Mitochondrial functional specialization in glycolytic and oxidative muscle fibers: tailoring the organelle for optimal function

In skeletal muscle, two major types of muscle fibers exist: slow-twitch oxidative (type I) fibers designed for low-intensity long-lasting contractions, and fast-twitch glycolytic (type II) fibers designed for high-intensity short-duration contractions. Such a wide range of capabilities has emerged through the selection across fiber types of a narrow set of molecular characteristics suitable to achieve a specific contractile phenotype. In this article we review evidence supporting the existence of distinct functional phenotypes in mitochondria from slow and fast fibers that may be required to ensure optimal muscle function. This includes differences with respect to energy substrate preferences, regulation of oxidative phosphorylation, dynamics of reactive oxygen species, handling of Ca2+, and regulation of cell death. The potential physiological implications on muscle function and the putative mechanisms responsible for establishing and maintaining distinct mitochondrial phenotype across fiber types are also discussed.