Reduction of glutamine synthetase activity in astroglia exposed in culture to low levels of inorganic lead.
Astroglia serve as a site of lead (Pb) deposition in Pb-exposed animals. Thus, the potential exists for astroglial function to be impaired by elevated intracellular Pb levels. We previously showed that the specific activity of glutamine synthetase (GS), an astroglial enzyme with a key role in glutamate and ammonia metabolism in the brain, is reduced in fetal guinea pigs exposed to low levels of lead. This observation indicates either a direct effect of Pb on astroglia or an indirect systemic effect. The purpose of the present study was to test the hypothesis that Pb directly reduces GS activity in astroglia. Cultured rat astroglia were fed three times per week with medium containing 0, 0.25, 0.5, or 1 microM Pb acetate for 7, 14, or 21 days. Trypan blue dye exclusion, cell number, and GS activity were measured. Lead treatment reduced the ability of cells to exclude trypan blue in a dose- and time-dependent manner, with the greatest reduction (28%) on day 21 in the group treated with 1 microM Pb. Cell numbers were reduced a maximum of 15% on day 15, but were unaffected on days 7 and 21. The effects of Pb exposure on GS activity were much more pronounced. For example, cultures exposed to 0.25 of 1 microM Pb for 7 days showed, respectively, 17 and 52% reduction in activity from the control value. Slightly greater reductions were measured on days 14 and 21. The much greater sensitivity in vitro of GS activity than dye exclusion or cell numbers to low level lead supports a specific enzymatic inhibition. In order to test the possibility that Pb might directly inhibit GS activity Pb-treated cells, we also measured the effect of Pb on GS activity in cytosolic extracts of the cells. Pb inhibited GS activity in a dose-dependent manner ranging from 27% inhibition at 0.1 nM to 67% at 0.1 microM and 100% at 10 microM Pb. These results indicate that astroglial function is vulnerable to Pb exposure, even at low lead levels.
Sierra
E M
EM
Department of Veterinary Anatomy, Texas A & M University, College Station 77843.
Tiffany-Castiglioni
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D000818:Animals; D001253:Astrocytes / Q000187:drug effects* / Q000201:enzymology; D002452:Cell Count; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D003600:Cytosol / Q000187:drug effects / Q000201:enzymology; D004305:Dose-Response Relationship, Drug; D005974:Glutamate-Ammonia Ligase / Q000187:drug effects* / Q000378:metabolism; D007854:Lead / Q000633:toxicity*; D051381:Rats; D011919:Rats, Inbred Strains; D014343:Trypan Blue
D007854:Lead; D005974:Glutamate-Ammonia Ligase; D014343:Trypan Blue