Cooperative effects of calcium on myofibrillar ATPase of normal and hypertrophied heart.

The ATPase of myofibrils from normal and hypertrophied rat heart was studied in relation to the Ca2+-dependence of the reaction. The experimental data were fitted to theoretical curves based on the Hill equation. A positive cooperativity (n = 1.6) was found for the activation curve of the ATPase from normal heart. For hypertrophied heart no significant change in either the cooperativity or the Ca2+-sensitivity of troponin was detected. This indicates that the troponin-tropomyosin system is not altered during pressure-overload hypertrophy. However, it should be noted that no precautions were taken to control the state of phosphorylation of contractile proteins during the preparation of myofibrils.