Increased Tolerance of the Chronically Hypoxic Immature Heart to Ischemia

Abstract

Background Hypoxia from birth in immature rabbits increases the tolerance of isolated hearts to ischemia compared with age-matched normoxic rabbits. We determined whether this increased tolerance to ischemia was due to an alteration in the ATP-sensitive potassium (K _ATP ) channel and whether increased K _ATP channel activation was associated with increases in intracellular lactate. Methods and Results Isolated immature rabbit hearts (7 to 10 days old) were perfused with bicarbonate buffer at 39°C in the Langendorff mode at a constant pressure. Saline-filled latex balloons were placed in the left and right ventricles for measurement of developed pressure. A K _ATP channel agonist (bimakalim) or a K _ATP channel antagonist (glibenclamide) was added 15 minutes before a global ischemic period of 18 minutes, followed by 35 minutes of reperfusion. Rabbits raised from birth in hypoxic conditions (F io ₂ =0.12) displayed significantly enhanced recovery of developed pressure. The right ventricle was more tolerant of ischemia than the left ventricle in normoxic and hypoxic hearts. Bimakalim (1 μmol/L) increased the recovery of left ventricular developed pressure in normoxic hearts to values not different from those of hypoxic controls (43±3% to 67±5%) and slightly increased developed pressure in hypoxic hearts (67±5% to 72±5%). Glibenclamide (3 μmol/L) abolished the cardioprotective effect of hypoxia (67±5% to 43±5%). Constant-flow studies indicated that the effects of bimakalim and glibenclamide were independent of their actions on coronary flow. Ventricular lactate and lactate dehydrogenase concentrations were elevated in hypoxic hearts compared with normoxic control hearts. Conclusions Increased tolerance to ischemia exhibited by chronically hypoxic rabbit hearts is associated with increased activation of the K _ATP channel. This increased K _ATP activity may be the result of increased intracellular concentrations of lactate.