The effects of exercise training versus intensive insulin treatment on skeletal muscle fibre content in type 1 diabetes mellitus rodents

Abstract

Background: Intensive-insulin treatment (IIT) strategy for patients with type 1 diabetes mellitus (T1DM) has been associated with sedentary behaviour and the development of insulin resistance. Exercising patients with T1DM often utilize a conventional insulin treatment (CIT) strategy leading to increased insulin sensitivity through improved intramyocellular lipid (IMCL) content. It is unclear how these exercise-related metabolic adaptations in response to exercise training relate to individual fibre-type transitions, and whether these alterations are evident between different insulin strategies (CIT vs. IIT).

Purpose: This study examined glycogen and fat content in skeletal muscle fibres of diabetic rats following exercise-training.

Methods: Male Sprague-Dawley rats were divided into four groups: Control-Sedentary, CIT- and IIT-treated diabetic sedentary, and CIT-exercised trained (aerobic/resistance; DARE). After 12 weeks, muscle-fibre lipids and glycogen were compared through immunohistochemical analysis.

Results: The primary findings were that both IIT and DARE led to significant increases in type I fibres when compared to CIT, while DARE led to significantly increased lipid content in type I fibres compared to IIT.

Conclusions: These findings indicate that alterations in lipid content with insulin treatment and DARE are primarily evident in type I fibres, suggesting that muscle lipotoxicity in type 1 diabetes is muscle fibre-type dependant.

Keywords: Exercise; Insulin treatment; Intramyocellular lipids; Muscle glycogen; Skeletal muscle fibre; Type 1 diabetes mellitus.

Fig. 1

Examples of serial sections with three histochemical stains used in fibre quantification, from left to right: a Metachromatic myosin ATPase (stain to identify muscle fibre type), b Glycogen Periodic Acid Schiff (stain to identify high/low glycogen content), and c Oil Red O (stain for high/low IMCL content). See Additional file 1 for quantification example. Note that Oil Red O-stained neutral lipids appear to primarily exist around the internal periphery of the cell membrane, suggesting a possible role for subcellular localization of lipid droplets in the utility of IMCL

Fig. 2

Complete blood glucose and body weight data are presented in (a) and (b), respectively. These data were collected for 17 male rats across four groups throughout the duration of the study. † denotes the start of streptozotocin injections to induce type 1 diabetes; ‡ denotes the start of insulin treatment; and ¶ denotes the start of exercise regimen. a Mean non-fasted weekly BG measures (mmol/L). All data presented as mean ± SEM. * denotes P ≤ 0.0001. Insulin was administered through subcutaneous implants, and pellet dosage was adjusted throughout the study to maintain desired blood glucose ranges b Mean weekly body mass measures (g). All data presented as mean ± SEM. * denotes P = 0.0059, ** denotes P < 0.0001. The four groups compared were Control Sedentary (CS); diabetic with conventional insulin therapy (DCT); diabetic with intensive insulin therapy (DIT); and diabetic with combined exercise training and conventional insulin therapy (DARE). Note that the body mass of all three diabetic groups was significantly lower than the mass of the CS group (P < 0.0001)

Fig. 3

The fibre-type specific metabolic profiles of 3630 muscle fibres from 17 male rats across four groups. The top, middle, and bottom rows present data for type I (a-c), type IIa (d-f), and type IIb myofibres (g-h) respectively. The columns from left to right present data on the percentage of high neutral-IMCL containing fibres (as assessed via Oil Red O staining) (a,d,g), the percentage of high-glycogen containing fibres (as assessed via glycogen periodic acid Schiff staining) (b,e,h), and finally the overall percentage of each row’s muscle fibre as a percentage of the total fibres quantified (fibre type assessed via metachromatic myosin ATPase staining) (c,f,i). The four groups are Control Sedentary (CS); diabetic with conventional insulin therapy (DCT); diabetic with intensive insulin therapy (DIT); and diabetic with combined exercise training and conventional insulin therapy (DARE). All data are expressed as mean ± SEM. * denotes 0.05 > P > 0.005; ** denotes P = 0.0047; *** denotes P = 0.0017; **** denotes P = 0.0003; ***** denotes P < 0.0001. Note that all changes in IMCL storage occurred within the type I oxidative fibres, and that there are a greater percentage of these type I myofibres in the DIT and DARE groups when compared to control and DCT

Fig. 4

The whole-muscle metabolic profiles of 3630 muscle fibres from 17 male rats across four groups. The four groups are Control Sedentary (CS); diabetic with conventional insulin therapy (DCT); diabetic with intensive insulin therapy (DIT); and diabetic with combined exercise training and conventional insulin therapy (DARE). a The percentage of total fibres across all fibre types identified as containing high neutral IMCL stores via Oil Red O staining. b The percentage of total muscle fibres across all fibre types identified as containing high glycogen stores via glycogen periodic acid Schiff staining. All data are expressed as mean ± SEM. ** denotes P = 0.0094; *** denotes P = 0.0002. Note that the DARE group had significantly more high-IMCL containing fibres than each of the other groups, which is indicative of a well-documented result of exercise training known as the athlete’s paradox. Taken in conjunction with Fig. 3, it appears that this phenomenon occurs as a result of changing IMCL storage specifically within type I oxidative fibres