Abstract

In a previous study we showed that hypothermia of 30 degrees C can expand the time during which retinal neurons in vitro can have their metabolism inhibited without adverse effects. In isolated chick retinae, the first signs of acute toxicity resulting from mild, partial, pharmacological inhibition of metabolism are N-methyl-D-aspartate (NMDA)-mediated histological swelling and gamma-aminobutyric acid release. More prolonged or severe inhibition of metabolism results in involvement of non-NMDA glutamate receptors and voltage-dependent Na+ channels. In this study we examine early cellular events that may be associated with hypothermic protection. The early cellular events thought to follow metabolic stress involve a decrease in ATP, reduced activity of the Na+, K(+)-ATPase, which renders ion leakage unopposed, degradation of the membrane potential and subsequent activation of ionotropic glutamate receptors and voltage-dependent Na+ channels, which leads to acute toxicity. Reduction by hypothermia of the rate of loss of ATP was shown, In past work, to only partially account for neuroprotection. In the present study, inhibition of the Na+, K(+)-ATPase with 10 microM ouabain for 30 min at 37 degrees C led to acute toxicity that was similar to the toxicity produced by severe metabolic stress, i.e., primarily excitotoxic and mediated by NMDA receptors and secondarily involving non-NMDA receptors and voltage-dependent Na+ channels. Swelling and increased gamma-aminobutyric acid release were first evident at 15 min of incubation with ouabain at 37 degrees C. Hypothermia (30 degrees C) delayed the onset of acute excitotoxicity caused by ouabain. This protection was independent of an involvement with ATP loss, because ouabain treatment did not reduce ATP levels. Protection against ouabain suggests that hypothermia can intervene at steps subsequent to decreased Na+, K(+)-ATPase activity. In contrast, reducing the temperature to 30 degrees C did not attenuate NMDA-mediated secondary excitotoxicity caused by lowering of the membrane potential with increasing extracellular K+ concentrations (32-55 mM). However, hypothermia of 30 degrees C was able to reduce the rate of ouabain-induced 86Rb efflux. The findings described above suggest that a critical site of action for hypothermic protection is at a step between decreased Na+, K(+)-ATPase activity and degraded membrane potential, specifically, slowing of the rate of ion leakage.