Effect of tibial bone resection on the development of fast- and slow-twitch skeletal muscles in foetal sheep

To determine if longitudinal bone growth affects the differentiation of fas t- and slow-twitch muscles, the tibial bone was sectioned at 90 days gestat ion in foetal sheep so that the lower leg was permanently without structura l support. At 140 days (term is similar to 147 days) the contractile proper ties of whole muscles, activation profiles of single fibres and ultrastruct ure of fast- and slow-twitch muscles from the hindlimbs were studied. The c ontractile and activation profiles of the slow-twitch soleus muscles were s ignificantly affected by tibial bone resection (TIBX). The soleus muscles f rom the TIBX hindlimbs showed: (1) a decrease in the time to peak of the tw itch responses from 106.2 +/- 10.7 ms (control, n = 4) to 65.1 +/- 2.48 ms (TIBX, n = 5); (2) fatigue profiles more characteristic of those observed i n the fast-twitch muscles; and (3) Ca2+- and Sr2+-activation profiles of sk inned fibres similar to those from intact hindlimbs at earlier stages of ge station. In the FDL, TIBX did not significantly change whole muscle twitch contraction time, the fatigue profile or the Ca2+- and Sr2+-activation prof iles of skinned fibres. Electron microscopy showed an increased deposition of glycogen in both soleus and FDL muscles. This study shows that the devel opment of the slow-twitch phenotype is impeded in the absence of the physic al support normally provided by the tibial bone. We suggest that longitudin al stretch is an important factor in allowing full expression of the slow-t witch phenotype.