ACTIVE ACCUMULATION OF GLUTAMATE AND ASPARTATE BY INSECT EPIDERMAL-CELLS

Similar and high concentrations of most amino acids are present in the blood plasma and tissues of insects. Plasma levels of the dicarboxyla ted amino acids glutamate and aspartate, however, are much lower than tissue levels. Plasma concentrations may be regulated by tissues such as the epidermis which is able to accumulate and retain glutamate and aspartate against strong concentration gradients. When larval epidermi s from the mealworm Tenebrio molitor was exposed to 250 muM concentrat ions of these two amino acids in saline, tissue concentrations climbed within 3 h from 7.3 to 45 mM for glutamate and from 0.9 to 31 mM for aspartate. At this concentration the uptake rates for glutamate and as partate were estimated to be 5.5 and 4.7 pmol amino acid/mm2 tissue/mi n. Tissue swelling occurs during amino acid accumulation, implying tha t the epidermis is osmotically stressed by the treatment. Other protei n amino acids tested at micromolar concentrations were not concentrate d by the epidermis. Glutamate and aspartate taken up by the epidermis were converted into other amino acids, the dominant transamination pro ducts being aspartate on tissue exposure to glutamate, and glutamate a nd alanine on exposure to aspartate. The synthesis of small amounts of beta-alanine, but not of glutamine, asparagine or proline, was also d etected. Glutamate is an important neurotransmitter, and aspartate a p otent glutamate agonist, at the neuromuscular synapse in insects. The primary role of the transport system for glutamate and aspartate may b e to keep their plasma levels below the synaptic threshold, rather tha n to supply the epidermis with its metabolic requirements for these am ino acids.