Cross-bridge kinetics in respiratory muscles.

In respiratory muscles, force generation and shortening depend on the cyclical interaction of actin and myosin (cross-bridge cycling). During cross-bridge cycling, adenosine triphosphate (ATP) is hydrolysed. The globular head region of the myosin heavy chain (MyHC) possesses both the binding site to actin and the site for ATP hydrolysis. Therefore, the MyHC is both a structural and enzymatic protein. Different isoforms of MyHC are expressed in skeletal muscle fibres, and these MyHC isoforms provide mechanical and metabolic diversity. In the present study, the relationships between MyHC isoform expression in single rat diaphragm muscle fibres and their mechanical and energetic properties were evaluated. The expression of MyHC isoforms in single diaphragm muscle fibres was identified using electrophoretic and immunohistochemical techniques. Cross-bridge cycling kinetics in diaphragm muscle fibres clearly depend on MyHC isoform expression, and these differences are interpreted in the context of Huxley's two-state cross-bridge model. It is concluded that the unique mechanical and energetic properties of myosin heavy chain isoforms are designed to accomplish different motor behaviours of the diaphragm muscle, and that, as a result of these unique properties, a selective recruitment of diaphragm muscle fibres is essential to avoid fatigue.