Bolus response to DN and OM measured by contractility indices. Analysis of LV load–independent contractility indices 1 min after infusion of either vehicle control (DMSO), DN at 3 mg/kg (DN Low), DN at 6 mg/kg (DN High), OM at 0.75 mg/kg (OM Low), or OM at 1.5 mg/kg (OM High). (A) Representative effects on the relationship between ESP and ESV at baseline (Pre) and after (Post) drug infusion. The dashed line is the linear fit of the data. (B) Quantification of the absolute change (∆) in the slope of the ESPVR. (C) Representative effects on the relationship between the maximum rate of LV pressure change (dP/dt_max) and EDV at baseline (Pre) and after (Post) drug infusion. The dashed line is the linear fit of the data. (D) Quantification of the absolute change in the slope of the dP/dt_max-EDV relationship. †P < 0.05 compared with the DMSO control group, *P < 0.05, **P < 0.01, ns P > 0.05 as determined by ANOVA and Tukey’s HSD. Exact P values are as follows: (B) DN Low–DN High, P = 1.0; OM Low–OM High, P = 0.88; DN Low–OM Low, P = 0.59; DN High–OM High, P = 0.96; DMSO–DN Low, P = 0.022; DMSO–DN High, P = 0.045; DMSO–OM Low, P = 0.00071; DMSO–OM High, P = 0.013. (D) DN Low–DN High, P = 0.0074; OM Low–OM High, P = 0.028; DN Low–OM Low, P = 1.0; DN High–OM High, P = 0.93; DMSO–DN Low, P = 0.99; DMSO–DN High, P = 0.0019; DMSO–OM Low, P = 1.0; DMSO–OM High, P = 0.011. Group sizes are as follows: (B) DMSO, N = 7; DN Low, N = 10; DN High, N = 9; OM Low, N = 8, OM High, N = 10. (D) DMSO, N = 8; DN Low, N = 9; DN High, N = 7; OM Low, N = 8, OM High, N = 10. ESV, end-systolic volume.