The role of glial cells in regulation of neurotransmitter amino acids in the external environment. II. Mechanism of aspartate transport.

Active, high-affinity (Km = 4.4 microM) D-aspartate transport in C6 astrocytoma cells has been investigated. Uptake of radioactive D-aspartate was competitively inhibited by L-aspartate (Ki = 8.5 microM) and L-glutamate (Ki = 0.95 mM) and was essentially independent of pH between 6.2 and 7.8. The rate of uptake of labeled D-aspartate and its maximum accumulation ratio, [Asp]i/[Asp]e increased as the second power of the transmembrane electrical potential (measured by the potassium concentration gradient, [K+]i/[K+]e) which indicates that aspartate is transported with a net charge of +2. Aspartate transport rate and gradient also increased as the second power of the sodium concentration gradient, [Na+]e/[Na+]i, indicating that two Na+ are transported inward with each aspartate. The maximum gradient measured from total intra- and extracellular concentrations of aspartate showed the same dependence on electrical potential and sodium concentration gradient as that determined from the distribution of [3H]D-aspartate. This indicates that energy for aspartate uptake is provided by a combination of transmembrane electrical potential and sodium concentration gradient. At physiological [Na+]e (140 mM) and [K+]e (3.5-5 mM) the energy available for aspartate uptake substantially exceeded the maximum aspartate gradient. It is suggested that aspartate uptake by C6 cells is kinetically prevented from attaining high concentration differences and that the excess of driving forces over accumulation ratio ensures that glial high-affinity transport systems for amino acid neurotransmitters function in vivo predominantly in the direction of net uptake.