Fiber types and size in equine skeletal muscle.

This research study investigates the proportions of different types and sizes of muscle fibers in equine skeletal muscles using standard histochemical processes involving myosin adenosine triphosphatase (ATPase) and reduced nicotinamide adenine dinucleotide-tetrazolium reductase (NADH-TR).

Approach and Staining Methods

• The researchers used sections from horses’ semitendinosus muscle, freezing them for study. These were examined using ATPase and NADH-TR, both essential molecular components involved in muscle contractions and metabolism.
• ATPase helps to identify different muscle fiber types since it shows different reactions in various fiber types. About 70% of the fibers were ATPase positive, categorizing them as alpha fibers. The rest, 30%, were found to be ATPase negative, or beta fibers.
• Further to this, the ATPase activity intensity was varied, leading to a division of alpha fibers into two subtypes: alpha positive and alpha intermediate.
• NADH-TR staining, on the other hand, helps in determining the type of metabolism ongoing within the muscle fibers. Fibers reacting positively to NADH-TR represent red fibers that mainly have aerobic metabolism, while those giving negative results are white fibers whose metabolism is primarily anaerobic.

Fiber Types and Metabolism

• All the beta fibers, which were ATPase negative, demonstrated a positive reaction with NADH-TR staining, categorizing them as red fibers with aerobic metabolism. This matches the expected characteristic of beta R fibers.
• All alpha positive fibers were ATPase positive and gave negative reactions with NADH-TR. Hence, they could be classified as white fibers corresponding to alpha W fibers.
• Most alpha intermediate fibers also presented this same reaction, although there was a range of alpha intermediate fibers that gave an intermediate reaction with NADH-TR. These could be classified as alpha R fibers that haven’t transformed into alpha W fibers yet.

Fiber Size

• The study found that alpha positive fibers were relatively larger in diameter, around 7 to 10 micrometers more than either beta or alpha intermediate fibers.

Unresolved Questions

• While the study provided important insights, it did not clarify the relationship between the variation in ATPase reaction intensity and the speed of muscle fiber contraction.