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NEUROHUMORAL CONTROL OF CARDIOVASCULAR FUNCTION

Myofilament calcium sensitivity does not affect cross-bridge activation-relaxation kinetics

¹Center for Cardiovascular Research, Department of Physiology and Biophysics, University of Illinois at Chicago, Chicago, IL; and ²Department of Physiology, University of Florence, Florence, Italy

Submitted 2 September 2006 ; accepted in final form 30 October 2006

We employed single myofibril techniques to test whether the presence of slow skeletal troponin-I (ssTnI) is sufficient to induce increased myofilament calcium sensitivity (EC₅₀) and 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 Tn was partially replaced for either recombinant cardiac Tn(cTn) or Tn composed of recombinant cTn-T (cTnT) and cTn-C (cTnC), and recombinant ssTnI (ssTnI-chimera Tn). Tn exchange was performed in rigor solution (0.5 mg/ml Tn; 20°C; 2 h) and confirmed by SDS-PAGE. 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 1) incorporation of ssTnI into the fast skeletal sarcomere is sufficient to induce increased myofilament Ca²⁺ sensitivity, and 2) 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.

single myofibril; contraction kinetics; muscle mechanics; troponin; regulation

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