Figure 9. HcTnI-C27 peptide reduces Ca²⁺ sensitivity in skinned rat cardiac muscle strips without decreasing maximum force production.

Ca²⁺-activated isometric force of skinned rat papillary muscle at sarcomere lengths (SL) of 2.0 μm and 2.3 μm in the absence or presence of 20 μM HcTnI-C27 peptide was plotted as Hill fitted force-pCa curves normalized to the maximum force at pCa 4.5. At sarcomere length of 2.0 μm, HcTnI-C27 peptide resulted in a small right-shift of the force-pCa curve (P = 0.055 in one-tail Student t test) (A). At sarcomere length of 2.3 μm, however, the addition of HcTnI-C27 peptide significantly decreased Ca²⁺ sensitivity and cooperativity (n), completely diminishing the Ca²⁺-sensitization effect of increasing sarcomere length from 2.0 μm to 2.3 μm. Values are presented as mean ± SE. N = 4 for SL 2.0 μm group and n = 3 for SL 2.3 μm group. The bar graphs show that the maximum force production was not affected by the addition of HcTnI-C27 peptide. Statistical analysis was done using paired Student’s t test. pCa50, Ca²⁺ concentration for 50% maximum force. *P < 0.05 vs the SL 2.0 μm control; ^#P < 0.05 vs the SL 2.3 μm baseline in the absence of HcTnI-C27 peptide; ^§P < 0.05 vs SL 2.0 μm control in one-tail Student t test.