Fig. 5.

Effect of MHC composition on tension cost in rat skinned myocardium. The rate of ATP utilization was measured in the skinned myocardium from euthyroid (∼60% α-MHC, n = 10; □), hyperthyroid (100% α-MHC, n = 12; •), and hypothyroid (0% α-MHC, n = 10; ○) rats and plotted against Ca²⁺-activated isometric force. The slope of the line is equal to tension cost. Force and ATPase were measured simultaneously in solutions of varying free Ca²⁺ concentrations (pCa 4.5 and 5.4–5.8). Data are means ± SE.