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. In the presence of a stimulant (such as 10−8 M 5-HT) they can absorb KC1 from fluid in the lumen at rates as high as 0·5 /μ mol.min−1 cm−2.

  3. The solution absorbed is strongly hyperosmotic (900 mM KC1) so that the fluid in the lumen becomes hypo-osmotic (as low as 75 mOsm) and its flow is not much slowed.

  4. The rate of absorption of KC1 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. Rubidium ions are partly effective substitutes for potassium ions in the system, but sodium ions are not absorbed at measurable rates.

  6. The wall of the lower tubule has a much lower permeability top-amino-hippuric acid, xylose, sucrose and inulin than has the upper tubule - as expected in a system able to maintain a large osmotic concentration difference.

*

This fluid contained 138 mM KCl; 8.5 mM MgCl2; 2 mM CaCl2; 10.2 mM NaHCO2: 4.3 mM NaH2PO4 and 34 mM glucose.

© 1975 by Company of Biologists
1975
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