In a paper on regional specialization in the lower Malpighian tubule of Rhodnius prolixus, Maddrell (1978) reported that active recovery of potassium chloride from the tubular fluid is confined to the lowest region of the tubule, and is inducible by 5-HT. He also reported that the gross influx of potassium, measured by movement of K42 from the bathing solution into the lumen, shows a 200-fold stimulation by 5-HT in this part of the tubule, but not in the non-transporting region. He could not explain the effect.

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