Absence of meal-induced insulin sensitization (AMIS) in aging rats is associated with cardiac dysfunction that is protected by antioxidants

Hepatalin: the missing link in prediabetes, obesity, and type 2 diabetes

Hepatalin is a hormone secreted by the liver in response to pulses of insulin after a mixed nutrient meal, but only if the liver receives two permissive synergistic feeding signals from the stomach. Hepatalin stimulates glucose uptake and storage as glycogen in skeletal muscle, heart, and kidney but not liver, intestines, or adipocytes. Insulin acts primarily on liver and fat. Reduced hepatalin action results in postprandial hyperglycemia, compensatory elevation of insulin secretion, and a resultant shift in partitioning of nutrient energy storage from glycogen in muscle, to fat. Chronic hepatalin suppression leads to a predictable chronology of dysfunctions, first diagnosable as Absence of Meal-induced Insulin Sensitization (AMIS) which progresses to prediabetes, adiposity, and type 2 diabetes. The focus on nutrient partitioning and the role of hepatalin allows AMIS to be diagnosed, prevented, and treated, including through the use of lifestyle interventions.

Fatty Liver and Fatty Heart—Where do They Stand in the AMIS Syndrome?

Meal-induced insulin sensitization (MIS) refers to the augmented glucose uptake response to insulin following a meal. Absence of MIS (AMIS) causes significant decrease in post-meal glucose disposal leading to postprandial hyperglycemia, hyperinsulinemia, hyperlipidemia, adiposity, increased free radical stress, and a cluster of progressive metabolic, vascular, and cardiac dysfunctions referred to as the AMIS syndrome. We tested the hypothesis that fat accumulation in the liver and heart is part of the AMIS syndrome. Questions examined in the study: (1) Is prediabetic fat accumulation in the heart and liver a component of the AMIS syndrome? (2) Is fatty liver a cause or consequence of peripheral insulin resistance? (3) Is early cardiac dysfunction in the AMIS syndrome attributable to fat accumulation in the heart? and (4) Can the synergistic antioxidant cocktail SAMEC (S-adenosylmethionine, vitamin E, and vitamin C), known to benefit MIS, affect cardiac and hepatic triglyceride levels? Four animal models of AMIS were used in aged male Sprague-Dawley rats (52 weeks ± sucrose ± SAMEC), compared with young controls (nine weeks). Fat accumulation in the heart was not significant and therefore cannot account for the early cardiac dysfunction. Hepatic triglycerides increased only in the most severe AMIS model but the small changes correlated with the much more rapidly developing peripheral adiposity. Systemic adiposity represents an early stage, whereas accumulation of cardiac and hepatic triglycerides represents a late stage of the prediabetic AMIS syndrome. Fat accumulation in the liver is a consequence, not a cause, of AMIS. SAMEC protected against the sucrose effects on whole body adiposity and hepatic lipid accumulation.

A synergistic, balanced antioxidant cocktail, protects aging rats from insulin resistance and absence of meal-induced insulin sensitization (AMIS) syndrome

A series of in vivo and in vitro studies using animal and human models in the past 15 years have demonstrated that approximately 55% (~66% in humans) of the glucose disposal effect of an i.v. injection of insulin in the fed state is dependent on the action of a second hormone, hepatic insulin sensitizing substance (HISS), which is released from the liver and stimulates glucose uptake in muscle, heart and kidneys. Sensitization of the insulin response by a meal through release of HISS is called meal-induced insulin sensitization (MIS). Absence of HISS action results in postprandial hyperglycemia, hyperinsulinemia, hyperlipidemia, adiposity, increased free radical stress and a cluster of progressive metabolic and cardiovascular dysfunctions referred to as the AMIS (absence of meal-induced insulin sensitization) syndrome. Reduced HISS release accounts for the insulin resistance that occurs with aging and is made worse by physical inactivity and diets high in sucrose or fat. This brief review provides an update of major metabolic disturbances associated with aging due to reduction of HISS release, and the protection against these pathological changes in aging animals using a balanced synergistic antioxidant cocktail SAMEC (S-adenosylmethionine, vitamins E and C). The synergy amongst the components is consistent with the known benefits of antioxidants supplied by a mixed diet and acting through diverse mechanisms. Using only three constituents, SAMEC appears suitable as an antioxidant specifically targeting the AMIS syndrome.

Obesity as an Early Symptom of the AMIS Syndrome

We review evidence that the AMIS (Absence of Meal-induced Insulin Sensitization) syndrome describes a paradigm fundamental to development of obesity. The hypoglycemic response to a pulse of insulin is doubled after a meal as a result of Hepatic Insulin Sensitizing Substance (HISS), released from the liver to act selectively on muscle, heart and kidney. In the absence of HISS action, the hypoglycemic response to insulin is the same as in the fasted state, and only half of what it should be. Postprandial hyperglycemia ensues, with compensatory hyperinsulinemia, resultant hyperlipidemia and elevated free radical stress. Storage of nutrient energy shifts from glycogen in muscle to fat. Chronic AMIS results in adiposity, occurs with age, is accelerated with sucrose supplement, and prevented by a synergistic antioxidant. Exercise reverses AMIS, as do pharmaceuticals that mimic the "feeding signals". The AMIS syndrome develops as a sequence of pathologies based on the consequences of absence of HISS action, including adiposity as the earliest symptom. Cardiac dysfunction, hypertension, hypercholesterolemia, and fatty liver are related to lack of HISS action. The AMIS syndrome hypothesis is mechanistic-based and accounts for the major pathologies associated with prediabetes, obesity, diabetes and metabolic syndrome. AMIS can be diagnosed, prevented and treated.

Interaction of antioxidants and exercise on insulin sensitivity in healthy and prediabetic rats

Meal-induced insulin sensitization (MIS) describes the augmented postprandial response to insulin through action of the hepatic insulin sensitizing substance (HISS). HISS-action is impaired in insulin resistance associated with aging and type 2 diabetes, but could be preserved by the antioxidant cocktail SAMEC, along with voluntary exercise. In this study, we tested whether antioxidant supplementation during voluntary training would interact with the effects of exercise on HISS-mediated glucose uptake in healthy and prediabetic rats. The 7-day voluntary running-wheel training was used as an exercise intervention. SAMEC supplementation was provided only during the 7-day training session. The rapid insulin sensitivity test (RIST) was conducted to determine insulin- and HISS-dependent glucose uptake in 14-week-old healthy rats, and sucrose-induced insulin-resistant rats, with or without exercise in the presence or absence of SAMEC supplementation. The postprandial insulin sensitivity was increased by exercise, primarily through enhancement of the HISS-dependent glucose uptake, which remained unaffected by SAMEC. SAMEC supplementation did not either harm or add benefit to the positive effects of exercise on insulin sensitivity in healthy or prediabetic rats. While SAMEC alone was a demonstrated preventive against the progressive loss of HISS action in previous studies, short-term supplementation in this study did not reverse the established disease state.

Lifestyle impact on meal-induced insulin sensitization in health and prediabetes: A focus on diet, antioxidants, and exercise interventions

The augmented whole-body glucose uptake response to insulin during the postprandial state is described as meal-induced insulin sensitization (MIS). MIS occurs when the presence of food in the upper gastrointestinal tract activates 2 feeding signals (activation of hepatic parasympathetic nerves and elevation of hepatic glutathione level), and causes insulin to release hepatic insulin sensitizing substance (HISS), which stimulates glucose uptake in skeletal muscle, heart, and kidneys. HISS action results in nutrient storage, primarily as glycogen. Impairment of HISS release results in the absence of meal-induced insulin sensitization (AMIS), which causes postprandial hyperglycemia and hyperinsulinemia, and chronically leads to the progression to a cluster of metabolic, vascular, and cardiac dysfunctions, which we refer to as components of the AMIS syndrome. Manipulation of the MIS process in health and in disease, by pharmacological and nonpharmacological interventions, is outlined in this review. High fat or sugar supplemented diet reduces MIS; exercise elevates MIS; and antioxidants protect MIS against reductions associated with diet and age.

Mitochondria and the aging heart

The average human life span has markedly increased in modern society largely attributed to advances in medical and therapeutic sciences that have successfully reduced important health risks. However, advanced age results in numerous alterations to cellular and subcellular components that can impact the overall health and function of an individual. Not surprisingly, advanced age is a major risk factor for the development of heart disease in which elderly populations observe increased morbidity and mortality. Even healthy individuals that appear to have normal heart function under resting conditions, actually have an increased susceptibility and vulnerability to stress. This is confounded by the impact that stress and disease can have over time to both the heart and vessels. Although, there is a rapidly growing body of literature investigating the effects of aging on the heart and how age-related alterations affect cardiac function, the biology of aging and underlying mechanisms remain unclear. In this review, we summarize effects of aging on the heart and discuss potential theories of cellular aging with special emphasis on mitochondrial dysfunction.

Cardioprotective effects of metformin and vildagliptin in adult rats with insulin resistance induced by a high-fat diet

Insulin resistance has been shown to be associated with cardiac sympathovagal imbalance, myocardial dysfunction, and cardiac mitochondrial dysfunction. Whereas metformin is a widely used antidiabetic drug to improve insulin resistance, vildagliptin is a novel oral antidiabetic drug in a group of dipeptidyl peptidase-4 inhibitors in which its cardiac effect is unclear. This study aimed to determine the cardiovascular effects of metformin and vildagliptin in rats with insulin resistance induced by high-fat diet. Male Wistar rats were fed with either a normal diet or high-fat diet (n =24 each) for 12 wk. Rats in each group were divided into three subgroups to receive the vehicle, metformin (30 mg/kg, twice daily), or vildagliptin (3 mg/kg, once daily) for another 21 d. Heart rate variability (HRV), cardiac function, and cardiac mitochondrial function were determined and compared among these treatment groups. Rats exposed to a high-fat diet developed increased body weight, visceral fat, plasma insulin, cholesterol, oxidative stress, depressed HRV, and cardiac mitochondrial dysfunction. Metformin and vildagliptin did not alter body weight and plasma glucose levels but decreased the plasma insulin, total cholesterol, and oxidative stress levels. Although both metformin and vildagliptin attenuated the depressed HRV, cardiac dysfunction, and cardiac mitochondrial dysfunction, vildagliptin was more effective in this prevention. Furthermore, only vildagliptin prevented cardiac mitochondrial membrane depolarization caused by consumption of a high-fat diet. We concluded that vildagliptin is more effective in preventing cardiac sympathovagal imbalance and cardiac dysfunction, as well as cardiac mitochondrial dysfunction, than metformin in rats with insulin resistance induced by high-fat diet.

Exercise-induced cardiac performance in autoimmune (type 1) diabetes is associated with a decrease in myocardial diacylglycerol

One of the fundamental biochemical defects underlying the complications of diabetic cardiovascular system is elevation of diacylglycerol (DAG) and its effects on protein kinase C (PKC) signaling. It has been noted that exercise training attenuates poor cardiac performance in Type 1 diabetes. However, the role of PKC signaling in exercise-induced alleviation of cardiac abnormalities in diabetes is not clear. We investigated the possibility that exercise training modulates PKC-βII signaling to elicit its beneficial effects on the diabetic heart. bio-breeding diabetic resistant rats, a model reminiscent of Type 1 diabetes in humans, were randomly assigned to four groups: 1) nonexercised nondiabetic (NN); 2) nonexercised diabetic (ND); 3) exercised nondiabetic; and 4) exercised diabetic. Treadmill training was initiated upon the onset of diabetes. At the end of 8 wk, left ventricular (LV) hemodynamic assessment revealed compromised function in ND compared with the NN group. LV myocardial histology revealed increased collagen deposition in ND compared with the NN group, while electron microscopy showed a reduction in the viable mitochondrial fraction. Although the PKC-βII levels and activity were unchanged in the diabetic heart, the DAG levels were increased. With exercise training, the deterioration of LV structure and function in diabetes was attenuated. Notably, improved cardiac performance in training was associated with a decrease in myocardial DAG levels in diabetes. Exercise-induced benefits on cardiac performance in diabetes may be mediated by prevention of an increase in myocardial DAG levels.

Exercise enhancement of hepatic insulin-sensitising substance-mediated glucose uptake in diet-induced prediabetic rats

The sensitisation of insulin action in response to a meal (i.e. meal-induced insulin sensitisation, MIS) represents one of the major means of increased glucose disposal in peripheral tissues during the postprandial state. MIS occurs when the release of hepatic insulin-sensitising substance (HISS) stimulates skeletal muscle glucose uptake. Our previous study had demonstrated that the HISS pathway is impaired in age-associated insulin resistance, and in the rats which were part of that study, voluntary exercise improved the response to insulin by restoring HISS action. The present study tests the hypothesis that voluntary exercise would reverse insulin resistance in diet-induced models of insulin resistance, and that the benefits are attributed through the improvement in HISS action. In this study, two experimental diets, a high-fat diet (for 4 weeks) and 35 % sucrose solution (for 9 and 16 weeks), were used to induce insulin resistance in rats. These rats were assigned to the exercise/no-exercise intervention. The effect of 7 d voluntary running-wheel exercise was determined by measuring insulin- and HISS action in the exercised rats and comparing them with the non-exercised controls. Voluntary exercise reversed insulin resistance, caused by dietary manipulation, through restoration of the HISS action. The direct insulin action was not changed by either diet or exercise. The metabolic improvements and reduced adiposity correlated with the extent of reversal of HISS action induced by exercise. Exercise improves insulin sensitivity in diet-induced insulin resistance primarily by restoration of HISS-mediated glucose uptake.