Ca-dependence of accumulation of monoamines into synaptosomes and its inhibition by copper

Effects of various concentrations of extracellular Ca on the high affinity uptake of dopamine (DA), noradrenaline (NA) and serotonin (5-HT) as well as the effects of Ca on Cu-induced uptake inhibition were studied by using striatal, cortical and hypothalamic synaptosomes, respectively. The spontaneous release of all three amines from preloaded synaptosomes by Cu increased slightly with increasing external Ca up to 2.5 mM. The sensitivity to Ca of the accumulation of the amines into synaptosomes decreased in the order of DA greater than NA greater than 5-HT. Below 0.3 mM Ca, DA uptake increased due to concurrent decreased release of DA from synaptosomes. The pattern was similar but less evident for NA while 5-HT uptake and its spontaneous release did not change upon buffer Ca at all. The uptake inhibition of none of the amines depended on extracellular Ca. Zn reversed the Cu-induced inhibition of 5-HT uptake but increased additively the inhibition of DA uptake and that of NA uptake in vitro. The potency of Cu to inhibit uptake decreased when the protein concentration in the incubation medium was increased. Cysteine, added concurrently with Cu, did not protect uptake from the toxic influence of Cu. The results so far suggest that the primary mechanism in the Cu-induced inhibition of uptake is blocking the membrane uptake carrier. The release of an accumulated amine from synaptosomes may rather be secondary and result from the inhibition of the reuptake.