cTnI-ND,Tnni3 ^−/− hearts show higher positive systolic response to increase of preload than WT control. (A) WT and cTnI-ND,Tnni3^−/− hearts both showed positive response of LVP_max to increase of preload from 3 to 10 mmHg. cTnI-ND,Tnni3^−/− hearts maintained high LVP_max at higher preload of 15–20 mmHg, whereas WT controls showed decreases. (B) This trend was confirmed by measuring LVP_dev (LVP_max – LVP_min). (C) The LV maximum systolic velocity (+dP/dt_max) of cTnI-ND,Tnni3^−/− and WT hearts both positively responded to increase of preload from 3 to 10 mmHg while cTnI-ND,Tnni3^−/− hearts had detectably lower + dP/dt_max than that of WT hearts. The difference diminished when preload was above 10 mmHg. At higher preload, +dP/dt_max of WT hearts became lower than that at 10 mmHg whereas it continued rising in cTnI-ND,Tnni3^−/− hearts. The values are mean ± SEM. n = 5–6 mice in WT and 8–9 mice in cTnI-ND,Tnni3^−/− groups. ^#P < 0.05 vs. WT in Student’s t test. *P < 0.05 vs. WT in two-way ANOVA with post-hoc method of Tukey’s test at preload from 10 to 20 mmHg; ^aP < 0.05, ^aaP < 0.01, and ^aaaP < 0.001 vs. that at 10 mmHg preload; ^bbP < 0.01 and ^bbbP < 0.001 vs. that at 12.5 mmHg preload; ^ccP < 0.01 and ^cccP < 0.001. ↑ and ↓ indicate increase and decrease in the comparisons.