Influence of the thyroid state on the calcium transient in ventricular muscle

The purpose of this study was to determine the influence of thyroid hormone on tension development and the intracellular calcium transient in mammalian ventricular muscle. A hyperthyroid (H) state was induced in ferrets by subcutaneous injection of L-thyroxine, 0.3 mg/kg daily, for 2-3 weeks. One-half of the age matched control group (C) were injected with vehicle. Aequorin was loaded into the cells of ferret papillary muscles by a chemical procedure. The muscles were stimulated at 0.33 Hz and isometric tension and the calcium transient were simultaneously recorded at 30 degrees C. Peak isometric tension in mN/mm2 (+/- SD) was 15.4 +/- 7.2 and 16.2 +/- 7.9 for C (n = 8) and H (n = 9) respectively. The time to peak tension and time to 80% relaxation from peak of tension were reduced by 22% and 28% respectively in H compared to C. After stimulation, the calcium transient reached a maximum in 56 +/- 6 ms in C and in 47 +/- 5 ms in H. The time to 80% decay of the peak calcium transient was 95 +/- 8 ms and 68 +/- 5 ms for C and H respectively. The ratio of the aequorin luminescence at the peak of the calcium transient over the calculated maximum luminescence, Lmax, were compared and they were not different. At 22 degrees C Log (L/Lmax) was -3.3 +/- 0.1 in C (n = 4) and -3.4 +/- 0.3 in H (n = 3). These results indicate that the thyroid state influences the time course of the calcium transient and are consistent with the abbreviation in the duration of contraction that is observed in the hyperthyroid state.