Pro- and macroglycogenolysis: relationship with exercise intensity and duration

We examined the net catabolism of two pools of glycogen, proglycogen (PG) and macroglycogen (MG), in human skeletal muscle during exercise. Male subjects (n = 21) were assigned to one of three groups. Group I exercised 45 min at 70% maximal O-2 uptake ((V) over dot(2max)) and had muscle biopsies at rest, 15 min, and 45 min. Group 2 exercised at 85% (V) over dot(2 max) to exhaustion (45.4 +/- 3.4 min) and had biopsies at rest, 10 min, and exhaustion, Group 3 performed three 3-min bouts of exercise at 100% (V) over dot(2 max) separated by 6 min of rest. Biopsies were taken at rest and after each bout. Group I had small MG and PG net glycogenolysis rates (ranging from 3.8 +/- 1.0 to 2.4 +/- 0.6 mmol glucosyl units kg(-1) . min(-1)) that did not change over time. In group 2, the MG glycogenolysis rate remained low and unchanged over time, whereas the PG rate was initially elevated (11.3 +/- 2.3 mmol glucosyl units . kg(-1) . min(-1)) and declined (P less than or equal to 0.05) with time. During the first 10 min, PG concentration ([PG]) declined (P less than or equal to 0.05), whereas MG concentration ([MG]) did not. Similarly, in group 3, in both the first and the second bouts of exercise [PG] declined (P 0.05) and [MG] did not, although by the end of the second exercise period the [MG] was lower (P less than or equal to 0.05) than the rest level. The net catabolic rates for PG in the first two exercises were 22.6 +/- 6.8 and 21.8 +/- 8.2 mmol glucosyl units kg(-1) . min(-1), whereas the corresponding values for MG were 17.5 +/- 6.0 and 10.8 +/- 5.6. The MG pool appeared to be more resistant to mobilization, and, when activated, its catabolism was inhibited more rapidly than that of PG. This suggests that the metabolic regulation of the two pools must be different.