Abstract 440: Improved Recovery of Reperfused Mouse Hearts Expressing Slow Skeletal Muscle TnI is Related to Reduced ATP Loss During Ischemia
Previous work shows that transgenic (TG) murine hearts expressing the slow skeletal isoform of cTnI (ssTnI) tolerate acidosis and ischemia/reperfusion (I/R) better than hearts expressing the adult form of troponin I, cardiac TnI (cTnI). We studied the bioenergetic basis for these protective advantages of ssTnI expression. High energy phosphate content was determined via 31P NMR in isolated hearts perfused with oxygenated, Krebs Henseleit buffer containing 0.4 mM plamitate + 5 mM glucose and then subjected to global, zero-flow ischemia at 37O for 10 minutes followed by 30 min reperfusion. Left ventricular (LV) function was measured from a fluid filled balloon in the LV. Prior to ischemia, no differences in LV function or energetic state (PCr/ATP) were evident between groups. Despite the already known improvements in contractile recovery of the ssTnI hearts during reperfusion, no differences in ATP content or energy charge (phosphocreatine/ATP) were observed. But surprisingly, during the actual ischemic insult the TG ssTnI hearts (n=6) displayed attenuated ATP loss compared to NTG hearts (n=5). After 0 –2 minutes of ischemia, ATP in NTG hearts dropped to 74±9% (mean±SD) of the preischemic content, while ssTnI hearts retained 100% ATP (P<0.05). ssTnI hearts continued to preserve ATP at significantly higher levels than NTG throughout the subsequent 10 min ischemia: at 3 min ischemia ATP levels were NTG = 73±11% vs. ssTnI =91±9%; at 5 min NTG = 57±13 % vs. ssTnI = 95±10%; at 7 min NTG = 44±6% vs. ssTnI = 74±9%; at 9 min NTG =44±3% vs. ssTnI = 52±7%. Therefore, ssTnI expression in cardiac myofilaments is associated with a preservation of intracellular energy stores during ischemia. These findings support the conclusion that the beneficial effects of ssTnI expression on contractile function during ischemia/reperfusion are, in part, attributed to improved energy maintenance in the ischemic myocardium expressing ssTnI. The shift toward expression of the slow skeletal muscle TnI appears to induce a metabolic shift within the cardiomyocyte toward a more ischemic-resistant energy metabolism, such as that of characterized by skeletal muscle in comparison to normal cardiac muscle.