Inhibition of glutamate uptake by unconjugated bilirubin in cultured cortical rat astrocytes: Role of concentration and pH

The molecular basis of bilirubin toxicity to nerve cell function is still u nclear. Since astrocytes are the main transporters of synaptically released glutamate and impaired glutamate uptake results in neuronal death, we inve stigated the effect of unconjugated bilirubin (UCB) on [H-3]glutamate uptak e in cultured rat astrocytes and the role of bilirubin ionization on toxici ty. Astrocytes were incubated for 5-15 min, with UCB concentrations from 17 to 342 mu M and UCB/albumin molar ratios of 0.2-3.0, at pH 7.0, 7.4, and 8 .0. Exposure of astrocytes for 15 min to 85.5 mu M UCB and 28.5 mu M albumi n resulted in a 63.1% decrease of glutamate uptake (p < 0.01). Interestingl y, the effect demonstrated to be correlated with the UCB/albumin molar rati o (r = -0.986, p < 0.01) and a significant decrease was observed for a UCB/ albumin molar ratio as; low as 0.8. Inhibition of glutamate transport was a lso pH-dependent as it occurred at 7.4 (p < 0.05) and 8.0 (p < 0.01), but n ot at 7.0, suggesting that the monoanionic species of UCB accounted for the inhibition. These findings indicate that UCB, and more precisely the monoa nionic species, impairs a crucial function of astrocytes such as glutamate transport and support a potential role of astrocyte function in the pathoge nesis of UCB-related brain damage (kernicterus). (C) 1999 Academic Press.