Absorption of Fluid by the Midgut of Rhodnius

Isolated midguts of 5th-instar Rhodnius prolixus will transport fluid from the lumen that is close to iso-osmotic with the luminal contents.

The transported fluid contains sodium and chloride ions as its major constituents.

Fluid transport can be attributed to active transport of sodium ions from the lumen. The haemolymph side of the epithelium, towards which transport is directed, is at a potential positive with respect to the lumen; this potential difference is greatly increased if the lumen contains only impermeant anions, and the rate of fluid transport is strongly dependent on the concentration of sodium ions in the luminal fluid.

The rate of fluid transport is increased approximately six times by treatment with 5-hydroxytryptamine (2 × 10⁻⁷ M) or cyclic AMP (2 × 10⁻³ M). The transepithelial potential is increased by such treatment but the major effects are on the short-circuit current, which increases by about five times, and on the electrical resistance of the epithelium, which falls to about a quarter of its earlier value.