Increased mitochondrial glycerol-3-phosphate acyltransferase protein and enzyme activity in rat epididymal fat upon cessation of wheel running

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.

Preclinical rodent models of physical inactivity-induced muscle insulin resistance: challenges and solutions

Physical inactivity influences the development of muscle insulin resistance yet is far less understood than diet-induced muscle insulin resistance. Progress in understanding the mechanisms of physical inactivity-induced insulin resistance is limited by a lack of an appropriate preclinical model of muscle insulin resistance. Here, we discuss differences between diet and physical inactivity-induced insulin resistance, the advantages and disadvantages of the available rodent inactivity models to study insulin resistance, and our current understanding of the mechanisms of muscle insulin resistance derived from such preclinical inactivity designs. The burgeoning rise of health complications emanating from metabolic disease presents an alarming issue with mounting costs for health care and a reduced quality of life. There exists a pressing need for more complete understanding of mechanisms behind the development and progression of metabolic dysfunction. Since lifestyle modifications such as poor diet and lack of physical activity are primary catalysts of metabolic dysfunction, rodent models have been formed to explore mechanisms behind these issues. Particularly, the use of a high-fat diet has been pervasive and has been an instrumental model to gain insight into mechanisms underlying diet-induced insulin resistance (IR). However, physical inactivity (and to some extent muscle disuse) is an often overlooked and much less frequently studied lifestyle modification, which some have contended is the primary contributor in the initial development of muscle IR. In this mini-review we highlight some of the key differences between diet- and physical inactivity-induced development of muscle IR and propose reasons for the sparse volume of academic research into physical inactivity-induced IR including infrequent use of clearly translatable rodent physical inactivity models.

Effects of exercise cessation on adipose tissue physiological markers related to fat regain: A systematic review

Tissues usually super compensate during the period that follow physical exercise. Although this is widely accepted for muscle and glycogen, the compensatory effect is not usually applied to fat tissues. Notwithstanding, evidence for this has been present since the 1970s when it was first suggested that the increased lipogenic activity in response to training might be an adaptation that enables to restore an energy reserve that can be used in times of need. In this context, the present review aimed to summarize information about the effect of detraining on fat metabolism and the physiological responses associated with fat regain. A systematic search on PubMed and Scielo was performed using "training cessation," "detraining," "exercise detraining," and "exercise cessation" combined with "fat tissue," "adipose tissue," "adipose metabolism," and "fat metabolism," as descriptors. From 377 results, 25 were included in this review, 12 humans and 13 rodents, resulting in a sample of 6772 humans and 613 animals. The analysis provided evidence for fat super compensation, as well as differences in humans and rodents, among different protocols and possible mechanisms for fat gain after exercise cessation. In summary, exercise cessation appears to increase the ability of the adipose tissue to store energy. However, caution should be taken, especially regarding conclusions based on investigations on humans, considering the multiple factors that could affect fat metabolism.

Role of Inactivity in Chronic Diseases: Evolutionary Insight and Pathophysiological Mechanisms

This review proposes that physical inactivity could be considered a behavior selected by evolution for resting, and also selected to be reinforcing in life-threatening situations in which exercise would be dangerous. Underlying the notion are human twin studies and animal selective breeding studies, both of which provide indirect evidence for the existence of genes for physical inactivity. Approximately 86% of the 325 million in the United States (U.S.) population achieve less than the U.S. Government and World Health Organization guidelines for daily physical activity for health. Although underappreciated, physical inactivity is an actual contributing cause to at least 35 unhealthy conditions, including the majority of the 10 leading causes of death in the U.S. First, we introduce nine physical inactivity-related themes. Next, characteristics and models of physical inactivity are presented. Following next are individual examples of phenotypes, organ systems, and diseases that are impacted by physical inactivity, including behavior, central nervous system, cardiorespiratory fitness, metabolism, adipose tissue, skeletal muscle, bone, immunity, digestion, and cancer. Importantly, physical inactivity, itself, often plays an independent role as a direct cause of speeding the losses of cardiovascular and strength fitness, shortening of healthspan, and lowering of the age for the onset of the first chronic disease, which in turn decreases quality of life, increases health care costs, and accelerates mortality risk.

Curcumin limits weight gain, adipose tissue growth, and glucose intolerance following the cessation of exercise and caloric restriction in rats

Weight regain, adipose tissue growth, and insulin resistance can occur within days after the cessation of regular dieting and exercise. This phenomenon has been attributed, in part, to the actions of stress hormones as well as local and systemic inflammation. We investigated the effect of curcumin, a naturally occurring polyphenol known for its anti-inflammatory properties and inhibitory action on 11β-HSD1 activity, on preserving metabolic health and limiting adipose tissue growth following the cessation of daily exercise and caloric restriction (CR). Sprague-Dawley rats (6-7 wk old) underwent a "training" protocol of 24-h voluntary running wheel access and CR (15-20 g/day; ~50-65% of ad libitum intake) for 3 wk ("All Trained") or were sedentary and fed ad libitum ("Sed"). After 3 wk, All Trained were randomly divided into one group which was terminated immediately ("Trained"), and two detrained groups which had their wheels locked and were reintroduced to ad libitum feeding for 1 wk. The wheel locked groups received either a daily gavage of a placebo ("Detrained + Placebo") or curcumin (200 mg/kg) ("Detrained + Curcumin"). Cessation of daily CR and exercise caused an increase in body mass, as well as a 9- to 14-fold increase in epididymal, perirenal, and inguinal adipose tissue mass, all of which were attenuated by curcumin ( P < 0.05). Insulin area under the curve (AUC) during an oral glucose tolerance test, HOMA-IR, and C-reactive protein (CRP) were elevated 6-, 9-, and 2-fold, respectively, in the Detrained + Placebo group vs. the Trained group (all P < 0.05). Curcumin reduced insulin AUC, HOMA-IR, and CRP vs. the placebo group (all P < 0.05). Our results indicate that curcumin has a protective effect against weight regain and impaired metabolic control following a successful period of weight loss through diet and exercise, perhaps via inhibition of glucocorticoid action and inflammation. NEW & NOTEWORTHY Weight regain after dieting and exercise is a common phenomenon plaguing many individuals. The biological mechanisms underlying weight regain are incompletely understood and are likely multifactorial. In this paper, we examined the metabolic implications of curcumin, a compound known for its anti-inflammatory properties and inhibitory action on the enzyme 11β-HSD1, in a rodent model of adiposity rebound after the cessation of diet and exercise.

Glucocorticoid antagonism limits adiposity rebound and glucose intolerance in young male rats following the cessation of daily exercise and caloric restriction

Severe caloric restriction (CR), in a setting of regular physical exercise, may be a stress that sets the stage for adiposity rebound and insulin resistance when the food restriction and exercise stop. In this study, we examined the effect of mifepristone, a glucocorticoid (GC) receptor antagonist, on limiting adipose tissue mass gain and preserving whole body insulin sensitivity following the cessation of daily running and CR. We calorically restricted male Sprague-Dawley rats and provided access to voluntary running wheels for 3 wk followed by locking of the wheels and reintroduction to ad libitum feeding with or without mifepristone (80 mg·kg(-1)·day(-1)) for 1 wk. Cessation of daily running and CR increased HOMA-IR and visceral adipose mass as well as glucose and insulin area under the curve during an oral glucose tolerance test vs. pre-wheel lock exercised rats and sedentary rats (all P < 0.05). Insulin sensitivity and glucose tolerance were preserved and adipose tissue mass gain was attenuated by daily mifepristone treatment during the post-wheel lock period. These findings suggest that following regular exercise and CR there are GC-induced mechanisms that promote adipose tissue mass gain and impaired metabolic control in healthy organisms and that this phenomenon can be inhibited by the GC receptor antagonist mifepristone.

Rapid Alterations in Perirenal Adipose Tissue Transcriptomic Networks with Cessation of Voluntary Running

In maturing rats, the growth of abdominal fat is attenuated by voluntary wheel running. After the cessation of running by wheel locking, a rapid increase in adipose tissue growth to a size that is similar to rats that have never run (i.e. catch-up growth) has been previously reported by our lab. In contrast, diet-induced increases in adiposity have a slower onset with relatively delayed transcriptomic responses. The purpose of the present study was to identify molecular pathways associated with the rapid increase in adipose tissue after ending 6 wks of voluntary running at the time of puberty. Age-matched, male Wistar rats were given access to running wheels from 4 to 10 weeks of age. From the 10^th to 11^th week of age, one group of rats had continued wheel access, while the other group had one week of wheel locking. Perirenal adipose tissue was extracted, RNA sequencing was performed, and bioinformatics analyses were executed using Ingenuity Pathway Analysis (IPA). IPA was chosen to assist in the understanding of complex ‘omics data by integrating data into networks and pathways. Wheel locked rats gained significantly more fat mass and significantly increased body fat percentage between weeks 10–11 despite having decreased food intake, as compared to rats with continued wheel access. IPA identified 646 known transcripts differentially expressed (p < 0.05) between continued wheel access and wheel locking. In wheel locked rats, IPA revealed enrichment of transcripts for the following functions: extracellular matrix, macrophage infiltration, immunity, and pro-inflammatory. These findings suggest that increases in visceral adipose tissue that accompanies the cessation of pubertal physical activity are associated with the alteration of multiple pathways, some of which may potentiate the development of pubertal obesity and obesity-associated systemic low-grade inflammation that occurs later in life.

A step-defined sedentary lifestyle index: <5000 steps/day

Step counting (using pedometers or accelerometers) is widely accepted by researchers, practitioners, and the general public. Given the mounting evidence of the link between low steps/day and time spent in sedentary behaviours, how few steps/day some populations actually perform, and the growing interest in the potentially deleterious effects of excessive sedentary behaviours on health, an emerging question is "How many steps/day are too few?" This review examines the utility, appropriateness, and limitations of using a reoccurring candidate for a step-defined sedentary lifestyle index: <5000 steps/day. Adults taking <5000 steps/day are more likely to have a lower household income and be female, older, of African-American vs. European-American heritage, a current vs. never smoker, and (or) living with chronic disease and (or) disability. Little is known about how contextual factors (e.g., built environment) foster such low levels of step-defined physical activity. Unfavorable indicators of body composition and cardiometabolic risk have been consistently associated with taking <5000 steps/day. The acute transition (3-14 days) of healthy active young people from higher (>10 000) to lower (<5000 or as low as 1500) daily step counts induces reduced insulin sensitivity and glycemic control, increased adiposity, and other negative changes in health parameters. Although few alternative values have been considered, the continued use of <5000 steps/day as a step-defined sedentary lifestyle index for adults is appropriate for researchers and practitioners and for communicating with the general public. There is little evidence to advocate any specific value indicative of a step-defined sedentary lifestyle index in children and adolescents.

Differential changes in vascular mRNA levels between rat iliac and renal arteries produced by cessation of voluntary running

Early vascular changes at the molecular level caused by adoption of a sedentary lifestyle are incompletely characterized. Herein, we employed the rodent wheel-lock model to identify mRNAs in the arterial wall that are responsive to the acute transition from higher to lower levels of daily physical activity. Specifically, we evaluated whether short-term cessation of voluntary wheel running alters vascular mRNA levels in rat conduit arteries previously reported to have marked increases (i.e. iliac artery) versus marked decreases (i.e. renal artery) in blood flow during running. We used young female Wistar rats with free access to voluntary running wheels. Following 23 days of voluntary running (average distance of ∼15 km per night; ∼4.4 h per night), rats in one group were rapidly transitioned to a sedentary state by locking the wheels for 7 days (n = 9; wheel-lock 7 day rats) or remained active in a second group for an additional 7 days (n = 9; wheel-lock 0 day rats). Real-time PCR was conducted on total RNA isolated from iliac and renal arteries to evaluate expression of 25 pro-atherogenic and anti-atherogenic genes. Compared with the iliac arteries of wheel-lock 0 day rats, iliac arteries of wheel-lock 7 day rats exhibited increased expression of TNFR1 (+19%), ET1 (+59%) and LOX-1 (+31%; all P < 0.05). Moreover, compared with renal arteries of wheel-lock 0 day rats, renal arteries of wheel-lock 7 day rats exhibited decreased expression of ETb (-23%), p47phox (-32%) and p67phox (-19%; all P < 0.05). These data demonstrate that cessation of voluntary wheel running for 7 days produces modest, but differential changes in mRNA levels between the iliac and renal arteries of healthy rats. This heterogeneous influence of short-term physical inactivity could be attributed to the distinct alteration in haemodynamic forces between arteries.

Potential clinical translation of juvenile rodent inactivity models to study the onset of childhood obesity

According to the latest data from the Center for Disease Control and Prevention 17%, or 12.5 million, of children and adolescents aged 2-19 years in the United States are obese. Physical inactivity is designated as one of the actual causes of US deaths and undoubtedly contributes to the obesity epidemic in children and adults. Examining the effects of inactivity on physiological homeostasis during youth is crucial given that 58% of children between the ages 6-11 yr old fail to obtain the recommended 60 min/day of physical activity and 92% of adolescents fail to achieve this goal [Troiano et al. Med Sci Sports Exerc. 40, 2008]. Nonetheless, invasive mechanistic studies in children linking diminished physical activity with metabolic maladies are lacking for obvious ethical reasons. The rodent wheel lock (WL) model was adopted by our laboratory and others to study how different organ systems of juvenile rats respond to a cessation of daily physical activity. Our WL model houses rats in cages equipped with voluntary running wheels starting at 28 days of age. After a certain period of voluntary running (3 to 6 wk), the wheels are locked, thus preventing the rats' primary source of physical activity. The studies discussed herein suggest that obesity-associated maladies including skeletal muscle insulin resistance, hypothalamic leptin resistance, fatty acid oxidation impairments in skeletal muscle and adipose tissue, nonalcoholic fatty liver disease, and endothelial dysfunction are initiated in juvenile animals that are restrained from voluntary exercise via WL. The use of the juvenile rodent WL or other inactivity models will continue to provide a powerful clinical translational tool that can be used for primordial prevention of human childhood obesity.

Physical inactivity as the culprit of metabolic inflexibility: evidence from bed-rest studies

Although it is no longer debatable that sedentary behaviors are an actual cause of many metabolic diseases, the physiology of physical inactivity has been poorly investigated for this purpose. Along with microgravity, the physiological adaptations to spaceflights require metabolic adaptations to physical inactivity, and that is exceedingly well-simulated during the ground-based microgravity bed-rest analogs. Bed rest thus represents a unique model to investigate the mechanisms by which physical inactivity leads to the development of current societal chronic diseases. For decades, however, clinicians and physiologists working in space research have worked separately without taking full awareness of potential strong mutual questioning. This review summarizes the data collected over the last 60 years on metabolic adaptations to bed rest in healthy subjects. Our aim is to provide evidence that supports the hypothesis that physical inactivity per se is one of the primary causes in the development of metabolic inflexibility. This evidence will focus on four main tenants of metabolic inflexiblity: 1) insulin resistance, 2) impaired lipid trafficking and hyperlipidemia, 3) a shift in substrate use toward glucose, and 4) a shift in muscle fiber type and ectopic fat storage. Altogether, this hypothesis places sedentary behaviors upstream on the list of factors involved in metabolic inflexibility, which is considered to be a primary impairment in several metabolic disorders such as obesity, insulin resistance, and type 2 diabetes mellitus.

Design and synthesis of small molecule glycerol 3-phosphate acyltransferase inhibitors

The incidence of obesity and other diseases associated with an increased triacylglycerol mass is growing rapidly, particularly in the United States. Glycerol 3-phosphate acyltransferase (GPAT) catalyzes the rate-limiting step of glycerolipid biosynthesis, the acylation of glycerol 3-phosphate with saturated long-chain acyl-CoAs. In an effort to produce small molecule inhibitors of this enzyme, a series of benzoic and phosphonic acids was designed and synthesized. In vitro testing of this series has led to the identification of several compounds, in particular 2-(nonylsulfonamido)benzoic acid (15g), possessing moderate GPAT inhibitory activity in an intact mitochondrial assay.

Cessation of daily wheel running differentially alters fat oxidation capacity in liver, muscle, and adipose tissue

Physical inactivity is associated with the increased risk of developing chronic metabolic diseases. To understand early alterations caused by physical inactivity, we utilize an animal model in which rats are transitioned from daily voluntary wheel running to a sedentary condition. In the hours and days following this transition, adipose tissue mass rapidly increases, due in part to increased lipogenesis. However, whether a concurrent decrease in fatty acid oxidative capacity (FAO) in skeletal muscle, liver, and adipose tissue occurs during this period is unknown. Following 6 wk of access to voluntary running wheels (average distance of approximately 6 km a night), rats were rapidly transitioned to a sedentary state by locking the wheels for 5 h (WL5) or 173 h (WL173). Complete ([(14)C]palmitate oxidation to (14)CO(2)) and incomplete ([(14)C]palmitate oxidation to (14)C-labeled acid soluble metabolites) was determined in isolated mitochondrial and whole homogenate preparations from skeletal muscle and liver and in isolated adipocytes. Strikingly, the elevated complete FAO in the red gastrocnemius at WL5 fell to that of rats that never ran (SED) by WL173. In contrast, hepatic FAO was elevated at WL173 above both WL5 and SED groups, while in isolated adipocytes, FAO remained higher in both running groups (WL5 and WL173) compared with the SED group. The alterations in muscle and liver fat oxidation were associated with changes in carnitine palmitoyl transferase-1 activity and inhibition, but not significant changes in other mitochondrial enzyme activities. In addition, peroxisome proliferator-activated receptor coactivator-1alpha mRNA levels that were higher in both skeletal muscle and liver at WL5 fell to SED levels at WL173. This study is the first to demonstrate that the transition from high to low daily physical activity causes rapid, tissue-specific changes in FAO.

Lipolysis and fatty acid metabolism in men and women during the postexercise recovery period

We sought to determine whether lipolysis, fatty acid (FA) mobilization, and plasma FA oxidation would remain elevated for hours following isoenergetic exercise bouts of different intensities. Ten men and eight women received a primed-continuous infusion of [1,1,2,3,3-(2)H(5)]glycerol and continuous infusion of [1-(13)C]palmitate to measure glycerol and plasma FA kinetics. On Day 1 (D1), participants were studied under one of three different conditions, assigned in random order: (1) before, during and 3 h after 90 min of exercise at 45% V(O2)peak (E45), (2) before, during and 3 h after 60 min of exercise at 65% V(O2)peak (E65), and (3) in a time-matched sedentary control trial (C). For each condition, participants were studied by indirect calorimetry the following morning as well (D2). Rate of appearance (Ra) of glycerol (Ra(GL)) increased above C during exercise in men and women (P < 0.05), was higher in E45 than E65 in men (P < 0.05), and was not different between exercise intensities in women. During 3 h of postexercise recovery, Ra(GL) remained significantly elevated in men (P < 0.05), but not women. FA Ra (Ra(FA)) increased during exercise in men and women and was higher in E45 than E65 (P < 0.05), and remained elevated during 3 h of postexercise recovery in both sexes (P < 0.05), but with a greater relative increase in men than women (P < 0.05). Plasma FA oxidation (Rox) increased during exercise with no difference between intensities, and it remained elevated during 3 h of postexercise recovery in both sexes (P < 0.05). Total lipid oxidation (Lox) was elevated in both sexes (P < 0.05), but more in men during 3 h of postexercise recovery on D1 (P < 0.05) and remained elevated on D2 in men (P < 0.05), but not in women. There were no differences between E45 and E65 for postexercise energy substrate turnover or oxidation in men and women as energy expenditure of exercise (EEE) was matched between bouts. We conclude that the impact of exercise upon lipid metabolism persists into recovery, but that women depend more on lipid during exercise whereas, during recovery, lipid metabolism is accentuated to a greater extent in men.

Inactivity induces increases in abdominal fat

Previously, inducing inactivity for 53 h after 21 days of voluntary running resulted in a 25 and 48% increase in epididymal and omental fat pad weights, respectively, while rats continued to eat more than a group that never had access to a running wheel (J Physiol 565: 911-925, 2005). We wanted to test the hypothesis that inactivity, independent of excessive caloric intake, could induce an increase in fat pad mass. Twenty-one-day-old rats were given access to voluntary running wheels for 42-43 days so that they were running approximately 9 km/day in the last week of running, after which wheels were locked for 5, 53, or 173 h (WL5, WL53, WL173) before the rats were killed. During the 53 and 173 h of inactivity, one group of animals was pair fed (PF) to match sedentary controls, whereas the other continued to eat ad libitum (AL). Epididymal and retroperitoneal fat masses were significantly increased in the WL173-PF vs. the WL5 group, whereas epididymal, perirenal, and retroperitoneal fat masses were all significantly increased in the WL173-AL group compared with the WL5 group. Additionally, hyperplasia, and not hypertrophy, of the epididymal fat mass was responsible for the increase at WL173-AL as demonstrated by a significant increase in cell number vs. WL5, with no change in cell diameter or volume. Thus two important findings have been elucidated: 1) increases in measured abdominal fat masses occur in both AL and PF groups at WL173, and 2) adipocyte expansion via hyperplasia occurred with an ad libitum diet following cessation of voluntary running.

Effects of ingesting a high-fat diet upon exercise-training cessation on fat accretion in the liver and adipose tissue of rats

The purpose of the present study was to determine if exercise trained rats might benefit from protection against fat accumulation in response to an obesity stimulus initiated upon training cessation. Two groups of female rats were either treadmill trained for 8 weeks (DTr) or remained sedentary (Sed). They were then submitted either to a high-fat diet (HF; 42 E%) or kept on a standard diet (SD; 12.5 E% lipids) for another 6 weeks while remaining sedentary. Fat accumulation in liver and adipocytes along with fat-cell diameter and plasma free fatty acid (FFA) levels were measured 0, 2, and 6 weeks after training cessation. Immediately after the training period (t = 0), DTr rats exhibited similar body mass and higher dietary intake but smaller body fat content (4 fat pads) compared with Sed rats. DTr rats, under both diets, exhibited higher gains in body fat than Sed rats (DTr vs. Sed, 71% vs. 8% and 132% vs. 55% for SD and HF, respectively), such that fat mass in all 4 depots was similar to Sed rats 6 weeks after training cessation. Despite higher adipocyte fat accretion, liver lipid infiltration was not increased in DTr animals and plasma FFA levels were lower throughout the detraining period. In addition, plasma leptin levels remained lower in DTr animals throughout the detraining period under the HF diet condition. The present results indicate that previously exercise trained rats are not protected against adipocyte fat accumulation whether they ingest a standard or a high-fat diet.