A quantitative analysis of the subcellular distribution of human skeletal muscle glycogen
A few qualitative investigations suggested that subcellular distribution of human skeletal muscle glycogen is heterogeneous and that the glycogen particles in specific sites may be associated with distinct metabolic roles. This thesis investigated the subcellular distribution of human skeletal muscle glycogen. We developed a transmission electron microscopy (TEM) based technique to quantify the subcellular glycogen volume with respect to particle size and number in different subcellular compartments and fibre types. The variability for the various measures was less than 4% and the TEM-derived glycogen data had a strong correlation with that determined biochemically (r2 = 0.96, p < 0.001). Our results showed that: (1) the distribution of glycogen is heterogeneous with respect to subcellular locations and fibre types. The concentration of glycogen is greater in the subsarcolemmal than the myofibrillar compartment. Glycogen particles are smaller and more numerous in the subsarcolemmal than the myofibrillar compartment. Particles are smaller in the type I than type II fibres. The proportion of myofibrillar glycogen located in the intra-myofibrillar compartment is greater in type I than type II fibres; (2) glycogen particle sizes are normally distributed (diameter 8 to 44 [eta]m) in each subcellular location, all fibre types, and throughout a range from low to high glycogen concentrations; (3) both prolonged-moderate and short-intense exercises cause a preferential depletion of myofibrillar over subsarcolemmal glycogen; (4) short-intense exercise leads to the preferential net degradation of larger glycogen particles, while the net degradation of smaller ones appears minimal; (5) the glycogen resynthesis during the first 4h post-exercise is principally caused by an increase in the number of glycogen particles, while the glycogen increase in the remainder of recovery (48h) is characterised by an increase in the volume of individual particles; (6) following a prolonged-moderate intensity exercise, the net rate of individual particle synthesis appears to be independent from the net rate of glycogen synthesis. Taken together, these observations are in agreement with the emerging concept of the role of subcellular compartmentalisation in the control of glycogen metabolism, and add another level of complexity to glycogen metabolism, which currently can only be addressed by use of ultrastructural analysis.