km values for basal and insulin-stimulated glucose transport were further characterized in the epitrochlearis isolated from transgenic mice overexpressing the GLUT-1 isoform in the sarcolemma and their nontransgenic littermates. The apparent Km for basal 2-DG transport in the transgenic muscle (9 mM) was not significantly different from the apparent Km for insulin-stimulated transport in the control muscle (10 mM). The present study provides evidence that insulin and contractions, either alone or in combination, increase glucose transport activity in skeletal muscle by increasing Vmax, with no significant change in Km. Our results also suggest that, in intact skeletal muscle, the Km for basal glucose transport (a process mediated primarily by GLUT-1) is similar to the Km values for stimulated transport, mediated predominantly by GLUT-4.

Defective regulation of energy metabolism in mdx-mouse skeletal muscles

Our previous finding of a reduced energy metabolism in slow- and fast-twitch skeletal muscle fibres from the murine model of Duchenne muscular dystrophy (the mdx mouse) led us to examine the importance of intracellular glucose availability for a normal energy turnover. To this end, basal and KCl-stimulated (20.9 mM total extracellular K+) rates of glucose uptake (GUP) and heat production were measured in isolated, glucose-incubated (5 mM) soleus and extensor digitorum longus muscles from mdx and control C57B1/10 mice, in the presence and in the absence of insulin (1.7 nM). Under all conditions and for both muscle types, glucose uptake values for mdx and control muscles were similar although heat production was lower in mdx muscles. The marked stimulation of GUP by insulin in both mdx and control muscles had only minor effects on heat production. In contrast, glucose deprivation or inhibition of glycolysis with 2-deoxy-D-glucose (5 mM) significantly decreased heat production in control muscles only, which attenuated, although did not suppress, the difference in basal heat production between mdx and control muscles. Stimulation of heat production by a short-chain fatty acid salt (octanoate, 2 mM) was significantly less marked in mdx than in control muscles. Increased cytoplasmic synthesis of CoA by addition of 5 mM pantothenate (vitamin B5) increased the thermogenic response to glucose more in mdx than in control muscles. We conclude that the low energy turnover in mdx-mouse muscle fibres is not due to a decrease of intracellular glucose availability, but rather to a decreased oxidative utilization of glucose and free fatty acids. We suggest that some enzyme complex of the tricarboxylic acid cycle or inefficiency of CoA transport in the mitochondria could be involved.

Acute effects of phorbol esters on the protein-synthetic rate and carbohydrate metabolism of normal and mdx mouse muscles

1. mdx mice do not express dystrophin, the product of the gene which is defective in Duchenne and Becker muscular dystrophy. We have previously shown that protein-synthetic rates (ks) are increased in mdx mouse muscles [MacLennan & Edwards (1990) Biochem. J. 268, 795-797]. 2. The tumour-promoting stereoisomer of phorbol 12,13-didecanoate (4 beta-PDD) acutely increased the ks of muscles from mdx and wild-type (C57BL/10) mice incubated in vitro in the absence of insulin. The effects of 4 beta-PDD are presumably mediated by activation of protein kinase C (PKC). 3. The muscle glycogen concentrations of mdx mice were higher than those of C57BL/10 mice. Studies performed in vivo and in vitro suggested that the effect might be at least partially due to increased rate of glycogen synthesis in mdx muscle. 4. 4 beta-PDD increased the glycogen-synthetic rates rates of C57BL/10, but not mdx, muscles incubated in vitro in the absence of insulin. 5. In muscles from both species incubated in the absence of insulin, treatment with 4 beta-PDD also induced increased rates of glucose uptake and lactate production. Kinetic studies of C57BL/10 and mdx muscles suggested that 4 beta-PDD raised the Vmax. of glucose uptake, but did not alter the Km for the process. 6. The possible role of PKC in controlling the protein and carbohydrate metabolism of normal and mdx mouse muscles is discussed.

Glucose transport in skeletal muscle membrane vesicles from control and exercised rats

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.

A compartmental model to quantitate in vivo glucose transport in the human forearm

Glucose transport is a critical step in the control of glucose disposal that, until presently, has not been quantitated in vivo in humans. We have employed the perfused forearm and euglycemic insulin-clamp techniques in combination with a dual-tracer injection to measure basal and insulin-mediated glucose transport in six normal subjects. L-[3H]glucose, which is not transported, was used to trace extracellular glucose kinetics; 3-O-[14C]-methyl-D-glucose, transportable but not metabolizable, was used to monitor glucose movement across the cell membrane. After bolus intra-arterial injection of the two tracers, plasma samples were obtained every 15-30 s for 10 min from a deep forearm vein to determine the washout curves. A linear compartmental model was developed that accounts for blood flow heterogeneity. It consists of three parallel, two-compartment chains merging into the sampling compartment to which cellular compartments are appended. A priori identifiability analysis was performed. The uniquely identifiable parameterization includes the transport rate constants of glucose into and out of the cell. The model was identified using nonlinear least-squares parameter estimation. Transport parameters are estimated with very good precision, and their reproducibility is satisfactory. The model also allows the estimation of the mean arteriovenous transit times of both the extracellular and the transported tracer. The compartmental model provides a novel approach to investigate glucose transport in vivo in humans.

Characteristics of acidic, basic and neutral amino acid transport in the perfused rat hindlimb

1. We have employed a paired-tracer isotope dilution technique in a perfused rat hindlimb preparation to obtain information on the kinetics of transport across the sarcolemmal membranes of acidic, neutral and basic amino acids. 2. We have defined the characteristics of the saturable transport of amino acids normally regarded as paradigm substrates for the A, ASC, L, y+(basic) and the dicarboxylic amino acid transport systems. Their maximal transport capacities (Vmax, nmol min-1 (g muscle)-1 and substrate concentrations for half-maximal transport (Km, mM) of representative amino acid substrates are as follows: 2-aminoisobutyrate (AIB), Vmax = 15 +/- 7, Km = 1.26 +/- 0.6; alanine, Vmax = 332 +/- 53, Km = 3.9 +/- 0.9; serine, Vmax = 410 +/- 61, Km 3.4 +/- 0.5; leucine, Vmax = 2800 +/- 420, Km = 20 +/- 2; lysine, Vmax = 136 +/- 46, Km = 2.1 +/- 1.3; glutamate, Vmax = 86 +/- 6, Km = 1.05 +/- 0.05; proline, Vmax = 196 +/- 48, Km = 4.1 +/- 0.6. 3. Glycine uptake was faster than expected on the basis of diffusion but was not saturable and showed uptake that could be best described by a first-order rate constant of 0.07 +/- 0.003 min-1. 4. We have attempted to discriminate kinetically between possible routes of entry for an amino acid on the basis of competitive and non-competitive interaction between substrates potentially sharing common routes. On this basis, the major routes of alanine entry appear to be via the ASC and L systems with the A system playing a quantitatively minor role. Glutamate and aspartate appear to be transported exclusively by a dicarboxylate amino acid carrier. The branched-chain amino acids (BCAA) and the aromatic amino acid, phenylalanine, are almost equivalent substrates for an L-like system. 5. Insulin had no detectable effect on the uptake of paradigm substrates for ASC, L, y+, the dicarboxylic amino acid or glycine transport systems. 6. Transport of serine and lysine was Na+ dependent. Lysine transport apparently occurred with a stoichiometry of 2 Na+: 1 lysine. With the exception of alanine, whose transport was partially Na+ dependent, all other amino acids examined in the present study were transported in a Na+-independent manner.

L(+)-lactate transport in perfused rat skeletal muscle: kinetic characteristics and sensitivity to pH and transport inhibitors

We have examined lactate uptake (as the rate of net muscle lactate accumulation) and unidirectional inward transport (measured by a paired-tracer dilution method) in muscle of the perfused skinned rat hindlimb. Inhibition of tracer influx (fractional uptake at 1 mM L(+)-lactate, 43.3 +/- 3.1% but only 32.9 +/- 1.8% at 50 mM lactate) suggested some competition between tracer and native forms of the carboxylate for transport. D(-)-lactate (50 mM) did not inhibit uptake of tracer L(+)-lactate. Pyruvate (25 mM), but none of five other monocarboxylates, inhibited uptake of tracer lactate, by 22% (P less than 0.01). Altering perfusate pH from 7.4 to 6.8 caused a 36% increase (P less than 0.001) in the unidirectional L(+)-lactate transport at 1 mM L(+)-lactate, whereas increasing pH to 7.7 reduced transport by 18% (P less than 0.01). Tracer lactate influx was inhibited by 500 microM 4-acetamido-4'-isothiocyanostilbene (SITS) (19%), 5 mM alpha-cyano-4-hydroxycinnamic acid (CIN) (20-30%), 1 mM amiloride (27%) and by a thiol group reagent p-chloromercuribenzenesulphonic acid (pCMBS) (26%). Overall the results indicate that at least two processes are involved in the transfer of lactate: one, saturable, with a Vmax of 0.84 mumol.min-1.g-1 and an apparent Km of 21 mM was sensitive to SITS, CIN, and a thiol group reagent; the other was non-saturable and insensitive to SITS and CIN with an apparent rate constant of 0.1 min-1.

Mechanism of insulin action on glucose transport in rat skeletal muscle

This study was designed to examine the effect of insulin stimulation on glucose transport in rat skeletal muscle. Sarcolemmal vesicles (SL) were isolated from the gastrocnemius-plantaris and quadriceps muscles from insulin-stimulated and control groups. The insulin-stimulated group received an intravenous insulin injection (1 U/kg) 10 min before isolation. The early time course of specific D-glucose transport was linear through 2 s. Michaelis-Menten kinetics at 1.5 s indicated that the Vmax for glucose transport was increased after insulin stimulation compared with controls (4,424 +/- 668 vs. 1,366 +/- 124 pmol.mg protein -1.s-1), whereas the Km remained unchanged (19.4 +/- 0.6 vs. 21.6 +/- 3.1 mM). Scatchard plots for the D-glucose-inhibitable class of cytochalasin B binding sites indicated that insulin stimulation increased the number of binding sites in the SL vesicles (9.3 +/- 0.6 vs. 5.5 +/- 0.3 pmol/mg protein) without altering the Kd (48 +/- 3 vs. 46 +/- 3 nM). That the increase in Vmax was greater than the increase in cytochalasin B binding sites indicates that insulin stimulation caused an increase in the turnover rate of existing transport molecules as well as an increase in the total number of SL glucose transport molecules.

Characteristics of L-glutamine transport in perfused rat skeletal muscle

1. We have investigated glutamine transport in the perfused rat hindlimb using the paired-tracer isotope dilution technique. 2. Uptake of L-glutamine was stereospecific, saturable, sodium dependent, insulin sensitive and pH insensitive in the physiological range. The maximum capacity of transport (Vmax) under normal perfusate conditions at 37 degrees C, 145 mM-Na+ and in the absence of insulin was 1156 +/- 193 nmol min-1 g-1 with transport being half-maximal at a perfusate glutamine concentration of 9.25 +/- 1.15 mM. 3. The kinetics of Na+ dependence strongly suggested co-transport of Na+ and glutamine with a stoichiometry of 1:1; furthermore, Na+ activated the carrier without any change in the concentration of glutamine at which transport was half-maximal, i.e. a 'Vmax effect' rather than a 'Km effect'. 4. The characteristics of glutamine transport, especially its substrate specificity and the pattern of competitive and non-competitive inhibition of glutamine transport by other amino acids, suggest that it is mediated by a carrier or carriers for which asparagine and histidine are also suitable substrates. 5. The characteristics of muscle glutamine transport are related but distinct from those of system N identified in hepatocytes; we suggest that they are sufficiently distinct to justify the identification of a new variant of mammalian amino acid transport systems which may be identified by the symbol Nm. 6. The kinetic characteristics of system Nm are such that glutamine is likely to be the most rapidly exchanging amino acid across the muscle membrane at physiological intra- and extracellular glutamine concentrations. Its hormone and ion sensitivities are likely to be important in the physiological modulation of whole-body glutamine metabolism and also during derangements observed in disease and after injury.

Characteristics of a glutamine carrier in skeletal muscle have important consequences for nitrogen loss in injury, infection, and chronic disease

A carrier for glutamine, identified in rat muscle, has properties in terms of kinetics, ion dependence and hormone sensitivity, and effects of endotoxin and branched-chain aminoacids that point to an important function in the control of whole-body aminoacid metabolism. The existence of a link between the size of the glutamine pool in muscle and the rate of muscle protein synthesis raises possibilities for therapeutic interventions to limit protein loss in injury, sepsis, and chronic disease.

Glucose turnover in response to exercise during high- and low-FIO2 breathing in man

The purpose of this study was to assess whether breathing high or low concentrations of O2 could affect glucose turnover during exercise in man. Ten healthy subjects performed two constant work-rate exercise tests, one when the fraction of inspired O2 (FIO2) was 0.15 and the other at the same work rate but when the FIO2 was 0.80. The work rate for each subject was chosen so that blood lactate would be elevated during hypoxia, but would be lower during hyperoxia. Glucose appearance (Ra) and disappearance (Rd) were measured using the primed, constant infusion of [3-3H]glucose. Although the work rate was the same during hypoxia and hyperoxia in each subject, hypoxic exercise was accompanied by a significantly larger rest to exercise increase in Rd (delta Rd) compared with hyperoxia by 265%. Similarly, delta Ra was greater during hypoxia than during hyperoxia by 188%. Lactate to pyruvate ratios were significantly higher during hypoxic exercise suggesting a shift in the cell redox to a more reduced state. Insulin and glucagon were not affected by the FIO2, but both epinephrine and norepinephrine were increased during hypoxic exercise, which may explain the increase in Ra. The regulation of blood glucose during exercise in vivo appears to be dependent on the availability of oxygen to the working muscle cells.

Characteristics of rat hindlimbs perfused with erythrocyte- and albumin-free medium

The isolated rat hindlimb was perfused with Krebs-bicarbonate buffer without erythrocytes and albumin in a flow-through mode at 32 degrees C, and the viability and metabolic characteristics of perfused skeletal muscle were examined. 1) With the flow rate at 15 ml X min-1 X leg-1, glucose and O2 uptake, lactate release, lactate-to-pyruvate ratio in effluent, and tissue creatine phosphate and adenine nucleotides remained constant at rest during perfusion for 90 min. The twitch tension changed little over perfusion. 2) When the leg was stimulated at a frequency below 0.5 Hz, the standard flow rate adequately delivered O2 to the perfused leg. Sciatic nerve stimulation enhanced glucose uptake in the absence of insulin. 3) The stimulatory effect of insulin on glucose uptake was observed with a concentration as low as 0.1 mU/ml, and maximal effect was at approximately mU/ml, with a nearly eightfold increase in glucose uptake. 4) Epinephrine and isoproterenol at a concentration of 0.5 nM stimulated lactate release, with maximal effect at 5 nM. The response to catecholamines was reversible and reproducible with a single preparation during the perfusion period of 120 min. The results indicated that the perfusion of hindlimb with a hemoglobin- and albumin-free medium is a convenient and reliable tool for the biochemical investigations of the integral function of hindlimb skeletal muscle.

Additive effects of prior exercise and insulin on glucose and AIB uptake by rat muscle

After exercise of moderate intensity the ability of insulin to stimulate the uptake of glucose and alpha-aminoisobutyric acid (AIB) in perfused rat muscle is enhanced in a parallel fashion. The present study was designed to examine the effect of intense exercise on the subsequent uptake of these substrates. For this purpose, rats fed ad libitum were run on a treadmill for 50 min at high intensity and glucose and AIB uptake by muscle were then assessed in the isolated perfused hindquarter preparation. In confirmation of previous studies, 30 min after such exercise the absolute rate of glucose uptake in the presence of 20,000 microU/ml of insulin was greater due to additive effects of insulin and prior exercise. A novel finding was that 150 min postexercise the rate of glucose uptake was still increased in the presence of a supramaximal concentration of insulin, but entirely due to an increase in insulin responsiveness. The uptake of AIB and its response to insulin in general paralleled that of glucose. The results indicate that both glucose and AIB uptake by skeletal muscle in the presence of a supramaximal concentration of insulin are increased after intense exercise. They suggest that this is initially due to an additive effect of insulin and exercise and later due to an increase in insulin responsiveness. The findings are compatible with the notion that after exercise insulin is able to recruit or activate glucose (and possibly AIB) transporters in muscle, that it does not affect in the resting state.

Blood flow distribution in tissues of perfused rat hindlimb preparations

Aerobic muscle metabolism during concentrations requires adequate blood flow and oxygen delivery. Since the perfused rat hindquarter (HQ) has become widely used for muscle stimulation, we examined the blood flow distribution, using 15 microns radiolabeled microspheres, and oxygen consumption of the HQ, using different commonly used perfusion protocols. Perfusion via the abdominal aorta resulted in well-matched (r = 0.90) blood flows between tissues of both hindlimbs that were proportional to total perfusion inflow. Blood flows to the high-oxidative fast-twitch and slow-twitch red muscle sections were three- to fourfold greater than flows to sections of low-oxidative fast-twitch white muscle. However, a large fraction (28%) of the total inflow went to the trunk region, even though all apparent arterial branches to the trunk region were ligated. This trunk mass accounts for at least 40% of the total metabolic responses of the HQ and diverts a large blood flow that is often presumed to supply the hindlimbs. As a result, muscle performance of the distal hindlimb muscle during stimulation can be inordinately poor. Ligation of the iliac artery to the contralateral limb improves blood flow to the remaining hindlimb but does not eliminate trunk blood flow. In contrast, perfusion via the femoral artery restricted 95% of the inflow to the single hindlimb, thereby reducing the tissue mass perfused. Blood flow to the distal limb musculature was high, resulting in an enhanced muscle performance. Thus single hindlimb perfusion provides a preparation where the contracting muscle is a large fraction of the total tissue, and the venous effluent better reflects the metabolic events in the contracting muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane transport in relation to net uptake of glucose in the perfused rat hindlimb. Stimulatory effect of insulin, hypoxia and contractile activity

The paired-tracer dilution method applied to the perfused rat hindlimb model was used to study glucose transport in relation to net glucose uptake in skeletal muscle tissue. 2-deoxyglucose was used as an analogue for glucose, since this eliminates the problem with release of labelled metabolites. The affinity of 2-deoxyglucose for the glucose carrier was shown to be indistinguishable from that of glucose. An insulin dose-response study showed maximal stimulation of glucose uptake and transport at 0.1 unit/l, and 75% of maximal stimulation at 0.01 unit of insulin/l. Hypoxia and contractile activity stimulated the 2-deoxyglucose transport rate similarly, and the stimuli were not additive, suggesting a common mechanism. The presence of insulin did not increase the effect of hypoxia or contractile activity, indicating no permissive effect of insulin. The 2-deoxyglucose transport rate was closely correlated with and always higher than that of glucose uptake, demonstrating that the transport is never rate-limiting for the net glucose uptake and that both processes are regulated together. Significant correlations between the 2-deoxyglucose transport rate and the intramuscular concentration of phosphocreatine suggest regulation of the glucose utilization by the energy state of the skeletal muscle tissue.

Mechanisms of insulin resistance in non-insulin-dependent (type II) diabetes

Characteristic of both obesity and non-insulin-dependent diabetes mellitus, insulin resistance is triggered at the level of the target tissue and can be induced by three general categories of causes: (1) an abnormal beta cell secretory product, (2) circulating insulin antagonists, or (3) a target tissue defect in insulin action. Decreased numbers of insulin receptors and a post-receptor defect in insulin action both play relative roles in insulin resistance. A general trend, however, indicates that as insulin resistance increases, the post-receptor defect becomes more prominent. Impaired glucose uptake and subsequent increased hepatic glucose oxidation in non-insulin-dependent diabetes mellitus are major contributing factors to fasting hyperglycemia.

A method to quantify glucose utilization in vivo in skeletal muscle and white adipose tissue of the anaesthetized rat

A quantitative method allowing determination of glucose metabolism in vivo in muscles and white adipose tissue of the anaesthetized rat is presented. A tracer dose of 2-deoxy[3H]glucose was injected intravenously in an anaesthetized rat and the concentration of 2-deoxy[3H]glucose was monitored in arterial blood. After 30-80 min, three muscles, the soleus, the extensor digitorum longus and the epitrochlearis, periovarian white adipose tissue and brain were sampled and analysed for their content of 2-deoxy[3H]glucose 6-phosphate. This content could be related to glucose utilization during the same time period, since (1) the integral of the decrease of 2-deoxy[3H]glucose in arterial blood was known and (2) correction factors for the analogue effect of 2-deoxyglucose compared with glucose in the transport and phosphorylation steps were determined from experiments in vitro. Glucose utilization was then measured by this technique in the tissues of post-absorptive rats in the basal state (0.1 munit of insulin/ml of plasma) or during euglycaemic-hyperinsulinaemic glucose clamp (8 munits of insulin/ml of plasma) and of 48 h-starved rats. Results corresponded qualitatively and quantitatively to the known physiological characteristics of the tissues studied.

Dose-response curves for in vivo insulin sensitivity in individual tissues in rats

A technique is described for examining in vivo insulin action on glucose utilization in individual tissues in the intact conscious rat. Indices of tissue glucose metabolic rate Rg' and of the percentage of total glucose uptake incorporated into specific storage products (Cf) are derived from tissue analysis after bolus administration of 2-[3H]deoxyglucose and [14C]glucose during the plateau phase of the euglycemic clamp. The effects of insulin elevation have been examined in several tissues. Rg' in diaphragm increased 10-fold over basal (maximal) with a half-maximal sensitivity (ED50) of 150 mU/l. This was similar to the ED50 for net whole body glucose utilization of 133 mU/l. In adipose tissue Rg' increased by twofold and Cf into lipids by sixfold; both were near maximal at 150 mU/l (ED50 of 60 mU/l). A small but significant insulin effect (Rg' increased 2-fold) was found in lung. Insulin did not significantly increase Cf into total liver lipids or glycogen. The methodology described here significantly increases the usefulness of the glucose clamp technique in the study of insulin action. Dose-response curves for insulin action during the euglycemic clamp vary considerably among different target tissues in the rat.

Skeletal muscle fibers in suspension: a new approach to the study of oxidative and glycolytic metabolism in differentiated muscle

A preparation of suspended fibers from m. flexor digitorum brevis of the rat was characterized with respect to morphological features, and its relevance for the study of muscular metabolism investigated. The activities of oxidative (palmitate and pyruvate oxidation) and glycolytic (lactate formation) pathways were enhanced in myofiber suspensions when compared to intact whole muscle. The rate of glycolysis was stimulated about two-fold by insulin in both the myofiber suspensions and intact muscle. Parameters of oxidative metabolism responded similarly to metabolic effectors in the myofiber suspensions and in intact muscle. It is concluded that the myofiber suspensions have distinct advantages over intact muscle for biochemical and pharmacological studies.