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1 Dept. of Physiology & Biophysics, Univ. of Illinois at Chicago, Chicago, Illinois, United States
2 Physiology, University of Florence, Florence, Italy
* To whom correspondence should be addressed. E-mail: pdetombe{at}uic.edu.
We employed single myofibril techniques to test a) whether presence of slow skeletal troponin I (ssTnI) is sufficient to induce increased myofilament calcium sensitivity (EC50), and b) whether modulation of EC50 affects the dynamics of force development. Studies were performed using rabbit psoas myofibrils activated by rapid solution switch and in which troponin was partially replaced for either recombinant cardiac troponin (cTnI) or troponin composed of recombinant cTnT and cTnC, and recombinant ssTnI (ssTnI-chimera Tn). Tn exchange was performed in rigor solution (0.5 mg/ml Tn; 20°C; 2 hr) and confirmed by SDS gel analysis. cTnI exchange induced a decrease in EC50; ssTnI-chimera Tn exchange induced a further decrease in EC50 (in µM: endogenous Tn, 1.35 ± 0.08; cTnI, 1.04 ± 0.13; ssTnI-chimera Tn, 0.47 ±0.03). EC50 was also decreased by application of 100 µM Bepridil (control: 2.04 ± 0.03 µM; Bepridil 1.35 ± 0.03 µM). Maximum tension was not different between any groups. Despite marked alterations in EC50, none of the dynamic activation-relaxation parameters were affected under any condition. Our results show that a) incorporation of ssTnI into the fast skeletal sarcomere is sufficient to induce increased myofilament Ca2+ sensitivity; b) the dynamics of actin-myosin interaction do not correlate with EC50. This result suggests that intrinsic cross-bridge cycling rate is not altered by the dynamics of thin filament activation.
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