cTnI-ND enhances LV relaxation. (A) cTnI-ND,Tnni3^−/− and WT mouse hearts showed similar relaxation velocities (–dP/dt_min) and increases when preload increased from 3 to 8 mmHg. At higher preloads of 12.5–20 mmHg, LV –dP/dt_min decreases which was significantly less in cTnI-ND,Tnni3^−/− hearts than WT control, rendering significantly higher LV relaxation velocity of cTnI-ND,Tnni3^−/− hearts. (B) Consistently, the increase in LVEDP due to increase of preload was significantly less in cTnI-ND,Tnni3^−/− hearts than that in WT hearts. Values are mean ± SEM. n = 6 mice in WT and 9 mice in cTnI-ND,Tnni3^−/− groups for data in A and B. *P < 0.05, **P < 0.01, and ***P < 0.001 vs. WT in Student’s t test; ^#P < 0.05 and ^###P < 0.001 vs. WT in two-way ANOVA with post-hoc method of Tukey’s test. (C and D) Data from the LVEDV and LVEDP of P-V loops were fitted with linear correlation to demonstrate the significantly different slopes of WT and cTnI-ND,Tnni3^−/− groups for the effect of cTnI-ND on LVEDP volume relationship (LVEDVR, calculated using the average slopes values; D). n = 4 for WT and 8 for cTnI-ND,Tnni3^−/− groups for data in C and D. *P < 0.05 vs WT in Student’s t test.