Martin, J.E.: Advances Through the Insect Hindgut

How are insects able to maintain internal osmotic and ionic balance in the face of environmental challenges? Since early studies by Phillips which established desert locusts actively conserve water and produce a hyperosmotic excreta, the question of how homeostasis is achieved has been the central focus of his work. Utilizing isolated preparations of hindgut segments, John�s laboratory has developed a detailed model of hindgut function and regulation. The anterior segment (ileum)reabsorbs the bulk of entering fluid and delivers an isosmotic fluid to the rectum. Local osmosis and ion recycling allow the rectum to elaborate a hyposmotic absorbate and produce strongly hyperosmotic excreta. 90% of water is recycled due in part to lateral membrane/mitochondrial complexes present in the rectum. Metabolic support of transport is provided by luminal amino acids. Numerous solute transporters have been characterized and localized in the hindgut and there is evidence that many are hormonally regulated. Various tissue homogenates stimulate ion transport and are antidiuretic factors. One factor, ion transport peptide (ITP) has been purified from locust corpora cardiaca and a partial amino acid sequence obtained. A partial cDNA corresponding to this sequence was amplified and extended by polymerase chain reaction to give a complete cDNA which predicted a 72 amino acid peptide with 3 disufide bridges and an amidated C terminus. Synthesis of the predicted peptide resulted in a peptide whose biological activity was identical to that of ITP purified from CC. Expression of ITP and mutant ITP peptides in a Drosphila cell line indicate residues important for binding and signal transduction. There is now evidence for peptides structurally related to ITP in Coleoptera, Lepidoptera, and Diptera as well as Orthoptera and it will be very exciting to determine their functions in whole animals.