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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.
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