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The regulation of glucose metabolism: Implications and considerations for the assessment of glucose homeostasis in rodents
Abstract and Figures
The incidence of insulin resistance and type 2 diabetes (T2D) is increasing at alarming rates. In the quest to understand the underlying causes of and to identify novel therapeutic targets to treat T2D, scientists have become increasingly reliant on the use of rodent models. Here we provide a discussion on the regulation of rodent glucose metabolism, highlighting key differences and similarities that exist between rodents and humans. In addition, some of the issues and considerations associated with assessing glucose homeostasis and insulin action are outlined. We also discuss the role of the liver versus skeletal muscle in regulating whole-body glucose metabolism in rodents, emphasizing the importance of defective hepatic glucose metabolism in the development of impaired glucose tolerance, insulin resistance and T2D.
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... These models facilitate the observation of the intricate aspects underlying the pathologies involved [90,91]. Rodents are commonly employed as animal models to explore these abnormalities due to their ready availability, short generation interval, small size, and relatively low costs [92,93]. According to animal studies, SFN could reduce glucose production and inhibit gluconeogenesis, improve glucose tolerance and insulin resistance to modulate glucose metabolism homeostasis in mouse ( Table 2) and rat models (Table 3) [23,24]. ...
Abnormal glucose homeostasis is associated with metabolic syndromes including cardiovascular diseases, hypertension, type 2 diabetes mellitus, and obesity, highlighting the significance of maintaining a balanced glucose level for optimal biological function. This highlights the importance of maintaining normal glucose levels for proper biological functioning. Sulforaphane (SFN), the primary bioactive compound in broccoli from the Cruciferae or Brassicaceae family, has been shown to enhance glucose homeostasis effectively while exhibiting low cytotoxicity. This paper assesses the impact of SFN on glucose homeostasis in vitro, in vivo, and human trials, as well as the molecular mechanisms that drive its regulatory effects. New strategies have been proposed to enhance the bioavailability and targeted delivery of SFN in order to overcome inherent instability. The manuscript also covers the safety evaluations of SFN that have been documented for its production and utilization. Hence, a deeper understanding of the favorable influence and mechanism of SFN on glucose homeostasis, coupled with the fact that SFN is abundant in the human daily diet, may ultimately offer theoretical evidence to support its potential use in the food and pharmaceutical industries.
... 24 A maior concentração de TNF-α e os valores estereológicos do grupo S comparado aos grupos AE e PR sugerem processo inflamatório e alterações teciduais decorrentes do sedentarismo. [23][24][25][26][27] Estudos prévios 3,4 Tabela 4. Valores do menor diâmetro das fibras musculares (µm), análise da dimensão fractal (UA) e concentrações (%) de glicogênio, lipídios e TNF-α dos músculos sóleo e gastrocnêmio após o período experimental. observaram redução da expressão de IL-1β hepática pelo treinamento aeróbico. ...
Introduction
Physical exercise can be an alternative for preventing and treating the harmful effects of obesity, mainly inflammatory effects on skeletal muscle and liver tissues. However, no consensus exists regarding this purpose's best physical training model.
Objective
Evaluate morphological, metabolic, and inflammatory alterations in rats’ skeletal and hepatic muscle tissues caused by aerobic and resistance training.
Methods
24 Wistar rats were divided into sedentary (S), aerobic (AE), and resistance training (R) groups. Blood glucose, total cholesterol, and serum triglycerides were measured periodically. After euthanasia, body mass was measured to calculate the total mass gain during the experiment. High-density lipoprotein (HDL) was measured. Adipose tissue was extracted to calculate its percentage relative to body mass and the liver, soleus, and gastrocnemius muscles for morphological analyses and concentrations of glycogen, lipids, and Tumor Necrosis Factor α (TNF-α). The Kruskall-Wallis test and Dunn's post-test were performed for statistical analysis, adopting p<0.05.
Results
Both training models reduced the percentage of adipose tissue, body mass gain, and hepatic TNF-α concentration (p<0.05). AE increased serum HDL, gastrocnemius fiber diameter and reduced the fractal dimension in the soleus (p<0.05). R reduced blood glucose and serum and liver lipids, increased liver and soleus glycogen concentrations, increased gastrocnemius fiber diameter, and decreased TNF-α (p<0.05).
Conclusion
Both training models reduced body mass, relative visceral adipose tissue, serum total cholesterol concentration, and liver inflammation. However, resistance training was more effective in promoting metabolic effects in the liver and skeletal muscle and reducing muscle inflammation in rats. Level of Evidence V; Expert Opinion.
Keywords:
Resistance Training; Liver; Muscle, Skeletal; Tumor Necrosis Factor-alpha
... 24 The higher concentration of TNF-α and the stereological values of the S group compared to the AE and PR groups suggest an inflammatory process and tissue alterations resulting from a sedentary lifestyle. [23][24][25][26][27] Table 4. Values of the smallest diameter of muscle fibers (µm), fractal dimension analysis (UA) and concentrations (%) of glycogen, lipids and TNF-α in the soleus and gastrocnemius muscles after the experimental period. Previous studies 3,4 have observed a reduction in hepatic IL-1β expression due to aerobic training. ...
Introduction
Physical exercise can be an alternative for preventing and treating the harmful effects of obesity, mainly inflammatory effects on skeletal muscle and liver tissues. However, no consensus exists regarding this purpose's best physical training model.
Objective
Evaluate morphological, metabolic, and inflammatory alterations in rats’ skeletal and hepatic muscle tissues caused by aerobic and resistance training.
Methods
24 Wistar rats were divided into sedentary (S), aerobic (AE), and resistance training (R) groups. Blood glucose, total cholesterol, and serum triglycerides were measured periodically. After euthanasia, body mass was measured to calculate the total mass gain during the experiment. High-density lipoprotein (HDL) was measured. Adipose tissue was extracted to calculate its percentage relative to body mass and the liver, soleus, and gastrocnemius muscles for morphological analyses and concentrations of glycogen, lipids, and Tumor Necrosis Factor α (TNF-α). The Kruskall-Wallis test and Dunn's post-test were performed for statistical analysis, adopting p<0.05.
Results
Both training models reduced the percentage of adipose tissue, body mass gain, and hepatic TNF-α concentration (p<0.05). AE increased serum HDL, gastrocnemius fiber diameter and reduced the fractal dimension in the soleus (p<0.05). R reduced blood glucose and serum and liver lipids, increased liver and soleus glycogen concentrations, increased gastrocnemius fiber diameter, and decreased TNF-α (p<0.05).
Conclusion
Both training models reduced body mass, relative visceral adipose tissue, serum total cholesterol concentration, and liver inflammation. However, resistance training was more effective in promoting metabolic effects in the liver and skeletal muscle and reducing muscle inflammation in rats. Level of Evidence V; Expert Opinion.
Keywords:
Resistance Training; Liver; Muscle, Skeletal; Tumor Necrosis Factor-alpha
Diabetes mellitus type 1 (T1D) and type 2 (T2D) develop due to dysfunction of the Langerhans islet β-cells in the pancreas, and this dysfunction is mediated by oxidative, endoplasmic reticulum (ER), and mitochondrial stresses. Although the two types of diabetes are significantly different, β-cell failure and death play a key role in the pathogenesis of both diseases, resulting in hyperglycemia due to a reduced ability to produce insulin. In T1D, β-cell apoptosis is the main event leading to hyperglycemia, while in T2D, insulin resistance results in an inability to meet insulin requirements. It has been suggested that autophagy promotes β-cell survival by delaying apoptosis and providing adaptive responses to mitigate the detrimental effects of ER stress and DNA damage, which is directly related to oxidative stress. As people with diabetes are now living longer, they are more susceptible to a different set of complications. There has been a diversification in causes of death, whereby a larger proportion of deaths among individuals with diabetes is attributable to nonvascular conditions; on the other hand, the proportion of cancer-related deaths has remained stable or even increased in some countries. Due to the increasing cases of both T1D and T2D, these diseases become even more socially significant. Hence, we believe that search for any opportunities for control of this disease is an overwhelmingly important target for the modern science. We focus on two differences that are characteristic of the development of diabetes’s last periods. One of them shows that all-cause death rates have declined in several diabetes populations, driven in part by large declines in vascular disease mortality but large increases in oncological diseases. Another hypothesis is that some T2D medications could be repurposed to control glycemia in patients with T1D.
Glucagon's ability to promote hepatic glucose production has been known for over a century with initial observations touting this hormone as a diabetogenic agent. However, glucagon receptor agonism (when balanced with an incretin, including glucagon-like peptide 1 (GLP-1) to dampen glucose excursions) is now being developed as a promising therapeutic target in the treatment of metabolic diseases, like metabolic dysfunction-associated steatotic disease/metabolic dysfunction-associated steatohepatitis (MASLD/MASH), and may also have benefit for obesity and chronic kidney disease. Conventionally regarded as the opposing tag-team partner of the anabolic mediator insulin, glucagon is gradually emerging as more than just a 'catabolic hormone'. Glucagon action on glucose homeostasis within the liver has been well characterized. However, growing evidence, in part thanks to new and sensitive 'omics' technologies, has implicated glucagon as more than just a 'glucose liberator'. Elucidation of glucagon's capacity to increase fatty acid oxidation while attenuating endogenous lipid synthesis speaks to the dichotomous nature of the hormone. Furthermore, glucagon action is not limited to just glucose homeostasis and lipid metabolism, as traditionally reported. Glucagon plays key regulatory roles in hepatic amino acid and ketone body metabolism, as well as mitochondrial turnover and function, indicating broader glucagon signaling consequences for metabolic homeostasis mediated by the liver. Here we examine the broadening role of glucagon signaling within the hepatocyte and question the current dogma, to appreciate glucagon as more than just that 'catabolic hormone'.
Hyperglucagonemia is a hallmark of type 2 diabetes (T2DM), yet the role of elevated plasma glucagon (P-GCG) to promote excessive post-absorptive glucose production and contribute to hyperglycemia in patients with this disease remains debatable. We investigated the acute action of P-GCG to safeguard/support post-absorptive endogenous glucose production (EGP) and euglycemia in healthy Zucker control lean (ZCL) rats. Using male Zucker diabetic fatty (ZDF) rats that exhibit the typical metabolic disorders of human T2DM, such as excessive EGP, hyperglycemia, hyperinsulinemia and hyperglucagonemia, we examined the ability of hyperglucagonemia to promote greater rates of post-absorptive EGP and hyperglycemia. Euglycemic or hyperglycemic basal insulin (INS-BC) and glucagon (GCG-BC) clamps were performed in the absence or during an acute setting of glucagon deficiency (GCG-DF, ~10% of basal), either alone or in combination with insulin deficiency (INS-DF, ~10% of basal). Glucose appearance, disappearance, and cycling rates were measured using [2- ³ H] and [3- ³ H]-glucose. In ZCL rats, GCG-DF reduced levels of hepatic cyclic AMP, EGP, and plasma glucose (PG) by 50, 32, and 50%, respectively. EGP fell in the presence GCG-DF and INS-BC, but under GCG-DF and INS-DF, EGP and PG increased two- and three-fold, respectively. GCG-DF revealed the hyperglucagonemia present in ZDF rats lacked the ability to regulate hepatic intracellular cyclic AMP levels and glucose flux, since EGP and PG levels fell by only 10%. We conclude that the liver in T2DM suffers from resistance to all three major regulatory factors, glucagon, insulin, and glucose, thus leading to a loss of metabolic flexibility.
Type 2 diabetes mellitus is characterised by resistance of peripheral tissues to insulin and a relative deficiency of insulin secretion. To find out which is the earliest or primary determinant of disease, we used a minimum model of glucose disposal and insulin secretion based on intravenous glucose tolerance tests to estimate insulin sensitivity (SI), glucose effectiveness (ie, insulin-independent glucose removal rate, SG), and first-phase and second-phase beta-cell responsiveness in normoglycaemic offspring of couples who both had type 2 diabetes. 155 subjects from 86 families were followed-up for 6-25 years. More than 10 years before the development of diabetes, subjects who developed the disease had lower values of both SI (mean 3.2 [SD 2.4] vs 8.1 [6.7] 10(-3) I min-1 pmol-1 insulin; p < 0.0001) and SG (1.6 [0.9] vs 2.3 [1.2] 10(-2) min-1, p < 0.0001) than did those who remained normoglycaemic). For the subjects with both SI and SG below the group median, the cumulative incidence of type 2 diabetes during the 25 years was 76% (95% confidence interval 54-99). By contrast, no subject with both SI and SG above the median developed the disease. Subjects with low SI/high SG or high SI/low SG had intermediate risks. Insulin secretion, especially first phase, tended to be increased rather than decreased in this prediabetic phase and was appropriate for the level of insulin resistance. The development of type 2 diabetes is preceded by and predicted by defects in both insulin-dependent and insulin-independent glucose uptake; the defects are detectable when the patients are normoglycaemic and in most cases more than a decade before diagnosis of disease.
Muscle and hepatic insulin resistance are two major defects of non-insulin-dependent diabetes mellitus. Dietary factors may be important in the etiology of insulin resistance. We studied progressive changes in the development of high-fat–diet–induced insulin resistance in tissues of the adult male Wistar rat. In vivo insulin action was compared 3 days and 3 wk after isocaloric synthetic high-fat or high-starch feeding (59 and 10% cal as fat, respectively). Basal and insulin-stimulated glucose metabolism were assessed in the conscious 5- to 7-h fasted state with the euglycemic clamp (600 pM insulin) with a [3-3H]-glucose infusion. Fat feeding significantly reduced suppressibility of hepatic glucose output by insulin after both 3 days and 3 wk of diet (P < 0.01). However, a significant impairment of insulin-mediated peripheral glucose disposal was only present after 3 wk of diet. Further in vivo [3H]-2-deoxyglucose uptake studies supported this finding and demonstrated adipose but not muscle insulin resistance after 3 days of high-fat feeding. Muscle triglyceride accumulation due to fat feeding was not significant at 3 days but had doubled by 3 wk in red muscle ( P < 0.001) compared with starch-fed controls. By 3 wk, high-fat—fed animals had developed significant glucose intolerance. We concludethat fat feeding induces insulin resistance in liver and adipose tissue before skeletal muscle with early metabolic changes favoring an oversupply of energy substrate to skeletal muscle relative to metabolic needs. This may generate later muscle insulin resistance.
Glucose production and utilization and activities of key enzymes involved in liver and muscle glucose metabolism were studied in post-absorptive streptozotocin-diabetic rats after 12 h of severe hyperglycaemia (17.5±0.5 mmol/l) and insulinopenia (5±1 ΜU/ml). Basal glucose production was increased: 36.6±3.0 mg·kg·min−1, vs 24.4±2.5 in controls (p<0.05); liver glycogen concentration was decreased by 40% (p<0.05); liver phosphoenolpyruvate carboxykinase and glucose-6-phosphatase activities were increased by 375 and 156%, respectively (p<0.001 and <0.01). During a euglycaemic clamp at a plasma insulin level of 200 ΜU/ml, glucose production was totally suppressed in controls, but persisted at 20% of basal in diabetic rats. In these rats, glucose production was suppressed at a plasma insulin level of 2500 ΜU/ml. Basal whole body glucose utilization rate, 2-deoxy-1-[3H]-d-glucose ([3H]-2dG) uptake by muscles and muscle glycogen concentrations were similar in both groups, as well as total and active forms of pyruvate dehydrogenase and glycogen synthase activities. During the euglycaemic clamp, the total body glucose utilization rates and [3H]-2dG uptake by muscles were similar in control and diabetic rats at a plasma insulin level of 200 ΜU/ ml, but lower in diabetic rats at a plasma insulin level of 2500 ΜU/ml. We conclude 1) in recent-onset severely insulinopenic rats, an excessive glucose production via gluconeogenesis prevailed, mainly accounting for the concomitant hyperglycaemia. This excess glucose output cannot be attributed to liver insulin resistance: the gluconeogenic pathway is physiologically less sensitive than glycogenolysis to the inhibition by insulin. 2) Glucose utilization was apparently normal under hyperglycaemic conditions and at a lower insulin plateau of the euglycaemic clamp but suboptimal in the presence of maximal insulin concentrations, suggesting an early appearance of peripheral insulin resistance.
Methods for the quantification of beta-cell sensitivity to glucose (hyperglycemic clamp technique) and of tissue sensitivity to insulin (euglycemic insulin clamp technique) are described. Hyperglycemic clamp technique. The plasma glucose concentration is acutely raised to 125 mg/dl above basal levels by a priming infusion of glucose. The desired hyperglycemic plateau is subsequently maintained by adjustment of a variable glucose infusion, based on the negative feedback principle. Because the plasma glucose concentration is held constant, the glucose infusion rate is an index of glucose metabolism. Under these conditions of constant hyperglycemia, the plasma insulin response is biphasic with an early burst of insulin release during the first 6 min followed by a gradually progressive increase in plasma insulin concentration. Euglycemic insulin clamp technique. The plasma insulin concentration is acutely raised and maintained at approximately 100 muU/ml by a prime-continuous infusion of insulin. The plasma glucose concentration is held constant at basal levels by a variable glucose infusion using the negative feedback principle. Under these steady-state conditions of euglycemia, the glucose infusion rate equals glucose uptake by all the tissues in the body and is therefore a measure of tissue sensitivity to exogenous insulin.