The effect of insulin treatment and exercise modality on skeletal muscle fiber size in streptozotocin-induced type 1 diabetic rats
Severe Type 1 Diabetes Mellitus (T1DM) is known to have several negative effects on skeletal muscle mass, a condition known as diabetic myopathy. One of these effects is the atrophy of the glycolytic muscle fibers. However, the role of intensity of insulin treatment and exercise modality in attenuating this loss in fiber cross-sectional area (CSA) specifically has yet to be determined. The purpose of this investigation was to determine the effects of 12 weeks of differing intensity of insulin therapy and exercise modality on the CSA of plantaris muscle fibers, identified by myosin heavy chain (MHC) of STZ-induced T1DM rats. Rats were divided into control sedentary (CS), T1DM-sedentary (DCT - conventional versus DIT- intensive insulin therapy, blood glucose 9-15mmol/l and 7-9mmol/l, respectively) and T1DM-exercised (DRE- resistance, DHE-endurance, DCE-combined training, blood glucose 9-15mmol/l) groups. Exercises consisted of repeated weighted ladder climbs (50%, 75%, 90%, and 100% pre-determined max. carrying capacity, 1.1 m, 80°) or treadmill running (27 m/min, 6% grade, 1hr) 5 days/week for 12 weeks. DCE animals performed resistance and endurance exercise on alternating days. Following 12 weeks, diabetic sedentary (DCT, DIT) and diabetic exercised (DRE, DHE, DCE) groups did not differ in body weight (BW) or plantaris whole-muscle weight, except where DIT BW was significantly greater than DRE (P0.05) or aerobic exercise (DHE) in some instances (all fibers expressing fast MHC’s except MHCIIx/IIb and MHCiib – P0.05). Surprisingly, DIT exhibited significantly larger MHCIIa and fewer hybrid fibers than any other group. MHCI fibers were unaffected by either diabetes or exercise, however, exercised animals displayed changes in percent fiber composition, including a significant proportion of hybrid fibers, that appeared to indicate a shift in fiber type particularly from MHCIIx to MHCIIa. Resistance exercise, whether alone (DRE) or in conjunction with aerobic training (DCE) resulted in significantly larger MHCIIx and MHCIIb than CS. Treadmill running (DHE) resulted in less hypertrophy of fibers expressing fast MHC than the other exercised groups. These findings indicate that conventional insulin therapy is sufficient to maintain muscle mass and fiber CSA, while intensive insulin therapy and exercise with conventional insulin therapy induce differential changes in fiber percent composition and fiber CSA increases.
type 1 diabetes mellitus, diabetic myopathy, skeletal muscle, insulin, exercise, muscle fiber cross-sectional area
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