Dependence of Prolonged Water Absorption by in Vitro Locust Rectum on Ion Transport

1. An in vitro preparation of the locust rectum, consisting of an everted cannulated sac, is described. After the first hour, the rates of ion and water transport, the electropotential difference and tissue volume do not change significantly over the next 4 h, i.e. a steady-state situation is approximated. 2. The activities and properties of these rectal sacs are reasonably similar to those in vivo; they produce hyposmotic absorbate, exhibit rectification of osmotic flow, and absorb water from pure sucrose solution for 1 h. Inhibition of absorption by KCN + IAA and also ouabain indicates metabolic dependence. 3. Analysis of absorbate shows that tissue ions are secreted to the haemocoel side when water is absorbed over the first hour from pure sucrose solutions. Prolonged transport of water (i.e. from second to fifth hour) requires the presence of ions on the lumen but not the haemocoel side. This is the first direct evidence that water absorption in the insect rectum is coupled to ion transport as previously postulated. The location of the proposed ion recycling suggested by these observations is discussed.