Cross-bridge kinetics of rabbit single extraocular and limb muscle fibers

PURPOSE: To gain insights into the functional significance of myosin heavy-chain (MyHC) heterogeneity by comparing the mechanical kinetic properties of single rabbit extraocular muscle (EOM) fibers with those of limb fibers. EOMs are known to contain developmental and EOM-specific MyHCs in addition to those present in limb muscles, and MyHCs profoundly influence muscle mechanics. METHODS: Isometric cross-bridge kinetics were analyzed in Ca(2+)-activated single glycerinated fibers from rabbit EOM and limb fast and slow muscles at 15 degrees C by means of mechanical perturbation analysis. The plots of stiffness and phase against frequency display a characteristic frequency, f(min), at which stiffness is minimum, and phase shift is zero. The value of f(min) is independent of Ca(2+) or force level but reflects the kinetics of cross-bridge cycling. RESULTS: Analysis of 121 limb fast fibers gave f(min) values ranging from 10 to 26 Hz. f(min) for the 10 slow soleus fibers was 0.5 Hz. Analysis of 170 EOM fibers gave f(min) values in the range for fast limb fibers, but in addition yielded f(min) values below (4-9 Hz) and above (27-33 Hz) this range. CONCLUSIONS: The wider range of mechanical kinetic characteristics in EOM fibers compared with limb fibers is likely due to the expression of developmental (low f(min)) and EOM-specific (high f(min)) MyHCs in addition to isoforms present in adult limb muscles. The considerable diversity of functional characteristics in EOM fibers is likely to be important for rotating the eyeball at various speeds during tracking and for executing saccades over a wide range of angles.