Introduction: 

Serine/threonine protein phosphatase calcineurin B (CnB) and thyroid hormones are known to modulate skeletal muscle fiber phenotype also in response to different exercise patterns. Previous studies determine the relationship predominant by cell cultures or animal models, whereas the involvement of these parameters in human skeletal muscle has received much less attention. The aim of the study was to analyze the role of CnB and thyroid hormones in a load-dependent endurance exercise induced fiber type transformation in 10 male cyclists (17.3 ± 0.5yr).

Methods: 

We investigated the training period (TP) and the competitive season (CS). Before (PRE) and after (POST) CS muscle biopsies were obtained from the vastus lateralis muscle and CnB was analyzed immunohistochemically. Serum levels of total (T3) and free (fT3) triiodothyronine were analyzed using electrochemiluminescence immunoassays.

Results: 

Throughout CS the relative distribution of fiber type I did not change (P > 0.05; PRE: 63.5 ± 4.87 vs. POST: 56.1 ± 10.26%). The % fiber type IIX increased significantly (P < 0.001; PRE: 10.5 ± 7.78 vs. POST: 16.4 ± 6.50%) whereas % fiber type IIA remained unchanged (P > 0.05; PRE: 26.0 ± 7.01 vs. POST: 27.5 ± 10.24%). CnB was decreased (P < 0.001) by ~46% (PRE: 29.9 ± 5.65 vs. POST: 16.1 ± 4.59 DU). Ratio fT3/T3 increased significantly (P < 0.01) by ~8% (PRE: 0.00309 ± 0.00018 vs. POST: 0.00333 ± 0.00018 ng·ml^-1).

Conclusions: 

Both CnB and thyroid hormones are affected strongly in response to load-dependent endurance exercise. A change of exercise regimen from TP to CS by a reduction of volume and increase of intensity leads to a significant increase of thyroid hormones and a significant decrease of CnB. We suggest that both parameters induce the slow to fast fiber type shift by an increase of IIX fibers.