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Aging Reduces Succinate Dehydrogenase Activity in Rat Type IIx/IIb Diaphragm Muscle Fibers.

M. Fogarty,Natalia Marin Mathieu,C. Mantilla,G. Sieck

Published 2019 in Journal of applied physiology

ABSTRACT

In aged rats, diaphragm muscle (DIAm) reduced specific force and fiber cross sectional area - sarcopenia, is selective for vulnerable type IIx and/or IIb DIAm fibers, with type I and IIa fibers being resilient. In humans, the oxidative capacity (as measured by maximum succinate dehydrogenase activity, SDHmax) of fast type muscle is reduced with aging, with slow muscle being unaffected. We hypothesized that in aged Fischer rats DIAm exhibiting sarcopenia, reduced SDHmax activity would occur in type IIx and/or IIb fibers. Rats obtained from the NIA colony (6-, 18- and 24-months old), euthanized and ~2 mm wide DIAm strips were obtained. For SDHmax and fiber type assessments, DIAm strips were stretched (~lo), fresh frozen in isopentane and sectioned on a cryostat at 6 μm. The SDHmax, quantified by intensity of nitro blue tetrazolium diformazan precipitation was assessed in a fiber type specific manner, by comparing serial sections labelled with myosin heavy chain (MyHC) antibodies differentiating type I (MyHCslow), IIa (MyHC2A) and IIx and/or IIb fibers. Isometric DIAm force and fatigue was assessed in DIAm strips using muscle stimulation with supramaximal pulses at a variety of frequencies (5-100 Hz) delivered in 1 s trains. By 24-months, DIAm sarcopenia was apparent and SDHmax in type IIx and/or IIb fibers activity was reduced ~35%, compared to 6-month old control DIAm. These results underscore the remarkable fiber type selectivity of type IIx and/or IIb fibers to age-associated perturbations and suggest that reduced mitochondrial oxidative capacity is associated with DIAm sarcopenia.

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