Fiber isometric tension redevelopment rate (k(TR)) was measured during submaximal and maximal activations in glycerinated fibers from rabbit psoas muscle. In fibers either containing endogenous skeletal troponin C (sTnC) or reconstituted with either purified cardiac troponin C (cTnC) or sTnC, graded activation was achieved by varying [Ca2+]. Some fibers were first partially, then fully, reconstituted with a modified form of cTnC (aTnC) that enables active force generation and shortening in the absence of Ca2+. k(TR) was derived from the half-time of tension redevelopment. In control fibers with endogenous sTnC, k(TR) increased nonlinearly with [Ca2+], and maximal k(TR) was 15.3 +/- 3.6 s(-1) (mean +/- SD; n = 26 determinations on 25 fibers) at pCa 4.0. During submaximal activations by Ca2+, k(TR) in cTnC reconstituted fibers was approximately threefold faster than control, despite the lower (60%) maximum Ca2+-activated force after reconstitution. To obtain submaximal force with aTnC, eight fibers were treated to fully extract endogenous sTnC, then reconstituted with a mixture of aTnC and cTnC (aTnC:cTnC molar ratio 1:8.5). A second extraction selectively removed cTnC. In such fibers containing aTnC only, neither force nor k(TR) was affected by changes in [Ca2+]. Force was 22 +/- 7% of maximum control (mean +/- SD; n = 15) at pCa 9.2 vs. 24 +/- 8% (mean +/- SD; n = 8) at pCa 4.0, whereas k(TR) was 98 +/- 14% of maximum control (mean +/- SD; n = 15) at pCa 9.2 vs. 96 +/- 15% (mean +/- SD; n = 8) at pCa 4.0. Maximal reconstitution of fibers with aTnC alone increased force at pCa 9.2 to 69 +/- 5% of maximum control (mean +/- SD; n = 22 determinations on 13 fibers) and caused a small but significant reduction of k(TR) to 78 +/- 8% of maximum control (mean +/- SD; n = 22 determinations on 13 fibers); neither force nor k(TR) was significantly affected by Ca2+ (pCa 4.0). Taken together, we interpret our results to indicate that k(TR) reflects the dynamics of activation of individual thin filament regulatory units and that modulation of k(TR) by Ca2+ is effected primarily by Ca2+ binding to TnC.
