Effects of ammonia in vitro on endogenous taurine efflux and cell volume in rat cerebrocortical minislices: Influence of inhibitors of volume-sensitive amino acid transport

Rat cerebrocortical minislices were incubated with physiological saline in the absence or presence of 5 mM ammonium acetate ("ammonia") and/or inhibit ors of osmosensitive amino acid transport: 50 mu M niflumic acid and 100 mu M N-ethyl-maleimide for 60 min, with medium changes after 20 min and 40 mi n. The efflux of endogenous taurine, glutamate and glutamine was assayed by high-performance liquid chromatography, and steady-state cell volumes were monitored in the slices with the [C-14]inulin method. In the absence of am monia, niflumic acid abolished taurine efflux bur did not affect glutamate or glutamine efflux at all time-points, and increased cell volume at 20 min and 60 min. N-Ethyl-maleimide increased taurine, glutamine and glutamate e fflux at 20 min and 40 min, inhibited taurine and glutamine efflux at 60 mi n, and increased cell volume at 20 min. Ammonia strongly Stimulated taurine (by 380% at 20 min), and only moderately glutamate (30% at 20 min) or glut amine efflux (76% at 20 min). Ammonia increased cell volume above the contr ol level at all time-points. Niflumic acid inhibited, but did not abolish a mmonia-dependent taurine and glutamine efflux. and did not change glutamate efflux. The effects of ammonia + niflumic acid on cell volume did not diff er from the effects of each compound separately. N-Ethyl-maleimide inhibite d ammonia-dependent efflux of all three amino acids except for stimulation of glutamate efflux at 20 min. N-Ethyl-maleimide + ammonia decreased the ce ll volumes more than did each compound separately. It is concluded that although ammonia-induced taurine efflux is accompanied by an increase in cell volume, the underlying mechanism is not simply a ce ll volume regulatory response normally observed in hypoosmotic stress. Incr eased efflux of taurine, which is an inhibitory amino acid and a cell membr ane protectant, may serve to counteract the deleterious effects of increase d excitatory transmission accompanying acute hyperammonemic insult. (C) 199 9 IBRO, Published by Elsevier Science Ltd.