To characterize the relationship between force production and Ca2+ occupancy of
troponin C, investigators have related peak intracellular Ca2+, measured with a variety of Ca2(+)-indicators, and peak force during twitches. Inherent in the force-[Ca2+] relationship is the responsiveness of the myofilaments to Ca2+ which can be altered by different pharmacological manipulations. In this study we compared the force-[Ca2+] relationship obtained in
aequorin-injected papillary muscles and
saponin skinned trabeculae from control, right ventricular pressure-overload
hypertrophy (POH), and
hyperthyroid ferret hearts. In POH, the twitch and [Ca2+]i transient were prolonged as compared to control. Force-[Ca2+] relationships from skinned fiber preparations were superimposable between control and POH. The peak force-peak [Ca2+]i relationship in intact muscles from POH was shifted to the left as compared to control. In
hyperthyroid hearts, the twitch and [Ca2+]i were abbreviated. Force-[Ca2+]i relationships from skinned fiber preparations were superimposable between control and thyrotoxic hearts. The peak force-peak [Ca2+]i relationship in intact muscles from
hyperthyroid hearts was shifted to the right as compared to control. Our findings indicate that time course changes in the
calcium transient artifacturally shift the peak force-peak
calcium relationship in a predictable manner. Therefore, this relationship can not be used to address changes at the level of the myofilaments as previously suggested.