Elevated perfusate [Na+] increases contractile dysfunction during ischemia and reperfusion

Figure 1. Experimental protocol design. (A) Sequential perfusion of 145 mM Na+ and 155 mM Na+ perfusates (n = 5), data presented in Figs. 2 and 3B. (B) SEA0400 perfusion (n = 5), data presented in Figs. 3 and 4. (C) Independent perfusion of 145 mM Na+ and 155 mM Na+ perfusates (n = 16 and n = 13, respectively, for baseline measurements, n = 11 and n = 8 hearts from each group were subjected to the ischemia reperfusion protocol). (D) Definitions of experimental measurements. Representative pressure trace from one heart perfused with the 145 Na+ perfusate defining left-ventricular developed pressure (LVDP; LVDP = Cyclic systolic pressure maximum – cyclic diastolic pressure minimum), time to peak ischemic contracture, amplitudes of ischemic and reperfusion contracture at return of cardiac perfusion.

Figure 2. Increasing perfusate Na+ depresses LVDP, independent of changes in flow rate or heart rate. (A) Representative paired left ventricular (LV) pressure traces from a single heart during 145 mM [Na+]o (145 Na+) and 155 mM [Na+]o (155 Na+) perfusion. (B) LVDP was significantly reduced in paired comparisons during perfusion with 155 relative to 145 mM [Na+]o. (C) Flow rate was not significantly reduced when hearts were perfused with 155 relative to 145 mM [Na+]o. (D) Heart rate did not significantly change when varying [Na+]o during baseline conditions. n = 5, *Indicates p < 0.05 as determined by paired Student’s t-test.

Figure 3. NCX inhibition (with 1 µM SEA0400) mitigates the negative inotropic effect of increasing [Na+]o. (A) NCX inhibition with SEA0400 significantly increases LVDP at baseline with 145 mM Na+. (B) SEA0400 treatment prior to elevating [Na+]o mitigates the negative inotropic effect of elevated [Na+]o. n = 5, *Indicates p < 0.05 as determined by paired Student’s t-test.

Figure 4. Elevating [Na+]o accelerates the onset of ischemic contracture. Pre-ischemic treatment with SEA0400 further accelerates the onset of ischemic contracture. (A) Representative traces of left-ventricular pressure during 30-min no-flow global ischemia for the 145 Na+, 155 Na+, and 155 Na+ + SEA0400 groups (amplitude full scale normalized for visualization purposes; EDP’—Eq. (1)). (B) Time to peak ischemic contracture for all groups. (C) Diastolic amplitude at time of peak ischemic contracture. (n = 11, 8, and 5 for 145 Na+, 155 Na+, and 155 Na+ + SEA0400, perfusates respectively, * indicates p < 0.05 as determined by a one-way ANOVA with Dunnett’s correction for multiple comparisons to the 155 Na+ group).

Figure 5. Elevated [Na+]o increases contractile dysfunction during reperfusion in hearts with a return of intrinsic rhythm. (A) Stacked bar graphs summarizing the return of electrical rhythm during reperfusion. (B) Peak EDP immediately upon reperfusion (reperfusion contracture). In the 145 mM Na+ group, hearts that eventually returned to an intrinsic rhythm during reperfusion had a significantly lower EDP than those developing arrhythmias or asystole; there was no difference in EDP between the 145 mM Na+ and 155 mM Na+ groups when only comparing EDP from hearts with returned intrinsic rhythm. (C) LVDP at 30-min of reperfusion is significantly higher in the 145 Na+ group relative to the 155 Na+ group, consistent with baseline LVDP values. Data from arrhythmic and asystolic hearts excluded for (C). *Indicates p < 0.05 as determined by unpaired student’s t-test.