1. Physics of Living Systems
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Distinct contributions of the thin and thick filaments to length-dependent activation in heart muscle

  1. Xuemeng Zhang
  2. Thomas Kampourakis
  3. Ziqian Yan
  4. Ivanka Sevrieva
  5. Malcolm Irving
  6. Yin-Biao Sun  Is a corresponding author
  1. King’s College London, United Kingdom
  2. King's College London, United Kingdom
Research Article
Cite this article as: eLife 2017;6:e24081 doi: 10.7554/eLife.24081
5 figures and 2 tables

Figures

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.

https://doi.org/10.7554/eLife.24081.003
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.

https://doi.org/10.7554/eLife.24081.004
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.

https://doi.org/10.7554/eLife.24081.006
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).

https://doi.org/10.7554/eLife.24081.007
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.

https://doi.org/10.7554/eLife.24081.009

Tables

Table 1

Ca2+-dependence of force and the orientation parameter <P2> and the effects of sarcomere length. Mean ± SEM. pCa50 and nH are fitted parameters of Hill equation. Δ<P2> , changes in <P2> during Ca2+-activation from pCa 6 to 4.5. Comparisons: between sarcomere lengths 1.9 and 2.3 μm (t test, two-tailed; *p<0.05).

https://doi.org/10.7554/eLife.24081.005

BR-cTnC-CBR-cTnC-E
SL (μm)1.92.31.92.3
Force
(mN/mm2)22.7 ± 1.832.4 ± 1.5*23.2 ± 3.533.1 ± 6.0*
pCa505.37 ± 0.035.50 ± 0.03*5.37 ± 0.025.49 ± 0.03*
nH4.03 ± 0.204.29 ± 0.22*4.12 ± 0.244.33 ± 0.16*
<P2
pCa505.37 ± 0.025.48 ± 0.01*5.34 ± 0.025.45 ± 0.02*
nH3.28 ± 0.163.42 ± 0.103.59 ± 0.253.24 ± 0.18
at pCa 60.108 ± 0.0050.113 ± 0.0050.283 ± 0.0090.310 ± 0.010*
at pCa 4.50.028 ± 0.0040.036 ± 0.003*0.182 ± 0.0070.208 ± 0.008*
Δ<P2−0.081 ± 0.005−0.077 ± 0.005−0.101 ± 0.007−0.101 ± 0.006
n = 5n = 5
Table 2

Effects of force inhibition by 25 μM blebbistatin on Ca2+-dependence of force and the cTnC orientation parameter <P2>. Mean ± SEM. pCa50 and nH are fitted parameters of Hill equation. Δ<P2>, changes in <P2> during Ca2+-activation from pCa 6 to 4.5. Comparisons (paired t test, two-tailed): before and after addition of blebbistatin (*p<0.05); between sarcomere lengths 1.9 and 2.3 μm in the presence of blebbistatin (#p<0.05).

https://doi.org/10.7554/eLife.24081.008

BR-cTnC-CBR-cTnC-E
SL (μm)1.91.92.31.91.92.3
25 μM Blebbistatin++++
Force
pCa505.39 ± 0.03  5.39 ± 0.04  
nH3.72 ± 0.23  4.23 ± 0.20  
<P2>
pCa505.40 ± 0.015.32 ± 0.02 *5.41 ± 0.03 #5.35 ± 0.025.28 ± 0.03 *5.36 ± 0.02 #
nH3.29 ± 0.102.74 ± 0.20 *2.49 ± 0.153.57 ± 0.222.90 ± 0.19 *2.60 ± 0.17
at pCa 6.00.091 ± 0.0040.088 ± 0.0050.086 ± 0.0040.276 ± 0.0100.282 ± 0.010 *0.305 ± 0.009 #
at pCa 4.50.009 ± 0.0020.016 ± 0.0020.024 ± 0.0030.168 ± 0.0080.202 ± 0.007 *0.225 ± 0.006 #
Δ<P2>0.082 ± 0.0040.069 ± 0.004 *0.062 ± 0.003 #0.098 ± 0.0070.080 ± 0.004 *0.080 ± 0.005
n = 5n = 7

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