Effects of heat stress on metabolism of high-energy phosphates - Comparison of normothermic and hypothermic ischemia

Background. Alterations in metabolic pathways may contribute to the cardiop rotective effects of heat stress (HS). We investigated the effects of HS on ATP and phosphocreatine (PCr) levels in the ischemic rat myocardium, after both normothermic and hypothermic ischemia. Methods. Two protocols were used: (1) normothermic ischemia (20 min at 37 d egrees C) with no myocardial protection (n=6 HS; n=6 control); (2) hypother mic ischemia (4 hrs at 4 degrees C) after cardioplegic arrest (n=6 HS; n=6 control). ATP and PCr levels in the heart were measured using P-31 nuclear magnetic resonance spectroscopy. Results. At the end of normothermic ischemia, ATP levels were better mainta ined in HS hearts (C vs HS: 4.51+/-0.66 vs 7.81+/-1.06 mu mol/g dry wt+/-SE M, p=0.04). A trend for higher ATP content in HS hearts was observed after 40 min of reperfusion (C vs HS: 11.7+/-1.5 vs 16.9+/-2.0 mu mol/g dry wt+/- SEM, p=0.09). PCr content was also higher at the end of 40 minutes of reper fusion in HS hearts (C vs HS: 46.4+/-2.9 vs 56.9+/-3.0 mu mol/g dry wt+/-SE M, p=0.03). After prolonged hypothermic ischemia under cardioplegic arrest, heat stress again led to better preservation of ATP levels at the end of i schemia (C vs HS: 5.71+/-0.88 vs 9.23+/-1.38 mu mol/g dry wt+/-SEM, p=0.05) and after 40 minutes of reperfusion (C vs HS: 16.8+/-1.4 vs 24.6+/-2.8 mu mol/g dry wt+/-SEM, p=0.03). PCr levels were also better maintained at the end of ischemia (C vs HS: 4.87+/-0.77 vs 12.4+/-3.0 mu mol/g dry wt+/-SEM, p=0.03) and after 40 minutes of reperfusion in HS hearts (C vs HS: 55.1+/-7 .0 vs 79.8+/-7.3 mu mol/g dry wt+/-SEM, p=0.03). Conclusions. Heat stress induces changes in the energy profile of the heart which results in better preservation of ATP and phosphocreatine levels. Th ese changes could be observed after brief normothermic ischemia and also af ter prolonged hypothermic ischemia under cardioplegic arrest, mimicking con ditions of preservation for cardiac transplantation.