Cross-bridge and calcium behavior in ferret papillary muscle in different thyroid states

X-ray diffraction studies were made using synchrotron radiation on ferret r ight ventricular papillary muscle under three different thyroid states: eut hyroidism, hyperthyroidism, and hypothyroidism, The latter two states were induced by treatment with L-thyroxine and methimazole, respectively. The X- ray equatorial reflections were recorded at a time resolution of 10 ms to s tudy the mass movement of myosin crossbridges from thick to thin filaments. The myosin isomer content was measured by gel electrophoresis which showed that V3 isomer was predominant in euthyroid muscle and 27% of myosin was V 1 isomer in hyperthyroid muscle. The intracellular free Ca concentration wa s measured by using the aequorin method. The peak Ca concentration was simi lar in all three states, but in the hypothyroid state the time-to-peak was longer and the decay was slower. The time-to-peak of twitch tension was sho rter in hyperthyroidism and longer in hypothyroidism than in euthyroidism. The different time courses of twitch tension in different thyroid states ac companied a cross-bridge movement which closely followed the tension develo pment. In hyperthyroidism, the cross-bridge movement significantly preceded tension development, suggesting that hyperthyroid myosin (V1) has a longer latency period between the shift to the vicinity of the thin filament and force development.