Hereditary dominance of fast-twitch fibers in skeletal muscles and relation of thyroid hormone under physiological conditions in rats.

To investigate whether muscle fiber compositions of various skeletal muscles were correlated with each other and whether muscle fiber type composition was related to the serum thyroid hormone level, fast-twitch fiber dominant rats (FFDR), which had a higher percentage of fast-twitch (FT) fibers in the deep portion of the lateral head of the gastrocnemius muscle (GAS), were compared to a control group. The FFDR group was obtained by the selective breeding for high-percentage FT fibers in the GAS, and the control group was obtained by random breeding from the same base population. The percentages of FT fibers of the GAS, soleus (SOL), vastus intermedius (VI), biceps brachii (BB), rectus abdominis (RA) and extensor digitorum longus (EDL) muscles were analyzed. The percentages of FT fibers of the GAS, SOL, VI and BB in the FFDR group were significantly higher than in the control group (p<0.05), and percent FT fibers of the GAS were significantly correlated with those of the SOL (r = 0.62, p<0.01), VI (r = 0.45, p <0.05) and BB (r = 0.43, p<0.05). On the other hand, no significant differences between FFDR and the control group and no significant correlations with the GAS were observed in percent FT fibers of the RA and EDL. The total 3,3',5-triiodo-L-thyronine (T3) level in FFDR was not significantly different from the control group and was not significantly correlated with the percentage of FT fibers. These data suggest that the percentages of FT fibers of various skeletal muscles are genetically correlated with each other, but some FT muscles are not because they might be close to the upper limits of the genetic adaptive range. In addition, percentages of FT fibers are independent of the total T3 level under physiological conditions.