Myosin isoform fiber type and fiber architecture in the tail of the Virginia opossum (Didelphis virginiana)

Abstract

Myosin heavy chain (MHC) isoforms are the primary determinants of muscle fiber contractile properties, and therefore the distribution of slow and fast MHC fibers reflects functional specializations of muscles. Muscle architecture and fiber type are well studied properties in limb muscles, however, much less is understood about these properties in axial muscles, particularly those of tail musculature. Opossums are an interesting lineage in this context, as muscle fiber type has been studied in only two opossum marsupials and fiber typing of the tail in one species was limited to histochemical classification of fibers as either slow or fast. To expand on these previous studies, MHC isoform fiber type and their regional distribution (proximal/middle/distal) were determined in the tail of the Virginia opossum (Didelphis virginiana). Fiber type was determined by a combination of Myosin ATPase histochemistry, immunohistochemistry and SDS-PAGE. Results indicate a predominance of the fast MHC-2A and 2X isoforms in each region of the tail. The presence of two fast isoforms, in addition to the slow MHC-1 isoform, was confirmed by SDS-PAGE analysis. The overall MHC isoform fiber type distribution for the tail was: 25% MHC-1, 71% MHC-2A/2X hybrid, and 4% MHC-1/2A hybrid. Oxidative MHC-2A/2X isoform fibers were found to be relatively large in cross-section compared to slow, oxidative MHC-1 and MHC-1/2A hybrid fibers. A large percentage of fast MHC-2A/2X hybrids fibers may be suggestive of a evolutionary transition in MHC isoform distribution (slow-to-fast fiber type) in the tail musculature of an opossum with primarily a terrestrial locomotor habit vs. arboreal lifestyle.