Hypothermia-induced cardioprotection using extended ischemia and early reperfusion cooling

Abstract

timing of therapeutic hypothermia for cardiac ischemia is unknown. Our prior work suggests that ischemia with rapid reperfusion (I/R) in cardiomyocytes can be more damaging than prolonged ischemia alone. Also, these cardiomyocytes demonstrate protein kinase C (PKC) activation and nitric oxide (NO) signaling that confer protection against I/R injury. Thus we hypothesized that hypothermia will protect most using extended ischemia and early reperfusion cooling and is mediated via PKC and NO synthase (NOS). Chick cardiomyocytes were exposed to an established model of 1-h ischemia/3-h reperfusion, and the same field of initially contracting cells was monitored for viability and NO generation. Normothermic I/R resulted in 49.7 Ϯ 3.4% cell death. Hypothermia induction to 25°C was most protective (14.3 Ϯ 0.6% death, P Ͻ 0.001 vs. I/R control) when instituted during extended ischemia and early reperfusion, compared with induction after reperfusion (22.4 Ϯ 2.9% death). Protection was completely lost if onset of cooling was delayed by 15 min of reperfusion (45.0 Ϯ 8.2% death). Extended ischemia/early reperfusion cooling was associated with increased and sustained NO generation at reperfusion and decreased caspase-3 activation. The NOS inhibitor N -nitro-L-arginine methyl ester (200 M) reversed these changes and abrogated hypothermia protection. In addition, the PKC⑀ inhibitor myr-PKC⑀ v1-2 (5 M) also reversed NO production and hypothermia protection. In conclusion, therapeutic hypothermia initiated during extended ischemia/early reperfusion optimally protects cardiomyocytes from I/R injury. Such protection appears to be mediated by increased NO generation via activation of protein kinase C⑀; nitric oxide synthase.

Intra-arrest cooling with delayed reperfusion yields higher survival than earlier normothermic resuscitation in a mouse model of cardiac arrest

“…13 Other animal work has demonstrated that the earlier hypothermia therapy is instituted after an ischemic insult, the more likely survival and surrogate outcome measures will be improved. 10, 19-21 Finally, human investigations have suggested that the induction of hypothermia is a time-sensitive intervention and that injury pathways may be activated very shortly after ROSC.…”

“…9,10 Cellular ischemiareperfusion experiments have observed deleterious oxidant generation as well as apoptotic activation immediately after reperfusion, and intra-ischemia cooling blunts these damaging processes. [11][12][13] It is unknown to what degree initial cooling should be given priority over reperfusion and ROSC, if at all. This is particularly controversial if the induction of TH requires a delay in circulatory resuscitation and restoration of perfusion.…”

Therapeutic hypothermia cardioprotection via Akt- and nitric oxide-mediated attenuation of mitochondrial oxidants

“…Recently, it has been demonstrated that significant levels of oxidant stress in the heart following cardiac arrest originate from the mitochondria within minutes of reperfusion (11). In isolated cardiomyocytes, simulated I/R induces mitochondrial reactive oxygen species (ROS) generation, contractile dysfunction, mitochondrial release of cytochrome c, caspase activation, opening of the mitochondrial permeability transition pore, and ultimately cardiomyocyte death (4,29,40).…”

“…These cells also display significant cardioprotection that is associated with enhanced nitric oxide (NO) generation when TH is induced at end-ischemia and extended until 1 h into reperfusion (29). Possible mediators of this increased NO include Akt, a survival kinase that enhances cardioprotective NO generation from endothelial NO synthase (NOS3) (14).…”

Does mild hypothermia protect against reperfusion injury? The debate continues

“…It was not determined whether it was the additional 5 min of hypothermic ischemia or the additional hour of hypothermic reperfusion that caused the salvage. Cultured chick cardiomyocytes undergoing simulated ischemia were cooled to 25°C just before reoxygenation and were protected [24]. However, in that study cooling only to temperatures below the range of mild hypothermia was evaluated, and such a degree of cooling could not be tolerated in patients without external circulatory support.…”