Distinct contributions of the thin and thick filaments to length-dependent activation in heart muscle

Figure 1.. Sarcomere length-force relationships in rat cardiac trabeculae. (a) Passive force (○) and maximum Ca2+-activated force (●) at five sarcomere lengths (n = 6 trabeculae). Force is normalised to maximum force measured at SL 1.9 μm (T/T0). (b) pCa-force relationships at sarcomere lengths 1.9 μm (○) and 2.3 μm (●). The data were summarised from experiments for Figure 2a and b and fit to the Hill Equation (n = 10). Dotted line is the Hill curve at SL 2.3 μm normalised to its maximum force. Increases in SL resulted in increases in maximum Ca2+-activated force and Ca sensitivity (leftward shift of pCa-force curve). Error bars denote SEM. DOI: http://dx.doi.org/10.7554/eLife.24081.003

Figure 2.. Ca2+-dependence of force and troponin orientation, <P2>, in cardiac trabeculae. (a, c) BR-cTnC-C; (b, d) BR-cTnC-E. Open symbols, SL 1.9 μm; filled symbols, SL 2.3 μm. Error bars denote SEM. DOI: http://dx.doi.org/10.7554/eLife.24081.004

Figure 3.. Effects of sarcomere length on maximum Ca2+-activated force, orientation (<P2>) of cTnC-BR probes in solutions with the highest [Ca2+] where trabeculae remained relaxed (pCa 6) and the maximum [Ca2+] (pCa 4.5), and the Hill parameter pCa50. (a, c, e) BR-cTnC-C; (b, d, f) BR-cTnC-E. Open bars, SL 1.9 μm; grey bars, SL 2.3 μm. Mean ± SEM (n = 5). The statistical significance of differences was assessed using a two-tailed paired t-test. DOI: http://dx.doi.org/10.7554/eLife.24081.006

Figure 4.. Effects of active force inhibition by 25 μM blebbistatin on the orientation of cTnC probes and their length-dependence. (a, c) BR-cTnC-C; (b, d) BR-cTnC-E. (a–b) Continuous lines denote Hill fits to data at SL 1.9 μm (data not shown for clarity). Circles denote <P2> in the presence of blebbistatin at SL of 1.9 μm (○) and 2.3 μm (●). (c–d) Fitted Hill parameter, pCa50, for the control at 1.9 μm SL (white) and in the presence of blebbistatin at 1.9 μm SL (gray) and 2.3 μm (black). Mean ± SEM (n = 5–7). DOI: http://dx.doi.org/10.7554/eLife.24081.007

Figure 5.. Force and orientation (<P2>) of the BSR-RLC-BC probe in relaxing (pCa 6.6) and activating (pCa 4.5) solution at sarcomere lengths of 1.9 (white) and 2.3 μm (gray). Mean ± SEM (n = 5). Statistical significance was assessed using a two-tailed paired t-test. DOI: http://dx.doi.org/10.7554/eLife.24081.009