Mechanical properties of skinned single fibers of identified types from rat diaphragm.

Maximum isometric tension (Po), maximum velocity of shortening (Vmax), and tension-pCa (i.e., -log[Ca2+]) relationships were determined in single skinned fibers from rat diaphragm. Histochemistry (myosin-ATPase) and sodium dodecyl sulfate (SDS) gel electrophoresis were performed on these same fibers to determine fiber type and protein composition. Physiologically fast fibers were found to have larger cross-sectional areas (CSA) and produced more tension per CSA and were less sensitive to [Ca2+] than physiologically slow fibers. Fast fibers were typed histochemically as type II and contained myosin heavy chains (MHC) and light chains (LC) of the fast type, whereas the slow fibers contained slow MHC and LC. There were also corresponding differences in the regulatory protein composition of these two fiber types. The histochemical sections confirmed a significant fiber size difference between the type IIa and IIb fibers. When fiber size was used to separate the fast fibers into two groups, type IIb fibers were found to have significantly greater Vmax and tension per CSA than the type IIa fibers. Although there were no noticeable differences in MHC composition between the type IIa and IIb fibers, there were some differences in the myosin LC and regulatory protein content.