Physical training of lean and genetically obese Zucker rats: effect on fat cell metabolism

Biochemical adaptations in white adipose tissue following aerobic exercise: from mitochondrial biogenesis to browning

Our understanding of white adipose tissue (WAT) biochemistry has evolved over the last few decades and it is now clear that WAT is not simply a site of energy storage, but rather a pliable endocrine organ demonstrating dynamic responsiveness to the effects of aerobic exercise. Similar to its established effects in skeletal muscle, aerobic exercise induces many biochemical adaptations in WAT including mitochondrial biogenesis and browning. While past research has focused on the regulation of these biochemical processes, there has been renewed interest as of late given the potential of harnessing WAT mitochondrial biogenesis and browning to treat obesity and type II diabetes. Unfortunately, despite increasing evidence that innumerable factors, both exercise induced and pharmacological, can elicit these biochemical adaptations in WAT, the underlying mechanisms remain poorly defined. Here, we begin with a historical account of our understanding of WAT exercise biochemistry before presenting detailed evidence in favour of an up-to-date model by which aerobic exercise induces mitochondrial biogenesis and browning in WAT. Specifically, we discuss how aerobic exercise induces increases in WAT lipolysis and re-esterification and how this could be a trigger that activates the cellular energy sensor 5' AMP-activated protein kinase to mediate the induction of mitochondrial biogenesis and browning via the transcriptional co-activator peroxisome proliferator-activated receptor gamma co-activator-1 alpha. While this review primarily focuses on mechanistic results from rodent studies special attention is given to the translation of these results, or lack thereof, to human physiology.

Exercise, glucose transport, and insulin sensitivity

Physical exercise can be an important adjunct in the treatment of both non-insulin-dependent diabetes mellitus and insulin-dependent diabetes mellitus. Over the past several years, considerable progress has been made in understanding the molecular basis for these clinically important effects of physical exercise. Similarly to insulin, a single bout of exercise increases the rate of glucose uptake into the contracting skeletal muscles, a process that is regulated by the translocation of GLUT4 glucose transporters to the plasma membrane and transverse tubules. Exercise and insulin utilize different signaling pathways, both of which lead to the activation of glucose transport, which perhaps explains why humans with insulin resistance can increase muscle glucose transport in response to an acute bout of exercise. Exercise training in humans results in numerous beneficial adaptations in skeletal muscles, including an increase in GLUT4 expression. The increase in muscle GLUT4 in trained individuals contributes to an increase in the responsiveness of muscle glucose uptake to insulin, although not all studies show that exercise training in patients with diabetes improves overall glucose control. However, there is now extensive epidemiological evidence demonstrating that long-term regular physical exercise can significantly reduce the risk of developing non-insulin-dependent diabetes mellitus.

Effects of exercise training on in vivo insulin action in individual tissues of the rat

It has previously been suggested that exercise training leads to increased whole body insulin sensitivity. However, the specific tissues and metabolic pathways involved have not been examined in vivo. By combining the euglycemic clamp with administration of glucose tracers, [3H]2-deoxyglucose (2DG), [14C]glucose, and [3H]glucose, in vivo insulin action at the whole body level and within individual tissues has been assessed in exercise-trained (ET, running 1 h/d for 7 wk) and sedentary control rats at four insulin doses. Whole body insulin sensitivity was significantly increased in ET. In addition, the skeletal muscles, soleus, red and white gastrocnemius, extensor digitorum longus (EDL), and diaphragm all showed increased sensitivity of insulin-stimulated 2DG uptake with training. With the exception of EDL, no significant difference in insulin-mediated glycogen synthesis between control and ET could be found. Therefore, the increased insulin-induced 2DG uptake observed in muscle following training is apparently directed towards glucose oxidation. In ET animals, adipose tissue exhibited a significant increase in insulin-mediated 2DG uptake and [14C]glucose incorporation into free fatty acids but there was no difference from control in any parameters measured in lung or liver. EDL and white gastrocnemius, which are not primarily involved during exercise of this type, also demonstrated increased insulin sensitivity following training. In conclusion, exercise training results in a marked increase in whole body insulin sensitivity related mainly to increased glucose oxidation in skeletal muscle. This effect may be mediated by systemic as well as local factors and is likely to be of therapeutic value in pathological conditions exhibiting insulin resistance.

Increased insulin sensitivity and responsiveness of glucose metabolism in adipocytes from female versus male rats

This study was undertaken to examine whether there were sex-associated differences in the action of insulin on glucose metabolism in adipocytes. Insulin binding and the dose-response curves for glucose transport (assessed by measuring the cell-associated radioactivity after 15-s incubation with 50 microM [6-14C]glucose) and [U-14C]glucose (5 mM) metabolism into CO2 and lipids were compared in retroperitoneal adipocytes from age-matched (84 d) male and female rats. In addition, the activity of fatty acid synthetase, one of the key lipogenic enzymes, was determined. Fat cell size was not significantly larger in females than in males (0.238 vs. 0.209 microgram lipid per cell). At insulin concentrations less than or equal to 1.6 nM, adipocytes from females bound significantly more insulin than did adipocytes from males, due to an increased apparent affinity of the receptors for insulin. Accordingly, the sensitivity of glucose transport to insulin was greater in females than in males: insulin concentration eliciting half-maximal stimulation (ED50) = 0.19 nM vs. 0.41 nM. At maximal insulin stimulation the rates of glucose transport (12 times the basal values) were similar in the two sexes. In contrast, the maximal effect of insulin on glucose conversion to CO2 plus lipids was much greater in the adipocytes from females than males (increment over basal: 472 vs. 249 nmol/10(6) cells per 2 h). Fatty acid synthesis contributed approximately 40% of the incremental difference between the two types of adipocytes, while glyceride-glycerol synthesis contributed less than 10%. The insulin dose-response curves for adipocytes from females were shifted to the left for all the metabolic pathways investigated. The mean ED50 for total glucose metabolism in females was 50% of that in males (0.07 nM vs. 0.15 nM). Marked sex-associated differences in the action of insulin on glucose metabolism were also observed in subcutaneous inguinal adipocytes (increment over basal: 137 and 56 nmol/10(6) cells per 2 h, ED50 = 0.13 nM and 0.30 nM in females and males, respectively). The intracellular capacity to metabolize glucose through the fatty acid synthesis pathway, as assessed by FAS activity, was higher in adipocytes from females than in those from males and was greater in retroperitoneal than in inguinal adipocytes. Furthermore, by plotting the individual data, a highly significant correlation (r = 0.92, P less than 0.001) was found between the absolute effect of insulin on glucose metabolism at maximal stimulation and the fatty acid synthetase activity of the cells. These results indicate that the response of glucose metabolism to insulin in adipocytes from female as compared with male rats is characterized by two main features: (a) an increased sensitivity primarily due to an increase in insulin binding, and (b) an increased responsiveness closely associated with a postreceptor increase in the lipogenic capacity of the cell. These findings might be relevant to the differential disposition of male and female rats to develop fatness.

Effect of insulin on glucose transport and metabolism in isolated fat-cells of gonadal adipose tissue from mature age-matched male and female rats

Insulin action on glucose transport and metabolism was studied in paraovarian adipocytes from 3-month-old female rats and compared with insulin action in epididymal adipocytes from closely age-matched males. At maximal insulin concentrations the stimulations of 2-deoxyglucose uptake (4-fold the basal value) and of [U-14C]glucose incorporation into CO2 and total lipids (3- and 2-fold the basal values respectively) were similar in adipocytes from rats of both sexes. At submaximal insulin concentrations (less than 0.2 nM) the ability of paraovarian adipocytes to transport and to metabolize glucose was higher than that of epididymal adipocytes; accordingly an increase in insulin binding was observed in paraovarian adipocytes as compared with epididymal adipocytes. These results show that paraovarian adipocytes from mature female rats were highly responsive to insulin, and exhibited a higher sensitivity to the hormone than did epididymal adipocytes from male rats of the same age.

Sensitivity to insulin of glycolysis and glycogen synthesis of isolated soleus-muscle strips from sedentary, exercised and exercise-trained rats

The half-maximal stimulation of the rates of glycolysis and glycogen synthesis in soleus-muscle strips from sedentary animals occurred at a concentration of insulin of about 100 microunits/ml. In soleus-muscle strips from exercise-trained rats (5 weeks of treadmill training), half-maximal stimulation of the rate of glycolysis occurred at about 10 microunits of insulin/ml, whereas that for glycogen synthesis occurred between 10 and 100 microunits of insulin/ml. The sensitivity of glycolysis to insulin after exercise training is similar to that of adipose tissue from sedentary animals. This finding suggests that, in sedentary animals, the effects of normal changes in insulin concentration may affect muscle primarily indirectly via the anti-lipolytic effect on adipose tissue, whereas after training insulin may effect the rate of glycolysis in muscle directly. A single period of exercise did not change the sensitivity of glycolysis in soleus muscle to insulin, nor probably that of glycogen synthesis. It is suggested that the improvement in insulin sensitivity of glycolysis in muscle caused by exercise-training could account, in part, for the well-established improvement in glucose tolerance and insulin sensitivity observed in man and rats after exercise-training.