Morphological changes in the mechanically unloaded myocardial cell.

The role of stretch and/or tension in maintaining the structural integrity of the myocardial cell was investigated in 16 cats. Right ventricular papillary muscles were studied 1-28 days after transection of the chordae tendinae and compared to adjacent intact papillary muscles. A progressive atrophy, as shown by decreased mean cell cross-sectional area, occurred; by the 28th day mean cardiocyte area was only 28% of the control value. The earliest ultrastructural changes (one day after surgery) were primarily focal and included disorientation of contractile filaments and loss of Z-line substance. During the first week, vacuolation, loss of contractile filaments and infiltration of macrophages and fibroblasts were characteristic. By the second week a massive loss of contractile substance, disappearance of the sarcoplasmic reticulum, and a marked increase in connective tissue occurred. Leptomere structures, membrane alterations, and phagocytosis were the most typical changes between the second and fourth week. Hydroxyproline assays on papillary muscles unloaded for three days showed a 38% increase in connective tissue, indicating an early increase in connective tissue/muscle mass associated with mechanical unloading. It is concluded that the cardiocyte is extremely dependent upon mechanical loading, i.e., stretch and/or tension. Mechanical unloading (tenotomy) results in rapid and marked cellular degeneration which exceeds that observed in skeletal muscle following either disuse or denervation.