An investigation into the role of calcium in the modulation of rat synaptosomal d-[ 3H]aspartate transport by docosahexaenoic acid

The effect of the polyunsaturated fatty acid cis-4,7,10,13,16,19-docosahexaenoic acid (DHA) on the high-affinity, sodium-dependent uptake of d-[ 3H]aspartate into purified rat brain synaptosomes was examined. Incubation of the synaptosomes with 20 μM DHA caused over 50% inhibition of the maximum velocity ( V max) of d-[ 3H]aspartate transport. This inhibition was significantly potentiated by pre-exposure of the synaptosomes to the fatty acid for 10 min prior to the start of the transport assay. Less highly unsaturated fatty acids such as arachidonic acid ( cis-5,8,11,14-eicosatetraenoic acid), linolenic acid ( cis-9,12,15-octadecatrienoic acid) and oleic acid ( cis-9-octadecenoic acid) were significantly less potent than DHA. Removal of extracellular calcium, or reduction of the intracellular calcium concentration using the intracellular calcium chelator BAPTA/AM (10 μM), did not reduce the inhibition caused by DHA. On the other hand, an increase in the concentration of intracellular calcium mediated by thapsigargin (25 μM) or the calcium ionophore A23187 (10 or 100 nM) led to a reduction in the rate of d-[ 3H]aspartate transport in the absence of DHA. The CaM kinase II inhibitor, KN-93, reduced d-[ 3H]aspartate uptake independently of whether DHA was also present, but had no effect on the inhibition of d-[ 3H]aspartate uptake by either A23187 or thapsigargin. We conclude that whereas DHA inhibits synaptosomal d-[ 3H]aspartate uptake in a calcium-independent manner, a calcium-based mechanism exists that can also modulate glutamate transporter activity.