Exercise at anaerobic threshold intensity and insulin secretion by isolated pancreatic islets of rats

Exercise and inactivity as modifiers of β cell function and type 2 diabetes risk

Exercise and regular physical activity are beneficial for the prevention and management of metabolic diseases such as obesity and type 2 diabetes, whereas exercise cessation, defined as deconditioning from regular exercise or physical activity that has lasted for a period of months to years, can lead to metabolic derangements that drive disease. Adaptations to the insulin-secreting pancreatic β-cells are an important benefit of exercise, whereas less is known about how exercise cessation affects these cells. Our aim is to review the impact that exercise and exercise cessation have on β-cell function, with a focus on the evidence from studies examining glucose-stimulated insulin secretion (GSIS) using gold-standard techniques. Potential mechanisms by which the β-cell adapts to exercise, including exerkine and incretin signaling, autonomic nervous system signaling, and changes in insulin clearance, will also be explored. We will highlight areas for future research.

Impacts of an Exercise Intervention on the Health of Pancreatic Beta-Cells: A Review

There is an increasing consensus that exercise is a medicine and that regular exercise can effectively improve and prevent metabolic diseases such as diabetes. Islet cells are the endocrine of the pancreas and vital to the development of diabetes. Decades of developmental research in exercise intervention and the health of islet cells confirmed that exercise exerts beneficial effects on the function, proliferation, and survival rate of islet cells. However, the precise exercise reference scheme is still elusive. To accomplish this goal, we searched and analyzed relevant articles, and concluded the precise exercise prescription treatments for various species such as humans, rats, and mice. Each exercise protocol is shown in the tables below. These exercise protocols form a rich pipeline of therapeutic development for exercise on the health of islet cells.

Combined Effects of High-Intensity Aerobic Exercise Training and Ziziphus jujuba Extract on Tissue Nesfatin-1 in Rats

Nesfatin-1 is involved in metabolic/feeding regulation and prevention of cardiovascular disease. Previous studies have shown that exercise and herb supplementation can influence nesfatin-1 concentration. The present study investigated the effects of high-intensity training (HIT) and Ziziphus jujuba (ZJ) extract on tissue nesfatin-1 in rats. Twenty-eight female rats were randomly assigned to one of four groups i.e. 1) Saline-Control (SC), 2) Saline-High Intensity Training (ST), 3) Ziziphus jujuba-Control (ZJC), and 4) Ziziphus jujuba-High Intensity Training (ZJT). Rats performed exercise on a treadmill and/or administered supplements intragastrically for 6 weeks, depending on group category. Seventy-two hours after the last training session, rats were anesthetized. Blood, hypothafi 2lamus tissue, heart and gastrocnemius muscles were sent to the laboratory for analyses. Significantly higher nesfatin-1 gene expression and concentration and ATP concentration were found in trained rat. HIT increased plasma High Density Lipoprotein (HDL) and insulin concentration and reduced plasma Triglyceride (TG) and cortisol. ZJ increased tissue nesftain-1 gene expression and concentration while only increasing heart ATP. The combination of exercise and ZJ showed an additive effect compared to each intervention alone on hypothalamus, heart and gastrocnemius NUCB2 gene expression, heart and gastrocnemius nesfatin-1 concentration, plasma HDL and cortisol concentration. The authors recommend both interventions as a means to improve cardiovascular health in rats with further work needed to confirm similar findings in homo sapiens.

Response of Liver Metabolic Pathways to Ketogenic Diet and Exercise Are Not Additive

PURPOSE

Studies suggest ketogenic diets (KD) produce favorable outcomes (health and exercise performance); however, most rodent studies have used a low-protein KD, which does not reflect the normal- to high-protein KD used by humans. Liver has an important role in ketoadaptation due to its involvement in gluconeogenesis and ketogenesis. This study was designed to test the hypothesis that exercise training (ExTr) while consuming a normal-protein KD (NPKD) would induce additive/synergistic responses in liver metabolic pathways.

METHODS

Lean, healthy male C57BL/6J mice were fed a low-fat control diet (15.9% kcal protein, 11.9% kcal fat, 72.2% kcal carbohydrate) or carbohydrate-deficient NPKD (16.1% protein, 83.9% kcal fat) for 6 wk. After 3 wk on the diet, half were subjected to 3-wk treadmill ExTr (5 d·wk, 60 min·d, moderate-vigorous intensity). Upon conclusion, metabolic and endocrine outcomes related to substrate metabolism were tested in liver and pancreas.

RESULTS

NPKD-fed mice had higher circulating β-hydroxybutyrate and maintained glucose at rest and during exercise. Liver of NPKD-fed mice had lower pyruvate utilization and greater ketogenic potential as evidenced by higher oxidative rates to catabolize lipids (mitochondrial and peroxisomal) and ketogenic amino acids (leucine). ExTr had higher expression of the gluconeogenic gene, Pck1, but lower hepatic glycogen, pyruvate oxidation, incomplete fat oxidation, and total pancreas area. Interaction effects between the NPKD and ExTr were observed for intrahepatic triglycerides, as well as genes involved in gluconeogenesis, ketogenesis, mitochondrial fat oxidation, and peroxisomal markers; however, none were additive/synergistic. Rather, in each instance the interaction effects showed the NPKD and ExTr opposed each other.

CONCLUSIONS

An NPKD and an ExTr independently induce shifts in hepatic metabolic pathways, but changes do not seem to be additive/synergistic in healthy mice.

Eugenol Improves Insulin Secretion and Content of Pancreatic Islets from Male Mouse

Glucose homeostasis is required for control of insulin secretion. Phenolic compounds improved glucose-stimulated insulin secretion (GSIS). Eugenol is a phenolic compound that may increase GSIS. So, it was decided to investigate the effect of eugenol on the insulin secretion and content of pancreatic islets from the male mice. In this experimental study, 3-month-old NMRI mice (20-25 g) were prepared. The pancreatic islets of Langerhans were isolated using the collagenase digestion method and divided into 12 groups: glucose 2.8, 5.6, and 16.7 mM, glucose 2.8 mM + eugenol 50, 100, and 200 µM, glucose 5.6 mM + eugenol 50, 100, and 200 µM, and glucose 16.7 mM + eugenol 50, 100, and 200 µM. The islet's insulin secretion and content were measured after 1 hour and 24 hours incubation at 37°C, respectively, by the ELISA assays method and related commercial kit. Present results showed that all doses of eugenol increased islet's insulin secretion and content in the medium containing glucose concentrations 2.8, 5.6, and 16.7 mM (P < 0.05). In conclusion, eugenol as a phenolic compound increased insulin secretion and content of pancreatic islets. The moderate dose of this compound enhanced insulin secretion during hypo- and hyperglycemic conditions, as well as a high dose of eugenol, increased insulin content. Finally, present research suggested that the administration of eugenol 100 µM was suitable for the early stage of T2DM as well as eugenol 200 µM for the advanced stage of this disease.

Effects of leucine supplementation and resistance training on myopathy of diabetic rats

Leucine supplementation and resistance training positively influence the protein translation process and the cell signaling mTOR (mammalian target of rapamycin) pathway that regulates muscle protein balance and muscle remodeling, and thus may be therapeutic to diabetic myopathy. However, the effect of a combined intervention has not been well studied. Forty male Wistar rats were divided into five groups, control (C), diabetic control (D), diabetic + trained (DT), diabetic + L-leucine (DL), diabetic + L-leucine + trained (DLT). The supplementation of 5% leucine in chow, and resistance training were conducted for 8 weeks postweaning of rats. The extensor digitorum longus was used to assess signaling proteins involved in muscle protein synthesis, and the gastrocnemius and soleus were used for determination of muscle weight. Blood samples were collected for biochemical assays. Strength and ambulation tests were employed to evaluate motor performance. Results showed that both leucine supplementation and resistance training elevated the activity of mTOR-p70S6K in diabetic rats (P < 0.05). Moreover, though leucine supplementation in combination with resistance training demonstrated synergistic effects on p70S6K (P < 0.05), both treatments were capable of recovering motor performance (P < 0.05). In conclusion, 5% leucine supplementation combined with resistance training has the potential to attenuate muscle loss and motor performance decrements in diabetic rats, at least in part through increased protein synthesis.

Evaluation of Diabetogenic Mechanism of High Fat Diet in Combination with Arsenic Exposure in Male Mice

Obesity is a main reason of type 2 diabetes and also chronic exposure to arsenic (As) can produce diabetic symptoms. In previous studies, the association between high-fat diet and arsenic in the incidence of diabetes was found, but the role of beta cells activity, liver mitochondrial oxidative stress, and hepatic enzymes (leptin, adiponectin and beta amylase) was unclear. Thus, present study was conducted to evaluate the diabetogenic mechanism of arsenic followed by concomitant administration of high-fat diet (HFD) in male mice. In this experimental study, the mice consumed with HFD or low-fat diet (LFD) while exposed to As 25 or 50 ppm in drinking water for 20 weeks. At the end of experiments, hyperglycemia, insulin resistance variables, lipid profile, hepatic enzymes, liver mitochondrial oxidative stress, islet insulin secretion, liver, and pancreas histopathology were evaluated in all mice by their own methods. Control HFD fed mice showed a significant increase in FBG, OGTT, HOMA-IR, ITT, lipid profile, leptin, β-amylase, liver mitochondrial oxidative stress, hepatic enzymes and decreased FPI, HOMA-β, adiponectin, and islet insulin secretion or content. However, exposure to HFD concomitant with Arsenic revealed an impressive reduction in FBG, FPI, HOMA-IR, HOMA-β, ITT, lipid profile, and islet insulin secretion or content. This exposure enhanced OGTT, leptin, adiponectin, liver mitochondrial oxidative stress, and hepatic enzymes. In conclusion, HFD and arsenic concomitant administration induced impairment of OGTT and islet insulin secretion or content through the mitochondrial oxidative stress.

The Lactate Minimum Test: Concept, Methodological Aspects and Insights for Future Investigations in Human and Animal Models

In 1993, Uwe Tegtbur proposed a useful physiological protocol named the lactate minimum test (LMT). This test consists of three distinct phases. Firstly, subjects must perform high intensity efforts to induce hyperlactatemia (phase 1). Subsequently, 8 min of recovery are allowed for transposition of lactate from myocytes (for instance) to the bloodstream (phase 2). Right after the recovery, subjects are submitted to an incremental test until exhaustion (phase 3). The blood lactate concentration is expected to fall during the first stages of the incremental test and as the intensity increases in subsequent stages, to rise again forming a “U” shaped blood lactate kinetic. The minimum point of this curve, named the lactate minimum intensity (LMI), provides an estimation of the intensity that represents the balance between the appearance and clearance of arterial blood lactate, known as the maximal lactate steady state intensity (iMLSS). Furthermore, in addition to the iMLSS estimation, studies have also determined anaerobic parameters (e.g., peak, mean, and minimum force/power) during phase 1 and also the maximum oxygen consumption in phase 3; therefore, the LMT is considered a robust physiological protocol. Although, encouraging reports have been published in both human and animal models, there are still some controversies regarding three main factors: (1) the influence of methodological aspects on the LMT parameters; (2) LMT effectiveness for monitoring training effects; and (3) the LMI as a valid iMLSS estimator. Therefore, the aim of this review is to provide a balanced discussion between scientific evidence of the aforementioned issues, and insights for future investigations are suggested. In summary, further analyses is necessary to determine whether these factors are worthy, since the LMT is relevant in several contexts of health sciences.

Effect of moderate and high intensity chronic exercise on the pancreatic islet morphometry in healthy rats: BDNF receptor participation

The function and morphology of β-cells is largely dependent on insulin demand. As β-cells cover a bigger cell proportion in pancreas islets, changes of insulin producer cells affect the whole pancreatic islet morphology. Growth factors as the neurotrophins regulate the pancreas physiology, besides; physical exercise increases insulin sensitivity, and further modifies brain derived neurotrophic factor (BDNF) concentration in plasma. The aim of this study was to investigate the effects of chronic exercise (running in a treadmill for 8 weeks) intensity on pancreatic islet morphometry in healthy state. The BDNF receptor effect on the pancreatic islet morphometry was also evaluated. Adult male Wistar rats were divided in 6 groups: Control (C); moderate intensity training (MIT); high intensity training (HIT) did not treat with BDNF receptor inhibitor (K252a), and C, MIT and HIT treated with K252a. The results shown that chronic exercise induces β-cells hypertrophy without BDNF receptor participation. On the other hand, the moderate exercise increases the number of β cells per islet; the last effect does not require TrkB participation. In sedentary conditions, the K252a treatment reduced the β-cell density. Exercise intensity has differential effects on pancreas islet morphometry in healthy model; furthermore, BDNF receptor plays a role to maintain the amount of β-cells in sedentary state.

Effects of butyric acid and arsenic on isolated pancreatic islets and liver mitochondria of male mouse

AIM

The aim of the present study was to evaluate the different doses of Butyric acid (BA) and Arsenic (As) in liver mitochondria oxidative stress and pancreatic islet insulin secretion of male mouse.

BACKGROUND

BA is found in many foods and As as a toxic metal is present in drinking water. They can induce oxidative stress in tissues.

METHODS

In this experimental study, Liver mitochondria were isolated by administration of the different centrifugation method and pancreatic islets were isolated by collagenase method. Mitochondria were incubated by BA (35, 75, 150, 300 μM) and As (20, 50, 100, 200 μM) as the islets were incubated by BA (250, 500, 1000, 1500 μM) and As (50, 100, 200 μM) for 1 hour. At the end of the experiment, mitochondrial viability and membrane potential, ROS, MDA, GSH and islets insulin secretion were measured by their specific methods.

RESULTS

BA and As administration increased mitochondrial levels of ROS, MDA and decreased GSH and pancreatic islet insulin secretion in a dose dependent manner (p<0.05). The doses of BA 75μM and As 100μM have been revealed the most mitochondria toxic concentrations. Also, the doses of 1000μM for BA and 100μM for As were considered as reducing concentrations for islets insulin secretion. Additionally, co administration of them intensified more these effects.

CONCLUSION

Alone or in combination administration of BA and As induced oxidative stress in liver mitochondria and decreased insulin secretion of pancreatic islets.

Potential role of skeletal muscle glucose metabolism on the regulation of insulin secretion

Pancreatic beta cells sense glucose flux and release as much insulin as required in order to maintain glycaemia within a narrow range. Insulin secretion is regulated by many factors including glucose, incretins, and sympathetic and parasympathetic tones among other physiological factors. To identify the mechanisms linking obesity-related insulin resistance with impaired insulin secretion represents a central challenge. Recently, it has been argued that a crosstalk between skeletal muscle and the pancreas may regulate insulin secretion. Considering that skeletal muscle is the largest organ in non-obese subjects and a major site of insulin- and exercise-stimulated glucose disposal, it appears plausible that muscle might interact with the pancreas and modulate insulin secretion for appropriate peripheral intracellular glucose utilization. There is growing evidence that muscle can secrete so-called myokines that can have auto/para/endocrine actions. Although it is unclear in which direction they act, interleukin-6 seems to be a possible muscle-derived candidate protein mediating such inter-organ communication. We herein review some of the putative skeletal muscle-derived factors mediating this interaction. In addition, the evidence coming from in vitro, animal and human studies that support such inter-organ crosstalk is thoroughly discussed.

Effects of glucocorticoids and exercise on pancreatic β-cell function and diabetes development

Peripheral insulin resistance and pancreatic β-cell dysfunction are hallmark characteristics of type 2 diabetes mellitus (T2DM). Several contributing factors have been proposed to promote these two defects in individuals with T2DM, including physical inactivity and chronic exposure to various psychosocial factors that increase the body's exposure to glucocorticoids, the main stress hormones in humans. Initially, β-cells have been shown to adapt to these stimuli, a phenomenon known as β-cell 'compensation'. However, long-term exposure to these physiologic and psychological stressors induces islet failure. Interestingly, glucocorticoids stimulate β-cell mass growth in parallel with promoting severe insulin resistance, the former being an important adaptive response to the latter. The direct relationship between glucocorticoids and β-cell dysfunction remains a controversial area of research. Elevations in circulating and/or tissue specific glucocorticoids have been associated with the development of obesity and T2DM in human and rodent models; however, the progression from insulin resistance to overt T2DM is highly disputed with respect to the in vivo and in vitro effects of glucocorticoids. Paradoxically, both intermittent physical stress and regular exercise alleviate insulin resistance and help to preserve β-cell mass, potentially by lowering glucocorticoid levels. Recent studies have begun to examine the mechanisms of intermittent and chronic glucocorticoid exposure and regular exercise in altering β-cell function. This review highlights recent discoveries on the physiological regulation of β-cells and diabetes development in conditions of elevated glucocorticoids, regular exercise and intermittent stress.

Physical exercise and pancreatic islets: acute and chronic actions on insulin secretion

Diabetes mellitus (DM) is a great public health problem, which attacks part of the world population, being characterized by an imbalance in body glucose homeostasis. Physical exercise is pointed as a protective agent and is also recommended to people with DM. As pancreatic islets present an important role in glucose homeostasis, we aim to study the role of physical exercise (chronic adaptations and acute responses) in pancreatic islets functionality in Wistar male rats. First, animals were divided into two groups: sedentary (S) and aerobic trained (T). At the end of 8 weeks, half of them (S and T) were submitted to an acute exercise session (exercise until exhaustion), being subdivided as acute sedentary (AS) and acute trained (AT). After the experimental period, periepididymal, retroperitoneal and subcutaneous fat pads, blood, soleus muscle and pancreatic islets were collected and prepared for further analysis. From the pancreatic islets, total insulin content, insulin secretion stimulated by glucose, leucine, arginine and carbachol were analyzed. Our results pointed that body adiposity and glucose homeostasis improved with chronic physical exercise. In addition, total insulin content was reduced in group AT, insulin secretion stimulated by glucose was reduced in trained groups (T and AT) and insulin secretion stimulated by carbachol was increased in group AT. There were no significant differences in insulin secretion stimulated by arginine and leucine. We identified a possible modulating action on insulin secretion, probably related to the association of chronic adaptation with an acute response on cholinergic activity in pancreatic islets.

Feed restriction and a diet's caloric value: The influence on the aerobic and anaerobic capacity of rats

BACKGROUND

The influence of feed restriction and different diet's caloric value on the aerobic and anaerobic capacity is unclear in the literature. Thus, the objectives of this study were to determine the possible influences of two diets with different caloric values and the influence of feed restriction on the aerobic (anaerobic threshold: AT) and anaerobic (time to exhaustion: Tlim) variables measured by a lactate minimum test (LM) in rats.

METHODS

We used 40 adult Wistar rats. The animals were divided into four groups: ad libitum commercial Purina® diet (3028.0 Kcal/kg) (ALP), restricted commercial Purina® diet (RAP), ad libitum semi-purified AIN-93 diet (3802.7 Kcal/kg) (ALD) and restricted semi-purified AIN-93 diet (RAD). The animals performed LM at the end of the experiment, 48 h before euthanasia. Comparisons between groups were performed by analysis of variance (p < 0,05).

RESULTS

At the end of the experiment, the weights of the rats in the groups with the restricted diets were significantly lower than those in the groups with ad libitum diet intakes. In addition, the ALD group had higher amounts of adipose tissue. With respect to energetic substrates, the groups subjected to diet restriction had significantly higher levels of liver and muscle glycogen. There were no differences between the groups with respect to AT; however, the ALD group had lower lactatemia at the AT intensity and higher Tlim than the other groups.

CONCLUSIONS

We conclude that dietary restriction induces changes in energetic substrates and that ad libitum intake of a semi-purified AIN-93 diet results in an increase in adipose tissue, likely reducing the density of the animals in water and favouring their performance during the swimming exercises.

Endurance training stimulates growth and survival pathways and the redox balance in rat pancreatic islets

Endurance training has been shown to increase pancreatic β-cell function and mass. However, whether exercise modulates β-cell growth and survival pathways signaling is not completely understood. This study investigated the effects of exercise on growth and apoptotic markers levels in rat pancreatic islets. Male Wistar rats were randomly assigned to 8-wk endurance training or to a sedentary control group. After that, pancreatic islets were isolated; gene expression and the total content and phosphorylation of several proteins related to growth and apoptotic pathways as well as the main antioxidant enzymes were determined by real-time polymerase chain reaction and Western blot analysis, respectively. Reactive oxygen species (ROS) production was measured by fluorescence. Endurance training increased the time to reach fatigue by 50%. Endurance training resulted in increased protein phosphorylation content of AKT (75%), AKT substrate (AS160; 100%), mTOR (60%), p70s6k (90%), and ERK1/2 (50%), compared with islets from control group. Catalase protein content was 50% higher, whereas ROS production was 49 and 77% lower in islets from trained rats under basal and stimulating glucose conditions, respectively. Bcl-2 mRNA and protein levels increased by 46 and 100%, respectively. Bax and cleaved caspase-3 protein contents were reduced by 25 and 50% in islets from trained rats, respectively. In conclusion, these results demonstrate that endurance training favors the β-cell growth and survival by activating AKT and ERK1/2 pathways, enhancing antioxidant capacity, and reducing ROS production and apoptotic proteins content.

Exercise increases insulin content and basal secretion in pancreatic islets in type 1 diabetic mice

Exercise appears to improve glycemic control for people with type 1 diabetes (T1D). However, the mechanism responsible for this improvement is unknown. We hypothesized that exercise has a direct effect on the insulin-producing islets. Eight-week-old mice were divided into four groups: sedentary diabetic, exercised diabetic, sedentary control, and exercised control. The exercised groups participated in voluntary wheel running for 6 weeks. When compared to the control groups, the islet density, islet diameter, and β-cell proportion per islet were significantly lower in both sedentary and exercised diabetic groups and these alterations were not improved with exercise. The total insulin content and insulin secretion were significantly lower in sedentary diabetics compared to controls. Exercise significantly improved insulin content and insulin secretion in islets in basal conditions. Thus, some improvements in exercise-induced glycemic control in T1D mice may be due to enhancement of insulin content and secretion in islets.