Fig. 1. Ca²⁺-dependent ³H-Gln transport in hippocampal neurons is stimulated by brief high K⁺-induced depolarization.

A relatively low concentration of ³H-Gln (20 µM) in the uptake medium is used to isolate a high affinity K⁺-stimulated Gln transport system in cultured hippocampal neurons. Abundant uptake of ³H-Gln by neurons is observed following 5 min incubation in normal Krebs buffer without Ca²⁺ (N buffer) but ³H-Gln transport is not significantly affected by inclusion of MeAIB (5 mM). The uptake of ³H-Gln increases ~25% by brief exposure (5 min) to high K⁺-depolarization (plus Ca²⁺) of axon terminals (K), which is blocked by inclusion of MeAIB (5 mM) in the uptake buffer (K-M). The MeAIB-sensitive portion of ³H-Gln transport increases ~8X fold following K⁺-depolarization (M sen, +Ca²⁺). The transport of ³H-Gln is conducted in N buffer at DIV 14. Uptake values obtained at 4°C were less than 5% and were subtracted. N, normal Krebs buffer without Ca²⁺ and with EGTA (1 mM). N-M, Same buffer with MeAIB (5 mM). K, Krebs buffer containing Ca²⁺ (1.2 mM) and with Na⁺ replaced with high K⁺ (60 mM, 5 min). K-M, Same K⁺-buffer with MeAIB (5 mM). M-sen, MeAIB-sensitive transport with same buffers. Data are the mean +/− S.E. values for n=3 independent experiments. F_5,18 = 59.4, p < 0.01 (main effect). *p = 0.04 (K vs K-M), ** p = 0.003 (M_sen+Ca vs M_sen-Ca), N.S. not significant.