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.