A study of the mechano-kinetic parameters of different isoforms of muscle myosin II in vivo

The performance of skeletal muscles contracting in isometric conditions or actively shortening against an external load exhibits a large variability: parameters such as speed of isometric force development, unloaded shortening velocity, maximum power output and ATPase activity vary from one muscle to the other according to the functional tasks of muscles. Muscles involved in maintenance of posture develop low power (low shortening speed for a given load) consuming ATP at low rate, while muscles involved in movement develop high power (high shortening speed for a given load) consuming ATP at high rate, whereas the efficiency of energy conversion is similar in both muscle types. The different biochemical and mechanical performances are attributed to the myosin heavy chain (MHC) isoform expressed in skeletal muscle: muscles responsible for the maintenance of posture (slow muscles) mostly contain fibres expressing the slow MHC isoform while those involved in movement (fast muscles) mostly contain fibres expressing the fast MHC isoform. The work described in this thesis is aimed at investigating in situ the mechanical and kinetic bases of the functional diversity of the isoforms of the myosin motor present in slow skeletal muscles in terms of both the properties of the single motor and of the motor ensemble in the half-sarcomere. For this the mechanical parameters of the myosin motor (force, stiffness and size and speed of the working stroke) and of the motor ensemble (number of motors attached to actin, kinetics underlying both the rate of isometric force development and the force-velocity relation) have been determined in demembranated fibres from a slow skeletal muscle of the rabbit, the soleus, and compared to those from a fast muscle, the psoas. Apart the ten time slower kinetics of both the working stroke and the motor attachment-detachment rates, the most relevant finding is that the slow myosin isoform has a three time smaller stiffness and develops a correspondingly three time smaller force than the fast isoform. In terms of a myosin-actin interaction model with a tight coupling between mechanical and biochemical steps, in contrast to the data in the literature, this finding predicts a three times smaller efficiency in energy conversion in the slow muscle. These results suggest that the stiffness of the myosin motor is a determinant of the isoform-dependent functional diversity between skeletal muscle types and opens the question on the molecular mechanism for the high efficiency of the slow muscle.