Mechanism of Inhibition of Active Potassium Transport in Isolated Midgut of Manduca Sexta by Bacillus Thuringiensis Endotoxin

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Abstract
After incubation at pH 10 or higher, Bacillus thuringiensis spores and endotoxin, at concentrations above 0·1 lU/ml, affected transport parameters in the isolated midgut of Manduca sexta larvae. (Toxic activity was lost during roughly 1 week at pH 11.) About 60% of the short-circuit current was inhibited, and the remainder was reversibly inhibited by anoxia. Electrical resistance was reduced by about 55% and oxygen uptake stimulated by about 30%. Influx of potassium from blood-side to lumen-side (‘active’ flux) was unaffected but flux in the reverse direction was nearly tripled. These results suggest that hydrolysis of the toxin yields an inhibitor of potassium transport, presumably a polypeptide. It is argued that inhibition is not primarily by uncoupling of oxidative phosporylation, but instead by interference with an active depression of the efflux of potassium from lumen-side to blood-side.