Muscle fibre type and dimension in genetically obese and lean Zucker rats.

Skeletal muscle structure and morphology may be altered in obesity. To study this further, muscles from six genetically obese (fa/fa) and six normal male rats were examined at 15 weeks of age. The gluteus medius, vastus lateralis and rectus abdominis muscles were dissected out and stained for histochemical fibre typing. In addition the fibre cross-sectional area was measured on a graphic tablet. The proportion of fast-twitch fibres was larger in the vastus lateralis and rectus abdominis muscles of the obese rats (P < 0.01); no difference was seen for the gluteus medius muscle. For the normal rats the cross-sectional area of the fast-twitch fibres was 2-3 times larger than the area of slow-twitch fibres in the same muscle. The cross-sectional area of the fast-twitch fibres in the obese rats was 40-47% less than in the control animals (P < 0.003), while no difference between the two groups was found for the slow-twitch fibre area. The data thus suggest that in the genetically obese rats the development of fast-twitch fibres was primarily affected. Moreover, in these animals some muscles may be more affected than others.