Effects of wheel running on glucose transporter (GLUT4) concentration in skeletal muscle of young adult and old rats.

We examined the effects of voluntary exercise on glucose transporter concentration in skeletal muscle from young adult and old female Long-Evans rats. Rats had free access to voluntary running wheels beginning at 4 months of age or remained sedentary. Exercising rats ran approximately 7.5, 6.2, 5.6 and 5.3 km/day during their 6th, 8th, 9th and 10th month of age, respectively. During the 23rd, 24th and 25th month of age running distance averaged 3.0, 2.8 and 2.4 km/day, respectively. At 10 and 25 months of age, glucose transporter protein concentration was assessed in epitrochlearis and flexor digitorum brevis muscles with a polyclonal antibody directed against the GLUT4 transporter isoform. GLUT4 protein concentration was not altered by the aging process (i.e., comparing 10- and 25-month-old rats) in either muscle type. Wheel running increased GLUT4 protein concentration by 45% in epitrochlearis muscles of 10-month-old rats relative to age-matched sedentary controls. The training-induced adaptation in GLUT4 protein was no longer present at age 25 months, probably because the running distance had declined by 50%. In the flexor digitorum brevis, exercise did not alter GLUT4 concentration at either 10 or 25 months, presumably due to insufficient recruitment of this muscle during wheel running as assessed by measurement of citrate synthase and hexokinase enzyme activities. Wheel running induced cardiac and soleus muscle hypertrophy in 10- and 25-month-old rats. In summary, voluntary wheel running can induce an increase in skeletal muscle GLUT4 protein concentration in adult rats. Older rats that run less exhibit cardiac and soleus muscle hypertrophy, but do not maintain an elevated GLUT4 protein concentration in the epitrochlearis muscle. Aging does not alter GLUT4 protein concentration in the epitrochlearis or FDB muscles.