Mitochondrial morphology and function varies across diaphragm muscle fiber types

In diaphragm muscle (DIAm), type I and Ita fibers are recruited to accomplish breathing, while type IIx/IIb fibers are recruited only during expulsive/straining behaviors. Thus, type I and Ita DIAm fibers are much more active (duty cycle of similar to 40 %) than type IIx/IIb fibers (duty cycle of <1%), which we hypothesized underlies intrinsic differences in mitochondrial structure and function. MitoTracker Green labeled mitochondria were imaged in 3-D using confocal microscopy. Mitochondrial volume density (MVD, per muscle fiber volume) was higher, and mitochondria were more filamentous in type I and Ita DIAm compared to type IIx/IIb fibers. The maximum velocity of the succinate dehydrogenase reaction (SDHmax), measured using a quantitative histochemical technique was found to be higher in type I and Ita DIAm fibers compared to type IIx/IIb fibers with and without normalizing for MVD. These results are consistent with fiber type differences in the intrinsic structural and functional properties of DIAm fibers and closely match differences in energetic demands.

Automated summary

The study found that type I and Ita DIAm fibers have higher mitochondrial volume density, more filamentous mitochondria, and higher maximum velocity of the succinate dehydrogenase reaction compared to type IIx/IIb fibers, indicating significant differences in structural and functional properties between fiber types.