Abstract
Major stores of glucose are found as glycogen in skeletal muscle and liver. Skeletal muscle is a heterogenous tissue, with cellular metabolic and contractile distinctions dependent on whether the cell (fibre) is slow-twitch (Type I) or fast-twitch (Type II). We hypothesised that proteins important for glycogen metabolism would be differentially abundant between these diverse fibres. We further hypothesised that the cellular location of these proteins would be different in muscle samples between control (CON) and individuals with type 2 diabetes (T2D). We dissected individual muscle fibre segments from vastus lateralis skeletal muscle biopsy samples from CON and T2D and used cell-type-specific approaches to address muscle heterogeneity. We measured glycogen and glycogen-related proteins by immunoblotting techniques. A lower proportion of Type I fibres was found in muscle in T2D compared with CON. AMPK-β2, glycogen branching enzyme (GBE), glycogen debranching enzyme (GDE), and glycogen phosphorylase (GP) were differentially localized between fibre types and in fibres from CON and T2D individuals. A key novel finding was that the majority of glycogen is loosely bound or cytosolic in location in human skeletal muscle. The proportion of this diffusible pool of glycogen was significantly lower in Type I fibres in T2D compared to CON. A hyperinsulinaemic, euglycaemic clamp in people with type 2 diabetes had no effect on the proportion of diffusible glycogen. We identify cell-type as an important consideration when assessing glycogen metabolism in muscle. Our findings demonstrate varying glucose handling abilities in specific muscle fibre types in type 2 diabetes. A model is presented to provide an overview of the cell-specific differences in glycogen metabolism in type 2 diabetes.