A pharmacological study of signal transduction mechanisms controlling fluid reabsorption and Ion transport in the locust rectum

Like most insects, locusts face severe regulatory challenges associated with arid habitats, high metabolic rates and high surface area to volume ratios. Therefore, the conservation of water, essential ions and metabolites is very important. Consequently, locusts regulate their hemolymph composition primarily by controlling epithelial transport in the excretory system. The locust excretory system is typical of many insects and consists of the Malpighian tubules and the hindgut. The Malpighian tubules secrete a primary isosmotic urine rich in KC1 and low in Na+ that contains most small hemolymph solutes, waste products and toxic plant chemicals. The hindgut (ileum and rectum) is responsible for the enormous changes in composition of the urine, by the selective reabsorption of water, ions and metabolites. Both the Malpighian tubules and the hindgut are under endocrine control. The purpose of this study was to investigate the involvement of second messengers in the control of fluid reabsorption (iv) and Cl- transport in the rectum of the desert locust, Schistocerca gregaria. Various agents known to block or activate specific signal transduction pathways were added to everted rectal sacs and short-circuited rectal flat-sheet bioassays. Cyclic AMP and its analogs were shown to stimulate rectal Jv and Cl-transport to the same extent as aqueous extracts of the nervous lobes of the corporacardiaca, suggesting that activation of the adenylate cyclase pathway is sufficient for maximal stimulation. It also appears that cGMP is involved, since its addition partially stimulated both Cl- and fluid reabsorption. External Ca2+ was not required for the maintenance or stimulation of rectal transport. However, intracellular Ca2+ was shown to influence the control of rectal transport. The role of intracellular Ca2+ appears to be quite complex and may vary with its relative concentration. Finally, it was found that Protein kinase C and the phosphotidylinositol cycle do not appear to be involved in the stimulation of rectal Cl- and fluid transport.Science, Faculty ofZoology, Department ofGraduat