Heat acclimation improves cardiac mechanics and metabolic performance during ischemia and reperfusion.

Cardiac mechanics and metabolic performance were studied in isolated perfused hearts of heat-acclimated (AC) rats (at 34 degrees C for 1 mo) and their age-matched controls (C). Diastolic and systolic pressures, coronary flow, and the appearance of ischemic contracture (IC) were measured during progressive graded ischemia, total ischemia (TI), and reperfusion. ATP, phosphocreatine, and intracellular pH were measured during TI and reperfusion with the use of 31P-nuclear magnetic resonance spectroscopy. Systolic pressure was greater in AC hearts than in C hearts (P < 0.0001). During 50% of perfusion pressure 15 and 46% of AC and C hearts, respectively, showed IC (P < 0.001). During 25% of perfusion pressure 85% of the hearts in both groups developed IC. The onset of IC in AC hearts was delayed compared with in C hearts. On reperfusion 93 and 66% of AC and C hearts, respectively, resumed contraction. Recovery of diastolic pressure was 78 and 36% for the AC and C hearts, respectively (P < 0.05). During TI ATP declined by 0.94 and 1.20 mumol/min in AC and C hearts, respectively, resulting in 21 +/- 2.8% preservation of the ATP pool in AC hearts after 30 min of TI (P < 0.001). The AC group also showed a delayed decline in intracellular pH (P < 0.001). The data suggest beneficial effects of heat acclimation on the heart, which were exhibited by greater pressure generation and by the emergence of protecting features during ischemia and reperfusion, possibly via energy-sparing mechanisms.