Date Published: November 10, 2016
Publisher: Public Library of Science
Author(s): Naoki Horii, Koji Sato, Noboru Mesaki, Motoyuki Iemitsu, Marcia B. Aguila.
Regular resistance exercise induces skeletal muscle hypertrophy and improvement of glycemic control in type 2 diabetes patients. Administration of dehydroepiandrosterone (DHEA), a sex steroid hormone precursor, increases 5α-dihydrotestosterone (DHT) synthesis and is associated with improvements in fasting blood glucose level and skeletal muscle hypertrophy. Therefore, the aim of this study was to investigate whether increase in muscle DHT levels, induced by chronic resistance exercise, can contribute to skeletal muscle hypertrophy and concomitant improvement of muscular glucose metabolism in type 2 diabetic rats. Male 20-week-old type 2 diabetic rats (OLETF) were randomly divided into 3 groups: sedentary control, resistance training (3 times a week on alternate days for 8 weeks), or resistance training with continuous infusion of a 5α-reductase inhibitor (n = 8 each group). Age-matched, healthy nondiabetic Long-Evans Tokushima Otsuka (LETO) rats (n = 8) were used as controls. The results indicated that OLETF rats showed significant decrease in muscular DHEA, free testosterone, DHT levels, and protein expression of steroidogenic enzymes, with loss of skeletal muscle mass and hyperglycemia, compared to that of LETO rats. However, 8-week resistance training in OLETF rats significantly increased the levels of muscle sex steroid hormones and protein expression of steroidogenic enzymes with a concomitant increase in skeletal muscle mass, improved fasting glucose level, and insulin sensitivity index. Moreover, resistance training accelerated glucose transporter-4 (GLUT-4) translocation and protein kinase B and C-ζ/λ phosphorylation. Administering the 5α-reductase inhibitor in resistance-trained OLETF rats resulted in suppression of the exercise-induced effects on skeletal muscle mass, fasting glucose level, insulin sensitivity index, and GLUT-4 signaling, with a decline in muscular DHT levels. These findings suggest that resistance training-induced elevation of muscular DHT levels may contribute to improvement of hyperglycemia and skeletal muscle hypertrophy in type 2 diabetic rats.
In 2014, 9% of adults 18 years and older around the world had diabetes, and diabetes was the direct cause of 1.5 million deaths [1, 2]. The beneficial effects of regular aerobic exercise for type 2 diabetics have been well established, which include restoration of glycemic control and reduced insulin resistance without muscle hypertrophy [3–6]. Additionally, regular resistance exercise reduces fasting glucose and glycosylated hemoglobin (HbA1c) of type 2 diabetic patients in randomized, controlled studies [3–6]. On a molecular level, an increase in expression and translocation of glucose transporter 4 (GLUT-4) in the skeletal muscle following resistance training contributes to improvement of glycemic control . Furthermore, decreased skeletal muscle mass is associated with a deterioration of insulin resistance , thus, the increase in muscle mass in response to resistance training may participate in improvement of glycemic control.
The ethical approval for this study was obtained from the Committee on Animal Care at the Ritsumeikan University. Male OLETF and Long-Evans Tokushima Otsuka (LETO) rats (6 weeks old) were obtained (Japan SLC, Shizuoka, Japan) and cared for according to the Guiding Principles for the Care and Use of Animals, based on the Declaration of Helsinki. The rats were housed individually in an animal facility under controlled conditions (12:12-h light:dark cycle, with the light period being from 8:00 A.M. to 8:00 P.M.), and were given access to water and fed normal chow (CE2; CLEA Japan, Tokyo, Japan). After 14 weeks, the 20-week-old OLETF rats were randomly divided into three groups (n = 8 each group): sedentary control (Con), resistance training (RT), or resistance training with continuous infusion of 5α-reductase inhibitor (RT+In). The 5α-reductase inhibitor (dutasteride; Sigma, Steinheim, Germany) was administrated continuously at 0.15 μL per h for 8 weeks via an implanted osmotic mini pump in subcutaneous adipose tissue (Model 2006; Alzet, Cupertino, CA) [16, 17]. The inhibitor (2 mg/kg) was dissolved into sesame oil and 200 μL was loaded into the pump . Additionally, nondiabetic, healthy, and age-matched LETO rats (n = 8) were used as controls. Post-treatment experiments in trained rats were performed 48 h after the last exercise session to avoid acute effects of exercise. All rats were fasted for 12 h and after measuring body weight, blood samples were obtained from the abdominal aorta under general anesthesia. After sacrifice, the heart, epididymal fat, soleus, gastrocnemius, and plantaris muscles were resected quickly, rinsed in ice-cold saline, weighed, and frozen in liquid nitrogen. The gastrocnemius muscle was used to evaluate the expressions of steroidogenic enzymes and hormones levels and perform histochemical analysis of the skeletal muscle, thus providing a steroidogenic status of the tissue.
Body weight and epididymal fat mass in the OLETF control rats were significantly higher, and soleus, plantaris, and gastrocnemius muscle mass and CSA in gastrocnemius muscle were significantly lower than those in LETO rats (Tables 1 and 2 and Fig 1). Compared to the OLETF control rats, resistance-trained OLETF rats showed decreased body weight and epididymal fat mass, and increased LV mass, soleus, plantaris, and gastrocnemius muscle mass and CSA in gastrocnemius muscle (Tables 1 and 2 and Fig 1). Compared to LETO rats, OLETF control rats showed significantly higher fasting blood glucose and insulin levels and significantly lower QUICKI values (Table 1). The resistance-trained OLETF rats had significantly decreased fasting glucose levels and significantly increased QUICKI values compared to OLETF control rats (Table 1). However, the OLETF rats that were resistance-trained and received the 5α-reductase inhibitor showed no improvement in fasting blood glucose levels or QUICKI, and no increase in skeletal muscle mass compared to the resistance-trained OLETF rats, but the effects of gastrocnemius CSA and fasting blood glucose levels on resistance training was not completely suppressed (Tables 1 and 2 and Fig 1). Moreover, the resistance training-induced cardiac hypertrophy was not affected by the 5α-reductase inhibitor (Table 1). Average dietary intake over the 8-week experimental period was significantly higher in three OLETF groups than that in the LETO group (Table 1), whereas no significant difference was observed in the dietary intake among the three OLETF groups (Table 1).
The results of this study demonstrated that chronic resistance training induced a significant increase in muscle tissue levels of DHEA, testosterone, and DHT; increased protein levels of steroidogenic enzymes; reduced fasting glucose level; improved insulin sensitivity index (QUICKI), and increased muscle mass and cross-sectional area in OLETF diabetic rats. The Akt/PKC-ζ/λ-GLUT-4 pathway was also upregulated in muscle after resistance training in the diabetic rats. Notably, resistance training-induced improvements in muscle mass and hyperglycemia with upregulation of GLUT-4-regulated signaling were suppressed by chronic inhibition of 5α-reductase. Furthermore, these changes in muscular DHT levels were significantly associated with muscle mass, fasting glucose level, insulin sensitivity index, and GLUT-4 translocation. Therefore, these results suggest that resistance training-induced elevations in the intramuscular synthesis of DHT were necessary to achieve improvement of hyperglycemia and muscle hypertrophy in type 2 diabetic rats.