Functional differences in type-I fibres from two slow skeletal muscles of rabbit |
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Authors: | Andruchov Oleg Andruchova Olena Wang Yishu Galler Stefan |
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Affiliation: | (1) Institut für Zoologie, Universität Salzburg, Hellbrunnerstrasse 34, 5020 Salzburg, Austria |
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Abstract: | The present study addressed the question of whether the slow fibres of mammalian skeletal muscle, containing the myosin heavy chain MHCI (type-I fibres), are a functionally homogeneous population. We compared various properties of Ca2+-activated, skinned, type-I fibres from the soleus and semitendinosus muscles of a rabbit. Soleus type-I fibres showed significantly faster kinetics of stretch activation, measured as the time-to-peak of the stretch-induced, delayed force increase, t3, than semitendinosus fibres (1239±438 ms, n=136, vs. 1600±409 ms, n=208 respectively) (means±SD, 22 °C). Similarly, the speed of unloaded shortening at 15 °C was faster in soleus than in semitendinosus fibres [0.79±0.16 fibre lengths (FL) s–1, n=44, vs. 0.65±0.15 FL s–1, n=35 respectively]. The kinetics of stretch activation were more temperature sensitive in semitendinosus than in soleus fibres. Finally, the generation of steady-state isometric force was more sensitive to Ca2+ in semitendinosus than in soleus fibres: [pCa50 (–log [Ca2+] for half-maximal activation) at 22 °C: 6.29±0.15, n=28, vs. 6.19±0.10, n=18 respectively]. These results suggest strongly that there is no functional homogeneity within type-I fibres of different muscles. The observed differences might reflect the existence of more than one functionally different slow myosin heavy chain isoforms or other modifications of contractile proteins. |
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Keywords: | Skeletal muscle Slow myosin heavy chain Soleus muscle Semitendinosus muscle Muscle mechanics Stretch activation Shortening velocity |
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