Changes in free-calcium levels and pH in synaptosomes during transmitter release

The free cytoplasmic Ca2+ concentration [( Ca2+]i) in rat brain synaptosomes estimated by the indicator quin 2 is 104 +/- 8 nM (S.D.) in artificial cerebrospinal fluid (1.2 mM Ca2+), but decreases at lower Ca2+ concentrations in the medium. The presence of quin 2 in the synaptosomes does not affect either the spontaneous release of transmitter (gamma-aminobutyric acid) or the release induced by K+ depolarisation. In quin 2-loaded synaptosomes, depolarisation by K+ causes an abrupt increase in [Ca]i (less than 2-fold) that is approximately proportional to the extent of depolarisation, whereas depolarisation by veratrine alkaloids produces a slow rise in [Ca]i. The increase in [Ca]i produced by K+ depolarisation does not occur in the absence of Ca2+ in the medium. The data are consistent with a direct correlation between [Cai] and transmitter release in functional synaptosomes. The pH in synaptosomes estimated by the indicator quene 1 is 7.04 +/- 0.07 and is stable in media containing 5 mM bicarbonate. The pH in synaptosomes was decreased by protoveratrine but not by K+ depolarisation.