Figure 7.

Relaxation kinetics and SL-resting-tension relationship of rabbit psoas myofibrils after exchange of endogenous TnI with a C-terminal truncated TnI form (cTnI_1-192) at 15°C. (A) Time course of full-force relaxation from maximal activation of native (unexchanged) myofibrils (Nat, black line) and myofibrils in which whole native Tn had been replaced (by exchange) by cTn, including either the full-length (cTnI_FL, gray line) or truncated (1–192 (cTnI_1-192), dark gray line) cTnI subunit, normalized and presented on an expanded timescale to show the kinetics of force relaxation. (B) Mean values of the duration of slow-phase relaxation. (C) Mean values of the rate of the slow phase of relaxation (slow k_REL). (D) Mean values of the rate of the fast phase of relaxation (fast k_REL). ^∗p < 0.01 versus the same parameter measured in cTnI_FL and native unexchanged myofibrils. The number of myofibrils is given above the bars. (E) Average SL-resting-tension relationships of myofibrils replaced with either cTnI_FL (gray solid circle) or cTnI_1-192 (dark gray square). BDM (10 mM) completely abolishes the difference in passive tension (open square, cTnI_1-192; open circle, cTnI_FL). Vertical and horizontal bars are SEM. Resting tension and mean SL were measured under quasi-steady-state conditions.