The essentiality of insulin and the role of glucagon in regulating glucose utilization and production during strenuous exercise in dogs

Interactions between insulin and exercise

The interaction between insulin and exercise is an example of balancing and modifying the effects of two opposing metabolic regulatory forces under varying conditions. While insulin is secreted after food intake and is the primary hormone increasing glucose storage as glycogen and fatty acid storage as triglycerides, exercise is a condition where fuel stores need to be mobilized and oxidized. Thus, during physical activity the fuel storage effects of insulin need to be suppressed. This is done primarily by inhibiting insulin secretion during exercise as well as activating local and systemic fuel mobilizing processes. In contrast, following exercise there is a need for refilling the fuel depots mobilized during exercise, particularly the glycogen stores in muscle. This process is facilitated by an increase in insulin sensitivity of the muscles previously engaged in physical activity which directs glucose to glycogen resynthesis. In physically trained individuals, insulin sensitivity is also higher than in untrained individuals due to adaptations in the vasculature, skeletal muscle and adipose tissue. In this paper, we review the interactions between insulin and exercise during and after exercise, as well as the effects of regular exercise training on insulin action.

Carbohydrate Intake in the Context of Exercise in People with Type 1 Diabetes

Although the benefits of regular exercise on cardiovascular risk factors are well established for people with type 1 diabetes (T1D), glycemic control remains a challenge during exercise. Carbohydrate consumption to fuel the exercise bout and/or for hypoglycemia prevention is an important cornerstone to maintain performance and avoid hypoglycemia. The main strategies pertinent to carbohydrate supplementation in the context of exercise cover three aspects: the amount of carbohydrates ingested (i.e., quantity in relation to demands to fuel exercise and avoid hypoglycemia), the timing of the intake (before, during and after the exercise, as well as circadian factors), and the quality of the carbohydrates (encompassing differing carbohydrate types, as well as the context within a meal and the associated macronutrients). The aim of this review is to comprehensively summarize the literature on carbohydrate intake in the context of exercise in people with T1D.

Co-ingestion of carbohydrate with leucine-enriched essential amino acids does not augment acute postexercise muscle protein synthesis in a strenuous exercise-induced hypoinsulinemic state

Strenuous exercise following overnight fasting increases fat oxidation during exercise, which can modulate training adaptation. However, such exercise induces muscle protein catabolism by decreasing blood insulin concentrations and increasing amino acid oxidation during the exercise. Leucine-enriched essential amino acids (LEAAs) enhance muscle protein synthesis (MPS) at rest and after exercise. However, it remains to be clarified if the co-ingestion of carbohydrate with LEAAs induces an additional increase in MPS, particularly in a hypoinsulinemic state induced by strenuous exercise. Eight-week-old male Sprague–Dawley rats were made to perform strenuous jump exercise (height 35 cm, 200 jumps, 3-s intervals), after which they ingested distilled water and 1 g/kg LEAAs with or without 1 g/kg of glucose. The fractional synthesis rate was determined by measuring the incorporation of l-[ring-²H₅]-phenylalanine into skeletal muscle protein. Immediately after the exercise, plasma insulin concentration was significantly lower than that at the basal level. Co-ingestion of glucose with LEAAs alleviated the reduction in plasma insulin concentration, while LEAA ingestion alone did not. LEAA administration with or without glucose led to a higher MPS compared with water administration (P < 0.05). However, the co-ingestion of glucose with LEAAs did not induce further increases in MPS compared with LEAA ingestion alone. Thus, the co-ingestion of glucose with LEAAs does not additionally increase MPS under a strenuous exercise–induced hypoinsulinemic state when glucose is co-ingested with a dose of LEAAs that maximally stimulates MPS.

The oral [(13)C]bicarbonate technique for measurement of short-term energy expenditure of sled dogs and their physiological response to diets with different fat:carbohydrate ratios

The oral [¹³C]bicarbonate technique (o¹³CBT) was assessed for the determination of short-term energy expenditure (EE) under field conditions. A total of eight Alaskan huskies were fed two experimental diets in a cross-over experiment including two periods of 3 weeks. Effects of diets on EE, apparent total tract digestibility (ATTD) and on plasma hormones, blood lactate and glucose were furthermore investigated. The percentages of metabolisable energy derived from protein (P), fat (F) and carbohydrates (C) were 26:58:16 in the PFC diet and 24:75:1 in the PF diet. Measurements of EE were performed in the post-absorptive state during rest. Blood samples were collected during rest and exercise and ATTD was determined after days with rest and with exercise. EE was higher (P < 0·01) in period 2 than in period 1 (68 v. 48 kJ/kg body weight^0·75 per h). The ATTD of organic matter, crude protein and crude fat was higher (P < 0·01) in the PF diet compared with the PFC diet, and lower (P < 0·01) for total carbohydrates. Exercise did not affect ATTD. Higher (P < 0·01) insulin-like growth factor 1 and leptin concentrations were measured when fed the PF diet compared with the PFC diet. Concentrations of insulin decreased (P < 0·01), whereas cortisol and ghrelin increased (P < 0·05), after exercise. There was no effect of diet on blood lactate and glucose, but higher (P < 0·001) lactate concentrations were measured in period 1 than in period 2. The results suggest that the o¹³CBT can be used in the field to estimate short-term EE in dogs during resting conditions. Higher ATTD and energy density of the PF diet may be beneficial when energy requirements are high.

Metabolic clearance rate is a more robust and physiological parameter for insulin sensitivity than glucose infusion rate in the isoglycemic glucose clamp technique

BACKGROUND

The metabolic clearance rate (MCR) of glucose has been defined as the value of the glucose infusion rate (GIR) divided by the glucose concentration and could be thus expected to be a robust marker at various glucose concentrations.

METHODS

We evaluated the validity of MCR compared with GIR in 15 healthy subjects and 38 type 2 diabetes patients. The glucose clamp technique was performed at two different glucose levels-isoglycemia (fasting plasma glucose [FPG]) of each subject and euglycemia (100 mg/dL), consecutively. GIR and MCR were obtained at both glucose levels, and ratios of those at isoglycemia to euglycemia were calculated.

RESULTS

Although there was no obvious relationship between FPG levels and GIR ratio, the MCR ratio showed a good linear regression with FPG levels (r=-0.652, P<0.0001). Furthermore, MCR at FPG was excellently (r=0.955) correlated with that at euglycemia in comparison with the modest correlation of GIR between the two plasma glucose levels (r=0.876).

CONCLUSIONS

GIR was rather variable and not proportional to clamped plasma glucose levels. MCR was a less variable parameter than GIR at various plasma glucose levels, and MCR at FPG in the isoglycemic clamp study could be substituted for GIR at the euglycemic clamp study.

Odyssey between Scylla and Charybdis through storms of carbohydrate metabolism and diabetes: a career retrospective

This research perspective allows me to summarize some of my work completed over 50 years, and it is organized in seven sections. 1) The treatment of diabetes concentrates on the liver and/or the periphery. We quantified hormonal and metabolic interactions involved in physiology and the pathogenesis of diabetes by developing tracer methods to separate the effects of diabetes on both. We collaborated in the first tracer clinical studies on insulin resistance, hypertriglyceridemia, and the Cori cycle. 2) Diabetes reflects insulin deficiency and glucagon abundance. Extrapancreatic glucagon changed the prevailing dogma and permitted precise exploration of the roles of insulin and glucagon in physiology and diabetes. 3) We established the critical role of glucagon-insulin interaction and the control of glucose metabolism during moderate exercise and of catecholamines during strenuous exercise. Deficiencies of the release and effects of these hormones were quantified in diabetes. We also revealed how acute and chronic hyperglycemia affects the expression of GLUT2 gene and protein in diabetes. 4) We outlined molecular and physiological mechanisms whereby exercise training and repetitive neurogenic stress can prevent diabetes in ZDF rats. 5) We and others established that the indirect effect of insulin plays an important role in the regulation of glucose production in dogs. We confirmed this effect in humans and demonstrated that in type 2 diabetes it is mainly the indirect effect. 6) We indicated that the muscle and the liver protected against glucose changes. 7) We described molecular mechanisms responsible for increased HPA axis in diabetes and for the diminished responses of HPA axis, catecholamines, and glucagon to hypoglycemia. We proposed a new approach to decrease the threat of hypoglycemia.

Antidiabetic effects of IGFBP2, a leptin-regulated gene

We tested whether leptin can ameliorate diabetes independent of weight loss by defining the lowest dose at which leptin treatment of ob/ob mice reduces plasma glucose and insulin concentration. We found that a leptin dose of 12.5 ng/hr significantly lowers blood glucose and that 25 ng/hr of leptin normalizes plasma glucose and insulin without significantly reducing body weight, establishing that leptin exerts its most potent effects on glucose metabolism. To find possible mediators of this effect, we profiled liver mRNA using microarrays and identified IGF Binding Protein 2 (IGFBP2) as being regulated by leptin with a similarly high potency. Overexpression of IGFBP2 by an adenovirus reversed diabetes in insulin-resistant ob/ob, Ay/a, and diet-induced obese mice, as well as insulin-deficient streptozotocin-treated mice. Hyperinsulinemic clamp studies showed a 3-fold improvement in hepatic insulin sensitivity following IGFBP2 treatment of ob/ob mice. These results show that IGFBP2 can regulate glucose metabolism, a finding with potential implications for the pathogenesis and treatment of diabetes.

Normal insulin response to short-term intense exercise is abolished in Type 2 diabetic patients treated with gliclazide

BACKGROUND

Physical activity is an essential component of diabetes management; however, exercise is associated with the risk for metabolic decompensation. The aim of the study was to analyze insulin response to the short-term intense exercise in middle-aged Type 2 diabetic patients treated with gliclazide.

MATERIALS AND METHODS

Fourteen Type 2 diabetic patients (47.9+/-1.6 years, mean+/-S.E.M.), treated with gliclazide, and 14 healthy controls (45.1+/-1.0 years) were submitted to standard graduated submaximal (90% HR(max)) exercise treadmill testing at 2 h after standardized breakfast. Serum glucose, insulin, proinsulin, C peptide, growth hormone, insulin-like growth factor-1, and cortisol concentrations; and plasma lactate, glucagon, epinephrine, and norepinephrine concentrations were measured during the periexercise period up to the 60th min of the recovery period.

RESULTS

Significant hemodynamic (heart rate, systolic, and diastolic blood pressure), metabolic (lactate concentration), and hormonal (epinephrine and norepinephrine levels) responses to the exercise were similar in patients and healthy subjects. Glucose, insulin, and proinsulin levels were higher in the diabetic group at the preexercise and at all the next analyzed time points. The insulin concentration increased during the postprandial period in both groups and decreased subsequently during the exercise only in the control group, without concurrent C peptide decline. The C peptide-to-insulin ratio increased during the exercise and decreased immediately postexercise only in the control group.

CONCLUSIONS

The initial decrease of the insulin serum concentration during short-term intense exercise in normal middle-aged men is primarily related to the increased clearance of the hormone. Normal insulin response to the exercise was abolished in Type 2 diabetic patients treated with gliclazide.

Is early and late post-meal exercise so different in type 1 diabetic lispro users?

To compare blood glucose (BG) responses during a 60 min moderate intensity exercise session performed in early or late postprandial periods. Nine generally well-controlled (HbA(1c): 7.3+/-0.1%) type 1 diabetic patients performed, at least one week apart, two exercise sessions, 60 (early exercise) and 180 min (late exercise) after a standardized breakfast. All subjects were using Humulin N (N) and Humalog (Lispro, LI) insulin. During exercise, the overall decrease in BG was 4.8+/-0.6 mmol/l and 3.6+/-0.8 mmol/l in early and late exercise, respectively (P=0.051). To prevent hypoglycemia, a dextrose infusion was initiated when BG reached 5 mmol/l. The quantity of dextrose infused was 6.2+/-3.0 g and 10.5+/-3.2g in early and late exercise, respectively (NS). The time free of dextrose infusion during exercise was 41.2+/-7.8 min and 31.7+/-7.5 min in early and late exercise, respectively (NS). In N-LI users, overall drop in BG during exercise tends to be greater in the early postprandial period. However, early and late exercise present similar quantity of dextrose infused and time free of dextrose infusion. Consequently, the similar risk of exercise-induced hypoglycemia suggests similar precautions in either exercise times.

Tracer-determined glucose fluxes in health and type 2 diabetes: basal conditions

The role of increases in basal glucose production (EGP) in the pathogenesis of hyperglycaemia in type 2 diabetes (DM2) has been controversial. It is proposed here that the differences arose from: (i) different patient populations at different stages in the evolution of the disease, (ii) a non-steady state due to diurnal variations in EGP, and measurements at different times of day, and (iii) differences in experimental techniques: tracers, priming strategies and methods of calculation. Methodologically we show that (i) non-steady-state methods and (ii) a one-compartment model with volume of distribution estimated from tracer data are necessary in DM2. Studies with sufficient data demonstrated diurnal variations in EGP, with the highest rates in the morning, normalizing by late afternoon. Metabolic clearance rate of glucose (MCR) remained constant. Long-standing DM2 demonstrated increases in glycaemia and relative decreases in morning EGP, probably feedback-induced. A falling MCR, partly secondary to glucotoxicity, likely induced the rise in baseline hyperglycaemia.

Invited review: effect of acute exercise on insulin signaling and action in humans

After a single bout of exercise, insulin action is increased in the muscles that were active during exercise. The increased insulin action has been shown to involve glucose transport, glycogen synthesis, and glycogen synthase (GS) activation as well as amino acid transport. A major mechanism involved in increased insulin stimulation of glucose uptake after exercise seems to be the exercise-associated decrease in muscle glycogen content. Muscle glycogen content also plays a pivotal role for the activity of GS and for the ability of insulin to increase GS activity. Insulin signaling in human skeletal muscle is activated by physiological insulin concentrations, but the increase in insulin action after exercise does not seem to be related to increased insulin signaling [insulin receptor tyrosine kinase activity, insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation (RS1), IRS-1-associated phosphatidylinositol 3-kinase activity, Akt phosphorylation (Ser(473)), glycogen synthase kinase 3 (GSK3) phosphorylation (Ser(21)), and GSK3alpha activity], as measured in muscle lysates. Furthermore, insulin signaling is also largely unaffected by exercise itself. This, however, does not preclude that exercise influences insulin signaling through changes in the spatial arrangement of the signaling compounds or by affecting unidentified signaling intermediates. Finally, 5'-AMP-activated protein kinase has recently entered the stage as a promising player in explaining at least a part of the mechanism by which exercise enhances insulin action.

Intense exercise has unique effects on both insulin release and its roles in glucoregulation: implications for diabetes

In intense exercise (>80% VO(2max)), unlike at lesser intensities, glucose is the exclusive muscle fuel. It must be mobilized from muscle and liver glycogen in both the fed and fasted states. Therefore, regulation of glucose production (GP) and glucose utilization (GU) have to be different from exercise at <60% VO(2max), in which it is established that the portal glucagon-to-insulin ratio causes the less than or equal to twofold increase in GP. GU is subject to complex regulation by insulin, plasma glucose, alternate substrates, other humoral factors, and muscle factors. At lower intensities, plasma glucose is constant during postabsorptive exercise and declines during postprandial exercise (and often in persons with diabetes). During such exercise, insulin secretion is inhibited by beta-cell alpha-adrenergic receptor activation. In contrast, in intense exercise, GP rises seven- to eightfold and GU rises three- to fourfold; therefore, glycemia increases and plasma insulin decreases minimally, if at all. Indeed, even an increase in insulin during alpha-blockade or during a pancreatic clamp does not prevent this response, nor does pre-exercise hyperinsulinemia due to a prior meal or glucose infusion. At exhaustion, GU initially decreases more than GP, which leads to greater hyperglycemia, requiring a substantial rise in insulin for 40--60 min to restore pre-exercise levels. Absence of this response in type 1 diabetes leads to sustained hyperglycemia, and mimicking it by intravenous infusion restores the normal response. Compelling evidence supports the conclusion that the marked catecholamine responses to intense exercise are responsible for both the GP increment (that occurs even during glucose infusion and postprandially) and the restrained increase of GU. These responses are normal in persons with type 1 diabetes, who often report exercise-induced hyperglycemia, and in whom the clinical challenge is to reproduce the recovery period hyperinsulinemia. Intense exercise in type 2 diabetes requires additional study.

5'-AMP-activated protein kinase phosphorylates IRS-1 on Ser-789 in mouse C2C12 myotubes in response to 5-aminoimidazole-4-carboxamide riboside

Exercise is known to increase insulin sensitivity and is an effective form of treatment for the hyperglycemia observed in type 2 diabetes. Activation of 5'-AMP-activated protein kinase (AMPK) by 5-aminoimidazole-4-carboxamide riboside (AICAR), exercise, or electrically stimulated contraction leads to increased glucose transport in skeletal muscle. Here we report the first evidence of a direct interaction between AMPK and the most upstream component of the insulin-signaling cascade, insulin receptor substrate-1 (IRS-1). We find that AMPK rapidly phosphorylates IRS-1 on Ser-789 in cell-free assays as well as in mouse C2C12 myotubes incubated with AICAR. In the C2C12 myotubes activation of AMPK by AICAR matched the phosphorylation of IRS-1 on Ser-789. This phosphorylation correlates with a 65% increase in insulin-stimulated IRS-1-associated phosphatidylinositol 3-kinase activity in C2C12 myotubes preincubated with AICAR. The binding of phosphatidylinositol 3-kinase to IRS-1 was not affected by AICAR. These results demonstrate the existence of an interaction between AMPK and early insulin signaling that could be of importance to our understanding of the potentiating effects of exercise on insulin signaling.

Hepatic alpha- and beta-adrenergic receptors are not essential for the increase in R(a) during exercise in diabetes

The purpose of this study was to determine the role of direct hepatic adrenergic stimulation in the control of endogenous glucose production (R(a)) during moderate exercise in poorly controlled alloxan-diabetic dogs. Chronically catheterized and instrumented (flow probes on hepatic artery and portal vein) dogs were made diabetic by administration of alloxan. Each study consisted of a 120-min equilibration, 30-min basal, 150-min moderate exercise, 30-min recovery, and 30-min blockade test period. Either vehicle (control; n = 6) or alpha (phentolamine)- and beta (propranolol)-adrenergic blockers (HAB; 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 suprapharmacological levels at the liver. Isotopic ([3-(3)H]glucose, [U-(14)C]alanine) and arteriovenous difference methods were used to assess hepatic function. Arterial plasma glucose was similar in controls (345 +/- 24 mg/dl) and HAB (336 +/- 23 mg/dl) and was unchanged by exercise. Basal arterial insulin was 5 +/- 1 mU/ml in controls and 4 +/- 1 mU/ml in HAB and fell by approximately 50% during exercise in both groups. Basal arterial glucagon was similar in controls (56 +/- 10 pg/ml) and HAB (55 +/- 7 pg/ml) and rose similarly, by approximately 1.4-fold, with exercise in both groups. Despite greater arterial Epi and NE levels in HAB compared with controls during the basal and exercise periods, exercise-induced increases in catecholamines from basal were similar in both groups. Gluconeogenic conversion from alanine and lactate and the intrahepatic efficiency of this process were increased by twofold during exercise in both groups. R(a) rose similarly by 2.9 +/- 0.7 and 2.7 +/- 1.0 mg. kg(-1). min(-1) at time = 150 min during exercise in controls and HAB. During the blockade test period, arterial plasma glucose and R(a) rose to 454 +/- 43 mg/dl and 11.3 mg. kg(-1). min(-1) in controls, respectively, but were essentially unchanged in HAB. The attenuated response to the blockade test in HAB substantiates the effectiveness of the hepatic adrenergic blockade. In conclusion, these results demonstrate that direct hepatic adrenergic stimulation does not play a role in the stimulation of R(a) during exercise in poorly controlled diabetes.

Exercise modulates postreceptor insulin signaling and glucose transport in muscle-specific insulin receptor knockout mice

Physical exercise promotes glucose uptake into skeletal muscle and makes the working muscles more sensitive to insulin. To understand the role of insulin receptor (IR) signaling in these responses, we studied the effects of exercise and insulin on skeletal muscle glucose metabolism and insulin signaling in mice lacking insulin receptors specifically in muscle. Muscle-specific insulin receptor knockout (MIRKO) mice had normal resting 2-deoxy-glucose (2DG) uptake in soleus muscles but had no significant response to insulin. Despite this, MIRKO mice displayed normal exercise-stimulated 2DG uptake and a normal synergistic activation of muscle 2DG uptake with the combination of exercise plus insulin. Glycogen content and glycogen synthase activity in resting muscle were normal in MIRKO mice, and exercise, but not insulin, increased glycogen synthase activity. Insulin, exercise, and the combination of exercise plus insulin did not increase IR tyrosine phosphorylation or phosphatidylinositol 3-kinase activity in MIRKO muscle. In contrast, insulin alone produced a small activation of Akt and glycogen synthase kinase-3 in MIRKO mice, and prior exercise markedly enhanced this insulin effect. In conclusion, normal expression of muscle insulin receptors is not needed for the exercise-mediated increase in glucose uptake and glycogen synthase activity in vivo. The synergistic activation of glucose transport with exercise plus insulin is retained in MIRKO mice, suggesting a phenomenon mediated by nonmuscle cells or by downstream signaling events.

Increased dependence of glucose production on peripheral insulin in diabetic depancreatized dogs

We have recently found that in nondiabetic dogs and humans, suppression of glucose production (GP) is mediated by both peripheral and hepatic effects of insulin. We have also found that both nonesterified fatty acids (NEFA) and glucagon are important determinants of the peripheral effect of insulin on GP. However, in moderately hyperglycemic depancreatized dogs, suppression of GP appeared to be mediated by peripheral but not hepatic insulin. In this latter study, insulin concentrations were in the high postprandial range (approximately 300 pmol/L) and suppression of GP may have been close to maximum. The aim of the present study was to determine whether GP can be regulated by hepatic insulin in depancreatized dogs at low insulin concentrations in the postabsorptive range. Depancreatized dogs were maintained at moderately hyperglycemic levels (approximately 10 mmol/L) by subbasal insulin infusions. In paired experiments, additional low-dose equimolar insulin infusions (0.75 pmol/kg x min) were administered peripherally (PER, n = 6) or portally (POR, n = 6) during glucose clamps. This resulted in a minimal increase in peripheral insulin levels, which was greater in PER versus POR, 29.0 +/- 3.7 versus 11.7 +/- 2.2 pmol/L. Also, we infused insulin peripherally at half this rate (1/2 PER, n = 6) to match the increase in peripheral insulin levels in POR (1/2 PER, 14.6 +/- 2.2) and thus obtain a selective POR versus 1/2 PER difference in hepatic sinusoidal insulin levels. PER suppressed GP more than POR (45.4% +/- 4.0% v 35.3% +/- 6.8%, P < .001), whereas POR did not suppress GP more than 1/2 PER (35.6% +/- 6.3%). Therefore, suppression of GP was proportional to peripheral rather than hepatic sinusoidal insulin levels, as in our previous study at higher insulin concentrations. In conclusion, during glucose clamps in moderately hyperglycemic depancreatized dogs, (1) suppression of GP was dominated by insulin's peripheral effects not only at postprandial but also postabsorptive insulin levels, and (2) we found no evidence for a hepatic effect of insulin in suppressing GP. We hypothesize that this effect is reduced in the depancreatized dog model of diabetes due to hepatic insulin resistance and/or hyperglycemia.

Role of adenosine in regulation of carbohydrate metabolism in contracting muscle

Adenosine production from AMP in the sarcoplasm and interstitial space of muscle is markedly enhanced during contractions. The produced adenosine may act as a 'local hormone' by binding to various types of adenosine receptors present in the membrane of adjacent cells, including skeletal muscle, vascular smooth muscle and neurons. Thus, interstitial adenosine may significantly contribute to regulation of muscle carbohydrate metabolism, both by adjusting metabolism and local blood flow to the energy needs imposed by a given degree of contratile activity on the muscle cell. The studies presented here demonstrate that endogenous adenosine via A1-adenosine receptors is able to directly stimulate insulin-mediated glucose transport in oxidative muscle cells during contractions. In addition, adenosine may further contribute to stimulation of muscle glucose uptake during contractions by increasing blood flow and thereby targetting glucose and insulin delivery to active muscle fibres. Furthermore, our findings demonstrate that adenosine via A1- and A2-receptors may inhibit glycogen breakdown in oxidative muscle tissue which during contractions is simultaneously exposed to insulin and beta-adrenergic stimulation. It is concluded that adenosine importantly contributes to regulation of carbohydrate metabolism in oxidative muscle fibers during contractions.

Prior muscular contraction enhances disposal of glucose analog in the liver and muscle

To noninvasively investigate in vivo glucose disposal in muscle and liver after exercise, 19F magnetic resonance spectroscopy (19F-MRS) was applied using 3-fluoro-3-deoxy-D-glucose (3FDG) as a metabolic probe. After 30 minutes of muscle contraction of rabbit hindlimb by a 1-Hz electrical stimulation, 3FDG 250 mg/kg was injected intravenously and 19F-MRS was performed on the postcontracted hindlimb or the liver. Rabbits subjected to muscle contraction showed 1.5- and 1.7-fold higher peaks for 3FDG signal intensity in the liver and muscle than those not subjected to it. 3FDG was converted to 3-fluoro-3-deoxy-gluconic acid (3FGA) in the muscle and liver, and 3FDG oxidation was not affected by muscle contraction. During intraportal 3FDG infusion for 120 minutes at a dose of 2 mg x kg-1 x min-1 after termination of muscle contraction, the postcontracted rabbits showed a continuous increase in the signal intensity of 3FDG and a 2.1-fold higher total signal intensity of 3FDG than those not subjected to muscle contraction. In conclusion, 19F-MRS allows direct noninvasive observation of 3FDG disposal in rabbit muscle and liver. The increased intensity of 3FDG in the liver after muscle contraction suggests that exercise enhances disposal of the glucose analog in the liver, as well as in muscle, and these effects persist for at least 2 hours after exercise.

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.

Acute stimulation of glucose metabolism in mice by leptin treatment

Leptin is an adipocyte hormone that functions as an afferent signal in a negative feedback loop regulating body weight, and acts by interacting with a receptor in the hypothalamus and other tissues. Leptin treatment has potent effects on lipid metabolism, and leads to a large, specific reduction of adipose tissue mass after several days. Here we show that leptin also acts acutely to increase glucose metabolism, although studies of leptin's effect on glucose metabolism have typically been confounded by the weight-reducing actions of leptin treatment, which by itself could affect glucose homoeostasis. We have demonstrated acute in vivo effects of intravenous and intracerebroventricular administrations of leptin on glucose metabolism. A five-hour intravenous infusion of leptin into wild-type mice increased glucose turnover and glucose uptake, but decreased hepatic glycogen content. The plasma levels of insulin and glucose did not change. Similar effects were observed after both intravenous and intracerebroventricular infusion of leptin, suggesting that effects of leptin on glucose metabolism are mediated by the central nervous system (CNS). These data indicate that leptin induces a complex metabolic response with effects on glucose as well as lipid metabolism. This response is unique to leptin, which suggests that new efferent signals emanate from the CNS after leptin treatment.

Influence of moderate physical exercise on insulin-mediated and non-insulin-mediated glucose uptake in healthy subjects

To establish the relative importance of insulin sensitivity and glucose effectiveness during exercise using Bergman's minimal model, 12 nontrained healthy subjects were studied at rest and during 95 minutes of moderate exercise (50% maximum oxygen consumption [VO2max]). Each subject underwent two frequently sampled intravenous glucose tolerance tests (FSIGTs) for 90 minutes, at rest (FSIGTr) and during exercise (FSIGTe). Plasma glucose, insulin, and C-peptide were determined. Insulin sensitivity (S(I)), glucose effectiveness at basal insulin (S(G)), insulin action [X(t)], and first-phase (phi1) and second-phase (phi2) beta-cell responsiveness to glucose were estimated using both minimal models of glucose disposal (MMg) and insulin kinetics (MMi). Glucose effectiveness at zero insulin (GEZI), glucose tolerance index (K(G)), and the area under the insulin curve (AUC(0-90)) were also calculated. Intravenous glucose tolerance improved significantly during physical exercise. During exercise, S(I) (FSIGTr v FSIGTe: 8.5 +/- 1.0 v 25.5 +/- 7.2 x 10(-5) x min(-1) [pmol x L(-1)]-1, P < .01), S(G) (0.195 +/- 0.03 v 0.283 +/- 0.03 x 10(-1) x min(-1), P < .05), and GEZI (0.190 +/- 0.03 v 0.269 +/- 0.04 x 10(-1) x min(-1), P < .05) increased; however, no changes in phi1 and phi2 were found. Despite a significant decrease in the insulin response to glucose (AUC0-90, 21,000 +/- 2,008 v 14,340 +/- 2,596 pmol x L(-1) x min, P < .01), insulin action [X(t)] was significantly higher during the FSIGTe. These results show that physical exercise improves mainly insulin sensitivity, and to a lesser degree, glucose effectiveness. During exercise, the insulin response to glucose was lower than at rest, but beta-cell responsiveness to glucose did not change.

Gestational diabetes and exercise: a survey

Exercise has long been accepted as an adjunctive nonmedical intervention in the management of diabetes in nonpregnant subjects. It is universally accepted that pregnancy is a diabetogenic event which could develop into gestational diabetes mellitus (GDM) in up to 12% of pregnant women. GDM, a carbohydrate intolerance of variable severity with onset or first recognition during pregnancy, involves a relative resistance to insulin. Exercise becomes thus a logical intervention, only recently offered as an adjunctive therapy to pregnant diabetics. This article reviews our current understanding of the role of exercise in the management of GDM.

A moderate decline in specific activity does not lead to an underestimation of hepatic glucose production during a glucose clamp

We have previously shown that modeling errors lead to underestimation of hepatic glucose production (HGP) during glucose clamps when specific activity (SA) declines markedly. We wished to assess whether the failure to keep SA constant substantially affects calculation of HGP during insulin infusion when glucose requirements to maintain the glucose clamp are moderate. Therefore, 150-minute hyperinsulinemic (5.4 pmol - kg (-1) - min (-1) clamps were performed in depancreatized dogs that were maintained hyperglycemic (approximately 10 mmol/L with either (l) unlabeled glucose infusate (COLD Ginf, n = 5) or (2) labeled glucose infusate (HOT Ginf, n = 6) containing high-performance liquid chromatography (HPLC purified [6-3H]glucose. Insulinemia and glucagonemia were similar between the two groups. Additionally, glucose infusion rates were equivalent with COLD and HOT Ginf, indicating comparable insulin effects on overall glucose metabolism. The SA decreased a maximum of 32% with COLD Ginf, but remained constant with HOT Ginf. HGP was suppressed equally with COLD or HOT Ginf treatments at each time point during the clamp (mean suppression during last hour of clamp, 69% +/- 4% and 69% +/- 5%, P = NS, COLD and HOT Ginf, respectively). We conclude that when glucose requirements are moderate and SA changes slowly, as in the diabetic dog, it is not necessary to keep SA perfectly constant to avoid significant modeling errors when calculating HPG during hyperinsulinemic clamps.

Exercise-induced hypoglycaemia in IDDM patients treated with a short-acting insulin analogue

In order to examine the effect of short-acting insulin analogue on the exercise-induced hypoglycaemia in insulin-dependent diabetes mellitus (IDDM) patients we compared the glycaemic response of 40 min cycle ergometer exercise performed either shortly (40 min) or later (180 min) after a breakfast meal and subcutaneous injection of either short-acting insulin analogue [Lys(B28) Pro(B29)] or soluble human insulin (Humulin Regular) in ten IDDM patients with long duration of the disease. Both preparations had been used 1 month before respective studies. Changes in blood glucose, insulin and counterregulatory hormones were assayed. As compared to human insulin, after the analogue injection the peak insulin concentration came earlier, was 56% higher (p < 0.05) and disappeared faster, and the postprandial blood glucose response was lower (p < 0.05). In the analogue-treated patients the exercise-induced hypoglycaemia was 2.2-fold greater (p < 0.01) during the early exercise, but 46% less (p < 0.05) during late exercise as compared to the treatment with human insulin. Serum insulin or analogue concentration at the beginning of the exercise correlated closely with the fall in blood glucose during exercise (r = 0.74, p < 0.01; r = 0.73, p < 0.02, respectively). In the analogue-treated patients, fasting serum glucagon and adrenalin concentrations were higher than during human insulin therapy (p < 0.05) and remained so throughout the study.(ABSTRACT TRUNCATED AT 250 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)

Adenosine receptors mediate synergistic stimulation of glucose uptake and transport by insulin and by contractions in rat skeletal muscle

The role of adenosine receptors in the regulation of muscle glucose uptake by insulin and contractions was studied in isolated rat hindquarters that were perfused with a standard medium containing no insulin or a submaximal concentration of 100 microU/ml. Adenosine receptor antagonism was induced by caffeine or 8-cyclopentyl-1,3-dipropylxantine (CPDPX). Glucose uptake and transport were measured before and during 30 min of electrically induced muscle contractions. Caffeine nor CPDPX affected glucose uptake in resting hindquarters. In contrast, the contraction-induced increase in muscle glucose uptake was inhibited by 30-50% by caffeine, as well as by CPDPX, resulting in a 20-25% decrease in the absolute rate of glucose uptake during contractions, compared with control values. This inhibition was independent of the rate of perfusate flow and only occurred in hindquarters perfused with insulin added to the medium. Thus, adenosine receptor antagonism inhibited glucose uptake during simultaneous exposure to insulin and contractions only. Accordingly, caffeine inhibited 3-O-methylglucose uptake during contractions only in oxidative muscle fibers that are characterized by a high sensitivity to insulin. In conclusion, the present data demonstrate A1 receptors to regulate insulin-mediated glucose transport in contracting skeletal muscle. The findings provide evidence that stimulation of sarcolemmic adenosine receptors during contractions is involved in the synergistic stimulation of muscle glucose transport by insulin and by contractions.

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.

Effects of subbasal insulin infusion on resting and exercise-induced glucose turnover in depancreatized dogs

beta-Adrenergic blockade suppressed lipolysis and normalized the exercise-induced increments in glucose uptake (GlcU) and metabolic clearance rate (MCR) in alloxan-diabetic dogs with residual insulin, but not in insulin-deprived depancreatized dogs even when combined with methylpalmoxirate (MP), which suppresses fatty acid oxidation. The effects of a minimal amount of insulin (as in the alloxan-diabetic dog), were studied in depancreatized, 24-h insulin-deprived dogs during rest and treadmill exercise (6 km/h, 10% slope) using a 1/4 basal insulin infusion (50 microU.kg-1.min-1, insulin, n = 6) alone, or with MP (20 mg.kg-1.day orally, 2.5 days, MP+insulin, n = 6). At rest, insulin decreased circulating fatty acids (31%) and Glc (13%) and increased GlcU and MCR (86 and 72%). Glc production was unaffected. MP plus insulin markedly suppressed hepatic fatty acid oxidation, decreased Glc (44%) and Glc production (50%), and markedly increased MCR (128%). The exercise-induced increments in MCR were markedly improved only by MP plus insulin but were still lower than in the propranolol-treated alloxan-diabetic dogs. Plasma Glc inversely correlated with the exercise-induced increase in MCR (r = -0.86). We conclude that 1) acute infusion of subbasal insulin improved GlcU in depancreatized dogs at rest but not during exercise; 2) inhibition of fatty acid oxidation combined with subbasal insulin improved the exercise-induced increase in MCR; and 3) the difference in GlcU and MCR between the MP plus insulin-treated depancreatized dogs and the beta-blockade-treated alloxan-diabetic dogs suggests a difference between acute and chronic effects of insulin.

Indirect effects of insulin in regulating glucose fluxes

Metabolism of fuels is driven by the energy demand of the organism and its regulation is influenced by many hormonal and metabolic factors. Insulin is of utmost importance in regulating glucose metabolism by promoting glucose uptake in the insulin-sensitive tissues for energy consumption and/or storage. The effects of insulin on glucose metabolism can be both direct and indirect. Ample evidence has indicated that insulin directly stimulates glucose transport systems in the target tissues. However, the changes in glucose fluxes can also be brought out by indirect effects of insulin which are produced secondary to the insulin-induced changes in other hormones and metabolites. In this chapter, we discussed a number of examples of insulin's indirect effects on glucose metabolism. We demonstrated that insulin can indirectly promote muscle glucose uptake during exercise by restraining the release and oxidation of fatty acids and decrease of hyperglycemia. We have presented some evidence for an indirect regulation of glucose cycling by insulin. We have also demonstrated the importance of the peripheral levels of insulin for insulin-induced inhibition of hepatic glucose production. This presumably indirect effects of peripheral insulin might consist of 1) suppression of the release of energy substrates and gluconeogenic precursors; and 2) suppression of glucagon secretion. In a carbachol-induced stress model, insulin is not required for a putatively neural regulation of an increase in systemic glucose uptake but a "permissive" effect of insulin is essential. These studies underscore the importance of the interactions between insulin and other hormones and metabolites as opposed to insulin's direct actions per se.

Importance of peripheral insulin levels for insulin-induced suppression of glucose production in depancreatized dogs

It is generally believed that glucose production (GP) cannot be adequately suppressed in insulin-treated diabetes because the portal-peripheral insulin gradient is absent. To determine whether suppression of GP in diabetes depends on portal insulin levels, we performed 3-h glucose and specific activity clamps in moderately hyperglycemic (10 mM) depancreatized dogs, using three protocols: (a) 54 pmol.kg-1 bolus + 5.4 pmol.kg-1.min-1 portal insulin infusion (n = 7; peripheral insulin = 170 +/- 51 pM); (b) an equimolar peripheral infusion (n = 7; peripheral insulin = 294 +/- 28 pM, P < 0.001); and (c) a half-dose peripheral infusion (n = 7), which gave comparable (157 +/- 13 pM) insulinemia to that seen in protocol 1. Glucose production, use (GU) and cycling (GC) were measured using HPLC-purified 6-[3H]- and 2-[3H]glucose. Consistent with the higher peripheral insulinemia, peripheral infusion was more effective than equimolar portal infusion in increasing GU. Unexpectedly, it was also more potent in suppressing GP (73 +/- 7 vs. 55 +/- 7% suppression between 120 and 180 min, P < 0.001). At matched peripheral insulinemia (protocols 2 and 3), not only stimulation of GU, but also suppression of GP was the same (55 +/- 7 vs. 63 +/- 4%). In the diabetic dogs at 10 mM glucose, GC was threefold higher than normal but failed to decrease with insulin infusion by either route. Glycerol, alanine, FFA, and glucagon levels decreased proportionally to peripheral insulinemia. However, the decrease in glucagon was not significantly greater in protocol 2 than in 1 or 3. When we combined all protocols, we found a correlation between the decrements in glycerol and FFAs and the decrease in GP (r = 0.6, P < 0.01). In conclusion, when suprabasal insulin levels in the physiological postprandial range are provided to moderately hyperglycemic depancreatized dogs, suppression of GP appears to be more dependent on peripheral than portal insulin concentrations and may be mainly mediated by limitation of the flow of precursors and energy substrates for gluconeogenesis and by the suppressive effect of insulin on glucagon secretion. These results suggest that a portal-peripheral insulin gradient might not be necessary to effectively suppress postprandial GP in insulin-treated diabetics.

Role of FFA-glucose cycle in glucoregulation during exercise in total absence of insulin

Muscle contraction in vitro increases glucose uptake (GU), independent of insulin, but in vivo, the exercise-induced increase in GU is impaired in insulin-deficient diabetic dogs. We wished to determine whether, in vivo, suppression of the free fatty acid (FFA)-glucose cycle with methylpalmoxirate (MP, inhibitor of FFA oxidation) alone or combined with propranolol (PRO, beta-blocker) could improve GU during exercise in the absence of insulin. We performed four groups of exercise experiments (6 km/h, 10% slope) in depancreatized insulin-deprived dogs: 1) control (n = 6); 2) MP treated (5 oral doses of 10 mg/kg, twice daily, n = 6); 3) treated with MP+octanoate (OCT; oxidation unaffected by MP, 27 mumol.kg-1.min-1 iv during exercise; n = 5); and 4) MP+PRO treated (5 micrograms.kg-1.min-1 iv during exercise, n = 6). MP abolished ketosis (inhibition of hepatic FFA oxidation), decreased basal glucose production (GP), and increased metabolic clearance of glucose (MCR). During exercise, MP attenuated the increment in GP (P < 0.01), which was reversed by OCT. MP did not affect the exercise-induced increase in GU and MCR. With MP+PRO, FFAs decreased and lactate did not rise during exercise. GP was not further suppressed, but GU and MCR were increased (P < 0.01) to 89 and 31% of normal, respectively. In insulin-deprived depancreatized dogs, glucose cycling was increased to a greater extent than GP, as in type II diabetes. By the end of exercise, glucose cycling increased (P < 0.05), but to a similar extent as GP.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucoregulation during rest and exercise in depancreatized dogs: role of the acute presence of insulin

To determine the effects of the presence of insulin in poorly controlled diabetes, depancreatized (PX) dogs (n = 5) were studied during rest and 150 min of exercise in paired experiments in which saline alone was infused (IDEF) and in which insulin was replaced intraportally (200 microU.kg-1.min-1) with glucose clamped at the levels in IDEF (IR+G). PX dogs (n = 4) were also studied with insulin, but glucose was allowed to fall (IR). Insulin was not detectable, 6 +/- 1 and 6 +/- 2 microU/ml in IDEF, IR+G, and IR. Plasma glucose was 470 +/- 47, 480 +/- 48, and 372 +/- 35 mg/dl at rest in IDEF, IR+G, and IR, respectively. Levels were unchanged with exercise in IDEF and IR+G, but fell by 139 +/- 13 mg/dl in IR. Basal glucose rate of appearance (Ra) was 7.0 +/- 0.9, 1.3 +/- 1.1, and 6.0 +/- 0.7 mg.kg-1.min-1 in IDEF, IR+G, and IR, respectively. Exercise elicited a rise in Ra in only IDEF. The rises in Rd and metabolic clearance rate in IDEF were reduced (delta 2.6 +/- 0.7 and delta 0.8 +/- 0.3 ml.kg-1.min-1 at 150 min) compared with IR+G (delta 5.3 +/- 1.9 and delta 1.7 +/- 0.2 ml.kg-1.min-1 at 150 min) and IR (delta 3.7 +/- 1.2 and delta 2.4 +/- 0.8 ml.kg-1.min-1). The insulin sensitivity of glucose utilization (Rd) was elevated by approximately 75% at 150 min. Basal glycerol was similar in IDEF and IR but was reduced by approximately 70% in IR+G. Glycerol rose similarly with exercise in IDEF and IR.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic glucagon sensitivity and fasting glucose concentration in normal dogs

We assessed hepatic glucagon sensitivity in overnight-fasted, conscious dogs. Six pancreatic replacement protocols were performed in each of five animals. Somatostatin was infused to inhibit endogenous insulin and glucagon, insulin was replaced intraportally at 200 microU.min-1.kg-1, and glucagon was infused intraportally at 0, 0.6, 1, 2, 5, or 20 ng.min-1.kg-1. One intravenous glucose tolerance test was also performed in each animal for measurement of insulin sensitivity (SI). During hormone replacement at a given glucagon dose, plasma glucose differed substantially among animals (P = 0.003). Therefore the dose required for restoration of euglycemia ("glucagon requirement") varied nearly sevenfold among animals, suggesting appreciable differences in glucagon sensitivity (GS). The latter was quantitated in individual animals as the initial slope of integrated glucose output vs. glucagon concentration. GS varied from 0.22 to 3.9 mg.kg-1.pg-1.ml among various animals and was inversely and significantly related to glucagon requirement. SI varied less (approximately 4-fold) and was not associated with glucagon requirement. These observations suggested that interanimal differences in glucose during hormone replacement were the result of substantial differences in GS. In addition, we found the GS of a given animal to be highly associated (P = 0.01) with its fasting glucose level. We conclude that GS varies substantially, and as such may be an important determinant of the fasting glucose level in normal animals.

Hepatic fuel metabolism during muscular work: role and regulation

The increased fuel demands of the working muscle necessitate that metabolic processes within the liver be accelerated accordingly. The sum of changes in hepatic glycogenolysis and gluconeogenesis are closely coupled to the increase in glucose uptake by the working muscle, due to the actions of the pancreatic hormones. The exercise-induced rise in glucagon and fall in insulin interact to stimulate hepatic glycogenolysis, whereas the increase in gluconeogenesis is determined primarily by glucagon action. The increment in gluconeogenesis is caused by increases in hepatic gluconeogenic precursor delivery and fractional extraction as well as in the efficiency of intrahepatic conversion to glucose. Glucagon stimulates the latter two processes. Epinephrine may become important in the regulation of hepatic glucose production during prolonged or heavy exercise when its levels are particularly high. On the other hand, there is no evidence that hepatic innervation is essential for the rise in hepatic glucose production during exercise. Nonesterified fatty acid (NEFA) delivery to, uptake of, and oxidation by the liver are accelerated during prolonged exercise, resulting in an increase in ketogenesis. The rate of the first two of these processes is largely determined by factors that stimulate fat mobilization. The third step is regulated by both NEFA delivery to and glucagon-stimulated fat oxidation within the liver. The increase in hepatic fat oxidation produces energy that fuels gluconeogenesis. The shuttling of amino acids to the liver provides carbon-based compounds that are used for gluconeogenesis, transfers nitrogen to the liver, and supplies substrate for protein synthesis. During exercise, metabolic events within the liver, which are regulated by hormone levels and substrate supply, integrate pathways of carbohydrate, fat, and amino acid metabolism. These processes function to provide substrates for muscular energy metabolism and conserve carbon in glucose and nitrogen in protein.

Mechanism of glucoregulatory responses to stress and their deficiency in diabetes

During exercise, increased energy demands are met by increased glucose production that occurs simultaneously with the increased glucose uptake. We had previously observed that, during exercise, metabolic clearance rate of glucose (MCR) increases markedly in normal, but only marginally in poorly controlled diabetic dogs. We wished to determine (i) whether in a more general model of stress matched increases in rate of appearance of glucose and MCR also occur, or if MCR is suppressed, as during catecholamine infusion; and (ii) whether diabetes affects stress-induced changes in rate of glucose appearance and MCR. Therefore, we injected carbachol (27 nmol/50 microliters), an analog of acetylcholine, intracerebroventricularly in seven conscious dogs before and after induction of alloxan diabetes. In normal dogs, plasma epinephrine and cortisol increased 4- to 5-fold, whereas norepinephrine and glucagon doubled. Plasma insulin, however, remained unchanged. Tracer-determined hepatic glucose production increased rapidly, but transiently, by 2.5-fold. This increment can be fully explained by the observed increments in the counterregulatory hormones. Surprisingly, however, MCR also promptly increased, and therefore, plasma glucose changed only marginally. After induction of diabetes, the animals were given intracerebroventricular carbachol while plasma glucose was maintained at moderate hyperglycemia (9.0 +/- 0.4 mM). Increments in counterregulatory hormones were similar to those seen in normal dogs, except for exaggerated norepinephrine release. Peripheral insulin levels were higher in diabetic than in normal dogs; however, MCR was markedly reduced and the lipolytic response to stress increased, indicating insulin resistance. Interestingly, the hyperglycemic response to stress was 6-fold greater in diabetic than normal animals, relating mainly to the failure of MCR to rise. Plasma lactate increased equivalently in diabetic and normal animals despite suppression of MCR in the diabetics, indicating either greater muscle glycogenolysis and/or impairment in glucose oxidation. We conclude that in this stress model MCR increases as in exercise in normal but not in diabetic dogs. We speculate that glucose uptake in stress could be mediated through an insulin-dependent neural mechanism.

Effect of stress on glucoregulation in physiology and diabetes

To examine the glucoregulatory responses to stress and their impact on diabetes, we used the following models of stress: A) Hypoglycemia; B) Epinephrine infusion; C) intracerebroventricular (ICV) injection of carbachol, an analog of acetylcholine. A) Hypoglycemia induces release of all counterregulatory hormones. During acute hypoglycemia, glucose production increases initially mainly due to glucagon release but eventually also due to a very large increment in catecholamines. In newborn dogs, neither epinephrine nor glucagon respond to a decrease in plasma glucose. This lack of a safeguard against hypoglycemia may indicate that the brain in pups is less dependent on a normal supply of glucose as a fuel, than in adult dogs. Counterregulation is enhanced when the effects of endogenous opiates are blocked by naloxone, indicating that endogenous opiates play a regulatory role during hypoglycemia. However, beta-endorphins which can be released with epinephrine during various stress situations, potentiate the peripheral effect of epinephrine. Glucoregulatory responses, even to slight changes in plasma glucose, are greatly enhanced during glucocorticoid treatment. This apparently reflects the greater sensitivity of the liver to glucagon. In diabetic dogs, similar to human diabetics, the glucagon response is abolished and the response of the catecholamines is partially decreased. On the basis of histological studies, we proposed that the deficient glucagon response in diabetes could be related to an increase in the somatostatin-glucagon ratio in the diabetic pancreas. This ratio is further augmented when normoglycemia is maintained with insulin. In response to a decrease in plasma glucose, there is a biphasic increment in glucose production in normal dogs, which is missing in diabetes. When normoglycemia is restored in diabetic dogs with phlorizin treatment, the second but not the first increment in glucose production is restored. We postulated, therefore, that the toxic effect of hyperglycemia, in addition to the lack of glucagon response, is the main reason why in diabetes, glucose production cannot respond promptly to a decrease in plasma glucose. The low rate of metabolic clearance of glucose seen in diabetes in the post-absorptive state, also reflects, at least in part, the toxic effect of glucose, because with acute normalization of glucose with phlorizin, metabolic glucose clearance substantially improves. Hyperglycemia is the main reason for the decreased number of glucose transporters in diabetic muscle. B) Epinephrine infusion in normal dogs mimics some effects of stress, in that it increases glucose production, inhibits metabolic glucose clearance and increases lipolysis. These metabolic effects of epinephrine are independent of glucagon release.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaction of exercise and insulin action in humans

To assess the interaction of exercise and insulin action, healthy males were studied with saline infusion (n = 5) or with a hyperinsulinemic euglycemic clamp (0.5, 1.0, 2.0, or 15.0 mU.kg-1.min-1; n = 5 at each dose) during rest (40 min), moderate-intensity cycle exercise (100 min), and recovery (100 min). Metabolism was assessed using isotopic methods and indirect calorimetry. During rest, exercise, and recovery with saline infusion, plasma glucose was unchanged, total glucose utilization (Rd) was 2.4 +/- 0.4, 4.9 +/- 0.2, and 2.6 +/- 0.2 mg.kg-1.min-1, and carbohydrate (CHO) oxidation (OX) was 1.4 +/- 0.3, 10.6 +/- 1.1, and 0.5 +/- 0.2 mg.kg-1.min-1. The glucose infusion, insulin-dependent Rd, and CHO OX increased synergistically when exercise and insulin clamps were combined. Exercise decreased (P less than 0.05) the half-maximal doses (ED50) and increased the maximal responses (Vmax) for insulin-dependent Rd and CHO OX. Estimates of insulin-independent Rd were 1.3 +/- 0.7, 4.1 +/- 1.3, and 1.9 +/- 0.7 mg.kg-1.min-1 and insulin-independent CHO OX were 1.2 +/- 0.9, 10.4 +/- 1.3, and 0.6 +/- 0.3 mg.kg-1.min-1 during rest, exercise, and recovery. Estimates during exercise were greater than those at rest (P less than 0.05). The total suppression of free fatty acids (FFA) and fat OX by insulin were elevated by exercise (P less than 0.05). In summary, exercise and insulin interact synergistically in stimulating Rd and CHO OX.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic hyperinsulinemia decreases insulin action but not insulin sensitivity

Hyperinsulinemia and insulin resistance are commonly seen in obese and non-insulin-dependent diabetes mellitus (NIDDM) patients, suggesting a causal link exists between hyperinsulinemia and insulin resistance. In a previous study, we demonstrated that chronic (28 days) intraportal hyperinsulinemia (50% increase in basal insulin levels) resulted in a decrease in insulin action as assessed by a one-step euglycemic hyperinsulinemic clamp. Since only one dose of insulin was used during the clamp, it was not possible to determine if the decrease in insulin action was due to a change in insulin sensitivity and/or maximal insulin responsiveness. In the present study, insulin resistance was induced as before, but insulin action was assessed in overnight fasted conscious dogs using a four-step euglycemic hyperinsulinemic clamp (1, 2, 10, and 15 mU/kg/min). Insulin responsiveness was assessed before the induction of chronic hyperinsulinemia (day 0), and after 28 days of hyperinsulinemia (day 28). Transhepatic glucose balance and whole-body glucose utilization were measured to allow assessment of both the hepatic and peripheral effects of insulin. Chronic hyperinsulinemia increased basal insulin levels from 13 +/- 2 to 21 +/- 4 microU/mL. After 4 weeks of chronic hyperinsulinemia, maximal insulin-stimulated glucose utilization was decreased 23% +/- 4% (P less than .05) and insulin sensitivity (ED50) was not significantly altered. During the four-step clamp, the liver was a major site of glucose utilization. The liver was responsible for 13% of the total glucose disposal rate on day 0 (2.9 mg/kg/min) at the highest insulin infusion rate (15 mU/kg/min; 2,000 microU/mL).(ABSTRACT TRUNCATED AT 250 WORDS)

Contractile activity increases plasma membrane glucose transporters in absence of insulin

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.

Physiological bases for the treatment of the physically active individual with diabetes

Substrate utilisation and glucose homoeostasis during exercise is controlled by the effects of precise changes in insulin, glucagon and the catecholamines. The important role these hormones play is clearly seen in people with diabetes, as the normal endocrine response is often lost. In individuals with insulin-dependent diabetes (IDDM), there can be an increased risk of hypoglycaemia during or after exercise or, conversely, there can be a worsening of the diabetic state if insulin deficiency is present. In contrast, it appears that people with non-insulin-dependent diabetes (NIDDM) can generally exercise without fear of a deleterious metabolic response. The exercise response both in healthy subjects and in those with diabetes is dependent on many factors such as age, nutritional status and the duration and intensity of exercise. Since there are so many variables which govern individual response to exercise, an exact exercise prescription for all people with diabetes cannot be made. There are many adjustments to the therapeutic regimen which an individual with IDDM can make in order to avoid hypoglycaemia during or after exercise. In general, a reduction in insulin dosage and the added ingestion and continual availability of carbohydrates are wise precautions. On the other hand, exercise should be postponed if blood glucose is greater than 2500 mg/L and ketones are present in the urine. As more is understood about the regulation of substrate metabolism during exercise, more refined therapeutic strategies can be defined. An understanding of the metabolic response to exercise is critical for generating an effective and safe training programme for all diabetic individuals who wish to be physically active.

Usefulness of anaerobic threshold in estimating intensity of exercise for diabetics

We examined the utility of the anaerobic threshold (AT) for quantifying the intensity of exercise that a diabetic patient is capable of handling. Thirteen diabetic patients treated with buformin exercised on a bicycle ergometer, and comparison was made with 20 healthy subjects matched for age and sex. The AT was determined from VO2 and VE with a personal computer. The intensity of exercise at the AT was 93 +/- 6 W in diabetic men and 80 +/- 10 W in diabetic women, values that were less than those of healthy subjects (P less than 0.05). There was a negative correlation between the intensity of exercise at the AT and the plasma concentration of buformin (P less than 0.01). There were no significant differences in either plasma lactic acid or pyruvic acid concentration at the AT between healthy subjects and diabetics. The plasma glucose at the AT or after exercise was lower than the baseline values in all subjects (P less than 0.01). The plasma insulin at the AT was lower than the baseline values in healthy subjects (P less than 0.01), but not in diabetics. There were no changes in plasma glucagon in any group. We concluded that determination of the AT is a simple, non-invasive procedure useful for ascertaining the optimal intensity of exercise for diabetics.

Sport and the diabetic child

The triad of insulin, diet and exercise has been the basis for treatment of diabetes for several decades. However, the choice of sporting activities for young diabetics requires an understanding of: (a) the energy metabolism and the adaptation to physical activity in the healthy; (b) the metabolic adaptation during physical activity in the diabetic child; and (c) the practical recommendations concerning diet and insulin that have to be learned by the children themselves. The healthy child utilises immediately available substrates, such as ATP and creatine phosphate in much the same fashion as the adult. However, the capacity for anaerobic degradation of glycogen and glucose seems limited in the muscles of children relative to that of adults. Consequently, the adaptation to resistance exercise should be undertaken with prudence in children and adolescents. In diabetic children, an adequate insulin therapy is required to allow the full benefit of muscular activity on glucose assimilation and to reach the same level of physical performance as the non-diabetic. In the case of insufficient metabolic control, exercise can provoke severe hypoglycaemic episodes, even after muscle activity has ceased, or increase glucose levels and lead to ketoacidosis. Regular physical training induces a reduction in postexercise proteinuria measured in diabetic adolescents but its role in metabolic control remains controversial. If a diabetic child or adolescent follows individual recommendations concerning diet and insulin, he or she can perform physical activity much the same as a young non-diabetic. These recommendations include: (a) self-measurement of blood glucose concentration before and after exercise; (b) ingestion of carbohydrates before, during, and after exercise; (c) reduction of the insulin dose during and immediately after exercise; and (d) not choosing an injection site involved with muscular work. The only prohibited sports are those which constitute a danger to the diabetic child by provoking an eventual hypoglycaemia. The best sports are those that require progressive physical effort and that are spread out over several hours.

Phlorizin-induced normoglycemia partially restores glucoregulation in diabetic dogs

The plasma concentration of glucagon (IRG), catecholamines, and hepatic glucose production (Ra) were followed in insulin-induced hypoglycemia in dogs before (normal) and at 14-21 and again at 89-119 days after the injection of alloxan (diabetic). Some diabetic dogs were also tested when euglycemia was restored by phlorizin. In the normal state plasma IRG and epinephrine were raised by a factor of 3 and 15, respectively. Ra increased in two phases, an early peak (350% basal) was followed by a plataeu at about twice basal. In diabetes, irrespective of its duration, plasma IRG was decreased in hypoglycemia, and the rise in plasma epinephrine was significantly reduced. Ra remained unchanged. In phlorizin-treated euglycemic diabetic dogs plasma IRG fell, and the response in plasma epinephrine remained blunted. There was no early rise in Ra, but the same elevated plateau was reached at the same time as in normal animals. In conclusion, the following is observed in diabetic dogs. 1) The sensitivity of alpha-cells to insulin is maintained, but that to hypoglycemia is lost. The concentration of plasma catecholamines is raised less than in normals. With no increase in plasma glucagon this rise is not sufficient to increase Ra. 2) Restoration of euglycemia with phlorizin does not restore normal IRG and epinephrine responses to hypoglycemia but restores the delayed increase of Ra. Thus the restoration of euglycemia in severely diabetic dogs partially restores the responses of the liver, but not of the alpha-cell or sympathetic discharge, to hypoglycemia.