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ABSTRACT: The possible involvement of blood-brain barrier (BBB) breakdown in the pathogenesis of thiamine deficiency encephalopathy was investigated in RBE4 cells, an immortalized rat brain endothelial cell line. The effects of thiamine deficiency produced by addition of pyrithiamine and by reduction of thiamine in the culture medium, on the metabolism and permeability of the RBE4 monolayer was examined. Pyrithiamine treatment in low thiamine medium (M199) for 7 days caused cytotoxic effects on RBE4 cells at all concentrations (10-50 microg/ml). Pyrithiamine caused a concentration- and time-dependent decrease in MTT reduction and a significant increase in glucose consumption and lactate production compared to controls. Pyrithiamine treatment for 3 days caused a significant decrease in MTT reduction at 50 microg/ml only. In contrast, increased glucose consumption and lactate production by the RBE4 cells was observed after treatment for 3 days with concentrations of 25 microg/ml pyrithiamine and above. The permeability of RBE4 cell monolayers to [14C]sucrose (Mw 342), but not FITC-dextran (Mw 4000) was significantly increased by treatment with pyrithiamine concentrations of 25 microg/ml and above for 3 days. These effects were not accompanied by detectable changes in F-actin distribution or content, although F-actin content was significantly reduced by 7 days exposure to pyrithiamine. These results suggest that metabolic and permeability changes in thiamine-deficient RBE4 cells may be important early events in thiamine-deficiency encephalopathy. The relative role of the BBB in the pathogenesis of thiamine deficiency is discussed.