Methylmercury has a selective effect on mitochondria in cultured astrocytes in the presence of [U-C-13]glutamate

The effect of methylmercury on glutamate metabolism was studied by C-13 mag netic resonance spectroscopy. Cerebral cortical astrocytes were pretreated with methylmercury, either 1 muM for 24 h, or 10 muM for 30 min, and subseq uently with 0.5 mM [U-C-13]glutamate for 2 h. Labeled glutamate, glutamine, aspartate and glutathione were present in cell extracts, and glutamine, as partate and lactate in the medium of all groups. HPLC analysis of these ami no acids showed no changes in concentrations between groups. Surprisingly, the amounts of [U-C-13]glutamate and unlabeled glucose taken up by the astr ocytes were unchanged. Furthermore, the amounts of most metabolites synthes ized from [U-C-13]glutamate were also unchanged in all groups. However, for mation of [U-C-13]lactate was decreased in the 10 muM methylmercury group. This was not observed for labeled aspartate. It is noteworthy that both [U- C-13]Iactate and [U-C-13]aspartate can only be derived from [U-C-13]glutama te via mitochondrial metabolism. [U-C-13]glutamate enters the tricarboxylic acid cycle (located in mitochondria) after conversion to 2-[U-C-13]oxoglut arate and [U-C-13]aspartate is formed from [U-C-13] oxaloacetate, as is [U- C-13]lactate. [U-C-13]lactate can also be formed from [U-C-13] malate. This differential effect on labeled aspartate and lactate indicates cellular co mpartmentation and thus selective vulnerability of mitochondria within the astrocytes to the effects of methylmercury. The decreased lactate productio n from glutamate might be detrimental to surrounding cells since lactate ha s been shown to be an important substrate for neurons. (C) 2001 Elsevier Sc ience B.V. All rights reserved.