Interaction between insulin and counterregulatory hormones in control of substrate utilization in health and diabetes during exercise

Sexual Dimorphism in Substrate Metabolism During Exercise

During aerobic exercise, women oxidize significantly more lipids and less carbohydrates than men. This sexual dimorphism in substrate metabolism has been attributed, in part, to the observed differences in epinephrine and glucagon levels between men and women during exercise. To identify the underpinning candidate physiological mechanisms for these sex differences, we developed a sex-specific multi-scale mathematical model that relates cellular metabolism in the organs to whole-body responses during exercise. We conducted simulations to test the hypothesis that sex differences in the exercise-induced changes to epinephrine and glucagon would result in the sexual dimorphism of hepatic metabolic flux rates via the glucagon-to-insulin ratio (GIR). Indeed, model simulations indicate that the shift towards lipid metabolism in the female model is primarily driven by the liver. The female model liver exhibits resistance to GIR-mediated glycogenolysis, which helps maintain hepatic glycogen levels. This decreases arterial glucose levels and promotes the oxidation of free fatty acids. Furthermore, in the female model, skeletal muscle relies on plasma free fatty acids as the primary fuel source, rather than intramyocellular lipids, whereas the opposite holds true for the male model.

A Narrative Literature Review on the Role of Exercise Training in Managing Type 1 and Type 2 Diabetes Mellitus

Diabetes mellitus (DM) is a metabolic disease characterized by chronic hyperglycemia associated with impaired carbohydrate, lipid, and protein metabolism, with concomitant absence of insulin secretion or reduced sensitivity to its metabolic effects. Patients with diabetes mellitus have a 30% more risk of developing heart failure and cardiovascular disease compared to healthy people. Heart and cardiovascular problems are the first cause of death worldwide and the main complications which lead to high healthcare costs. Such complications can be delayed or avoided by taking prescribed medications in conjunction with a healthy lifestyle (i.e., diet and physical activity). The American College of Sports Medicine and the American Diabetes Association recommend that diabetic people reduce total sedentary time by incorporating physical activity into their weekly routine. This narrative literature review aims to summarize and present the main guidelines, pre-exercise cardiovascular screening recommendations, and considerations for patients with diabetes and comorbidities who are planning to participate in physical activity programs.

Alterações morfométricas em músculo respiratório de ratos submetidos à imobilização de pata

A imobilização é um recurso frequentemente utilizado na prática clínica, sendo comum em patologias álgicas e nas fraturas. O objetivo deste estudo foi analisar a influência do processo de imobilização em músculo do sistema respiratório, o diafragma. O experimento foi efetuado com 12 ratos Wistar machos divididos em dois grupos, controle e imobilizado. O procedimento de imobilização foi realizado através de um método alternativo de imobilização por fita adesiva, sendo mantida por duas semanas. Analisou-se a morfometria das fibras do diafragma com coloração de hematoxilina e eosina. Ao compararmos o diâmetro médio das fibras musculares do diafragma dos animais imobilizados (47,15μm ± 0,329μm) em relação ao controle (54,67μm ± 0,396μm), encontramos diferença estatística entre os grupos (p < 0,0001). Considerando os dados encontrados, foi possível concluir que a imobilização de pata, no modelo utilizado, foi capaz de gerar hipotrofia da musculatura respiratória, assim como um quadro geral de redução de massa corporal do animal.

Multi-scale computational model of fuel homeostasis during exercise: effect of hormonal control

A mathematical model of the whole-body metabolism is developed to predict fuel homeostasis during exercise by using hormonal control over cellular metabolic processes. The whole body model is composed of seven tissue compartments: brain, heart, liver, GI (gastrointestinal) tract, skeletal muscle, adipose tissue, and "other tissues". Each tissue compartment is described by dynamic mass balances and major cellular metabolic reactions. The glucagon-insulin controller is incorporated into the whole body model to predict hormonal changes during exercise. Moderate [150 W power output at 60% of peak oxygen consumption (VO(2max))] exercise for 60 min was implemented by increasing ATP utilization rates in heart and skeletal muscle. Arterial epinephrine level was given as an input function, which directly affects heart and skeletal muscle metabolism and indirectly other tissues via glucagon-insulin controller. Model simulations were validated with experimental data from human exercise studies. The exercise induced changes in hormonal signals modulated metabolic flux rates of different tissues in a coordinated way to achieve glucose homeostasis, demonstrating the efficacy of hormonal control over cellular metabolic processes. From experimental measurements of whole body glucose balance and arterial substrate concentrations, this model could predict the dynamic changes of hepatic glycogenolysis and gluconeogenesis, which are not easy to measure experimentally, suggesting the higher contribution of glycogenolysis ( approximately 75%). In addition, it could provide dynamic information on the relative contribution of carbohydrates and lipids for fuel oxidation in skeletal muscle. Model simulations indicate that external fuel supplies from other tissue/organ systems to skeletal muscle become important for prolonged exercise emphasizing the significance of interaction among tissues. In conclusion, this model can be used as a valuable complement to experimental studies due to its ability to predict what is difficult to measure directly, and usefulness to provide information about dynamic behaviors.

Computational model of glucose homeostasis during exercise

A mathematical model of whole-body metabolism is developed to predict glucose homeostasis during exercise by using a hormonal controller over cellular metabolic processes. Model simulations were validated with experimental data from exercise studies in humans. The exercise-induced changes in hormonal signals modulated metabolic flux rates of various tissues in a coordinated way to maintain blood glucose constant. This study demonstrates the efficacy of a multi-tissue controller to accomplish blood glucose homeostasis by integrating the outputs of tissues under hormonal control. In conclusion, this model can be used as a valuable complement to experimental studies due to its ability to predict what is difficult to measure directly and to provide dynamic information about the system.

Therapeutic management of hyperglycaemic hyperosmolar syndrome

Hyperglycaemic hyperosmolar syndrome is a major acute complication of decompensated diabetes mellitus. It represents the second most common aetiology of diabetic coma and is associated with excess mortality. It is characterised by severe hyperglycaemia, hyperosmolality and dehydration in the absence of significant ketosis, afflicting principally middle-aged-to-elderly patients. Early clinical diagnosis and prompt treatment, consisting of fluid replacement, insulin therapy, restoration of electrolyte disturbances and management of concurrent illnesses may improve the outcome. This review provides an outline of the diagnostic approach of patients with manifestations of hyperglycaemic hyperosmolar syndrome and discusses the contemporary therapeutic recommendations.

Substrate utilization and glucose turnover during exercise of varying intensities in individuals with NIDDM

PURPOSE

This investigation was undertaken to examine substrate utilization and glucose turnover during exercise of varying intensities in NIDDM patients.

METHODS

Six male NIDDM patients (N) and six male controls (C) of similar age, body weight, % body fat, and VO2peak were studied in two experimental sessions administered in a randomized counterbalanced order. During each session the subjects cycled at a power output corresponding to 50% of VO2peak or 70% of VO2peak. Duration of exercise was adjusted so that energy expenditure (EE) was equal in both the 50% and 70% trials. Isotope infusion technique and indirect calorimetry were used to assess substrate utilization and glucose turnover during exercise.

RESULTS

Rates of carbohydrate (CHO) and lipid oxidation increased (P < 0.05) during both the 50% and 70% trials. Rates of CHO oxidation were greater (P < 0.05) during the 70% than during the 50% trial. However, rates of lipid oxidation were similar in the two trials. No differences in rates of CHO and lipid oxidation were observed in N and C. Rates of hepatic glucose production (Ra) and plasma glucose utilization (Rd) increased (P < 0.05) during exercise, and the increases were similar in the 50% and 70% trials. Ra did not differ between N and C. However, Rd was greater (P < 0.05) in N than in C. Plasma glucose concentration decreased (P < 0.05) in N, with the decrease being similar in the 50% and 70% trials. In contrast, plasma glucose concentration remained unchanged during both the 50% and 70% trials in C.

CONCLUSIONS

Exercise results in a greater increase in plasma glucose utilization in patients with NIDDM compared with that in normal individuals, and this increase mediates the decline in plasma glucose concentrations in patients with NIDDM. Under isocaloric conditions, the changes in plasma glucose utilization and plasma glucose concentrations are similar during exercise of varying intensities. Despite a greater glucose utilization, carbohydrate and fat oxidation are similar in the two groups and their relations to exercise intensity are not altered by NIDDM.

beta-adrenergic stimulation induces transient imbalance between myocardial substrate uptake and metabolism in vivo

At steady state, a balance is expected between net myocardial uptake of the principal exogenous carbon substrates and the rate at which these substrates are metabolized. Such a balance is present when the sum of the oxygen extraction ratios (OERs) for glucose, lactate, and free fatty acids (FFA) is near unity. We have previously observed that systemic administration of the beta-adrenergic agonist isoproterenol (Iso) induces a state of excess myocardial substrate uptake relative to the rate of substrate metabolism, reflected by a sum of OERs significantly >1.0. This occurs in conjunction with an Iso-stimulated increase in circulating insulin levels. The goal of the present study was to determine whether this excess substrate uptake depends on the effects of insulin and time. In open-chest anesthetized pigs, myocardial blood flow, substrate uptake, and oxygen consumption were measured at baseline and during systemic administration of Iso (0.08 microgram. kg-1. min-1 iv) under the following conditions: group 1 (n = 10), normal endogenous insulin release; group 2 (n = 10), inhibition of endogenous insulin release with somatostatin; group 3 (n = 7), at 45 and 90 min Iso; group 4 (n = 7), at 45 and 90 min Iso, with exogenous insulin given during the latter measurement. In group 1, plasma insulin rose fivefold with Iso while the sum of the OERs for glucose, lactate, and FFA increased from 0.92 +/- 0.21 at baseline to 1.57 +/- 0.17 with Iso (P < 0.01). In group 2, somatostatin blunted the increase in insulin with Iso and there was no significant change in the sum of OERs between baseline and Iso. In group 3, the sum of OERs increased from 0.95 +/- 0.11 at baseline to 1.69 +/- 0.20 at 45 min Iso (P < 0.01), similar to the response of group 1. However, the state of excess substrate uptake was transient; by 90 min Iso the sum of OERs declined to 0.69 +/- 0.21 (P < 0.05 vs. 45 min Iso). In group 4, excess substrate uptake could not be sustained at 90 min Iso despite administration of exogenous insulin. Systemic beta-adrenergic stimulation causes a transient condition of myocardial substrate uptake in excess of metabolism. Increased plasma insulin is necessary to produce this condition, but a high insulin level does not prolong the condition.

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.

Exercise tolerance is lower in type I diabetics compared with normal young men

The present investigation was conducted to study metabolic and hormonal responses to prolonged exercise to exhaustion in insulin-dependent diabetic subjects. Sixteen healthy subjects (control) and 15 diabetics with no-insulin administration for 12 hours were studied. They were submitted to short-term exercise to exhaustion on a cycle ergometer at 55% to 60% of maximum oxygen consumption (VO2max). Exercise tolerance was significantly lower in diabetic subjects (66 +/- 6.7 v 117 +/- 9.4 minutes), and glucose concentration was significantly higher in these subjects. At exhaustion, only diabetic subjects showed a significant decrease in glycemia (142 +/- 20 v 111 +/- 16 mg/dL). Lactate concentration increased significantly during exercise up to 30 minutes, but at exhaustion only control subjects showed a reduction. No significant difference in free fatty acid (FFA) concentrations was observed between the groups during a 30-minute exercise period; however, at exhaustion levels were significantly higher in control subjects. Prolactin and C-peptide concentrations were significantly lower in diabetic subjects, whereas glucagon concentration was higher. No significant differences between the groups were observed for cortisol and growth hormone (GH) concentrations. We conclude that (1) diabetic subjects show reduced exercise tolerance when no insulin is administered for 12 hours, and (2) exercise to exhaustion reduces serum glucose concentrations in insulin-dependent diabetics.

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.

Plasma glucose metabolism during exercise in humans

Plasma glucose is an important energy source in exercising humans, supplying between 20 and 50% of the total oxidative energy production and between 25 and 100% of the total carbohydrate oxidised during submaximal exercise. Plasma glucose utilisation increases with the intensity of exercise, due to an increase in glucose utilisation by each active muscle fibre, an increase in the number of active muscle fibres, or both. Plasma glucose utilisation also increases with the duration of exercise, thereby partially compensating for the progressive decrease in muscle glycogen concentration. When compared at the same absolute exercise intensity (i.e. the same VO2), reliance on plasma glucose is also greater during exercise performed with a small muscle mass, i.e. with the arms or just 1 leg. This may be due to differences in the relative exercise intensity (i.e. the %VO2peak), or due to differences between the arms and legs in their fitness for aerobic activity. The rate of plasma glucose utilisation is decreased when plasma free fatty acid or muscle glycogen concentrations are very high, effects which are probably mediated by increases in muscle glucose-6-phosphate concentration. However, glucose utilisation is also reduced during exercise following a low carbohydrate diet, despite the fact that muscle glycogen is also often lower. When exercise is performed at the same absolute intensity before and after endurance training, plasma glucose utilisation is lower in the trained state. During exercise performed at the same relative intensity, however, glucose utilisation may be lower, the same, or actually higher in trained than in untrained subjects, because of the greater absolute VO2 and demand for substrate in trained subjects during exercise at a given relative exercise intensity. Although both hyperglycaemia and hypoglycaemia may occur during exercise, plasma glucose concentration usually remains relatively constant. Factors which increase or decrease the reliance of peripheral tissues on plasma glucose during exercise are therefore generally accompanied by quantitatively similar increases or decreases in glucose production. These changes in total glucose production are mediated by changes in both hepatic glycogenolysis and hepatic gluconeogenesis. Glycogenolysis dominates under most conditions, and is greatest early in exercise, during high intensity exercise, or when dietary carbohydrate intake is high. The rate of gluconeogenesis is increased when exercise is prolonged, preceded by a restricted carbohydrate intake, or performed with the arms. Both glycogenolysis and gluconeogenesis appear to be decreased by endurance exercise training. These effects are due to changes in both the hormonal milieu and in the availability of hepatic glycogen and gluconeogenic precursors.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatially resolved changes in diabetic rat skeletal muscle metabolism in vivo studied by 31P-n.m.r. spectroscopy

Phase-modulated rotating-frame imaging (p.m.r.f.i.), a localization technique for 31P-n.m.r. spectroscopy, has been applied to obtain information on the heterogeneity of phosphorus-containing metabolites and pH in the skeletal muscle of control and streptozotocin-diabetic rats. Using this method, the metabolic changes in four spatially resolved longitudinal slices (where slice I is superficial and slice IV is deep muscle) through the ankle flexor muscles have been investigated at rest and during steady-state isometric twitch-contraction at 2 Hz. At rest, intracellular pH was lower, and phosphocreatine (PCr)/ATP was higher, throughout the muscle mass in diabetic compared with control animals. The change in PCr/ATP in diabetic muscle correlated with a decrease in the chemically determined ATP concentration. During the muscle stimulation period, the decrease in pH observed in diabetic muscle at rest was maintained, but not exacerbated, by the contractile stimulus. Stimulation of muscle contraction caused more marked changes in PCr/(PCr + Pi), PCr/ATP and Pi/ATP in the diabetic group. These changes were most evident in slice III, which contains the greatest proportion of fast glycolytic-oxidative (type IIa) fibres, in which statistically significant differences were observed for all metabolite ratios. The results presented suggest that some degree of heterogeneity occurs in diabetic skeletal muscle in vivo with respect to the extent of metabolic dysfunction caused by the diabetic insult and that regions of the muscle containing high proportions of type IIa fibres appear to be most severely affected.

Meal-feeding and physical effort. 1. Metabolic changes induced by exercise training

To evaluate the consequences of the combination of meal-feeding (which causes in the long term several adaptations that lead to saving stored energetic substrates), rats subjected to a 2-hr feeding/22-hr fasting schedule were forced to swim 30 min everyday at a fixed hour during four weeks. The results indicate that meal-fed exercised rats: 1) increase food intake above that found in the nonexercising and the corresponding (nonfood-restricted) controls; 2) did not lose weight (in contrast to the controls); 3) initially had a high glycogen mobilization but at the end of the fourth week started to save hepatic glycogen again, despite the intense energy demanding exercise; 4) maintained a slight hyperglycemia; 5) mobilized less free fatty acids than the nonexercising meal-fed rats, probably due to higher insulinemia; 6) had a lower content of ascorbic acid in the adrenal glands in comparison to the control exercising rats; this suggests that the exercise was less stressful in the latter group.

Meal-feeding and physical effort. 2. Metabolic changes induced by an acute exercise

In this study, rats under meal-feeding up to four weeks were submitted to a sudden exercise (swimming) to evaluate the effects of a behavior that requires mobilization of a large amount of energy on some physiological parameters already changed due to the chronic food-restriction. During exercise meal-fed rats: 1) increase the rate of gastric emptying; 2) maintain glycemia more steadily than the controls even during a long-lasting exercise; 3) maintain high liver glycogen concentration and its mobilization starts later on; 4) free fatty acid mobilization is lower than in the controls but during exercise do use much more; 5) keep more glycogen in the muscles (including the heart) than the controls but during the exercise do utilize much more; 6) are slightly less stressful (mainly after a longer exercise) than the controls as suggested by the adrenal ascorbic acid content.

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.

The effect of controlled feeding conditions on the metabolic characteristics of rats

To evaluate the effect of feeding conditions on blood glucose, insulin and free fatty acid concentrations, rats were maintained on a 2-hr feeding/22-hr fasting (regular-fasted) schedule for 4 weeks. The animals were then subjected to glucose or insulin loads immediately prior to the usual meal time. Animals fasted for only 22 hr (single-fasted) just before the experiments, and rats having access to food ad lib were similarly loaded and tested. The results demonstrate that the regular fasting regime induces certain metabolic alterations well described in the literature for the single-fasted-period to become more pronounced, specifically, a reduction in insulin secretion and a probably increase in peripheral responsiveness to this hormone. In addition, glucose loading was more effective in lowering plasma free fatty acid concentration in rats restricted to a regular fasting scheme.

Bioenergetic changes during contraction and recovery in diabetic rat skeletal muscle

Phosphorus nuclear magnetic resonance (31P-NMR) spectroscopy was used to assess the effects of hypoinsulinemia on skeletal muscle during contraction in vivo. Five groups of rats were studied: age-matched (CONA) and weight-matched (CONW) nondiabetic controls; rats given streptozotocin 21 days before study (UD); diabetic rats treated with insulin for 21 days (ITD); and insulin-treated diabetic rats with insulin treatment withheld for 72 h before study (IWD). Both UD and IWD had similar alterations in plasma substrate concentrations and an impairment in the rate of glycogen resynthesis after the stimulation protocol compared with ITD, CONA, and CONW. Pyruvate oxidation was decreased by 30-40% in mitochondria isolated from gastrocnemius of the UD group, whereas no significant decrease was observed for mitochondria from the IWD (or ITD) group(s). In UD, maintenance of gastrocnemius muscle isometric twitch tension at 1 Hz required exaggerated decreases in phosphocreatine (PCr) concentration and pH; at 5 Hz, muscle performance declined significantly, and intracellular pH decreased to lower values than observed for the control groups; during recovery, no impairment of PCr resynthesis was observed. We conclude that in skeletal muscle of UD 1) at 1 Hz there is an increased reliance on glycolytic mechanisms of ATP resynthesis and 2) at 5 Hz force failure may occur because of the decreased rate of pyruvate utilization.