Effect of acute exposure to ammonia on glutamate transport in glial cells isolated from the salamander retina

A rise of brain ammonia level, as occurs in liver failure, initially increa ses glutamate accumulation in neurons and glial cells. We investigated the effect of acute exposure to ammonia on glutamate transporter currents in wh ole cell clamped glial cells from the salamander retina. Ammonia potentiate d the current evoked by a saturating concentration of L-glutamate, and decr eased the apparent affinity of the transporter for glutamate. The potentiat ion had a Michaelis-Menten dependence on ammonia concentration, with a K-m of 1.4 mM and a maximum potentiation of 31%. Ammonia also potentiated the t ransporter current produced by D-aspartate. Potentiation of the glutamate t ransport current was seen even with glutamine synthetase inhibited, so ammo nia does not act by speeding glutamine synthesis, contrary to a suggestion in the literature. The potentiation was unchanged in the absence of Cl- ion s, showing that it is not an effect on the anion current gated by the gluta mate transporter. Ammonium ions were unable to substitute for Na+ in drivin g glutamate transport. Although they can partially substitute for K+ at the cation counter-transport site of the transporter, their occupancy of these sites would produce a potentiation of <1%. Ammonium, and the weak bases me thylamine and trimethylamine, increased the intracellular pH by similar amo unts, and intracellular alkalinization is known to increase glutamate uptak e. Methylamine and trimethylamine potentiated the uptake current by the amo unt expected from the known pH dependence of uptake, but ammonia gave a pot entiation that was larger than could be explained by the pH change, and som e potentiation of uptake by ammonia was still seen when the internal pH was 8.8, at which pH further alkalinization does not increase uptake. These da ta suggest that ammonia speeds glutamate uptake both by increasing cytoplas mic pH and by a separate effect on the glutamate transporter. Approximately two-thirds of the speeding is due to the pH change.