Contraction parameters, myosin composition and metabolic enzymes of the skeletal muscles of the Etruscan shrew Suncus etruscus and of the common European white-toothed shrew Crocidura russula (Insectivora : Soricidae)

In the Etruscan shrew, the isometric twitch contraction times of extensor d igitorum longus (EDL) and soleus muscles are shorter than in any other mamm al, allowing these muscles to contract at outstandingly high contraction fr equencies. This species has the highest mass-specific metabolic rate of all mammals and requires fast skeletal muscles not only for locomotion but als o for effective heat production and for an extremely high ventilation rate. No differences could be detected in the fibre type pattern, the myosin hea vy and light chain composition, or in the activity of the metabolic enzymes lactate dehydrogenase and citrate synthase of the two limb muscles, the ED L and the soleus, which in larger mammalian species exhibit distinct differ ences in contractile proteins and metabolic enzymes. All properties determi ned in EDL and soleus muscles of Suncus etruscus, as well as in the larger Crocidura russula, are typical for fast-oxidative fibres, and the same hold s for several other skeletal muscles including the diaphragm muscle of S. e truscus. Nevertheless, the EDL and soleus muscles showed different mechanic al properties in the two shrew species. Relaxation times and, in C. russula , time to peak force are shorter in the EDL than in the soleus muscle. This is in accordance with the time course of the Ca2+ transients in these musc les. Such a result could be due to different parvalbumin concentrations, to a different volume fraction of the sarcoplasmic reticulum in the two muscl es or to different Ca2+-ATPase activities. Alternatively, the lower content of cytosolic creatine kinase (CK) in the soleus compared with the EDL musc le could indicate that the observed difference in contraction times between these shrew muscles is due to the CK-controlled activity of their sarcopla smic reticulum Ca2+-ATPase.