CHANGES IN INTRACELLULAR PH ASSOCIATED WITH GLUTAMATE EXCITOTOXICITY

Excitotoxic neuronal injury is known to be associated with increases i n cytosolic calcium ion concentrations. However, it is not known if pe rturbations in other intracellular ions are also associated with gluta mate (GLU)-induced neuronal death. Accordingly, intracellular hydrogen ion concentrations were measured in cultured hippocampal neurons with the fluorescent dye BCECF during and after toxic exposures. Five minu te GLU applications produced an initial cytosolic acidification. Durin g the hour after GLU removal, intracellular pH (pH(i)) recovered stead ily, resulting in a rebound cytosolic alkalinization. Lowering extrace llular calcium depressed the initial GLU-induced acidification, sugges ting that the rapid acidification may result partly as a consequence o f calcium entry. An acidification-induced rebound alkalinization appea red to be activated by GLU exposure. Inhibitors of intracellular pH re gulation, harmaline, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic aci d (DIDS), and replacement of external Na+ with N-methyl-glucamine+ (NM G+), retarded the rate of recovery from GLU-induced acidification. The rapid acidification and rebound alkalinization could be mimicked by c hallenging neurons with elevated external K+ or replacement of externa l Na+ with NMG+. Two or more hours following toxic GLU exposure, hydro gen ion concentration did not stabilize at initial levels but progress ively increased. High K+ or Na+ removal did not produce this long-term acidification and were not toxic. The cumulative increase in intracel lular hydrogen ion may reflect the declining health of injured neurons and could contribute directly to neuronal death. Therefore, cytosolic acidification may act synergistically with increases in calcium conce ntration in mediating excitotoxicity.