The influence of 12 weeks high fat diet and low dose streptozotocin-induced diabetes on contractile properties of motor units in medial gastrocnemius muscle of rat

INTRODUCTION: Obesity and diabetes, with associated conditions termed as metabolic syndrome are growing problem worldwide. Consequences of impaired glucose metabolism affect the whole organism including neuromuscular system. Diabetic neuropathy influences predominantly sensory system and to smaller degree motor system. Additionally, skeletal muscles as involved in glucose metabolism are exposed to processes related to insulin resistance. AIM(S): The study was aimed to investigate effects of high fat diet and streptozotocin-induced diabetes on motor unit’s (MUs) contractile properties in rat medial gastrocnemius (MG) muscle. METHOD(S): Male rats weighting about 180 g were randomly assigned to 3 groups: C, untreated, control, on standard laboratory diet (n=10); HFD, on high fat diet for 12 weeks (n=10); and STZ, on high fat diet for 8 weeks, then injected with a single dose of STZ – 35 mg/kg (n=13). Contractile properties of MUs were investigated in electrophysiological experiments. RESULTS: Both interventions increased the glucose level in the blood but evoked no changes in MG mass. Proportions of the 3 MUs types (FF; fast fatigable, FR; fast resistant and S; slow) were not changed neither in HFD nor STZ compared to C but contractile properties differed significantly in HFD and/or STZ in relation to C. For both fast MU types the twitch time parameters in HFD and STZ were longer, and the twitch-to-tetanus ratio was higher in STZ. The force frequency curves were shifted to lower frequencies in HFD and STZ compared do C, and their slope increased in HFD compared to C group. Furthermore, for FR MUs the force potentiation was lower in STZ compared to C. For S MUs higher tetanus force in HFD compared to C was noted. CONCLUSIONS: Although high fat diet and low dose of streptozotocin have not changed the MG mass and MU proportions, the impaired glucose metabolism modified force-regulation mechanisms of fast MUs in studied muscle.