The effects of ß-adrenergic stimulation and exercise on NR4A3 protein expression in rat skeletal muscle.

ß-Adrenergic stimulation and exercise up-regulate the mRNA expression of nuclear receptor NR4A3, which is involved in the regulation of glucose and fatty acid utilization genes in skeletal muscle. The objective of our study was to examine the effects of ß-adrenergic stimulation and exercise on the expression of NR4A3 protein in rat skeletal muscle. A single subcutaneous injection of clenbuterol, which is a ß2-adrenergic receptor (ß2-AR) agonist, increased NR4A3 mRNA and protein expression in the fast-twitch glycolytic triceps muscle. On the other hand, an acute 3-h session of either treadmill running or swimming did not increase the NR4A3 protein level in the exercised muscle, although both treadmill running and swimming increased NR4A3 mRNA. Finally, loss of postural contractile activity because of hindlimb immobilization reduced NR4A3 mRNA and protein in the slow-twitch oxidative soleus muscle. These results suggest that: ß-adrenergic stimulation up-regulates not only NR4A3 mRNA but also NR4A3 protein in fast-twitch glycolytic muscle; exercise may increase NR4A3 mRNA but not NR4A3 protein in skeletal muscle; and local postural contractile activity plays a crucial role in maintaining NR4A3 protein expression level in postural muscle.

Muscle contractile activity regulates Sirt3 protein expression in rat skeletal muscles.

Sirt3, a member of the sirtuin family, is known to control cellular mitochondrial function. Furthermore, because sirtuins require NAD for their deacetylase activity, nicotinamide phosphoribosyltransferase (Nampt), which is a rate-limiting enzyme in the intracellular NAD biosynthetic pathway, influences their activity. We examined the effects of exercise training and normal postural contractile activity on Sirt3 and Nampt protein expression in rat skeletal muscles. Male rats were trained by treadmill running at 20 m/min, 60 min/day, 7 days/wk for 4 wk. This treadmill training program increased the Sirt3 protein expression in the soleus and plantaris muscles by 49% and 41%, respectively (P < 0.05). Moreover, a 4-wk voluntary wheel-running program also induced 66% and 95% increases in Sirt3 protein in the plantaris and triceps muscles of rats, respectively (P < 0.05). Treadmill-running and voluntary running training induced no significant changes in Nampt protein expression in skeletal muscles. In resting rats, the soleus muscle, which is recruited during normal postural activity, possessed the greatest expression levels of the Sirt3 and Nampt proteins, followed by the plantaris and triceps muscles. Furthermore, the Sirt3, but not Nampt, protein level was reduced in the soleus muscles from immobilized hindlimbs compared with that shown in the contralateral control muscle. These results demonstrated that 1) Sirt3 protein expression is upregulated by exercise training in skeletal muscles and 2) local postural contractile activity plays an important role in maintaining a high level of Sirt3 protein expression in postural muscle.

Role of local muscle contractile activity in the exercise-induced increase in NR4A receptor mRNA expression.

Exercise upregulates the expression of NR4A receptors, which are involved in regulation of glucose and fatty acid utilization genes in skeletal muscle. The aims of our study were 1) to determine the role of local contractile activity on NR4A mRNA expression in skeletal muscle during exercise; and 2) to elucidate the mechanisms underlying the induction of NR4A mRNA expression in response to muscle contractile activity. Rats were subjected to an acute 3-h low-intensity swimming or a 3-h low-intensity treadmill running as a model of endurance exercise. Low-intensity swimming increased NR4A1 and NR4A3 mRNA in triceps but not in soleus muscle. Conversely, low-intensity treadmill running increased NR4A1 and NR4A3 mRNA in soleus but not in triceps muscle. NR4A mRNA increased concomitantly with reduced postexercise muscle glycogen, suggesting that gene expression of NR4A receptors occurs in muscles recruited during exercise. Furthermore, in resting rats, an acute 1-h local electrical stimulation of a motor nerve to the tibialis anterior muscle caused increases in NR4A1 and NR4A3 mRNA relative to the contralateral control muscle of the same animals. On the other hand, after 6 h of hindlimb immobilization, NR4A1 and NR4A3 mRNA were reduced in immobilized soleus muscle relative to contralateral control muscle. In addition, both NR4A1 and NR4A3 mRNA in epitrochlearis muscle were increased after 6-h incubation with 0.5 mM 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside, which activates AMP-activated protein kinase. These results suggest that 1) local muscle contractile activity is required for increased expressions of NR4A1 and NR4A3 mRNA during exercise; and 2) muscle contractile activity-induced increases in NR4A1 and NR4A3 mRNA may be mediated by AMPK activation, at least in part.

Effect of acute high-intensity intermittent swimming on post-exercise insulin responsiveness in epitrochlearis muscle of fed rats.

Maximally insulin-stimulated glucose uptake in skeletal muscle, ie, insulin responsiveness, is reduced in fed animals as compared with fasted animals; but acute prior endurance exercise improves insulin responsiveness in the muscles of fed rats. The effect of acute prior sprint interval exercise on insulin responsiveness in the muscles of fed animals has not been clarified, and we therefore compared the effect of short high-intensity swimming as a model of sprint interval exercise on insulin responsiveness in the muscles of fed rats with the effect of prolonged low-intensity swimming as a model of endurance exercise. The fed rats were subjected to an acute bout of high-intensity intermittent swimming (HIS) or low-intensity continuous swimming (LIS). The HIS rats swam for eight 20-second periods with a weight equal to 18% of their body weight. The LIS rats swam with no load for 3 hours. HIS increased (P < .05) the phosphorylation of adenosine monophosphate-activated protein kinase (AMPK) Thr(172) and that of its downstream target acetyl-CoA carboxylase (ACC) Ser(79) 12.6- and 3.1-fold, respectively, whereas LIS increased them 3.8- and 1.9-fold, respectively, immediately after exercise compared with rested muscle. HIS and LIS increased the insulin responsiveness of 2-deoxyglucose uptake measured 4 hours after exercise by 39% and 41%, respectively, compared with rested muscles. These results show that very short (160 seconds) HIS exercise with greater AMPK activation increases the responsiveness of glucose uptake to insulin in the muscles of fed rats to a similar level observed after prolonged (3 hours) LIS exercise with lower AMPK activation. Therefore, it is suggested that an acute bout of sprint interval exercise that activates AMPK to a sufficiently high level can increase post-exercise insulin responsiveness on muscle glucose uptake irrespective of very short exercise duration.