Glycogenin protein level in muscle fiber types of stimulated rodent hind limb following changes in glycogen content

Abstract

Glycogenin (GN) may be degraded, may stay bound to partly‐catabolized glycogen or may form unglycosylated GN when glycogen content decreases. Oxidative and glycolytic muscle fibers may respond differently. One hind limb (40 Female Sprague Dawley rats) was stimulated to contract for 40 min to lower glycogen content. The soleus (SOL), red (RG) and white (WG) gastrocnemius muscles from unstimulated and stimulated limbs were examined for total and unglycosylated GN protein levels. Muscle tissue (corresponding unstimulated and stimulated) was grouped (n=6) by post‐stimulation glycogen content (WGI <40; WGII <100; RGI <100, RGII >100; SOL >100 μmol▸g dry wt.−1). GN increased (P≤0.05) in WGI (66%), RGI (33%), and RGII (100%), but was unchanged in WGII and SOL. In WGII, GN decreased in a low‐spin myofibrillar pellet fraction and increased in the soluble fraction (no net change). Unglycosylated GN was detected in stimulated W and RG muscles (up to 25% of total GN) and was most abundant in WGI (lowest glycogen content). In summary, no correlation existed between GN level and glycogen content post‐stimulation either across fiber types or within a fiber type. No net degradation of GN occurred in either W or RG muscles in response to significant reduction in glycogen content; new GN was always synthesized. Pre‐existing GN is likely retained in smaller granules or as unglycosylated protein. Funded by NSERC of Canada.