Changes in local capillarity of pure and hybrid MyHC muscle fiber types after nerve injury in rat extensor digitorum longus muscle (EDL)

Recently, we evaluated capillary indices without discrimination by fiber type in rat extensor digitorum longus muscle (EDL) 4 weeks after nerve cut (NC), after double nerve crush (double NCR) and in two controls, from the start (CON-1) and the end (CON-2) of the experiment. In the present study, we determined the capillary indices related to specific myosin heavy chain (MyHC) fiber types. Fiber-type composition and local capillarity were assessed from a single, composite, multicolor image, where different MyHC-fiber types and capillaries were shown simultaneously. Applying local capillary indices [the number of capillaries around fiber (CAF) and the CAF scaled to fiber perimeter (CAF/FP)], to specific MyHC-fiber types, we found changes relevant to neuro-muscular studies. In the NC group, only type-2x fibers had a significantly lower CAF, and in the double NCR group, only type-2a fibers had a higher CAF in comparison with both controls. Both types of nerve injury elicited two responses: a coupled regulation of fiber size and capillarity in the oxidative, type 2a fibers and a capillarity independent regulation of fiber size in the glycolytic type-2b fibers. All subtypes of type-2 fibers had a better capillary supply (higher CAF/FP) in the NC and double NCR than in CON-2. The highest improvement was observed in type-2b fibers; this change was mirrored in an oxidative shift only in the double NCR group. Adopting fiber-type-specific capillary indices improves data analysis of rat EDL muscle samples.