Altitude training is a common method used to enhance endurance performance in athletes. We have examined the interactive effect of exercise training and chronic hypoxic on
glycogen storage and
GLUT4 protein expression in cardiac muscles. Thirty-two male Sprague-Dawley rats were weight balanced and assigned to one of the following four groups: control, exercise,
hypoxia, and
hypoxia-exercise. Rats with hypoxic treatment (breathing 14% O(2) for 12 hr/d) were exposed under normobaric conditions. The training protocol consisted of swimming for two 3-hr periods per day for 4 weeks.
Glycogen content,
GLUT4 protein, and
mRNA of all rats were determined 16 hr
after treatments. Four-week exercise training without
hypoxia significantly elevated myocardial
glycogen level by 45%. The chronic hypoxic-exercise training elevated the myocardial
glycogen level by 67% above control level, significantly greater than the exercise group. Chronic
hypoxia, exercise training, and
hypoxia-exercise training significantly elevated
GLUT4 protein by 40-70% in cardiac muscles. Chronic
hypoxia significantly elevates the
GLUT1 protein level independent of exercise training. The new finding in this study was that GLUT4 gene expression in cardiac muscle can be stimulated by exercise training with
hypoxia treatments. This molecular adaptation appears to be associated with the observed increase in
glycogen storage of the muscle.