Substrate depletion in different types of muscle and in liver during prolonged running.

BALDWIN, K. M., J. S. REITMAN, R. L. TERJUNG, W. W, WINDER, AND J. 0. HOLLOSZY. Substrate depktion in dz’fmnt types of muscle and in liver during prolonged running. Am. J. Physiol. 2 2 5(5) : 1045-l 050. 1973,KDepletion of glycogen and triglyceride stores in red, white, and intermediate types of skeletal muscle and of glycogen in liver was studied in rats subjected to prolonged exercise. Groups of rats were killed after 15, 60, and 120 min of running up an 8’ incline at a) 1 mph, b) I mph with I min at 1.5 mph every 9 min, and c) alternate 30-set intervals at 0.5 and 1,5 mph. White muscle underwent little or no change in glycogen and triglyceride concentrations in response to the three exercise tests. In red muscle and intermediate muscle, the decrease in glycogen concentration ranged from 42 to 727, after 2 hr; glycogen decreased most sharply during the first 15 min of all three exercise tests and then stabilized or decreased more slowly. Triglyceride concentration in red muscle decreased approximately 4Oaj’, after 2 hr of all three exercise tests. Little or no change in triglyceride concentration occurred in white and intermediate muscle. Liver glycogen decreased progressively during all three exercise tests and was depleted approximately 85y0 after 2 hr. These results suggest that in the rat performing prolonged moderate exercise, a) white muscle contracts infrequently and b) liver glycogen can be a more important source of energy than muscle glycogen.