Effects of lowering extracellular and cytosolic pH on calcium fluxes, cytosolic calcium levels, and transmitter release in presynaptic nerve terminals isolated from rat brain.

Abstract

We examined the effects of extracellular and intracellular pH changes on the influx of radioactive 4~Ca, the concentration of ionized Ca (pCa0 as monitored with the Ca-sensitive fluorescent indicator fura-2, and the efflux of dopamine in presynaptic nerve endings (synaptosomes) isolated from rat brain corpora striata and preloaded with [3H]dopamine. Cytosolic pH (pHi) was monitored by loading the synaptosomes with the H+-sensitive fluorescent indicator 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) (see Nachshen, D. A., and P. Drapeau, 1988, Journal of General Physiology, 91:289-303). An abrupt decrease of the pH of the external medium, from 7.4 to 5.5, produced a slow decrease of pHi (over a 5-min period) from an initial value of 7.2 to a steady state level of-5.8. When 20 mM acetate was present in acidic media, pH~ dropped as fast as could be measured (within 2 s) to a level similar to that reached (more slowly) in the absence of acetate. It was therefore possible to lower pH~ over short time periods to different levels depending on whether or not acetate was present upon extracellular acidification. Extracellular acidification to pH 5.5 (in the absence of acetate) had no significant effect on pCai and dopamine release over a 30-s period (pHi = 6.4). Acidification in the presence of acetate lowered pHi to 5.8 without affecting pCa~, but dopamine efflux increased -20-fold. This increase in basal dopamine release was also observed in the absence of extracellular Ca. Thus, intraterminal, but not extracellular, acidification could stimulate the efflux of dopamine in a Caindependent manner. The high Q~0 (5.6) of acid-stimulated dopamine efflux in the presence of nomifensine (which blocks the dopamine carrier) was consistent with an activation of vesicular dopamine release by H § When synaptosomes were both depolarized for 2 s in high-K (77.5 mM) solutions and acidified (in the absence of acetate), there was a parallel block of 45Ca entry and evoked dopamine release (50% block at pH 6.0 with 0.2 mM external Ca). When

Roles of Na + –Ca 2+ exchange and of mitochondria in the regulation of presynaptic Ca 2+ and spontaneous glutamate release

“…We found that a fall in pHi after an ammonium pulse can signi¢cantly increase the sEPSC frequency. This result is in accordance with other studies showing that spontaneous neurotransmitter release is enhanced in synaptosomes during acidi¢cation (Drapeau & Nachshen 1988). Since the increase in sEPSC frequency that we have observed with FCCP and oligomycin was completely prevented when [Ca 2+ ] i changes were bu¡ered with BAPTA, we conclude that the small acidi¢cation was not important for the neurotransmitter release.…”

Regulation of acetylcholine release by intracellular acidification of developing motoneurons in Xenopus cell cultures

“…Increase in SSC frequency depends on the rise of cytoplasmic Ca¥ It has been reported that internal acidification results in Ca¥-independent release of neurotransmitters in rat brain synaptosomes (Drapeau & Nachshen, 1988). The current study demonstrates that cytosolic acidification increases intracellular Ca¥ and spontaneous ACh release.…”

The regulation of cytosolic pH in isolated presynaptic nerve terminals from rat brain.

“…terminals (Landau and Nachshen, 1975;Cohen and Van der Kloot, 1976;Drapeau and Nachshen, 1988). There is little information, however, about how pH~ is regulated in presynaptic nerve terminals.…”