Red-toothed shrews of subfamily Soricinae are small mammals with very high mass-specific metabolic rate. Owing to their high aerobic power they are interesting objects for studies concerning the limits and constraints of skeletal muscle adaptation. In order to clarify the correlation between metabolic rate and muscle properties, we have analyzed fiber types, fiber size, and myosin heavy chain composition of the common shrew (Sorex araneusj and compared them to those of rat (Rattus noruegicus).
Three distinct differences between shrew and rat muscles were noted. 1) The fibers of shrew muscles are exceptionally small in comparison to rat myofibers. 2) Electrophoretic and histochemical analysis showed that shrew muscles are composed of only fast fibers (fiber types IIB and IID), the slow type I fibers being totally absent. 3 ) The shrew muscles are much more homogenous than rat muscles in regard to myosin heavy chain and fiber type composition. The shrew diaphragm consists exclusively myosin heavy chain type IId (MHCIId), while masseter and soleus are composed 95% and 87% of MHCIId, respectively Other four studied muscles contain MHCIIb and MHCIId approximately in equal proportions.
The present findings show that shrew muscles are composed of small, highly aerobic, fast type I1 fibers, which may be sufficiently fatigue-resistant to function both as postural muscles and t o power fast and high frequency movements of limbs and diaphragm. 0 1995 Wiley-Liss, Inc.