Functional differences in type-I fibres from two slow skeletal muscles of rabbit.

The present study addressed the question of whether the slow fibres of mammalian skeletal muscle, containing the myosin heavy chain MHCI (type-I fibres), are a functionally homogeneous population. We compared various properties of Ca(2+)-activated, skinned, type-I fibres from the soleus and semitendinosus muscles of a rabbit. Soleus type-I fibres showed significantly faster kinetics of stretch activation, measured as the time-to-peak of the stretch-induced, delayed force increase, t(3), than semitendinosus fibres (1239+/-438 ms, n=136, vs. 1600+/-409 ms, n=208 respectively) (means+/-SD, 22 degrees C). Similarly, the speed of unloaded shortening at 15 degrees C was faster in soleus than in semitendinosus fibres [0.79+/-0.16 fibre lengths (FL) s(-1), n=44, vs. 0.65+/-0.15 FL s(-1), n=35 respectively]. The kinetics of stretch activation were more temperature sensitive in semitendinosus than in soleus fibres. Finally, the generation of steady-state isometric force was more sensitive to Ca(2+) in semitendinosus than in soleus fibres: [pCa(50) (-log [Ca(2+)] for half-maximal activation) at 22 degrees C: 6.29+/-0.15, n=28, vs. 6.19+/-0.10, n=18 respectively]. These results suggest strongly that there is no functional homogeneity within type-I fibres of different muscles. The observed differences might reflect the existence of more than one functionally different slow myosin heavy chain isoforms or other modifications of contractile proteins.