Etruscan shrew muscle: the consequences of being small.

The skeletal muscles of the smallest mammal, the Etruscan shrew Suncus etruscus, are functionally and structurally adapted to the requirements of an enormously high energy turnover. Isometric twitch contractions of the extensor digitorum longus (EDL) and soleus muscles are shorter than in any other mammal, allowing these muscles to contract at outstandingly high frequencies. The skeletal muscles of S. etruscus contract at up to 900 min(-1) for respiration, up to 780 min(-1) for running and up to 3500 min(-1) for shivering. All skeletal muscles investigated lack slow-twitch type I fibres and consist only of fast-twitch type IID fibres. These fibres are optimally equipped with properties enabling a high rate of almost purely oxidative metabolism: they have a small diameter, their citrate synthase activity is higher and their lactate dehydrogenase activity is lower than in the muscles of any other mammal and they have a rapid shortening velocity. Differences in isometric twitch contraction times between different muscles are, at least in part, probably due to differences in cytosolic creatine kinase activities.