Secretion by the Malpighian Tubules of Rhodnius. The Movements of Ions and Water

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Abstract
1. The Malpighian tubules of Rhodnius will secrete at a normal rate in solutions containing no potassium ions and the rate is unaffected by changes in the potassium concentration, when the balance of cationic concentration is made up by sodium ions. 2. In the absence of sodium ions the rate of secretion is much reduced. The addition of very small amounts of sodium-containing solution brings about an abrupt recovery in the rate and thereafter the rate is unaffected by further increases in the sodium concentration. 3. In solutions containing either sodium or potassium with choline making up the balance of monovalent cations, the rate of fluid secretion depends linearly on the concentration of either sodium or potassium. 4. The tubules will concentrate either sodium or potassium when they are present at low concentration in the bathing fluid, even in the face of a very much larger concentration of the other cation. This suggests that there are separate mechanisms for the handling of these two ions. 5. Secretion can be supported by a solution containing ammonium ions in place of sodium and potassium. The tubules behave, at least in the short term, as if they were unable to distinguish ammonium from potassium. 6. Chloride ions appear to play a special role in that only bromide ions and, to a limited extent, nitrate ions will substitute for them. The rate of secretion depends linearly on the chloride concentration. 7. The tubules secrete a fluid which is practically iso-osmotic to the bathing fluid. The rate of secretion depends inversely on the osmolarity of the bathing fluid. The rate of movement of solute is little affected by these changes of osmolarity. It appears that water movements follow, and are closely linked with, solute movements. 8. Copper, cyanide, iodoacetate and azide ions and 2,4-dinitrophenol all stop secretion when added to the bathing medium. Ouabain, acetazolamide and mammalian ADH all have no effect on the rate of secretion and ouabain has no effect on the composition of the secreted fluid. 5-Hydroxytryptamine (serotonin) will stimulate a rapid flow of secretion. 9. Apparent neurosecretory axons have been found which supply the tubules. They do not contain enough diuretic activity to do more than play a minor role in diuresis. They may contribute to the rapid onset of diuresis or may affect the Malpighian tubules in some other way. 10. The evidence suggests that the tubules function by secreting chloride, potassium and sodium ions into the lumen (it is speculated that this may conceivably involve the active transport of all three ions) and that water movements closely follow these ion movements so that a rapid flow of iso-osmotic fluid is achieved.