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1 Dept. of Anatomy and Physiology, University of Padova, Padova, Italy
2 Dept. of Experimental Veterinary Sciences, IIM, Italian Institute of Myology University of Padova, Padova, Italy
3 Dept. of Experimental Medicine, University of Pavia, Pavia, Italy
4 Dept. of Anatomy and Physiology, IIM, Italian Institute of Myology University of Padova, Padova, Italy
* To whom correspondence should be addressed. E-mail: carlo.reggiani{at}unipd.it.
This study aimed to achieve a definitive and unambiguos identification of fiber types in canine skeletal muscles and of myosin isoforms which are expressed therein. Correspondance of canine myosin isoforms with orthologs in other species as assessed by base sequence comparison was the basis for primers preparation and for expression analysis with RT-PCR. Expression was confirmed at protein level with histochemistry, immuno-histochemistry and SDS-PAGE combined together and showed that limb and trunk muscles of the dog express type 1, 2A and 2X-MHC isoforms and that the previously called type 2Dog fibers express the MHC-2X isoform. MHC-2A was found to be the most abundant isoforms in the trunk and limb muscle analysed. MHC-2X was expressed in most but not all muscles and more frequently in hybrid 2A-2X fibers than in pure 2X fibers. MHC-2B was restricted to specialized extra-ocular and laryngeal muscles, although 2B mRNA, but not 2B protein was occasionally detected in the semimembranosus muscle. Isometric tension and maximum shortening velocity (Vo) were measured in single fibers classified on the basis of their MHC isoform composition. Purified myosin isoforms were extracted from single muscle fibers and characterized by the speed (Vf) of actin filament sliding on myosin in an in vitro motility assay (IVMA). A close proportionality between Vo and Vf indicated that the diversity in Vo was due to the different myosin isoform composition. Vo increased progressively in the order 1/slow<2A<2X<2B, thus confirming the identification of the myosin isoforms and providing their first functional characterization of canine muscle fibers.
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