Active transport of sodium by the isolated midgut of Hyalophora cecropia.

Potassium is actively transported from blood-side to lumen of the isolated silkworm midgut (Harvey & Nedergaard, 1964). Rubidium is also actively transported and, when present in equal amounts, potassium and rubidium compete on an almost equal basis (Nedergaard & Harvey, 1968). Recently, Zerahn (1970) has found that caesium as well is actively transported, but the competition with potassium is unusual. We have now found that sodium and lithium are also actively transported by the midgut when potassium is not available. The competition between sodium and potassium is unusual and the active sodium transport is not inhibited by ouabain.