Exercise training does not compensate for age-related decrease in myocardial GLUT-4 content

We assessed the effects of age and endurance exercise training (treadmill running at 75% maximal running capacity, 1 h/day, 5 days/wk for 10 wk) on the total concentration of insulin-regulatable glucose transporters (GLUT-4) and GLUT-4 mRNA levels in the myocardium of male Fischer 344 rats aged 7, 15, and 25 mo. Myocardial GLUT-4 concentration was quantified with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting and detected with a polyclonal antibody to the GLUT-4 transporter. Myocardial GLUT-4 mRNA levels were quantified with slot-blot analysis and a cDNA probe for GLUT-4. Myocardial GLUT-4 concentration in the 25-mo group decreased 27 and 20% compared with the 7- and 15-mo group, respectively (P < 0.0001 and P < 0.003). GLUT-4 mRNA also decreased significantly in the 25-mo group compared with the 7-mo group (20% in the trained and 11% in the untrained group, P < 0.05). Endurance training did not significantly affect myocardial GLUT-4 concentrations in any age group despite a significant increase in GLUT-4 mRNA in the 7- and 25-mo trained groups. In conclusion, myocardial GLUT-4 protein levels in the rat are significantly decreased with age but are unaffected by 10 wk of treadmill running.