Secretion of Hypo-osmotic Fluid by the Lower Malpighian Tubules of Rhodnius Prolixus

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Abstract
The ability of Rhodnius to excrete an hypo-osmotic fluid at high rates can be attributed to the activity of the lower lengths of the Malpighian tubules. In the presence of a stimulant (such as 10−5 M 5-HT) they can absorb KCl from fluid in the lumen at rates as high as 0.5 μmol.min−1 cm−2. The solution absorbed is strongly hyperosmotic (900 mM KCl) so that the fluid in the lumen becomes hypo-osmotic (as low as 75 mOsm) and its flow is not much slowed. The rate of absorption of KCl from the lumen depends on the potassium content of fluid there; the highest rates are achieved when K-rich fluid is run through the tubule at high speed from a cannula. Rubidium ions are partly effective substitutes for potassium ions in the system, but sodium ions are not absorbed at measurable rates. The wall of the lower tubule has a much lower permeability to p-aminohippuric acid, xylose, sucrose and inulin than has the upper tubule -- as expected in a system able to maintain a large osmotic concentration difference.