Metabolic Effects of Hypothermia and Its Neuroprotective Effects on the Recovery of Metabolic and Electrophysiological Function in the Ischemic Retina in Vitro

Objective: Reduction in energy usage has been investigated as the mechanism by which hypothermia provides protection during ischemia. We describe experiments using hypothermia in the rabbit retina in vitro that show a correlation between hypothermia-induced reductions in energy usage and neuroprotection.

Methods: We examined energy metabolism and electrophysiological function under control/nonischemic conditions during 1 or 2 hours of "ischemia" (induced by decreasing glucose from 6 to 1 mmol/L and oxygen from 95 to 15%) and during 3 to 4 hours of "return-to-control" conditions. Glucose utilization and lactate production were measured as indices of energy metabolism, and light-evoked compound action potentials were monitored to assess functional recovery.

Results: Nonischemic retinas subjected to both mild (33 +/- 0.5 degrees C) and moderate (30 +/- 0.5 degrees C) hypothermia exhibited a decrease of 38% in the rate of glucose utilization and lactate production compared with normothermic retinas (36 +/- 0.5 degrees C) (analysis of variance, P < 0.001). In retinas that were made ischemic, mild or moderate hypothermia further reduced the rates of glucose utilization (18 and 39%, respectively) and lactate production (21 and 28%, respectively) (P < 0.001 for glucose, P < 0.01 for lactate). Retinas that had been mildly or moderately hypothermic during ischemia exhibited improved recovery of glucose utilization (65 and 57%, respectively) and lactate production (72 and 74%, respectively) compared with normothermic retinas (18% for glucose and 54% for lactate; repeated-measures analysis of variance, P < 0.001). Recovery of compound action potentials for retinas kept at 36, 33, and 30 degrees C was 15, 36, and 53%, respectively (repeated-measures analysis of variance, P < 0.001).

Conclusion: Our studies in an avascular neuronal model of ischemia demonstrate that hypothermia protects against ischemic injury. We interpret the smaller reductions in energy generation and usage caused by ischemia when the retinas were hypothermic as evidence that hypothermia had reduced energy requirements more than energy production, and we propose that this at least in part explains its protection.