Regulation of glucose kinetics during exercise by the glucagon-like peptide-1 receptor.

In response to oral glucose, glucagon-like peptide-1 receptor (Glp1r) knockout (Glp1r−/−) mice become hyperglycaemic due to impaired insulin secretion. Exercise also induces hyperglycaemia in Glp1r−/− mice. In contrast to oral glucose, exercise decreases insulin secretion. This implies that exercise-induced hyperglycaemia in Glp1r−/− mice results from the loss of a non-insulinotropic effect mediated by the Glp1r. Muscle glucose uptake (MGU) is normal in exercising Glp1r−/− mice. Thus, we hypothesize that exercise-induced hyperglycaemia in Glp1r−/− mice is due to excessive hepatic glucose production (HGP). Wild-type (Glp1r+/+) and Glp1r−/− mice implanted with venous and arterial catheters underwent treadmill exercise or remained sedentary for 30 min. [3-3H]glucose was used to estimate rates of glucose appearance (Ra), an index of HGP, and disappearance (Rd). 2[14C]deoxyglucose was used to assess MGU. Glp1r−/− mice displayed exercise-induced hyperglycaemia due to an excessive increase in Ra but normal Rd and MGU. Exercise-induced glucagon levels were ∼2-fold higher in Glp1r−/− mice, resulting in a ∼2-fold higher glucagon:insulin ratio. Since inhibition of the central Glp1r stimulates HGP, we tested whether intracerebroventricular (ICV) infusion of the Glp1r antagonist exendin(9­39) (Ex9) in Glp1r+/+ mice would result in exercise-induced hyperglycaemia. ICV Ex9 did not enhance glucose levels or HGP during exercise, suggesting that glucoregulatory effects of Glp1 during exercise are mediated via the pancreatic Glp1r. In conclusion, functional disruption of the Glp1r results in exercise-induced hyperglycaemia associated with an excessive increase in glucagon secretion and HGP. These results suggest an essential role for basal Glp1r signalling in the suppression of alpha cell secretion during exercise.

Effect of prior exercise on the partitioning of an intestinal glucose load between splanchnic bed and skeletal muscle.

Exercise leads to marked increases in muscle insulin sensitivity and glucose effectiveness. Oral glucose tolerance immediately after exercise is generally not improved. The hypothesis tested by these experiments is that after exercise the increased muscle glucose uptake during an intestinal glucose load is counterbalanced by an increase in the efficiency with which glucose enters the circulation and that this occurs due to an increase in intestinal glucose absorption or decrease in hepatic glucose disposal. For this purpose, sampling (artery and portal, hepatic, and femoral veins) and infusion (vena cava, duodenum) catheters and Doppler flow probes (portal vein, hepatic artery, external iliac artery) were implanted 17 d before study. Overnightfasted dogs were studied after 150 min of moderate treadmill exercise or an equal duration rest period. Glucose ([14C]glucose labeled) was infused in the duodenum at 8 mg/kg x min for 150 min beginning 30 min after exercise or rest periods. Values, depending on the specific variable, are the mean +/- SE for six to eight dogs. Measurements are from the last 60 min of the intraduodenal glucose infusion. In response to intraduodenal glucose, arterial plasma glucose rose more in exercised (103 +/- 4 to 154 +/- 6 mg/dl) compared with rested (104 +/- 2 to 139 +/- 3 mg/dl) dogs. The greater increase in glucose occurred even though net limb glucose uptake was elevated after exercise (35 +/- 5 vs. 20 +/- 2 mg/min) as net splanchnic glucose output (5.1 +/- 0.8 vs. 2.1 +/- 0.6 mg/kg x min) and systemic appearance of intraduodenal glucose (8.1 +/- 0.6 vs. 6.3 +/- 0.7 mg/kg x min) were also increased due to a higher net gut glucose output (6.1 +/- 0.7 vs. 3.6 +/- 0.9 mg/kg x min). Adaptations at the muscle led to increased net glycogen deposition after exercise [1.4 +/- 0.3 vs. 0.5 +/- 0.1 mg/(gram of tissue x 150 min)], while no such increase in glycogen storage was seen in liver [3.9 +/- 1.0 vs. 4.1 +/- 1.1 mg/(gram of tissue x 150 min) in exercised and sedentary animals, respectively]. These experiments show that the increase in the ability of previously working muscle to store glycogen is not solely a result of changes at the muscle itself, but is also a result of changes in the splanchnic bed that increase the efficiency with which oral glucose is made available in the systemic circulation.

Impact of suprapharmacological androgenic steroid administration on basal and insulin-stimulated glucose and amino acid metabolism.

Effects of androgenic steroids at doses used by athletes were studied in a canine model system in which dosage, diet, and activity were controlled. Dogs were treated with 19-nortestosterone (200 mg/wk intramuscularly) or vehicle and were studied at 18 (n = 4 in steroid and vehicle) or 32 (n = 6 in steroid and n = 4 in vehicle) days. A laparotomy was performed under general anesthesia 17 days before experimentation, and catheters were placed in an artery, portal vein, and hepatic vein. Studies consisted of an equilibration (120 minutes) and a control (40 minutes) period and a three-step immunoreactive insulin euglycemic clamp (1, 2, and 15 mU/kg.min). Step 1 was 150 minutes, and steps 2 and 3 were 90 minutes. Data were collected during the last 30 minutes of each step. Glucose and leucine kinetics were assessed with 3H-glucose and 14C-leucine. Plasma glucose in steroid and vehicle groups was 104 +/- 5 (mean +/- SE) versus 108 +/- 3 mg/dL and 100 +/- 5 versus 107 +/- 4 mg/dL at 18 and 32 days. Glucose turnover was similar at 18 days in steroid and vehicle groups (3.9 +/- 0.3 v 3.6 +/- 0.3 mg/kg.min, respectively), but was elevated in the steroid group at 32 days (5.4 +/- 0.5 v 3.2 +/- 0.4 mg/kg.min). Glucose infusion rates were lower in the steroid group with 15 mU/kg.min immunoreactive insulin at 32 days (15.0 +/- 1.1 v 21.2 +/- 1.4 mU/kg.min). Immunoreactive insulin-independent glucose utilization (Rd) was unaffected at 18 days of steroid treatment, but was increased by almost fourfold at 32 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between arterial and portal vein immunoreactive glucagon during exercise.

The importance of changes in glucagon in the regulation of hepatic glucose production (Ra) during exercise has been questioned, as an increase in arterial immunoreactive glucagon (IRG) is not always detectable. However, IRG in the portal vein (PV) and not in the artery is most relevant, as flow through PV is approximately 80% of liver blood flow. To assess the extent that arterial IRG reflects the levels the liver is exposed to in PV, dogs (n = 5) were implanted with catheters in a carotid artery, hepatic vein (HV), and PV. Dogs were studied > or = 16 days later during rest and 150 min of moderate treadmill exercise, with indocyanine green and [3-3H]glucose infused to assess hepatic plasma flow (HPF) and hepatic Ra. IRG was 66 +/- 7, 73 +/- 8, and 81 +/- 7 pg/ml in the artery, HV, and PV at rest; it rose at 10 and 150 min of exercise to 89 +/- 9 and 127 +/- 13 pg/ml in the artery, 106 +/- 17 and 186 +/- 21 pg/ml in HV, and, by considerably more, to 153 +/- 20 and 261 +/- 25 pg/ml in PV. HPF fell by approximately 30% with exercise. The fall in HPF accounted for < 11% of the increased arterial-to-PV IRG gradient during exercise, with increased splanchnic IRG release comprising the remainder. Ra was linearly related to IRG levels in the three vessels.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Role of glucose and insulin loads to the exercising limb in increasing glucose uptake and metabolism.

To assess the contributions of glucose load to the working hindlimb and local contraction-related events (changes related to the microvasculature and/or intrinsic muscle metabolic properties) to the exercise-induced increases in muscle glucose uptake and metabolism in vivo, dogs were studied with somatostatin infused to suppress insulin release, and glucose and insulin were replaced 1) during rest and treadmill exercise at rates that recreate limb glucose and insulin loads evident during exercise (n = 5), 2) at rest to selectively normalize the limb glucose load to rates present during exercise while retaining basal limb insulin loads (GL, n = 5), or 3) at rest to normalize both the limb glucose and insulin loads to those present during exercise (IGL, n = 5). Limb arteriovenous difference and isotopic ([U-14C]glucose) techniques were used to quantify muscle glucose uptake and metabolism. Limb glucose load rose from 819 +/- 141 mumol/min in the basal state to 1,568 +/- 190 mumol/min with exercise. Limb glucose loads were 1,423 +/- 88 and 1,502 +/- 165 mumol/min in GL and IGL. The limb insulin load rose from basal rates of 12.9 +/- 2.3 to 22.9 +/- 5.9 nmol/min during exercise. Limb insulin loads were similar to basal loads in GL (8.8 +/- 1.9 nmol/min) and exercise in IGL (28.2 +/- 5.5 nmol/min).(ABSTRACT TRUNCATED AT 250 WORDS)

Transcription of the rat skeletal muscle hexokinase II gene is increased by acute exercise.

A single bout of acute exercise increases hexokinase (HK) II mRNA and enzyme activity [R. M. O'Doherty, D. P. Bracy, H. Osawa, D. H. Wasserman, and D. K. Granner. Am. J. Physiol. 266 (Endocrinol. Metab. 29): E171-E178, 1994]. The present study addresses the mechanism of the increase in HK II mRNA. Male rats undertook a single bout of treadmill exercise and were then killed immediately or after a predetermined recovery period. The gastrocnemius/plantaris muscle complex, composed of mixed fiber types, was excised; the nuclei were isolated; and HK I, HK II, beta-actin, and alpha-tubulin gene transcription rates were measured. Genomic DNA and plasmid DNA were used as positive and negative controls, respectively. Immediately after the cessation of 30, 45, or 90 min of exercise, HK II gene transcription rates were 1.3 +/- 0.3-,2.9 +/- 0.3-, and 4.0 +/- 0.6-fold, respectively, above those of sedentary controls. The increases after 45 and 90 min of exercise were statistically significant (P < 0.01). One hour after the cessation of 30 min of exercise, HK II gene transcription was significantly increased (1.40 +/- 0.03-fold; P < 0.05). At all time points, transcription of the HK I, beta-actin, and alpha-tubulin genes was unchanged. We conclude that the exercise-induced increase in HK II gene transcription appears to play a major role in the increase of HK II mRNA and activity.

Carbohydrate metabolism during exercise: influence of circulating fat availability.

To examine the role of circulating fat in the regulation of carbohydrate metabolism, dogs were studied during rest and 90 min of moderate treadmill exercise with nicotinic acid infused to suppress lipolysis with (+Fat; n = 5) or without (-Fat; n = 5) Intralipid. Isotopic and hindlimb arteriovenous methods were used to assess metabolism. Plasma glucose was similar in both protocols during rest and exercise. Differences in insulin, catecholamines, and cortisol between groups were insignificant. Glucagon was approximately 50% greater during rest and exercise in -Fat. The following values represent those at 30 or 40 min of muscular work because peak responses were seen at these times. Arterial free fatty acid levels were 1,129 +/- 253 and 272 +/- 17 mu eq/l at rest and 756 +/- 145 and 269 +/- 51 mu eq/l with exercise in +Fat and -Fat, respectively. Glucose production was 4.2 +/- 0.3 and 5.0 +/- 0.4 mg.kg-1.min-1 at rest and 8.5 +/- 1.3 and 11.4 +/- 0.6 mg.kg-1.min-1 with exercise in +Fat and -Fat, respectively. Glucose utilization was 4.3 +/- 0.3 and 5.3 +/- 0.2 mg.kg-1.min-1 at rest and 9.2 +/- 1.2 and 12.7 +/- 0.8 mg.kg-1.min-1 with exercise in +Fat and -Fat, respectively. Significant glucose flux differences were present during rest and exercise. Limb glucose uptake rose similarly with exercise in +Fat (29 +/- 7 to 82 +/- 22 mumol/min) and -Fat (28 +/- 7 to 88 +/- 16 mumol/min). Arterial blood lactate was 50-100% greater in -Fat compared with that in +Fat.(ABSTRACT TRUNCATED AT 250 WORDS)

Gut and liver fat metabolism in depancreatized dogs: effects of exercise and acute insulin infusion.

Excessive circulating fat levels are a defining feature of poor metabolic control in diabetes. Splanchnic adipose tissue is a source of free fatty acids (FFA), and the liver is a key site of FFA utilization and the sole source of ketones. Despite the role of splanchnic tissues in fat metabolism, little is known about how these tissues respond to diabetes under divergent metabolic conditions. Therefore, splanchnic fat metabolism was studied in poorly controlled diabetes under two conditions. First, it was studied during exercise, a stimulus that enhances FFA flux. Second, it was studied while insulin was being acutely infused to achieve levels normally present during exercise, a treatment that may be expected to inhibit lipolysis. For this purpose, liver and gut arteriovenous differences were used during rest and 2.5 h of treadmill exercise in insulin-deficient (n = 6) and acutely insulin-infused (n = 4) depancreatized (PX) dogs. The data show that 1) exercise, in insulin-deficient PX dogs, leads to an increase in net FFA release from mesenteric fat that is equal in magnitude to the response in nondiabetic dogs; 2) net hepatic fractional FFA extraction is increased twofold during exercise in both insulin-deficient PX dogs and nondiabetic control dogs; 3) during exercise, approximately 40 and 75% of the FFA consumed by the liver is effectively transferred from fat stores mobilized from splanchnic adipose tissue in insulin-deficient PX and nondiabetic dogs, respectively; 4) hepatic ketogenic efficiency is elevated during rest three- to fourfold in insulin-deficient PX dogs compared with nondiabetic control dogs and remains elevated during exercise; and 5) surprisingly, acute insulin replacement is ineffective in normalizing net gut, hepatic, or splanchnic FFA or ketone body balances in PX dogs.

Role of Ca(2+) fluctuations in L6 myotubes in the regulation of the hexokinase II gene.

Expression of the hexokinase (HK) II gene in skeletal muscle is upregulated by electrically stimulated muscle contraction and moderate-intensity exercise. However, the molecular mechanism by which this occurs is unknown. Alterations in intracellular Ca(2+) homeostasis accompany contraction and regulate gene expression in contracting skeletal muscle. Therefore, as a first step in understanding the exercise-induced increase in HK II, the ability of Ca(2+) to increase HK II mRNA was investigated in cultured skeletal muscle cells, namely L6 myotubes. Exposure of cells to the ionophore A-23187 resulted in an approximately threefold increase in HK II mRNA. Treatment of cells with the extracellular Ca(2+) chelator EGTA did not alter HK II mRNA, nor was it able to prevent the A-23187-induced increase. Treatment of cells with the intracellular Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester (BAPTA-AM) also resulted in an approximately threefold increase in HK II mRNA in the absence of ionophore, which was similar to the increase in HK II mRNA induced by the combination of BAPTA-AM and A-23187. In summary, a rise in intracellular Ca(2+) is not necessary for the A-23187-induced increase in HK II mRNA, and increases in HK II mRNA occur in response to treatments that decrease intracellular Ca(2+) stores. Depletion of intracellular Ca(2+) stores may be one mechanism by which muscle contraction increases HK II mRNA.

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.

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.

Overexpression of hexokinase II increases insulinand exercise-stimulated muscle glucose uptake in vivo.

The hypothesis of this investigation was that glucose uptake would be increased in skeletal muscle of transgenic mice (TG) overexpressing hexokinase II (HK II) compared with their nontransgenic littermates (NTG) during euglycemic hyperinsulinemia and treadmill exercise. For insulin experiments, catheters were surgically implanted in the jugular vein and carotid artery for infusions and sampling, respectively. Conscious mice underwent experiments approximately 5 days later in which 4 mU. kg-1. min-1 insulin and variable glucose (n = 7 TG and n = 7 NTG) or saline (n = 5 TG and n = 4 NTG) was infused for 140 min. Over the last 40 min of the experiments, 2-deoxy-[3H]glucose ([2-3H]DG) was infused, after which muscles were removed. For the exercise experiments, jugular vein catheters were surgically implanted. Five days later, mice received a bolus of [2-3H]DG and then remained sedentary (n = 6 TG and n = 8 NTG) or ran on a motorized treadmill (n = 12 TG and n = 8 NTG) for 30 min. TG and NTG had similar muscle [2-3H]DG 6-phosphate ([2-3H]DGP) accumulation in the basal state (P > 0.05). In the hyperinsulinemic experiments, TG required approximately 25% more glucose to maintain euglycemia (P < 0.05), and muscle [2-3H]DGP accumulation normalized to infusate [2-3H]DG was similarly increased (P < 0.05). In the exercise experiments, muscle [2-3H]DGP accumulation was significantly greater in TG than NTG (P < 0.05). In conclusion, we did not detect an effect of HK II overexpression on muscle [2-3H]DGP accumulation under basal conditions. Hyperinsulinemia and exercise shift the control of muscle glucose uptake so that phosphorylation is a more important determinant of the rate of this process.

Gut proteolysis contributes essential amino acids during exercise.

Arteriovenous difference and tracer dilution techniques were utilized to determine the effect of exercise on whole body, gut, liver, and splanchnic leucine kinetics. Five postabsorptive dogs were infused with [1-13C]leucine and studied during rest, 90 min of moderate-intensity treadmill exercise (1st 45 min, early; last 45 min, late exercise), and 90 min of recovery. The whole body leucine rate of appearance (Rai; mumol.min 1.kg-1) increased from rest (3.33 +/- 0.11) during early (3.68 +/- 0.14) and late (4.24 +/- 0.27, P < 0.05) exercise and was 3.41 +/- 0.19 during recovery. Gut Ra increased from rest (0.64 +/- 0.08) during early (0.92 +/- 0.12) and late (1.30 +/- 0.20, P < 0.05) exercise and was 0.77 +/- 0.16 during recovery. Liver leucine Ra did not significantly change (P > 0.05). The whole body leucine rate of disappearance (Rd) paralleled whole body leucine Ra throughout. Leucine Rd across the gut, liver, and splanchnic bed, however, did not significantly change (P > 0.05), indicating an increase in leucine uptake outside of these regions. Because active skeletal muscle is likely the principal consumer of these amino acids, the data suggest that gut protein-derived amino acids are utilized for the attenuation of net muscle protein catabolism during and immediately following exercise.

Regulation of gluconeogenesis during rest and exercise in the depancreatized dog.

To assess the mechanism of the accelerated gluconeogenesis in the insulin-deficient state, chronically catheterized (carotid artery, portal vein, hepatic vein, vena cava) normal (C; n = 9) and depancreatized (PX; n = 7) dogs were studied during rest (40 min) and moderate exercise (150 min). Tracers ([14C]alanine, [3H]glucose) and dye were infused to measure determinants of gluconeogenesis in the gut and liver. Arterial levels, net gut output, hepatic load, and net hepatic uptake of alanine were similar in C and PX at rest. During exercise, alanine levels fell in C but rose approximately 100% in PX. Exercise did not affect gut output or liver uptake of alanine in C but increased these variables by approximately 50 and 100% in PX due to an increase in hepatic alanine load. Arterial lactate was similar at rest in C and PX but rose fourfold more in PX with exercise. Net gut lactate output was fivefold greater in PX during rest and exercise. Net hepatic lactate uptake was present in PX at rest, whereas net output was evident in C. In response to exercise, hepatic lactate uptake was increased further in PX due to a rise in hepatic lactate load. Net hepatic lactate uptake was not evident until the end of exercise in C. Net hepatic glycerol uptake was elevated at rest in PX and during the initial 60 min of exercise due to an elevated hepatic load. In contrast to the high rates of gut lactate and alanine output in PX, gut glycerol output was not present. Gluconeogenesis from lactate and alanine was 5- to 10-fold higher in PX than C during rest and exercise. At rest, this resulted, in part, from a twofold greater intrahepatic gluconeogenic efficiency. During exercise, the greater conversion occurred even though efficiency was not consistently greater. In summary, gluconeogenesis from alanine, lactate, and glycerol in the insulin-deficient diabetic state 1) is exaggerated at rest, due to an increased capacity for hepatic lactate extraction, increased hepatic precursor loads, and a greater gluconeogenic efficiency; 2) is accelerated further by exercise due to added increments in hepatic precursor loads; and 3) is exaggerated partly because of a greater net gut alanine and lactate output.

Sensitivity of exercise-induced increase in hepatic glucose production to glucose supply and demand.

It was hypothesized that the exercise-induced changes in glucoregulatory hormones and glucose production (Ra) occur as a result of a small deficit in glucose availability. To test this, 18-h fasted dogs performed 150 min of treadmill exercise with either the liver as the sole source of glucose (controls, n = 5) or with glucose infused from 0 to 50 min (period 1) and from 100 to 150 min (period 3) at rates designed to track the glucose utilization (Rd) response (ExoGlc, n = 5). The liver alone supplied glucose from 50 to 100 min (period 2). Isotopic and arteriovenous methods were used to assess Ra, Rd, and gluconeogenesis (GNG). Variable [3H]glucose infusion and frequent sampling were used to facilitate Ra measurements. Arterial glucose declined by -3.1 +/- 1.4, -4.3 +/- 2.9, and -6.4 +/- 3.7 mg/dl in periods 1-3 in controls (changes are mean values during each of the 50-min periods; P < 0.05). In ExoGlc, arterial glucose did not deviate from basal in periods 1 (+0.1 +/- 1.8 mg/dl) and 3 (+1.5 +/- 4.5 mg/dl) but fell from basal (P < 0.05) by the same amount as controls in period 2 (-5.7 +/- 2.1 mg/dl). Matching the Rd response with exogenous glucose led to increases in arterial and portal vein plasma insulin levels (P < 0.05) but did not affect glucagon, norepinephrine, epinephrine, and cortisol levels. Ra was elevated by 3.1 +/- 0.5, 4.0 +/- 1.1, and 4.7 +/- 1.1 mg.kg-1.min-1 in periods 1-3 in controls (P < 0.05). In ExoGlc, Ra rose by 0.0 +/- 0.4, 4.1 +/- 1.4 (P < 0.05), and 0.4 +/- 0.7 mg.kg-1.min-1, respectively, in periods 1-3. The rise in Ra was reduced in periods 1 and 3 of ExoGlc compared with controls (P < 0.02). GNG rose to approximately 250% basal in controls and did not respond with any significant difference in ExoGlc. In summary, the exercise-induced increases in counterregulatory hormones and GNG are present even when a deficit in glucose supply is eliminated by an exogenous glucose infusion. In contrast, the fall in insulin and the rise in hepatic glycogenolysis are greatly attenuated. The regulatory components affected by exogenous glucose predominate at the liver as deviations in plasma glucose of approximately 4% correspond to approximately 60% changes in Ra.(ABSTRACT TRUNCATED AT 400 WORDS)

Exercise-induced fall in insulin: mechanism of action at the liver and effects on muscle glucose metabolism.

To determine the importance of the fall in insulin on whole body glucose fluxes and muscle glucose metabolism during exercise, dogs ran on a motorized treadmill for 90 min at a moderate work rate with somatostatin (SRIF) infused to suppress insulin and glucagon and basal (B-INS; n = 6 dogs) or exercise-stimulated (S-INS; n = 8 dogs) insulin replacement. The fall in insulin during exercise potently stimulates glucose production at least in part by potentiating the actions of glucagon. To assess the hepatic effects of insulin in the absence of its potentiating effect on glucagon action, glucagon levels were not restored during SRIF infusion. At least 17 days before experimentation, dogs underwent surgery for chronic placement of sampling (carotid artery and femoral vein) and infusion (inferior vena cava and portal vein) catheters. Hindlimb blood flow was assessed by placement of a Doppler flow cuff on the external iliac artery. Whole body glucose production (Ra) and disappearance (Rd) were assessed with [3-3H]glucose, and hindlimb glucose uptake and metabolism were assessed with arterial-venous differences and [U-14C]glucose. Insulin levels were 69 +/- 6 and 61 +/- 7 pM at rest in B-INS and S-INS and 62 +/- 10 and 41 +/- 6 pM at 30 min of exercise. Glucose levels were clamped at euglycemic levels with an exogenous glucose infusion during rest and exercise in both groups. Exercise-induced increases in Ra, Rd, hindlimb glucose uptake, and hindlimb oxidative and nonoxidative glucose metabolism were not affected by maintenance of basil insulin levels during exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat skeletal muscle hexokinase II mRNA and activity are increased by a single bout of acute exercise.

This study addresses the potential role of skeletal muscle hexokinase (HK) II in the regulation of glucose uptake and metabolism in vivo. Male rats undertook a single bout of treadmill exercise and were then killed immediately or after a predetermined recovery period. Three muscles [soleus (Sol), gastrocnemius/plantaris (Gc), and white vastus] were excised, and HK II mRNA, GLUT-4 mRNA, total HK (HK I and HK II) and heat-stable HK (predominantly HK I) activities were assessed. Three hours after the cessation of a single bout of exhaustive exercise, HK II mRNA was significantly increased in all three muscles. Ninety or thirty minutes of exercise, with a 3-h recovery, increased Gc HK II mRNA to the same extent as exhaustive exercise, but 15 min of exercise had no effect. Gc HK II mRNA continued to increase up to 8 h after the cessation of 90 min of exercise but returned to basal by 24 h postexercise. In contrast to HK II mRNA, Gc GLUT-4 mRNA was unchanged at 0, 3, 8, and 24 h after the cessation of 90 min of exercise. Total HK activity was significantly increased in Sol and Gc, 8 and 24 h after the cessation of 90 min of exercise. Heat-stable HK activity was unchanged in all three muscles. The increase in total HK activity, inferred to be an increase of HK II, may be important in the persistence of the postexercise increase in insulin action.

Role of hepatic alpha- and beta-adrenergic receptor stimulation on hepatic glucose production during heavy exercise.

The role of catecholamines in the control of hepatic glucose production was studied during heavy exercise in dogs, using a technique to selectively block hepatic alpha- and beta-adrenergic receptors. Surgery was done > 16 days before the study, at which time catheters were implanted in the carotid artery, portal vein, and hepatic vein for sampling and the portal vein and vena cava for infusions. In addition, flow probes were implanted on the portal vein and hepatic artery. Each study consisted of a 100-min equilibration, a 30-min basal, a 20-min heavy exercise (approximately 85% of maximum heart rate), a 30-min recovery, and a 30-min adrenergic blockade test period. Either saline (control; n = 7) or alpha (phentolamine)- and beta (propranolol)-adrenergic blockers (Blk; n = 6) were infused in the portal vein. In both groups, epinephrine (Epi) and norepinephrine (NE) were infused in the portal vein during the blockade test period to create supraphysiological levels at the liver. Isotope ([3-3H]glucose) dilution and arteriovenous differences were used to assess hepatic function. Arterial Epi, NE, glucagon, and insulin levels were similar during exercise in both groups. Endogenous glucose production (Ra) rose similarly during exercise to 7.9 +/- 1.2 and 7.5 +/- 2.0 mg.kg-1.min-1 in control and Blk groups at time = 20 min. Net hepatic glucose output also rose to a similar rate in control and Blk groups with exercise. During the blockade test period, arterial plasma glucose and Ra rose to 164 +/- 5 mg/dl and 12.0 +/- 1.4 mg.kg-1.min-1, respectively, but were essentially unchanged in Blk. The attenuated response to catecholamine infusion in Blk substantiates the effectiveness of the hepatic adrenergic blockade. In conclusion, these results show that direct hepatic adrenergic stimulation does not participate in the increase in Ra, even during the exaggerated sympathetic response to heavy exercise.

Analysis of insulin-stimulated skeletal muscle glucose uptake in conscious rat using isotopic glucose analogs.

An isotopic method was used in conscious rats to determine the roles of glucose transport and the transsarcolemmal glucose gradient (TSGG) in control of basal and insulin-stimulated muscle glucose uptake. Rats received an intravenous 3-O-[3H]methylglucose (3-O-[3H]MG) infusion from -100 to 40 min and a 2-deoxy-[3H]glucose infusion from 0 to 40 min to calculate a glucose metabolic index (Rg). Insulin was infused from -100 to 40 min at rates of 0.0, 0.6, 1.0, and 4.0 mU.kg-1.min-1, and glucose was clamped at basal concentrations. The ratios of soleus intracellular to extracellular 3-O-[3H]MG concentration and soleus glucose concentrations were used to estimate the TSGG using principles of glucose counter-transport. Tissue glucose concentrations were compared in well-perfused, slow-twitch muscle (soleus) and poorly perfused, fast-twitch muscle (vastus lateralis, gastrocnemius). Data show that 1) small increases in insulin increase soleus Rg without decreasing TSGG, suggesting that muscle glucose delivery and phosphorylation can accommodate the increased flux; 2) due to a limitation in soleus glucose phosphorylation and possibly delivery, insulin at high physiological levels decreases TSGG, and at supraphysiological insulin levels the TSGG is not significantly different from 0; 3) maximum Rg is maintained even though TSGG decreases with increasing insulin levels, indicating that glucose transport continues to increase and is not rate limiting for maximal insulin-stimulated glucose uptake; and 4) muscle consisting of fast-twitch fibers that are poorly perfused exhibits a 35-45% fall in tissue glucose with insulin, suggesting that glucose delivery is a major limitation in sustaining the TSGG. In conclusion, control of glucose uptake is distributed between glucose transport and factors that determine the TSGG. Insulin stimulation of glucose transport increases the demands on the factors that maintain glucose delivery to the muscle membrane and glucose phosphorylation inside the muscle.