Objectives: The aim of the study is to test the hypothesis that tension-dependent change in the affinity of cardiac troponin-C influences the time courses of Ca2+ transients and tension in twitch contraction.
Methods: The Ca(2+)-sensitive photoprotein, aequorin, was microinjected into superficial cells of ferret papillary muscles and the Ca2+ transients and tension were simultaneously measured. The peak of developed tension was altered by changing the extracellular Ca2+ concentration, initial muscle length, and the application of 2,3-butanedione monoxime.
Results: In each maneuver, the decay time of Ca2+ transients was prolonged and the tension relaxation time was shortened when the peak of developed tension was decreased. In contrast, when the peak of developed tension was increased, the decay time of Ca2+ transients was shortened and the tension relaxation time was prolonged. The decay time of Ca2+ transients measured with different maneuvers was negatively correlated with the peak tension and the tension relaxation time was positively correlated with the tension peak.
Conclusions: The changes in the decay time of Ca2+ transients and the tension relaxation time indicate that developed tension modulates the affinity of troponin-C for Ca2+ in normal twitch contraction.