Abstract
The mechanism underlying the apparent cooperativity of the cardiac thin filament was investigated by using FRET to follow the N-domain opening of recombinant cardiac troponin C (N-cTnC opening) passively exchanged into rat myocardial fiber bundles and tested under a variety of experimental conditions. Calcium titrations conducted in the presence of crossbridge cycle modulators and tension recovery experiments revealed: 1) N-cTnC opening occurred just as “cooperatively” with or without crossbridge cycle activity; 2) the calcium sensitivity of N-cTnC opening is enhanced by crossbridge cycling; 3) the rigor state results in no cooperativity and 10% of the cTnC ensemble being apparently open under resting conditions; 4) the breaking of crossbridges preceding a tension recovery phase results in a slight relaxation of the FRET distance associated with N-cTnC opening, which recovers during tension recovery.
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