Developmental difference in the stimulation of cardiac myofibrillar Mg²⁺-ATPase activity by calmidazolium

We probed possible developmentally related changes in thin filament activity in rat hearts with the aid of calmidazolium (CDZ). CDZ is a calmodulin antagonist that also binds to troponin C and stimulates Ca²⁺ troponin C-dependent activation of cardiac myofibrillar contractile activity. In paired experiments, we compared the effects of 10, 30, 50, 70, and 100 μM CDZ on Mg³⁺-dependent ATPase activity of myofibrillar preparations from adult and neonatal rat hearts. Over the dose-response curve, the ATPase activity of neonatal myofibrils was significantly less stimulated than was the ATPase activity of the adult preparations. To know whether this difference in response to CDZ was related to differences in the thin or thick filaments, we studied hybrid adult and neonatal myofibrillar preparations. These hybrid myofibrils had native thin filaments, but the thick filaments were displaced with rabbit skeletal myosin. The relative insensitivity of the neonatal preparations to the effect of CDZ was retained in the hybrid myofibrils. This suggested that developmental transitions in the population of thin filament proteins are responsible for the difference between adult and neonatal myofibrils in their response to CDZ. Recently, we and others have reported developmental switching of troponin I isoforms in the rat heart. Since troponin I reacts strongly with troponin C in a Ca²⁺-dependent manner, we speculate that developmentaliy related changes in troponin I isoforms may contribute to the differential effect of CDZ in neonatal cardiac myofibrils.