The uptake and release of glutamate at the crayfish neuromuscular junction

The abdominal slow flexor muscle of the crayfish (Cambarus clarkii) was pre-incubated in deuterium-labelled glutamate (glutamate-d5) solution, and the release of glutamate, glutamate-d5 and aspartate in the bath solution was measured by mass fragmentography. The glutamate-d5 was taken up into the preparation and released together with endogenous glutamate. The resting release of glutamate-d5 was 8.2 +/- 0.96 pmol/10 min after the incubation with 0.5 mM-glutamate-d5, and those of endogenous glutamate and aspartate were 7.4 +/- 1.19 and 2.8 +/- 0.81 pmol/10 min, respectively. The release of glutamate-d5 was not significantly increased by nerve stimulation, while that of endogenous glutamate was increased by about 9.7 pmol/10 min above the resting release. The application of high-K solution induced a net increase of 1.9 pmol/10 min in glutamate-d5 release, 6.1 pmol/10 min in endogenous glutamate release and 7.9 pmol/10 min in aspartate release. The relative amount of glutamate-d5 accumulated into the preparation during pre-incubation with 0.5 mM-glutamate-d5 was a few per cent of the endogenous glutamate in the preparation. It is concluded that the exogenous glutamate is taken up mainly into a compartment which differs from that related to nerve evoked transmitter release and that high-K solution has effects on the amino acid release which differ from those of nerve stimulation.