1. 1.

    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.

  2. 2.

    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.

  3. 3.

    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.

  4. 4.

    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.

  5. 5.

    Rubidium ions are partly effective substitutes for potassium ions in the system, but sodium ions are not absorbed at measurable rates.

  6. 6.

    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.

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