RECOVERY FROM NMDA-INDUCED INTRACELLULAR ACIDIFICATION IS DELAYED ANDDEPENDENT ON EXTRACELLULAR BICARBONATE

A 30-s exposure to N-methyl-D-aspartate (NMDA) produced a dose-depende nt and long-lasting (10-20 min) reduction in intracellular pH in cultu red cortical neurons, detected by the fluorescent dye 2',7'-bis(carbox yethyl)-5(6)-carboxyfluorescein. This intracellular acidification coul d be blocked by addition of the NMDA antagonist, D-(-)-2-amino-5-phosp honovalerate, or by removal of extracellular Ca2+. Removal of extracel lular HCO3- markedly impaired recovery front NMDA-induced intracellula r acidification. Recovery was also impaired when 4,4'-diisothiocyanost ilbene-2,2'-disulfo acid or -acetamido-4'-isothiocyanostilbene-2,2'-di sulfonic acid, inhibitors of HCO3- transport, were added to the cultur es immediately after NMDA exposure. In contrast, the Na+/H+ exchange b locker, 5-(N-ethyl-N-isopropyl)amiloride, did not affect pH recovery. Removal of extracellular Cl- partially prevented pH recovery after NMD A stimulation. In addition, extracellular HCO3- increased intracellula r Na+ after NMDA exposure, consistent with HCO3- activation of a Na+-d ependent exchanger. These results demonstrate that stimulation of cort ical neuronal NMDA receptors is followed by long-lasting intracellular acidification and that the presence of extracellular HCO3- is importa nt in the subsequent recovery of normal intracellular pH, likely actin g at least in part via the Na+-dependent Cl-/HCO3- exchanger.