The fiber type transition of skeletal muscle is an intricate and essential physiological process in the body, significantly influencing both the function and metabolism of skeletal muscle. This phenomenon is not only affected by external environmental changes but also intricately regulated by internal physiological mechanisms. Therefore, exploring the physiological process of muscle fiber type transition holds considerable significance for the treatment of human neuromuscular disorders and the improvement of meat quality in livestock and poultry. It has been discovered that the cytokines secreted by skeletal muscle, i.e., myokines, play a role in the fiber type transition of skeletal muscle. Myokines mainly act on skeletal muscle in autocrine and paracrine forms to participate in signal transduction and regulate the fiber type transition of skeletal muscle. This paper reviews the functional differences among various muscle fiber types, expounds the effects and mechanisms of myokines in regulating the transition processes of these fiber types, and prospects the future research directions in this field. This review is expected to provide theoretical support for enhancing the meat quality of livestock and poultry and treating skeletal muscle-related diseases.
Humans ; Animals ; Cytokines/metabolism* ; Muscle Fibers, Skeletal/metabolism* ; Muscle, Skeletal/metabolism* ; Signal Transduction ; Muscle Fibers, Slow-Twitch/metabolism* ; Muscle Fibers, Fast-Twitch/metabolism* ; Myostatin/metabolism* ; Myokines

BACKGROUNDAge-associated decrease in type IIA/B human skeletal muscle fibers was detected in human biopsies in our previous study. The relationship between change in muscle fiber typing and bone mineral density (BMD) is, however, unknown either cross-sectionally or longitudinally. We therefore conducted a cross-sectional study to investigate their correlation using human muscle biopsies.

METHODSForty human subjects aged (53.4 ± 20.2) years were recruited. Histomorphometric parameters of their muscle biopsies were measured by ATPase staining and image analysis, including average area percentage, fiber number percentage, mean fiber area, and area percentage of connective tissues. Hip and spine BMD was measured by dual-energy X-ray absorptiometry. Partial correlation with adjusting age was performed.

RESULTSType IIB muscle fiber was found positively correlated with hip BMD irrespective to age and demonstrated significantly stronger relationship with BMD among all fiber types, in terms of its cross-sectional area (r = 0.380, P = 0.029) and size (r = 0.389, P = 0.025). Type IIA muscle fibers associated with hip BMD in mean fiber area only (r = 0.420, P = 0.015).

CONCLUSIONSType IIB muscle fiber may play an important role in maintaining bone quality. This may also be a relatively more sensitive fiber type of sarcopenia and osteoporosis. These findings further consolidate the muscle-bone relationship.

Adult ; Age Factors ; Aged ; Aged, 80 and over ; Bone Density ; physiology ; Female ; Humans ; In Vitro Techniques ; Male ; Middle Aged ; Muscle Fibers, Fast-Twitch ; cytology ; metabolism ; Prospective Studies ; Young Adult

OBJECTIVETo study the effect of testosterone propionate (TP) on the distribution pattern of calcitonin gene-related peptide (CGRP) in two types of motoneuron (Mn) pools in rats.

METHODThe double labeling of cholera toxin B subunit coupled with colloidal gold (CB-Au) retrograde identification combining with immunocytochemistry was mainly used to reveal the distribution pattern of CGRP-like immunoreactivity (CGRP-LI) and its changes in the motoneuron pools labeled by CB-Au.

RESULTTP injected intramuscularly 28 days later significantly decreased CGRP expression in Mn pool innervating extensor digitorum longus (EDL, fast-twitch), comparing with corresponding control and castration group respectively (P < 0.001), while no significant effect on Mn pools innervating soleus (SOL, slow-twitch, P > 0.05) was observed.

CONCLUSIONEDL-Mn pool is more sensitive to testosterone propionate than SOL-Mn pool in regulating CGRP expression.

Animals ; Calcitonin Gene-Related Peptide ; drug effects ; metabolism ; Male ; Motor Neurons ; drug effects ; metabolism ; Muscle Fibers, Fast-Twitch ; cytology ; drug effects ; Muscle Fibers, Slow-Twitch ; cytology ; drug effects ; Rats ; Rats, Wistar ; Testosterone Propionate ; pharmacology