STRUCTURE-FUNCTION RELATIONSHIP OF SOLEUS MUSCLE-FIBERS FROM THE RHESUS-MONKEY

Functional and structural properties of rhesus monkey skinned fibres w ere studied in order to examine the relationship between calcium/stron tium (Ca/Sr) activation characteristics and protein composition. The f ibres were classified according to their Ca/Sr affinity into slow (61% ) and fast groups (39%). According to the myosin isoform composition, two additional hybrid types were defined. Thus, four profiles were cha racterized: two corresponding to slow (S) and fast (F) isoforms and tw o corresponding to a mixed proportion of slow and fast isoforms. They were called hybrid slow (HS) or hybrid fast (HF) based on the predomin ant myosin isoform. Tension/pCa parameters and maximal shortening velo cities were determined. S fibres showed a higher pCa threshold and aff inity as well as shallower slopes of their tension/pCa curve than did F fibres. HS and HF fibres exhibited tension/pCa curves which were pos itioned close to those of S and F fibres, respectively. No significant difference was observed between S and HS fibres or between F and HF f ibres. Maximal shortening velocity values were higher for fibres expre ssing predominantly fast myosin isoforms. We suggest than when both S and F isoforms of myofibrillar proteins are expressed in a muscle fibr e, the functional properties are mainly governed by the predominant is oform.