Interaction of exercise and insulin action in humans

New model of glucose-insulin regulation characterizes effects of physical activity and facilitates personalized treatment evaluation in children and adults with type 1 diabetes

Accurate treatment adjustment to physical activity (PA) remains a challenging problem in type 1 diabetes (T1D) management. Exercise-driven effects on glucose metabolism depend strongly on duration and intensity of the activity, and are highly variable between patients. In-silico evaluation can support the development of improved treatment strategies, and can facilitate personalized treatment optimization. This requires models of the glucose-insulin system that capture relevant exercise-related processes. We developed a model of glucose-insulin regulation that describes changes in glucose metabolism for aerobic moderate- to high-intensity PA of short and prolonged duration. In particular, we incorporated the insulin-independent increase in glucose uptake and production, including glycogen depletion, and the prolonged rise in insulin sensitivity. The model further includes meal absorption and insulin kinetics, allowing simulation of everyday scenarios. The model accurately predicts glucose dynamics for varying PA scenarios in a range of independent validation data sets, and full-day simulations with PA of different timing, duration and intensity agree with clinical observations. We personalized the model on data from a multi-day free-living study of children with T1D by adjusting a small number of model parameters to each child. To assess the use of the personalized models for individual treatment evaluation, we compared subject-specific treatment options for PA management in replay simulations of the recorded data with altered meal, insulin and PA inputs.

A physical activity-intensity driven glycemic model for type 1 diabetes

BACKGROUND AND OBJECTIVE

The glucose response to physical activity for a person with type 1 diabetes (T1D) depends upon the intensity and duration of the physical activity, plasma insulin concentrations, and the individual physical fitness level. To accurately model the glycemic response to physical activity, these factors must be considered.

METHODS

Several physiological models describing the glycemic response to physical activity are proposed by incorporating model terms proportional to the physical activity intensity and duration describing endogenous glucose production (EGP), glucose utilization, and glucose transfer from the plasma to tissues. Leveraging clinical data of T1D during physical activity, each model fit is assessed.

RESULTS

The proposed model with terms accommodating EGP, glucose transfer, and insulin-independent glucose utilization allow for an improved simulation of physical activity glycemic responses with the greatest reduction in model error (mean absolute percentage error: 16.11 ± 4.82 vs. 19.49 ± 5.87, p = 0.002).

CONCLUSIONS

The development of a physiologically plausible model with model terms representing each major contributor to glucose metabolism during physical activity can outperform traditional models with physical activity described through glucose utilization alone. This model accurately describes the relation of plasma insulin and physical activity intensity on glucose production and glucose utilization to generate the appropriately increasing, decreasing or stable glucose response for each physical activity condition. The proposed model will enable the in silico evaluation of automated insulin dosing algorithms designed to mitigate the effects of physical activity with the appropriate relationship between the reduction in basal insulin and the corresponding glycemic excursion.

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.

Separating insulin-mediated and non-insulin-mediated glucose uptake during and after aerobic exercise in type 1 diabetes

Aerobic exercise in type 1 diabetes (T1D) causes rapid increase in glucose utilization due to muscle work during exercise, followed by increased insulin sensitivity after exercise. Better understanding of these changes is necessary for models of exercise in T1D. Twenty-six individuals with T1D underwent three sessions at three insulin rates (100%, 150%, 300% of basal). After 3-h run-in, participants performed 45 min aerobic exercise (moderate or intense). We determined area under the curve for endogenous glucose production (AUC_EGP) and rate of glucose disappearance (AUC_Rd) over 45 min from exercise start. A novel application of linear regression of R_d across the three insulin sessions allowed separation of insulin-mediated from non-insulin-mediated glucose uptake before, during, and after exercise. AUC_Rd increased 12.45 mmol/L (CI = 10.33-14.58, P < 0.001) and 13.13 mmol/L (CI = 11.01-15.26, P < 0.001) whereas AUC_EGP increased 1.66 mmol/L (CI = 1.01-2.31, P < 0.001) and 3.46 mmol/L (CI = 2.81-4.11, P < 0.001) above baseline during moderate and intense exercise, respectively. AUC_EGP increased during intense exercise by 2.14 mmol/L (CI = 0.91-3.37, P < 0.001) compared with moderate exercise. There was significant effect of insulin infusion rate on AUC_Rd equal to 0.06 mmol/L per % above basal rate (CI = 0.05-0.07, P < 0.001). Insulin-mediated glucose uptake rose during exercise and persisted hours afterward, whereas non-insulin-mediated effect was limited to the exercise period. To our knowledge, this method of isolating dynamic insulin- and non-insulin-mediated uptake has not been previously employed during exercise. These results will be useful in informing glucoregulatory models of T1D. The study has been registered at www.clinicaltrials.gov as NCT03090451.NEW & NOTEWORTHY Separating insulin and non-insulin glucose uptake dynamically during exercise in type 1 diabetes has not been done before. We use a multistep process, including a previously described linear regression method, over three insulin infusion sessions, to perform this separation and can graph these components before, during, and after exercise for the first time.

Hyperinsulinaemia and hyperglycaemia promote glucose utilization and storage during low- and high-intensity exercise

PURPOSE

The effect of hyperglycaemia with and without additional insulin was explored at a low and high intensity of exercise (40% vs 70% VO_2peak) on glucose utilization (GUR), carbohydrate oxidation, non-oxidative glucose disposal (NOGD), and muscle glycogen.

METHODS

Eight healthy trained males were exercised for 120 min in four trials, twice at 40% VO_2peak and twice at 70% VO_2peak, while glucose was infused intravenously (40%G; 70%G) at rates to "clamp" blood glucose at 10 mM. On one occasion at each exercise intensity, insulin was also infused at 40 mU/m²/per min (i.e. 40%GI and 70%GI). The glucose and insulin infusion began 30 min prior to exercise and throughout exercise. A muscle biopsy was taken at the end of exercise for glycogen analysis.

RESULTS

Hyperglycaemia significantly elevated plasma insulin concentration (p < 0.001), although no difference was observed between the exercise intensities. Insulin infusion during both mild and severe exercise resulted in increased insulin concentrations (p < 0.01) and GUR (p < 0.01) compared with glucose (40%GI by 25.2%; 70%GI by 26.2%), but failed to significantly affect carbohydrate, fat and protein oxidation. NOGD was significantly higher for GI trials at both intensities (p < 0.05) with storage occurring during both lower intensities (62.7 ± 19.6 g 40%GI; 127 ± 20.7 g 40%GI) and 70%GI (29.0 ± 20.0 g). Muscle glycogen concentrations were significantly depleted from rest (p < 0.01) after all four trials.

CONCLUSION

Hyperinsulinaemia in the presence of hyperglycaemia during both low- and high-intensity exercise promotes GUR and NOGD, but does not significantly affect substrate oxidation.

Exercise Metabolism: Fuels for the Fire

During exercise, the supply of adenosine triphosphate (ATP) is essential for the energy-dependent processes that underpin ongoing contractile activity. These pathways involve both substrate-level phosphorylation, without any need for oxygen, and oxidative phosphorylation that is critically dependent on oxygen delivery to contracting skeletal muscle by the respiratory and cardiovascular systems and on the supply of reducing equivalents from the degradation of carbohydrate, fat, and, to a limited extent, protein fuel stores. The relative contribution of these pathways is primarily determined by exercise intensity, but also modulated by training status, preceding diet, age, gender, and environmental conditions. Optimal substrate availability and utilization before, during, and after exercise is critical for maintaining exercise performance. This review provides a brief overview of exercise metabolism, with expanded discussion of the regulation of muscle glucose uptake and fatty acid uptake and oxidation.

Glucagon responses to exercise-induced hypoglycaemia are improved by somatostatin receptor type 2 antagonism in a rat model of diabetes

AIMS/HYPOTHESIS

Regular exercise is at the cornerstone of care in type 1 diabetes. However, relative hyperinsulinaemia and a blunted glucagon response to exercise promote hypoglycaemia. Recently, a selective antagonist of somatostatin receptor 2, PRL-2903, was shown to improve glucagon counterregulation to hypoglycaemia in resting streptozotocin-induced diabetic rats. The aim of this study was to test the efficacy of PRL-2903 in enhancing glucagon counterregulation during repeated hyperinsulinaemic exercise.

METHODS

Diabetic rats performed daily exercise for 1 week and were then exposed to saline (154 mmol/l NaCl) or PRL-2903, 10 mg/kg, before hyperinsulinaemic exercise on two separate occasions spaced 1 day apart. In the following week, animals crossed over to the alternate treatment for a third hyperinsulinaemic exercise protocol.

RESULTS

Liver glycogen content was lower in diabetic rats compared with control rats, despite daily insulin therapy (p < 0.05). Glucagon levels failed to increase during exercise with saline but increased three-to-six fold with PRL-2903 (all p < 0.05). Glucose concentrations tended to be higher during exercise and early recovery with PRL-2903 on both days of treatment; this difference did not achieve statistical significance (p > 0.05).

CONCLUSIONS/INTERPRETATION

PRL-2903 improves glucagon counterregulation during exercise. However, liver glycogen stores or other factors limit the prevention of exercise-induced hypoglycaemia in rats with streptozotocin-induced diabetes.

Timing of carbohydrate ingestion did not affect inflammatory response and exercise performance during prolonged intermittent running

Background

Carbohydrate ingestion during exercise is known to attenuate exercise-induced elevation of plasma IL-6 concentration. However, the influence of timing of carbohydrate ingestion remains unclear.

Purpose

The present study investigated the influence of different timing of carbohydrate ingestion during a simulated soccer game on exercise performance, metabolic and inflammatory responses.

Methods

Seven active males performed 3 exercise trials in a randomized order. The exercise consisted of two consecutive bouts of 45 min running (4–16 km/h), separated with 15 min rest period between bouts. The subjects ingested carbohydrate gel (1.0 g/kg) immediately before the first bout of exercise (ONE), immediately before first and second bouts of exercise (0.5 g/kg for each ingestion) (TWO) or placebo immediately before exercise (PLA) Time course changes of maximal jump height, peak power output during 6-s maximal pedaling, perceived fatigue and heart rate (HR) were monitored. Blood samples were also drawn to determine blood glucose, serum insulin, free fatty acid (FFA), myoglobin (Mb), creatine kinase (CK) and plasma IL-6 concentrations.

Results

Blood glucose and serum insulin concentrations were significantly higher in the ONE trial after first bout of 45 min exercise compared with PLA trial (P < 0.05), while serum FFA concentration was significantly elevated in PLA compared with ONE and TWO trials after second bout of exercise (P < 0.05). However, changes of jump height, peak power output during 6-s maximal pedaling, perceived fatigue, HR, or indirect muscle damage (Mb, CK) and inflammatory (IL-6) markers were not significantly different among three trials (P > 0.05).

Conclusions

The timing of carbohydrate ingestion did not affect exercise performance, exercise-induced muscle damage or inflammatory response during a simulated soccer game.

Exercise, GLUT4, and skeletal muscle glucose uptake

Glucose is an important fuel for contracting muscle, and normal glucose metabolism is vital for health. Glucose enters the muscle cell via facilitated diffusion through the GLUT4 glucose transporter which translocates from intracellular storage depots to the plasma membrane and T-tubules upon muscle contraction. Here we discuss the current understanding of how exercise-induced muscle glucose uptake is regulated. We briefly discuss the role of glucose supply and metabolism and concentrate on GLUT4 translocation and the molecular signaling that sets this in motion during muscle contractions. Contraction-induced molecular signaling is complex and involves a variety of signaling molecules including AMPK, Ca(2+), and NOS in the proximal part of the signaling cascade as well as GTPases, Rab, and SNARE proteins and cytoskeletal components in the distal part. While acute regulation of muscle glucose uptake relies on GLUT4 translocation, glucose uptake also depends on muscle GLUT4 expression which is increased following exercise. AMPK and CaMKII are key signaling kinases that appear to regulate GLUT4 expression via the HDAC4/5-MEF2 axis and MEF2-GEF interactions resulting in nuclear export of HDAC4/5 in turn leading to histone hyperacetylation on the GLUT4 promoter and increased GLUT4 transcription. Exercise training is the most potent stimulus to increase skeletal muscle GLUT4 expression, an effect that may partly contribute to improved insulin action and glucose disposal and enhanced muscle glycogen storage following exercise training in health and disease.

The physiological regulation of glucose flux into muscle in vivo

Skeletal muscle glucose uptake increases dramatically in response to physical exercise. Moreover, skeletal muscle comprises the vast majority of insulin-sensitive tissue and is a site of dysregulation in the insulin-resistant state. The biochemical and histological composition of the muscle is well defined in a variety of species. However, the functional consequences of muscle biochemical and histological adaptations to physiological and pathophysiological conditions are not well understood. The physiological regulation of muscle glucose uptake is complex. Sites involved in the regulation of muscle glucose uptake are defined by a three-step process consisting of: (1) delivery of glucose to muscle, (2) transport of glucose into the muscle by GLUT4 and (3) phosphorylation of glucose within the muscle by a hexokinase (HK). Muscle blood flow, capillary recruitment and extracellular matrix characteristics determine glucose movement from the blood to the interstitium. Plasma membrane GLUT4 content determines glucose transport into the cell. Muscle HK activity, cellular HK compartmentalization and the concentration of the HK inhibitor glucose 6-phosphate determine the capacity to phosphorylate glucose. Phosphorylation of glucose is irreversible in muscle; therefore, with this reaction, glucose is trapped and the uptake process is complete. Emphasis has been placed on the role of the glucose transport step for glucose influx into muscle with the past assertion that membrane transport is rate limiting. More recent research definitively shows that the distributed control paradigm more accurately defines the regulation of muscle glucose uptake as each of the three steps that define this process are important sites of flux control.

Exercise-related hypoglycemia in diabetes mellitus

Current recommendations are that people with Type 1 and Type 2 diabetes mellitus exercise regularly. However, in cases in which insulin or insulin secretagogues are used to manage diabetes, patients have an increased risk of developing hypoglycemia, which is amplified during and after exercise. Repeated episodes of hypoglycemia blunt autonomic nervous system, neuroendocrine and metabolic defenses (counter-regulatory responses) against subsequent episodes of falling blood glucose levels during exercise. Likewise, antecedent exercise blunts counter-regulatory responses to subsequent hypoglycemia. This can lead to a vicious cycle, by which each episode of either exercise or hypoglycemia further blunts counter-regulatory responses. Although contemporary insulin therapies cannot fully mimic physiologic changes in insulin secretion, people with diabetes have several management options to avoid hypoglycemia during and after exercise, including regularly monitoring blood glucose, reducing basal and/or bolus insulin, and consuming supplemental carbohydrates.

Four grams of glucose

Four grams of glucose circulates in the blood of a person weighing 70 kg. This glucose is critical for normal function in many cell types. In accordance with the importance of these 4 g of glucose, a sophisticated control system is in place to maintain blood glucose constant. Our focus has been on the mechanisms by which the flux of glucose from liver to blood and from blood to skeletal muscle is regulated. The body has a remarkable capacity to satisfy the nutritional need for glucose, while still maintaining blood glucose homeostasis. The essential role of glucagon and insulin and the importance of distributed control of glucose fluxes are highlighted in this review. With regard to the latter, studies are presented that show how regulation of muscle glucose uptake is regulated by glucose delivery to muscle, glucose transport into muscle, and glucose phosphorylation within muscle.

Pro/con debate: Is intensive insulin therapy targeting tight blood glucose control of benefit in critically ill patients?

You have decided to develop a protocol for insulin therapy in your intensive care unit (ICU). You wonder about the merit of using intensive insulin therapy (IIT) to maintain tight blood glucose control in your patients.

Exercise improves phosphatidylinositol-3,4,5-trisphosphate responsiveness of atypical protein kinase C and interacts with insulin signalling to peptide elongation in human skeletal muscle

We investigated if acute endurance-type exercise interacts with insulin-stimulated activation of atypical protein kinase C (aPKC) and insulin signalling to peptide chain elongation in human skeletal muscle. Four hours after acute one-legged exercise, insulin-induced glucose uptake was approximately 80% higher (N = 12, P < 0.05) in previously exercised muscle, measured during a euglycaemic-hyperinsulinaemic clamp (100 microU ml(-1)). Insulin increased (P < 0.05) both insulin receptor substrate (IRS)-1 and IRS-2 associated phosphatidylinositol (PI)-3 kinase activity and led to increased (P < 0.001) phosphorylation of Akt on Ser(473) and Thr(308) in skeletal muscle. Interestingly, in response to prior exercise IRS-2-associated PI-3 kinase activity was higher (P < 0.05) both at basal and during insulin stimulation. This coincided with correspondingly altered phosphorylation of the extracellular-regulated protein kinase 1/2 (ERK 1/2), p70S6 kinase (P70S6K), eukaryotic elongation factor 2 (eEF2) kinase and eEF2. aPKC was similarly activated by insulin in rested and exercised muscle, without detectable changes in aPKC Thr(410) phosphorylation. However, when adding phosphatidylinositol-3,4,5-triphosphate (PIP3), the signalling product of PI-3 kinase, to basal muscle homogenates, aPKC was more potently activated (P = 0.01) in previously exercised muscle. Collectively, this study shows that endurance-type exercise interacts with insulin signalling to peptide chain elongation. Although protein turnover was not evaluated, this suggests that capacity for protein synthesis after acute endurance-type exercise may be improved. Furthermore, endurance exercise increased the responsiveness of aPKC to PIP3 providing a possible link to improved insulin-stimulated glucose uptake after exercise.

Dynamic modeling of exercise effects on plasma glucose and insulin levels

BACKGROUND

Regulation of plasma glucose concentration for type 1 diabetic patients is challenging, and exercise is an added complication. From a metabolic prospective, the significant exercise-induced effects are increased glucose uptake rate by the working tissues, increased hepatic glucose release to maintain overall glucose homeostasis, and decreased plasma insulin concentration. During prolonged exercise, glucose levels drop significantly because of the decrease in hepatic glucose production. With the long-term goal of developing a closed-loop insulin delivery system operating under various physiological conditions, it is necessary to develop a model that is capable of predicting blood glucose concentration at rest and during physical activity.

METHODS

A minimal model developed previously was extended to include the major effects of exercise on plasma glucose and insulin levels. Differential equations were developed to capture the exercise-induced dynamics of plasma insulin clearance and the elevation of glucose uptake and hepatic glucose production rates. The decreasing liver glucose output resulting from prolonged exercise was modeled using an equation depending on exercise intensity and duration.

RESULTS

The exercise model successfully captured the glucose and insulin dynamics during short- and long-term exercise. Model predictions of glucose and insulin dynamics during the postexercise recovery period were also consistent with literature data.

CONCLUSION

The model successfully emulated the physiological effects of exercise on blood glucose and insulin levels. This extended model may provide a new disturbance test platform for the development of closed-loop glucose control algorithms.

Exercise under hyperinsulinaemic conditions increases whole-body glucose disposal without affecting muscle glycogen utilisation in type 1 diabetes

AIMS/HYPOTHESIS

We examined whole-body and muscle metabolism in patients with type 1 diabetes during moderate exercise at differing circulating insulin concentrations.

METHODS

Eight men (mean +/- SEM age 36.4 +/- 1.5 years; diabetes duration 11.3 +/- 1.4 years; BMI 24.6 +/- 0.7 kg/m(2); HbA(1c) 7.9 +/- 0.2% and VO(2) peak 44.5 +/- 1.2 ml kg(-1) min(-1)) with type 1 diabetes were studied on two occasions at rest (2 h) and during 45 min of cycling at 60% maximum VO(2) with insulin infused at the rate of either 15 (LO study) or 50 (HI) mU m(-2) min(-1) and blood glucose clamped at 8 mmol/l. Indirect calorimetry, insulin-glucose clamps and thigh muscle biopsies were employed to measure whole-body energy and muscle metabolism.

RESULTS

Fat oxidation contributed 15 and 23% to total energy expenditure during exercise in the HI and LO studies, respectively. The respective carbohydrate (CHO) oxidation rates were 31.7 +/- 2.7 and 27.8 +/- 1.9 mg kg(-1) min(-1) (p < 0.05). Exogenous glucose utilisation rate during exercise was substantially greater (p < 0.001) in the HI study (18.4 +/- 2.1 mg kg(-1) min(-1)) than in the LO study (6.9 +/- 1.2 mg kg(-1) min(-1)). Muscle glycogen content fell by approximately 40% during exercise in both trials. Muscle glycogen utilisation, muscle intermediary metabolism, and phosphorylation of protein kinase B/Akt, glycogen synthase kinase 3alpha/beta and extracellular signal-regulated protein kinase 1 and 2 proteins were no different between interventions.

CONCLUSIONS/INTERPRETATION

In patients with type 1 diabetes, exercise under peak therapeutic insulin concentrations increases exogenous glucose utilisation but does not spare muscle glycogen utilisation. A disproportionate increase in exogenous glucose utilisation relative to the increase in CHO oxidation suggests an increase in glucose flux through non-oxidative pathways.

Impact of glutamine supplementation on glucose homeostasis during and after exercise

The interaction of glutamine availability and glucose homeostasis during and after exercise was investigated, measuring whole body glucose kinetics with [3-3H]glucose and net organ balances of glucose and amino acids (AA) during basal, exercise, and postexercise hyperinsulinemic-euglycemic clamp periods in six multicatheterized dogs. Dogs were studied twice in random treatment order: once with glutamine (12 micromol.kg(-1).min(-1); Gln) and once with saline (Con) infused intravenously during and after exercise. Plasma glucose fell by 7 mg/dl with exercise in Con (P < 0.05), but it did not fall with Gln. Gln further stimulated whole body glucose production and utilization an additional 24% above a normal exercise response (P < 0.05). Net hepatic uptake of glutamine and alanine was greater with Gln than Con during exercise (P < 0.05). Net hepatic glucose output was increased sevenfold during exercise with Gln (P < 0.05) but not with Con. Net hindlimb glucose uptake was increased similarly during exercise in both groups (P < 0.05). During the postexercise hyperinsulinemic-euglycemic period, glucose production decreased to near zero with Con, but it did not decrease below basal levels with Gln. Gln increased glucose utilization by 16% compared with Con after exercise (P < 0.05). Furthermore, net hindlimb glucose uptake in the postexercise period was increased approximately twofold vs. basal with Gln (P < 0.05) but not with Con. Net hepatic uptake of glutamine during the postexercise period was threefold greater for Gln than Con (P < 0.05). In conclusion, glutamine availability modulates glucose homeostasis during and after exercise, which may have implications for postexercise recovery.

Gender differences in insulin action after a single bout of exercise

Effects of a single exercise bout on insulin action were compared in men (n = 10) and women (n = 10). On an exercise day, subjects cycled for 90 min at 85% lactate threshold, whereas on a rest (control) day, they remained semirecumbent. The period of exercise, or rest, was followed by a 3-h hyperinsulinemic-euglycemic clamp (30 mU.m(-2).min(-1)) and indirect calorimetry. Glucose kinetics were measured isotopically by using an infusion of [6,6-2H2]glucose. Glucose infusion rate (GIR) during the clamp on the rest day was not different between the genders. However, GIR on the exercise day was significantly lower in men compared with women (P = 0.01). This was mainly due to a significantly lower glucose rate of disappearance in men compared with women (P = 0.05), whereas no differences were observed in the endogenous glucose rate of appearance. Nonprotein respiratory quotient (NPRQ) increased significantly during the clamp from preclamp measurements in men and women on the rest day (P < 0.01). Exercise abolished the increase in NPRQ seen during the clamp on the rest day and tended to decrease NPRQ in men. Our results indicate the following: 1) exercise abolishes the usual increase in NPRQ observed during a hyperinsulinemic-euglycemic clamp in both genders, 2) men exhibit relatively lower whole body insulin action in the 3-4 h after exercise compared with women, and 3) gender differences in insulin action may be explained by a lower glucose rate of disappearance in the men after acute exercise. Together, these data imply gender differences in insulin action postexercise exist in peripheral tissues and not in liver.

Exercise augments the acute anabolic effects of intradialytic parenteral nutrition in chronic hemodialysis patients

Decreased dietary protein intake and hemodialysis (HD)-associated protein catabolism are among several factors that predispose chronic hemodialysis (CHD) patients to uremic malnutrition and associated muscle wasting. Intradialytic parenteral nutrition (IDPN) acutely reverses the net negative whole body and forearm muscle protein balances observed during the HD procedure. Exercise has been shown to improve muscle protein homeostasis, especially if performed with adequately available intramuscular amino acids. We hypothesized that exercise performance would provide additive anabolic effects to the beneficial effects of IDPN. We studied six CHD patients at two separate HD sessions: 1) IDPN administration only and 2) IDPN + exercise. Patients were studied 2 h before, during, and 2 h after an HD session by use of a primed constant infusion of l-[1-(13)C]leucine and l-[ring-(2)H(5)] phenylalanine. Exercise combined with IDPN promoted additive twofold increases in forearm muscle essential amino acid uptake (455 +/- 105 vs. 229 +/- 38 nmol.100 ml(-1).min(-1), P < 0.05) and net muscle protein accretion (125 +/- 37 vs. 56 +/- 30 microg.100 ml(-1).min(-1), P < 0.05) during HD compared with IDPN alone. Measurements of whole body protein homeostasis and energy expenditure were not altered by exercise treatment. In conclusion, exercise in the presence of adequate nutritional supplementation has potential as a therapeutic intervention to blunt the loss of muscle mass in CHD patients.

Suppression of endogenous glucose production by mild hyperinsulinemia during exercise is determined predominantly by portal venous insulin

Hyperinsulinemia during exercise in people with diabetes requiring exogenous insulin is a major clinical problem. The aim of this study was to assess the significance of portal vein versus arterial insulin to hepatic effects of hyperinsulinemia during exercise. Dogs had sampling (artery, portal vein, and hepatic vein) and infusion (vena cava and portal vein) catheters and flow probes (hepatic artery and portal vein) implanted >16 days before a study. Protocols consisted of equilibration (-130 to -30 min), basal (-30 to 0 min), and treadmill exercise (0-150 min) periods. Somatostatin was infused and glucagon and insulin were replaced in the portal vein to achieve basal arterial and portal vein levels at rest and simulated levels during the first 60 min of exercise. From 60 to 150 min of exercise, the simulated insulin infusion was sustained (C; n = 7), modified to selectively create a physiologic increment in arterial insulin (Pe; n = 7), or altered to increase arterial insulin as in Pe but with a concomitant increase in portal insulin (PePo; n = 7). Euglycemic clamps were performed in all studies. Portal and arterial insulin were 15 +/- 2 and 4 +/- 1 micro U/ml (mean +/- SE of all groups), respectively, at t = 60 min in all groups. Insulin levels were unchanged for the remainder of the exercise period in C. Arterial insulin was increased from 3 +/- 1 to 14 +/- 2 micro U/ml, whereas portal insulin did not change in Pe after t = 60 min. Arterial insulin was increased from 3 +/- 1 to 15 +/- 2 micro U/ml, and portal insulin was increased from 16 +/- 3 to 33 +/- 3 micro U/ml in PePo after t = 60 min. Endogenous glucose production (R(a)) rose similarly from basal during the first 60 min of exercise in all groups (mean +/- SE of all groups was from 2.2 +/- 0.1 to 6.8 +/- 0.5 mg. kg(-1). min(-1)). The increase in R(a) was sustained for the remainder of the exercise period in C. R(a) was suppressed by approximately 40%, but only after 60 min of hyperinsulinemia, and by approximately 20% after 90 min of hyperinsulinemia in Pe. In contrast, the addition of portal venous hyperinsulinemia caused approximately 90% suppression of R(a) within 20 min and for the remainder of the experiment in PePo. Measurements of net hepatic glucose output were similar to R(a) responses in all groups. Arterial free fatty acids (FFAs), a stimulus of R(a), were increased to 1,255 +/- 258 micro mol/l in C but were only 459 +/- 67 and 312 +/- 42 micro mol/l in Pe and PePo, respectively, by 150 min of exercise. Thus, during exercise, the exquisite sensitivity of R(a) to hyperinsulinemia is due entirely to portal venous hyperinsulinemia during the first 60 min, after which peripheral hyperinsulinemia may control approximately 20-40%, possibly as a result of inhibition of the exercise-induced increase in FFA.

Intradialytic parenteral nutrition improves protein and energy homeostasis in chronic hemodialysis patients

Decreased dietary protein intake and hemodialysis-associated protein catabolism are among several factors that predispose chronic hemodialysis (CHD) patients to protein calorie malnutrition. Since attempts to increase protein intake by dietary counseling are usually ineffective, intradialytic parenteral nutrition (IDPN) has been proposed as a potential therapeutic approach in malnourished CHD patients. In this study, we examined protein and energy homeostasis during hemodialysis in seven CHD patients at two separate hemodialysis sessions, with and without IDPN administration. Patients were studied 2 hours before, during, and 2 hours following a hemodialysis session, using a primed constant infusion of L-(1-(13)C) leucine and L-(ring-(2)H(5)) phenylalanine. Our results showed that IPDN promoted a large increase in whole-body protein synthesis and a significant decrease in whole-body proteolysis, along with a significant increase in forearm muscle protein synthesis. The net result was a change from an essentially catabolic state to a highly positive protein balance, both in whole-body and forearm muscle compartments. We conclude that the provision of calories and amino acids during hemodialysis with IDPN acutely reverses the net negative whole-body and forearm muscle protein balances, demonstrating a need for long-term clinical trials evaluating IDPN in malnourished CHD patients.

Prior exercise and the response to insulin-induced hypoglycemia in the dog

To test whether hepatic insulin action and the response to an insulin-induced decrement in blood glucose are enhanced in the immediate postexercise state as they are during exercise, dogs had sampling (artery, portal vein, and hepatic vein) catheters and flow probes (portal vein and hepatic artery) implanted 16 days before a study. After 150 min of moderate treadmill exercise or rest, dogs were studied during a 150-min hyperinsulinemic (1 mU.kg(-1).min(-1)) euglycemic (n = 5 exercised and n = 9 sedentary) or hypoglycemic (65 mg/dl; n = 8 exercised and n = 9 sedentary) clamp. Net hepatic glucose output (NHGO) and endogenous glucose appearance (R(a)) and utilization (R(d)) were assessed with arteriovenous and isotopic ([3-(3)H]glucose) methods. Results show that, immediately after prolonged, moderate exercise, in relation to sedentary controls: 1) the glucose infusion rate required to maintain euglycemia, but not hypoglycemia, was higher; 2) R(d) was greater under euglycemic, but not hypoglycemic conditions; 3) NHGO, but not R(a), was suppressed more by a hyperinsulinemic euglycemic clamp, suggesting that hepatic glucose uptake was increased; 4) a decrement in glucose completely reversed the enhanced suppression of NHGO by insulin that followed exercise; and 5) arterial glucagon and cortisol were transiently higher in the presence of a decrement in glucose. In summary, an increase in insulin action that was readily evident under euglycemic conditions after exercise was abolished by moderate hypoglycemia. The means by which the glucoregulatory system is able to overcome the increase in insulin action during moderate hypoglycemia is related not to an increase in R(a) but to a reduction in insulin-stimulated R(d). The primary site of this reduction is the liver.

Postexercise protein intake enhances whole-body and leg protein accretion in humans

PURPOSE

Exercise increases the use of amino acids for glucose production and stimulates the oxidation of amino acids and other substrates to provide ATP for muscular contraction, and thus the availability of amino acids and energy for postexercise muscle protein synthesis may be limiting. The purpose of this study was to determine the potential of postexercise nutrient intake to enhance the recovery of whole-body and skeletal muscle protein homeostasis in humans.

METHODS

Primed-continuous infusions of L-[1-13C]leucine and L-[ring-2H5]phenylalanine were initiated in the antecubital vein and blood was sampled from a femoral vein and a heated (arterialized) hand vein. Each study consisted of a 30-min basal, a 60-min exercise (bicycle at 60% VO2max), and a 180-min recovery period. Five men and five women were studied three times with an oral supplement administered immediately following exercise in random order: NO = 0, 0, 0; SUPP = 0, 8, 3; or SUPP+PRO = 10, 8, 3 g of protein, carbohydrate, and lipid, respectively.

RESULTS

Compared to NO, SUPP did not alter leg or whole-body protein homeostasis during the recovery period. In contrast, SUPP+PRO increased plasma essential amino acids 33%, leg fractional extraction of phenylalanine 4-fold, leg uptake of glucose 3.5-fold, and leg and whole-body protein synthesis 6-fold and 15%, respectively. Whereas postexercise intake of either NO or SUPP resulted in a net leg release of essential amino acids and net loss of whole-body and leg protein, SUPP+PRO resulted in a net leg uptake of essential amino acids and net whole-body and leg protein gain.

CONCLUSIONS

These findings suggest that the availability of amino acids is more important than the availability of energy for postexercise repair and synthesis of muscle proteins.

Postexercise nutrient intake timing in humans is critical to recovery of leg glucose and protein homeostasis

Although the importance of postexercise nutrient ingestion timing has been investigated for glycogen metabolism, little is known about similar effects for protein dynamics. Each subject (n = 10) was studied twice, with the same oral supplement (10 g protein, 8 g carbohydrate, 3 g fat) being administered either immediately (EARLY) or 3 h (LATE) after 60 min of moderate-intensity exercise. Leg blood flow and circulating concentrations of glucose, amino acids, and insulin were similar for EARLY and LATE. Leg glucose uptake and whole body glucose utilization (D-[6,6-2H(2)]glucose) were stimulated threefold and 44%, respectively, for EARLY vs. LATE. Although essential and nonessential amino acids were taken up by the leg in EARLY, they were released in LATE. Although proteolysis was unaffected, leg (L-[ring-2H(5)]phenylalanine) and whole body (L-[1-13C]leucine) protein synthesis were elevated threefold and 12%, respectively, for EARLY vs. LATE, resulting in a net gain of leg and whole body protein. Therefore, similar to carbohydrate homeostasis, EARLY postexercise ingestion of a nutrient supplement enhances accretion of whole body and leg protein, suggesting a common mechanism of exercise-induced insulin action.

Insulin and exercise differentially regulate PI3-kinase and glycogen synthase in human skeletal muscle

The purpose of this study was to determine the separate and combined effects of exercise and insulin on the activation of phosphatidylinositol 3-kinase (PI3-kinase) and glycogen synthase in human skeletal muscle in vivo. Seven healthy men performed three trials in random order. The trials included 1) ingestion of 2 g/kg body wt carbohydrate in a 10% solution (CHO); 2) 75 min of semirecumbent cycling exercise at 75% of peak O(2) consumption; followed by 5 x 1-min maximal sprints (Ex); and 3) Ex, immediately followed by ingestion of the carbohydrate solution (ExCHO). Plasma glucose and insulin were increased (P < 0.05) at 15 and 30 (Post-15 and Post-30) min after the trial during CHO and ExCHO, although insulin was lower for ExCHO. Hyperinsulinemia during recovery in CHO and ExCHO led to an increase (P < 0.001) in PI3-kinase activity at Post-30 compared with basal, although the increase was lower (P < 0. 004) for ExCHO. Furthermore, PI3-kinase activity was suppressed (P < 0.02) immediately after exercise (Post-0) during Ex and ExCHO. Area under the insulin response curve for all trials was positively associated with PI3-kinase activity (r = 0.66, P < 0.001). Glycogen synthase activity did not increase during CHO but was increased (P < 0.05) at Post-0 and Post-30 during Ex and ExCHO. Ingestion of the drink increased (P < 0.05) carbohydrate oxidation during CHO and ExCHO, although the increase after ExCHO was lower (P < 0.05) than CHO. Carbohydrate oxidation was directly correlated with PI3-kinase activity for all trials (r = 0.63, P < 0.001). In conclusion, under resting conditions, ingestion of a carbohydrate solution led to activation of the PI3-kinase pathway and oxidation of the carbohydrate. However, when carbohydrate was ingested after intense exercise, the PI3-kinase response was attenuated and glycogen synthase activity was augmented, thus facilitating nonoxidative metabolism or storage of the carbohydrate. Activation of glycogen synthase was independent of PI3-kinase.

Glucose infusion attenuates endogenous glucose production and enhances glucose use of horses during exercise

We examined the effects of increased glucose availability on glucose kinetics and substrate utilization in horses during exercise. Six conditioned horses ran on a treadmill for 90 min at 34 +/- 1% of maximum oxygen uptake. In one trial [glucose (Glu)], glucose was infused at a mean rate of 34.9 +/- 1.1 micromol. kg(-1). min(-1), whereas in the other trial [control (Con)] an equivalent volume of isotonic saline was infused. Plasma glucose increased during exercise in Glu (90 min: 8.3 +/- 1.7 mM) but was largely unchanged in Con (90 min: 5.1 +/- 0.4 mM). In Con, hepatic glucose production (HGP) increased during exercise, reaching a peak of 38.6 +/- 2.7 micromol. kg(-1). min(-1) after 90 min. Glucose infusion partially suppressed (P < 0.05) the rise in HGP (peak value 25.8 +/- 3.3 micromol. kg(-1). min(-1)). In Con, glucose rate of disappearance (R(d)) rose to a peak of 40.4 +/- 2.9 micromol. kg(-1). min(-1) after 90 min; in Glu, augmented glucose utilization was reflected by values for glucose R(d) that were twofold higher (P < 0.001) than in Con between 30 and 90 min. Total carbohydrate oxidation was higher (P < 0.05) in Glu (187.5 +/- 8.5 micromol. kg(-1). min(-1)) than in Con (159.2 +/- 7.3 micromol. kg(-1).min(-1)), but muscle glycogen utilization was similar between trials. We conclude that an increase in glucose availability in horses during low-intensity exercise 1) only partially suppresses HGP, 2) attenuates the decrease in carbohydrate oxidation during such exercise, but 3) does not affect muscle glycogen utilization.

Epinephrine inhibits exogenous glucose utilization in exercising horses

This study examined the effects of preexercise glucose administration, with and without epinephrine infusion, on carbohydrate metabolism in horses during exercise. Six horses completed 60 min of treadmill exercise at 55 +/- 1% maximum O(2) uptake 1) 1 h after oral administration of glucose (2 g/kg; G trial); 2) 1 h after oral glucose and with an intravenous infusion of epinephrine (0.2 micromol. kg(-1). min(-1); GE trial) during exercise, and 3) 1 h after water only (F trial). Glucose administration (G and GE) caused hyperinsulinemia and hyperglycemia ( approximately 8 mM). In GE, plasma epinephrine concentrations were three- to fourfold higher than in the other trials. Compared with F, the glucose rate of appearance was approximately 50% and approximately 33% higher in G and GE, respectively, during exercise. The glucose rate of disappearance was approximately 100% higher in G than in F, but epinephrine infusion completely inhibited the increase in glucose uptake associated with glucose administration. Muscle glycogen utilization was higher in GE [349 +/- 44 mmol/kg dry muscle (dm)] than in F (218 +/- 28 mmol/kg dm) and G (201 +/- 35 mmol/kg dm). We conclude that 1) preexercise glucose augments utilization of plasma glucose in horses during moderate-intensity exercise but does not alter muscle glycogen usage and 2) increased circulating epinephrine inhibits the increase in glucose rate of disappearance associated with preexercise glucose administration and increases reliance on muscle glycogen for energy transduction.

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.

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.

Islet transplantation in diabetic rats normalizes basal and exercise-induced energy metabolism

Transplantation of islets of Langerhans in diabetic rats normalizes resting glucose and insulin levels, but it remains unclear whether islet transplantation restores resting and exercise-induced energy metabolism. Therefore, we compared energy metabolism in islet transplanted rats with energy metabolism in normal controls and in streptozotocin-induced diabetic rats. Indirect calorimetry was applied before, during, and after moderate swimming exercise. Blood was sampled by means of a heart catheter for determination of nutrient and hormone concentrations. In islet transplanted rats, the results from indirect calorimetry and the nutrient and hormone concentrations were similar to the results in normal controls. In resting diabetic rats, insulin levels were very low, while glucose levels were exaggerated. Compared to resting controls, fat oxidation and energy expenditure were elevated, but carbohydrate oxidation was similar. Exercise increased energy expenditure and was similar in diabetic and control rats. Carbohydrate oxidation was lower and fat oxidation was higher in diabetic than in control rats. Exercise-induced increments in glucose, lactate and non-esterified fatty acid levels were the highest in diabetic rats. Thus, at rest, but not during exercise, insulin influences energy expenditure. Insulin reduces lipolysis and glycogenolysis. It enhances the relative contribution of carbohydrate oxidation and reduces fat oxidation to total energy expenditure, at rest and during exercise. Absence of insulin enhances anaerobic glycolytic pathways during exercise. It is concluded that in diabetic rats, islet transplantation of 50% of the normal pancreatic endocrine volume successfully normalizes insulin levels and hence energy metabolism at rest and during exercise.

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.