Aspartate transport in synaptosomes from rat brain.

Active transport of aspartate by neuronal synapses has been studied using preparations of synaptosomes from rat forebrain. The maximal accumulation of labeled D-aspartate [( Asp]i/[Asp]o) was observed to increase as the second power of the transmembrane electrical potential (measured by the potassium ion concentration gradient [( K+]i/[K+]o], which indicates that aspartate is transported with a net charge of +2. The aspartate gradient also increased as the second power of the sodium ion gradient, indicating that two Na+ are transported inward with each aspartate. Measurement of total intra- and extrasynaptosomal aspartate (L-aspartate) and the specific activities of each after adding labeled D-aspartate shows the presence of two intrasynaptosomal pools, a rapidly exchanging pool in which the L-aspartate is dependent on [Na+] and [K+] in the same manner as described for D-aspartate and a slowly exchanging pool essentially insensitive to [Na+] and [K+]. Maximal activation of uptake with 50 microM aspartate causes an alkalinization of the external medium which suggests that H+ is taken up with the aspartate (or OH- is ejected) to neutralize the negative charge of aspartate at physiological pH values. The data are consistent with the transport process being: Asp- + H+ + 2Na+ o = Asp- + H+ + 2Nai+. These stoichiometries can explain the energetics of both in vitro and in vivo uptake of aspartate by the high affinity, [Na+]- and [K+]-dependent system.