Overcontraction and excess actin filaments. Basic elements of hypertrophic cardiomyopathy

Endomyocardial biopsies were taken from the right ventricular aspect of the interventricular septum in three patients with hypertrophic cardiomyopathy and were subjected to electronmicroscopical examination. Longitudinal sections confirmed already well-established findings. In the transverse sections disarray in the arrangement of the actin filaments and expansion of the myosin lattice, indicating clear overcontraction, were observed. The number of actin filaments varied from seven to 14 per hexagon; a number exceeding 12, however, was found in only one case. From our findings we conclude that overcontraction leads to a progressive deviation of the action filaments during systole caused by double overlap. The majority of these mechanisms results in a "self-impeding contraction" of the fibres. Functionally the excess of actin filaments may provide a balance between the unequal forces of contraction.