The midgut of the American silkworm transports potassium actively from blood side to lumen. The active K transport was measured by short-circuiting the gut. The K transport is inhibited by Tl in the lumen solution. A Lineweaver-burk plot showed that the inhibition with Tl is competitive with the active K transport. The Km was found to be 5.7 +/− 0.7 and Ki = 0.1. It was conclude that Tl acts on the K pump which is located close to the lumen of the gut. When Tl was applied to the blood-side, only about half inhibition is found, yet, paradoxically, Tl concentration in the gut tissue was ten times as high as when Tl was applied to the lumen side.

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