Glutamate transport in large muscle fibres of Balanus nubilus

The uptake of glutamate and other acidic amino acids into barnacle single muscle fibres has been characterized. The uptake of glutamate consists of two components, one Na-independent and one Na-dependent. The Na-dependent uptake is saturable (half-maximal at 250 microM external glutamate) and is inhibited by a variety of analogues of which L-cysteate and D- and L-aspartate are the most potent. These amino acids are also transported into the muscle in a Na-dependent manner. The excitatory agonists kainate, quisqualate, and N-methyl-D-aspartate do not inhibit or affect uptake in any way. Progressive replacement of external Na by choline reduces uptake with very little effect on the apparent affinity for glutamate, suggesting that Na and glutamate bind to the transporter independently. The kinetics of activation are consistent with a requirement for at least two Na ions. Na activation of glutamate uptake can be inhibited by guanidinium with kinetics that are consistent with competitive inhibition at the Na binding site. Studies on the efflux of L-glutamate and other analogues have shown that efflux rates are only slightly increased by the removal of Na and do not seem to be affected in any clear manner by external levels of acidic amino acids.