EFFECTS OF ACUTE AND CHRONIC EXERCISE ON SKELETAL-MUSCLE GLUCOSE-TRANSPORT IN AGED RATS

The purpose of this study was to investigate the effects of acute and chronic exercise on skeletal muscle glucose transport in aged rats by using an isolated sarcolemmal membrane preparation. In 24-mo-old femal e Fischer 344 rats, a maximum dose of insulin increased glucose transp ort from 43 +/- 6 to 82 +/- 6 pmol . mg protein(-1). 15 s(-1). A 45-mi n bout of exhaustive treadmill running increased glucose transport to the same maximum level (88 +/- 5 pmol . mg protein(-1). 15 s(-1)). Eig ht weeks of progressive exercise training resulted in a 65% increase i n succinic dehydrogenase activity in hindlimb muscles and a 55% increa se in total cellular GLUT-4 content. Despite these biochemical adaptat ions, there was no change in either basal or maximum insulin-stimulate d glucose transport between control (43 +/- 6 and 82 +/- 6 pmol . mg p rotein(-1). 15 s(-1), respectively) and trained (42 +/- 2 and 82 +/- 8 pmol . mg protein(-1). 15 s respectively) animals. When hindlimb musc le succinate dehydrogenase activity and GLUT-4 content were compared f or both the combined sedentary and trained groups, a significant corre lation (r = 0.68) was obtained. This study demonstrates that the skele tal muscle glucose transport system of 24-mo-old rats is fully stimula ted by acute exercise and that, although GLUT-4 levels are increased i n aged animals after exercise training, this does not result in an enh ancement of maximal insulin-stimulated glucose transport. Thus increas es in GLUT-4 are not sufficient to improve muscle insulin responsivene ss with training.