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Changes in the contractile properties of motor units in functionally overloaded medial gastrocnemius muscle of the rat

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Krysciak K. , Drzymala-Celichowska H. , Krutki P. , Celichowski J.

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nr S

EN

The study aimed at examining the contractile properties of motor units (MUs) in medial gastrocnemius muscle subjected to 3 months compensatory overload, induced by bilateral tenotomy of its synergists (lateral gastrocnemius, plantaris and soleus). To assure that operated muscles were regularly voluntarily activated, surgical procedures were followed by keeping animals in wheelequipped cages and treadmill exercise. The final electrophysiological experiments were carried out on 106 MUs of the overloaded medial gastrocnemius muscle (OMG) and 88 MUs of the untreated, healthy gastrocnemius muscle (MG). Functionally isolated MUs were examined by electrical stimulation of thin filaments of the L4 – L5 ventral roots. MUs classification was based on 20 Hz tetanus index which divided MUs into fast and slow, whereas the fatigue index distinguished fast fatigable (FF) and fast resistant (FR) MUs. Results showed that both OMG mass and its relation to the body weight were higher in comparison to MG. MUs proportion was modified in response to altered functional demand and in OMG higher percentage of S and FF simultaneously with lower contribution of FR MUs were observed. Changes in MUs contractile properties of OMG in comparison to MG included: shorter half-relaxation time and lower twitch force in all types of MUs and in parallel higher (FR and S) or unchanged (FF) tetanus force. Changes in force parameters in OMG resulted in lower values of the twitch-to-tetanus ratio in all three types of MUs. Higher posttetanic potentiation was also noted for all MUs types in OMG. Changes in fatigue resistance were observed only in fast MUs: for FF type the mean value of the fatigue index was lower in OMG in comparison to MG, but for FR type this value was higher in OMG. In conclusion, the adaptation of the medial gastrocnemius muscle to overload included transformation of some MUs accompanied by changes in MUs contractile properties.

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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

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Krysciak K. , Drzymala-Celichowska H. , Krutki P. , Lochyński D. , Celichowski J.

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nr Suppl.1

EN

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.

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Sinusoidal mechanical responses from single motor units of rat medial gastrocnemius muscle

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Drzymala-Celichowska H. , Krysciak K. , Kaczmarek P. , Orizio C. , Krutki P. , Celichowski J.

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nr Suppl.1

EN

INTRODUCTION: The relationship between the output force and motor command depends on the intrinsic RESULTS: The mathematical decomposition of unfused tetanic contractions of FF and FR MUs into twitch-shape responses to consecutive stimuli was conducted. The decomposition indicated substantial changes predominantly in force and additionally in time parameters of successive twitch‑like components, responsible for a sag profile in tetanic curve. Namely, initially the force increased and the highest force was observed in a response to the 2nd–3rd stimulus for FF units, while after the 3rd–7th stimulus for FR MUs and later decreased leading to the sag. In the second series of experiments, a repeatability of the sag in tetanic contractions of the same MU in a muscle with preserved blood circulation and under ischemic conditions was tested. Sag restitution was present in muscles with the circulation preserved but it was prevented by occlusion of blood vessels, indicating that sag depends on an availability of an energy source which can be restituted under aerobic conditions. CONCLUSIONS: The study indicated that sag profile of unfused tetanic contractions is predominantly an effect of early increase in amplitudes of several initial responses followed by a decrease in their amplitudes and that these changes are stronger and longer in time scale in FR than in FF MUs. The results concerning repeatability of the phenomenon suggest that most probable source of energy for initial force increase is phosphocreatine.