Effects of Mild Versus Deep Hypothermia on a Cloned Human Brain Glutamate Transporter (GLT-1) Expressed in Chinese Hamster Ovary Cells

Abstract

Glutamate transporters, widely distributed in the brain and spinal cord, maintain extracellular glutamate concentrations below neurotoxic levels. In cerebral ischemia/anoxia, the glutamate transporter runs in reverse and releases glutamate into the extracellular space, causing irreversible neuronal damage. Although hypothermia reduces the elevation of extracellular glutamate concentration during cerebral ischemia/anoxia, little is known about the effect of hypothermia on the glutamate transporter. A human glial glutamate transporter (hGLT-1) cDNA was isolated by screening a human cerebral cortical library, and cloned cDNA was stably transfected in Chinese hamster ovary (CHO) cells. Effects of deep hypothermia (22 to 23 degrees C) on uptake and release of L-glutamate via hGLT-1 were investigated by whole-cell patch-clamp. The control study was performed at 34 to 35 degrees C. The hGLT-1 transporter had the capacity to take up extracellular L-glutamate under essentially physiological ionic conditions, whereas this transporter promoted release of L-glutamate under a nonphysiological condition mimicking complete ischemia. Deep hypothermia decreased a) uptake and b) release of L-glutamate via hGLT-1 to a) 4.8+/-4.8% (P < .01, n = 7) and b) 19.0+/-4.5% (P < .01, n = 15) of control values, respectively. The results suggest that deep hypothermia is a potent inhibitor of glutamate uptake by intact glial cells as well as glutamate release from glial cells under certain pathophysiological circumstances. The balance between these antagonistic effects of hypothermia may attenuate the elevation of the extracellular glutamate concentration during ischemia/anoxia.