Effects of exercise training on the relationship between insulin binding and insulin-stimulated tyrosine kinase activity in rat skeletal muscle

PGC-1α activation: a therapeutic target for type 2 diabetes?

Peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) has gained popularity as a very attractive target for diabetic therapies due to its role in lipid and glucose metabolism. Pharmacological activation of PGC-1α is thought to elicit health benefits. However, this notion has been questioned by increasing evidence, which suggests that insulin resistant is exacerbated when PGC-1α expression is far beyond normal physiological limits and is prevented under the condition of PGC-1α deficiency. This narrative review suggests that PGC-1α, as a master metabolic regulator, exerts roles in insulin sensitivity in a tissue-specific manner and in a physical activity/age-dependent fashion. When using PGC-1α as a target for therapeutic strategies against insulin resistance and T2DM, we should take these factors into consideration.Level of evidence: Level V, narrative review.

Differential regulation of PGC-1alpha expression in rat liver and skeletal muscle in response to voluntary running

BACKGROUND

The beneficial actions of exercise training on lipid, glucose and energy metabolism and insulin sensitivity appear to be in part mediated by PGC-1alpha. Previous studies have shown that spontaneously exercised rats show at rest enhanced responsiveness to exogenous insulin, lower plasma insulin levels and increased skeletal muscle insulin sensitivity. This study was initiated to examine the functional interaction between exercise-induced modulation of skeletal muscle and liver PGC-1alpha protein expression, whole body insulin sensitivity, and circulating FFA levels as a measure of whole body fatty acid (lipid) metabolism.

METHODS

Two groups of male Wistar rats (2 Mo of age, 188.82 +/- 2.77 g BW) were used in this study. One group consisted of control rats placed in standard laboratory cages. Exercising rats were housed individually in cages equipped with running wheels and allowed to run at their own pace for 5 weeks. At the end of exercise training, insulin sensitivity was evaluated by comparing steady-state plasma glucose (SSPG) concentrations at constant plasma insulin levels attained during the continuous infusion of glucose and insulin to each experimental group. Subsequently, soleus and plantaris muscle and liver samples were collected and quantified for PGC-1alpha protein expression by Western blotting. Collected blood samples were analyzed for glucose, insulin and FFA concentrations.

RESULTS

Rats housed in the exercise wheel cages demonstrated almost linear increases in running activity with advancing time reaching to maximum value around 4 weeks. On an average, the rats ran a mean (Mean +/- SE) of 4.102 +/- 0.747 km/day and consumed significantly more food as compared to sedentary controls (P < 0.001) in order to meet their increased caloric requirement. Mean plasma insulin (P < 0.001) and FFA (P < 0.006) concentrations were lower in the exercise-trained rats as compared to sedentary controls. Mean steady state plasma insulin (SSPI) and glucose (SSPG) concentrations were not significantly different in sedentary control rats as compared to exercise-trained animals. Plantaris PGC-1alpha protein expression increased significantly from a 1.11 +/- 0.12 in the sedentary rats to 1.74 +/- 0.09 in exercising rats (P < 0.001). However, exercise had no effect on PGC-1alpha protein content in either soleus muscle or liver tissue. These results indicate that exercise training selectively up regulates the PGC-1alpha protein expression in high-oxidative fast skeletal muscle type such as plantaris muscle.

CONCLUSION

These data suggest that PGC-1alpha most likely plays a restricted role in exercise-mediated improvements in insulin resistance (sensitivity) and lowering of circulating FFA levels.

Strategies for testing long-term transcutaneous amperometric glucose sensors

OBJECTIVES

Transcutaneous and embedded devices were developed for use in characterizing the in vivo performance of subcutaneously implanted glucose sensors. The devices were used as a portal for accessing electrochemical glucose sensors from the exterior. They were designed to prevent the sensors from being pulled out of the animals and the sensor leads from breaking. Development of the devices took into consideration rodent mobility, infection control, and animal welfare balanced with sensor durability, accessibility, and functionality.

METHODS

Our approach was developed over five animal protocols spanning a period of 6 months. A total of 68 sensors were implanted with 60 associated devices in 22 Sprague-Dawley outbred rats.

RESULTS

The average sensor lifetime was 11.2 +/- 3.1 days with a maximum of 56 days. All-cause sensor failure averaged one sensor per day. As implantation devices were modified, failure attributable to the device was decreased by 40%. The resulting devices showed good durability and allowed for easy sensor access and testing.

CONCLUSIONS

These data represent baseline sensor function against which future sensor improvements may be measured. The new devices and techniques described should be a valuable tool in the development of continuous glucose sensors.

Exercise training eliminates age-related differences in skeletal muscle insulin receptor and IRS-1 abundance in rats

Insulin resistance is common in old age, and exercise training can improve insulin sensitivity. The purpose of this study was to determine the influence of age (6 vs 26 months) and exercise training (10 weeks of treadmill running) on insulin signaling protein abundance in skeletal muscle from male Fisher 344 rats. Muscle levels of insulin receptor (IR), insulin receptor substrate-1 (IRS-1), phosphatidylinositol 3-kinase (PI3K), and Akt1, a serine-threonine kinase, were determined. IRS-1 was reduced with aging, IR and PI3K abundance was greater in old rats, and Akt1 was unchanged. IRS-1 was increased by training in old but not young rats, and IR was increased by training in young but not old rats. PI3K tended to increase and Akt1 did not change with training, regardless of age. Aging does not uniformly affect insulin signaling protein abundance, and exercise differentially alters IR and IRS-1 in young and old rats, thereby eliminating age-related differences in these proteins.

Effect of long-term exercise on gene expression of insulin signaling pathway intermediates in skeletal muscle

To elucidate the molecular mechanism underlying insulin sensitivity, we have thought to investigate gene expression of insulin signaling pathway intermediates in skeletal muscle from sedentary and endurance-trained rats. Adult male Sprague-Dawley rats were trained for 9 weeks on a treadmill; 30 m/min at a 6 degrees incline, 90 min/day, 5 days/week. The levels of PI 3-kinase, GLUT4, p70 S6 kinase and Ras mRNA were significantly increased by 89, 40, 38, and 47%, respectively, with running training; however, the Nck mRNA level was decreased by 24%. mRNA levels of SHP-2, Grb2, Sos, Shc, GAP, p62 and p90 S6 kinase were unaltered by running training. We have previously reported that endurance training increases mRNA levels of insulin receptor, IRS-1 and ERK1 in skeletal muscle of rats. Taken together, our data suggest that gene expression of the insulin signal pathway intermediates is modulated by endurance training that may be associated with alteration of insulin sensitivity.

Tissue-related changes in insulin receptor number and autophosphorylation induced by starvation and diabetes in rats

Insulin action is subject to regulation at the level of the insulin receptor and at postreceptor levels. Starvation and diabetes are often associated with insulin resistance for glucose metabolism in various tissues. In muscle, fat, and liver, we examined whether changes in the functionality of the insulin receptor correlated with changes in insulin action in the starved and diabetic state. Insulin-stimulated receptor autophosphorylation reflects an early physiologic step in transmission of the insulin signal, and for that reason, changes in autophosphorylation activity of the insulin receptor were used as a marker to determine the functionality of the insulin receptor. Glycoprotein fractions prepared from skeletal muscle, diaphragm, epididymal fat, and liver of control, 3-day starved, short-term 3-day (S) diabetic (streptozotocin, 70 mg/kg intravenously), and long-term 6-month (L) diabetic (neonatal streptozotocin 100 micrograms/g intraperitoneally) rats were used in this study. Receptor activity was monitored by measuring insulin-stimulated [gamma-32P]adenosine triphosphate (ATP) receptor autophosphorylation. In addition, to obtain information about whether changes in receptor autophosphorylation are related to changes in receptor number, relative numbers of high-affinity insulin receptors were determined by affinity cross-linking of [125I]insulin to the receptor alpha-chain and quantitation of the yield of labeled receptor alpha-chain. Control, starved, S diabetic, and L diabetic rats had plasma insulin and glucose levels of 294 +/- 42, 90 +/- 24, 48 +/- 12, and 216 +/- 30 pmol/L and 6.7 +/- 0.2, 4.1 +/- 0.2, 23.3 +/- 0.7, and 21.6 +/- 2.9 mmol/L, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in glucose and protein metabolism in animals subjected to simulated microgravity

Reduction of physical activity due to disease or environmental restraints, such as total bed rest or exposure to spaceflight, leads to atrophy of skeletal muscle and is frequently accompanied by alterations in food intake and the concentration of metabolic regulatory hormones such as insulin. Hindlimb suspension of laboratory rats, as a model for microgravity, also shows marked atrophy of gravity dependent muscles along with a reduced gain in body weight. Suspended rats exhibit enhanced sensitivity to insulin-induced glucose uptake when compared with normal control rats and resistance to insulin action when compared with control rats matched similarly for reduced body weight gain. These changes are accompanied by decreased insulin binding and tyrosine kinase activity in soleus but not plantaris muscle, unchanged glucose uptake by perfused hindlimb and decreased sensitivity but not responsiveness to insulin-induced suppression of net proteolysis in hindlimb skeletal muscle. These findings suggest that loss of insulin sensitivity during muscle atrophy is associated with decreased insulin binding and tyrosine kinase activity in atrophied soleus muscle along with decreased sensitivity to the effects of insulin on suppressing net protein breakdown but not on enhancing glucose uptake by perfused hindlimb.

Characterization of insulin receptor kinase activity and autophosphorylation in different skeletal muscle types

We have examined insulin binding, autophosphorylation, and tyrosine kinase activity in detergent-solubilized and wheat germ agglutinin-purified insulin receptor preparations from four rat muscles of different fiber composition (i.e., tensor fascia latae, soleus, vastus intermedius, and plantaris). Insulin binding activity was similar in three of the four muscles but lower in tensor fascia latae. No significant differences were noted in the affinity of insulin for its receptor from various muscle types. Insulin receptor tyrosine kinase activity measured in the absence (basal) and presence of insulin (0.3-300 nM) was comparable in all muscle types (normalized to the amount of insulin bound). Insulin sensitivity, measured as the dose of insulin required for half-maximal activation of kinase activity, was also similar in all muscle types. Likewise, incubation of receptor preparations with [gamma-32P]ATP, Mn2+, and insulin (0.25-100 nM) resulted in a dose-dependent autophosphorylation of the beta-subunit (relative molecular weight approximately 95 kDa) with similar kinetics in all muscle types. In conclusion, these results show that the functional behavior of the insulin receptor autophosphorylation-kinase system (in vitro) is not changed by alterations in muscle fiber composition, indicating that differences in insulin sensitivity between different skeletal muscle types is probably not due to modulation of the insulin receptor phosphorylation system.

Effects of insulin on carbohydrate and protein metabolism in voluntary running rats

The goal of this study was to assess the effects of voluntary running activity in rats on various aspects of carbohydrate and protein metabolism. After 6 wk of exercise training, rats (ET) were rested for 24 h and their hindquarters, along with those from sedentary control (SC) and dietary control (DC) rats, were perfused with 0, 60, 250, or 6,000 microU/ml insulin. At 0 insulin, glucose clearance was similar for all groups, and it was increased with added insulin. However, the insulin effect was 20-40% greater for ET rats at all insulin concentrations (P less than 0.05). Muscle glycogen deposition also increased with added insulin but showed muscle-specific differences. Specifically, glycogen content of the plantaris muscle was significantly higher in ET compared with SC or DC rats, whereas this pattern was reversed in soleus muscle. In plantaris muscle, insulin stimulated glucose 6-phosphate (G-6-P)-independent (-G-6-P) glycogen synthase activity only in SC and DC rats and increased its affinity for G-6-P at 250 microU/ml in all groups. In contrast, the -G-6-P synthase activity was not increased in soleus muscle and was actually decreased in all groups at 6,000 microU/ml. Tyrosine release was suppressed by insulin in all groups, but this effect was significantly greater at insulin levels of 60 microU/ml (P less than 0.02) in hindquarters from ET rats compared with SC and DC rats. Neither insulin nor exercise training decreased 3-methylhistidine release from perfused hindquarters.(ABSTRACT TRUNCATED AT 250 WORDS)

Exercise-enhanced activation of glycogen synthase in human skeletal muscle

The present study was undertaken to elucidate aspects of the regulatory mechanisms leading to enhanced glucose metabolism and insulin sensitivity of muscle after physical exertion. Biopsies were obtained from the vastus lateralis muscle of healthy volunteers before and after 60 min of bicycle exercise at 60% of their maximal aerobic capacity. Insulin binding to wheat germ agglutinin-purified muscle insulin receptors as well as basal and insulin-stimulated receptor kinase activity toward an exogenous substrate were unaltered by exercise. Muscle glycogen levels diminished from 3.35 +/- 0.26 to 1.85 +/- 0.13 mg/100 mg muscle (P less than 0.01) and the half-maximal activation constant of glycogen synthase for glucose 6-phosphate decreased from 0.62 +/- 0.05 to 0.25 +/- 0.02 mM (P less than 0.001). Total glycogen synthase activity was unchanged. In the absence of phosphatase inhibitors, glucose 6-phosphate-independent glycogen synthase activity of the crude enzyme extract increased during in vitro incubation. The initial rate of activation (through dephosphorylations) of glycogen synthase was 0.18 +/- 0.06 vs. 0.37 +/- 0.03 U.min-1.mg-1 protein before and after exercise, respectively (P less than 0.02). The total as well as the glycogen-associated phosphoprotein phosphatase activity was, however, unaffected by exercise.

Abnormal insulin metabolism by specific organs from rats with spontaneous hypertension

Spontaneously hypertensive rats (SHR) have been shown to be both insulin resistant and hyperinsulinemic after oral glucose administration or infusion of exogenous insulin during an insulin suppression test. To determine if this hyperinsulinemia may be due to decreased removal of insulin, the metabolic clearance (k) of insulin was measured in isolated perfused liver, kidney, and hindlimb skeletal muscle from SHR and Wistar-Kyoto (WKY) control rats. The data indicate that the k for insulin removal by liver was similar in SHR and WKY rats, averaging 287 +/- 18 and 271 +/- 10 microliters.min-1.g-1 liver, respectively. In contrast, the k for insulin removal by hindlimbs from SHR was decreased 37% (P less than 0.001) compared with WKY rats (8.6 +/- 0.5 vs. 13.7 +/- 0.7 microliters.min-1.g-1 muscle), and this decrease was not accompanied by decreased binding of insulin to its receptor in plantaris muscle. Although the removal of insulin by glomerular filtration was similar in SHR and WKY rats (653 +/- 64 microliters/min vs. 665 +/- 90 microliters.min-1.kidney-1), total insulin removal by kidney was significantly lower (P less than 0.05) in SHR (710 +/- 78 microliters/min) compared with WKY rats (962 +/- 67 microliters/min), due to decreased peritubular clearance of insulin in SHR (56 +/- 73 vs. 297 +/- 59 microliters/min, P less than 0.05). These findings suggest that the decreased clearance of insulin in SHR rats was possibly not due to impaired hepatic removal of insulin but rather to decreased removal by skeletal muscle and kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)