Key Points
- To determine whether normal myocardial structure, composition, and function depend directly on the maintenance of physiological mechanical loading rather than reduced loading conditions.
- Unloaded papillary muscles in experimental cats by transecting the chordae tendinae during right ventriculotomy under venous inflow occlusion, preserving blood supply, contraction rate, and innervation.
- Assessed myocardial ultrastructure, cell cross-sectional area, hydroxyproline content, mechanics, and energetics at 1 to 3 days post-surgery, with blood flow and norepinephrine measured in dogs.
- Myocardial unloading caused cell cross-sectional area to decrease to 66% of control and hydroxyproline concentration to increase to 138% of control at 3 days, alongside loss and disorientation of contractile filaments.
- Isometric and isotonic contractile function showed marked depression with negative shifts in force-velocity and length-tension curves, while passive stiffness increased significantly and oxygen consumption remained unchanged.
Cardiac structure, composition, and function are dynamically regulated by the myocyte loading environment, with unloading leading to rapid structural and functional deterioration.