Mechanism of inhibition of active potassium transport in isolated midgut of Manduca sexta by Bacillus thuringiensis endotoxin.

After incubation at pH 10 or higher, Bacillus thuringiensis spores and endotoxin, at concentrations above 0.1 IU/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 phosphorylation, but instead by interference with an active depression of the efflux of potassium from lumen-side to blood-side.