Responsiveness of Contractile Elements to Muscle Length Change in Hyperthyroid Ferret Myocardium

Summary

Hyperthyroidism induces cardiac hypertrophy, which alters the properties of contractile elements. However, the responsiveness of hyperthyroid myocardium to changes in muscle length (the cellular basis of Frank-Starling’s law of the heart) is poorly understood. In the present study, we measured the changes in tension and the Ca²⁺ transients monitored with aequorin in the papillary muscles excised from thyroxine-treated hyperthyroid ferrets (Hy) when muscle length was quickly changed during twitch contraction. We also measured the pCa-tension relationship at different muscle lengths in trabeculae treated with Triton X-100. The ratio of heart weight to body weight in Hy was significantly higher than that in age-matched euthyroid ferrets (Eu) (p < 0.001). Extra-Ca²⁺ (a transient increase in the intracellular Ca + concentration in response to quick length change) did not significantly differ in Hy and Eu. The values of pCa₅₀ reflecting the responsiveness of the myofilament to Ca²⁺, measured at sarcomere lengths of 2.3 and 1.9 µm, were identical at each sarcomere length in Hy and Eu. These results indicate that in the hypertrophied mycoardium induced by hyperthyroidism the responsiveness of the contractile elements to muscle length change is similar to that in euthyroid myocardium.