Fig. 11. Model of agNAT8 functions superimposed with patterns of its spatial expression and putative membrane localizations, along with specific physiological roles of particular epithelial areas and with a simplified Na⁺ ion recycling scheme. Light blue fill corresponds to epithelial areas with areas of hybridization and qPCR detection of agNAT8 transcripts. Dark blue outlines correspond with theoretically possible membrane localization of agNAT8. The posterior midgut is associated with apical Na⁺-dependent absorption of free amino acids from the midgut lumen into epithelial cells (green arrow). A complementary transporter for emitting amino acids via the basal membrane of the posterior midgut into the hemocoel is expected but presently unknown; also, it may not require an active transport mechanism since the intracellular concentration of free amino acids in this area is high vs the concentration in the hemolymph. In contrast, the cardia, salivary glands and gastric caeca clearly are secretory parts of the alimentary canal, which mediates the synthesis and apical secretion of various amino acids in the form of peptides and protein polymers. A basal accumulative transport mechanism is necessary here (green arrow), which is complemented by an apical secretion process. The disposable pool of sodium ions in the midgut lumen, which is involved in the transport process, is recycled via the area of anterior alkalinization (pH 11), shown as a pink gradient band in the anterior midgut lumen. PR, pharynx; SG, salivary glands; EO, oesophagus; CA, cardia; GC, gastric caeca; AMG, anterior midgut; PMG, posterior midgut; MT, Malpighian tubule; RG, rectal gland.