The enhanced Frank-Starling response of cTnI-ND,Tnni3 ^−/− hearts is augmented by β-adrenergic stimulation. Functions of ex vivo working heart were measured in the absence (baseline) or presence of ISO. (A) The stroke volume curves show that 10 nM ISO augmented the Frank-Starling responses of WT hearts and had much higher effect on the already enhanced response of cTnI-ND,Tnni3^−/− hearts at preload of 10–20 mmHg. (B) At 10 mmHg preload, 10 nM ISO produced a larger increase in contractile velocity (+dP/dt_max) in cTnI-ND,Tnni3^−/− hearts than that in WT hearts, whereas the effects on increasing relaxation velocity (–dP/dt_min) were similar in the two groups. Values are mean ± SEM, n = 6 mice in WT and 9 mice in cTnI-ND,Tnni3^−/− groups. ^&&P < 0.01 vs. control in two-way ANOVA with post-hoc method of Tukey’s test. **P < 0.01 and ***P < 0.001 vs. WT in Student’s t test. ^aaaP < 0.001 vs. stroke volume at 10 mmHg preload; ^bP < 0.05 and ^bbP < 0.01 vs. 12.5 mmHg, and ^##P < 0.01 vs. baseline in paired Student’s t test. ↑ and ↓ indicate increase and decrease in the comparisons.