[Cellular and molecular aspects of contraction in normal and hypertrophied heart]

As a striated muscle, myocardium is characterized by a succession of contractile units, the sarcomeres, whose shortening is a consequence of the sliding of filaments. The thin filament bears the troponin-tropomyosin regulatory system. The main protein of the thick filament, myosin, is a hinge for the sliding, a receptor for actin and a catalytic site for ATP hydrolysis. This ATPase activity resulted from the combination of several isoenzyme forms of this polymer. Identification of these isoenzymes needed special precautions due to the high insolubility of myosin. The ATPase correlated with the speed of shortening as, for example, shown during chronic cardiac overload. In this condition the heart adapted to change in work by both increasing its mass and reducing its speed of shortening. The former corresponded to a stimulation of protein synthesis which affected equally myosin and actin and which was very likely a consequence of enhanced mRNA synthesis. The latter corresponded to an isoenzyme change of myosin to the benefit of the low ATPase form. This correlated with the maximal speed of shortening estimated on papillary muscle.