Muscle glycogen content in type 2 diabetes mellitus

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Abstract
Muscle contains the largest reservoir of glycogen (Glyc), a depot that is closely regulated and with influence on insulin sensitivity. The current study examines muscle Glyc in type 2 diabetes mellitus (T2DM) and obesity and with respect to muscle fiber type, intramyocellular lipid content (IMCL), and mitochondrial function (oxidative enzyme activity; OX-Enz). There is increasing interest in the relation of IMCL and mitochondrial dysfunction with insulin resistance (IR), yet the association with muscle Glyc has not been examined with regard to these parameters. Using a quantitative histological approach specific to muscle fiber types, we assessed muscle Glyc, IMCL, and OX-Enz in vastus lateralis obtained by percutaneous biopsy in lean nondiabetic (L; n = 16), obese nondiabetic (Ob; n = 15), and T2DM volunteers ( n = 14). Insulin sensitivity was estimated using homeostasis model assessment (HOMA)-IR. Muscle Glyc was reduced in T2DM, a deficit evident for type IIa fibers, yet minor in types I and IIb fibers. Low Glyc in T2DM correlated with fasting hyperglycemia. Also, in T2DM and Ob, there was significantly higher IMCL and lower OX-Enz in all fiber types. The IMCL-to-OX-Enz ratio, especially for type I fibers, correlated strongly with IR. Similarly, a Glyc-to-OX-Enz ratio correlated with IR, particularly for type IIb fibers. This ratio tended to be higher in Ob and T2DM. In summary, there is decreased muscle Glyc in T2DM yet a disproportional Glyc-to-OX-Enz relationship that is related to IR, although not as robustly as the IMCL-to-OX-Enz ratio.