1. The short-circuited midgut from the larvae of Hyalophora cecropia can actively transport potassium and rubidium from blood side to lumen. In this paper it is demonstrated that caesium also can be actively transported.
2. Transport of potassium does not interfere with the metabolism of the midgut; nor does transport of caesium.
3. Potassium and rubidium compete for the transport mechanism in a one-to-one ratio but competition between potassium and caesium is more complicated. At high caesium:potassium ratios the active transport taking place is transport of caesium when the ratio is 2 there is more transport of potassium than of caseium; when the ratio is i there is no transport of caesium.
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