Muscle fiber size and chronic exposure to hypoxia

Sarcomere length is practically never considered when fiber size and dependent variables are compared between muscles or experimental conditions. Because of the direct dependence of fiber cross-sectional area on muscle shortening, it is imperative to normalize measurements of fiber size, and related variables (e.g. capillary number/mm2 of fiber) to sarcomere length. We examined the relationship between fiber cross-sectional area and sarcomere length in muscles of animals chronically exposed to high altitude (deer mice, Peromyscus maniculatus, native to 3800 m, and rats, Sprague-Dawley, kept at the same altitude for 5 months) compared to sea-level controls. We found no difference in fiber cross-sectional area, normalized to sarcomere length, between high altitude and control animals in either species. It has been demonstrated that sarcomere length can vary by as much as 30-44% and 43-76% in biopsy and perfusion-fixed muscles, respectively. Therefore, identifying relatively small changes in fiber size in response to a given experimental condition in such material without normalizing for sarcomere length is difficult if not impossible. Furthermore, if the conditions of the investigation induce differences in sarcomere length between experimental and control animals, artifactual changes in fiber cross-sectional area will be produced.